diff --git a/implementations/Crafter.Debug.Core-Codec.cpp b/implementations/Crafter.Debug.Core-Codec.cpp
new file mode 100644
index 0000000..c29759f
--- /dev/null
+++ b/implementations/Crafter.Debug.Core-Codec.cpp
@@ -0,0 +1,118 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+module Crafter.Debug.Core:Codec_impl;
+import std;
+import :Value;
+import :Codec;
+
+using namespace Crafter::Debug;
+
+namespace {
+    // "a string" or "an integer", for a kind as errors name it.
+    std::string WithArticle(ValueKind kind) {
+        const std::string_view name = KindName(kind);
+        if (kind == ValueKind::Null) return std::string(name);
+        const bool vowel = name.starts_with('a') || name.starts_with('i') || name.starts_with('o');
+        return std::format("{} {}", vowel ? "an" : "a", name);
+    }
+
+    std::size_t Distance(std::string_view a, std::string_view b) {
+        std::vector<std::size_t> previous(b.size() + 1);
+        std::vector<std::size_t> current(b.size() + 1);
+        for (std::size_t j = 0; j <= b.size(); ++j) previous[j] = j;
+        for (std::size_t i = 1; i <= a.size(); ++i) {
+            current[0] = i;
+            for (std::size_t j = 1; j <= b.size(); ++j) {
+                const std::size_t substitution = previous[j - 1] + (a[i - 1] == b[j - 1] ? 0 : 1);
+                current[j] = std::min({ previous[j] + 1, current[j - 1] + 1, substitution });
+            }
+            std::swap(previous, current);
+        }
+        return previous[b.size()];
+    }
+
+    std::string Join(std::span<const std::string_view> names) {
+        std::string joined;
+        for (const std::string_view name : names) {
+            if (!joined.empty()) joined += ", ";
+            joined += name;
+        }
+        return joined;
+    }
+}
+
+void DecodeContext::Fail(std::string message) {
+    issues.push_back({ Path(), std::move(message) });
+}
+
+std::string DecodeContext::Report() const {
+    std::string report;
+    for (const DecodeIssue& issue : issues) {
+        if (!report.empty()) report += "; ";
+        report += issue.path.empty() ? issue.message : std::format("{}: {}", issue.path, issue.message);
+    }
+    return report;
+}
+
+DecodeContext::Scope::~Scope() {
+    context.segments.pop_back();
+}
+
+DecodeContext::Scope DecodeContext::Field(std::string_view name) {
+    segments.emplace_back(name);
+    return Scope(*this);
+}
+
+DecodeContext::Scope DecodeContext::Index(std::size_t index) {
+    segments.push_back(std::format("[{}]", index));
+    return Scope(*this);
+}
+
+std::string DecodeContext::Path() const {
+    std::string path;
+    for (const std::string& segment : segments) {
+        if (!path.empty() && !segment.starts_with('[')) path += '.';
+        path += segment;
+    }
+    return path;
+}
+
+std::optional<std::string_view> Crafter::Debug::Suggest(std::string_view word, std::span<const std::string_view> candidates) {
+    const std::size_t allowed = word.size() <= 4 ? 1 : 2;
+    std::optional<std::string_view> best;
+    std::size_t bestDistance = allowed + 1;
+    for (const std::string_view candidate : candidates) {
+        const std::size_t distance = Distance(word, candidate);
+        if (distance < bestDistance) {
+            best = candidate;
+            bestDistance = distance;
+        }
+    }
+    return best;
+}
+
+std::string Crafter::Debug::Detail::Expected(std::string_view wanted, const Value& got) {
+    return std::format("expected {}, got {}", wanted, WithArticle(got.Kind()));
+}
+
+Value Crafter::Debug::Detail::Type(std::string_view type) {
+    Value description = Value::MakeObject();
+    description.Set("type", Value(type));
+    return description;
+}
+
+std::string Crafter::Debug::Detail::UnknownField(std::string_view key, std::span<const std::string_view> fields) {
+    if (fields.empty()) return "unknown field; this takes no arguments";
+    if (const auto suggestion = Suggest(key, fields)) {
+        return std::format("unknown field; did you mean '{}'? (fields: {})", *suggestion, Join(fields));
+    }
+    return std::format("unknown field (fields: {})", Join(fields));
+}
+
+std::string Crafter::Debug::Detail::UnknownName(std::string_view name, std::span<const std::string_view> names) {
+    if (const auto suggestion = Suggest(name, names)) {
+        return std::format("'{}' is not a name here; did you mean '{}'? (one of: {})", name, *suggestion, Join(names));
+    }
+    return std::format("'{}' is not a name here (one of: {})", name, Join(names));
+}
diff --git a/implementations/Crafter.Debug.Core-Value.cpp b/implementations/Crafter.Debug.Core-Value.cpp
new file mode 100644
index 0000000..83fb74e
--- /dev/null
+++ b/implementations/Crafter.Debug.Core-Value.cpp
@@ -0,0 +1,507 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+module Crafter.Debug.Core:Value_impl;
+import std;
+import :Value;
+
+using namespace Crafter::Debug;
+
+namespace {
+    // Nesting deeper than this is refused rather than recursed into: the
+    // parser is recursive, and its text comes from another process.
+    constexpr std::uint32_t maxDepth = 256;
+
+    bool IsDigit(char c) {
+        return c >= '0' && c <= '9';
+    }
+
+    void AppendUtf8(std::string& out, std::uint32_t code) {
+        if (code < 0x80) {
+            out += static_cast<char>(code);
+        } else if (code < 0x800) {
+            out += static_cast<char>(0xC0 | (code >> 6));
+            out += static_cast<char>(0x80 | (code & 0x3F));
+        } else if (code < 0x10000) {
+            out += static_cast<char>(0xE0 | (code >> 12));
+            out += static_cast<char>(0x80 | ((code >> 6) & 0x3F));
+            out += static_cast<char>(0x80 | (code & 0x3F));
+        } else {
+            out += static_cast<char>(0xF0 | (code >> 18));
+            out += static_cast<char>(0x80 | ((code >> 12) & 0x3F));
+            out += static_cast<char>(0x80 | ((code >> 6) & 0x3F));
+            out += static_cast<char>(0x80 | (code & 0x3F));
+        }
+    }
+
+    // The length of the valid UTF-8 sequence that starts at text[at], or 0
+    // when it is not one: a stray continuation byte, a truncated or overlong
+    // sequence, a surrogate, or a code point above U+10FFFF.
+    std::size_t Utf8SequenceLength(std::string_view text, std::size_t at) {
+        const auto byte = [&](std::size_t i) { return static_cast<std::uint8_t>(text[at + i]); };
+        const std::uint8_t lead = byte(0);
+        std::size_t length = 0;
+        std::uint8_t low = 0x80;
+        std::uint8_t high = 0xBF;
+        if (lead >= 0xC2 && lead <= 0xDF) {
+            length = 2;
+        } else if (lead >= 0xE0 && lead <= 0xEF) {
+            length = 3;
+            if (lead == 0xE0) low = 0xA0;
+            if (lead == 0xED) high = 0x9F;
+        } else if (lead >= 0xF0 && lead <= 0xF4) {
+            length = 4;
+            if (lead == 0xF0) low = 0x90;
+            if (lead == 0xF4) high = 0x8F;
+        } else {
+            return 0;
+        }
+        if (at + length > text.size()) return 0;
+        if (byte(1) < low || byte(1) > high) return 0;
+        for (std::size_t i = 2; i < length; ++i) {
+            if (byte(i) < 0x80 || byte(i) > 0xBF) return 0;
+        }
+        return length;
+    }
+
+    // `text` as a JSON string. Invalid UTF-8 becomes U+FFFD, so the output
+    // always parses, whatever bytes an app put in a name.
+    void AppendEscaped(std::string& out, std::string_view text) {
+        out += '"';
+        std::size_t i = 0;
+        while (i < text.size()) {
+            const std::uint8_t c = static_cast<std::uint8_t>(text[i]);
+            if (c < 0x80) {
+                switch (c) {
+                case '"': out += "\\\""; break;
+                case '\\': out += "\\\\"; break;
+                case '\b': out += "\\b"; break;
+                case '\f': out += "\\f"; break;
+                case '\n': out += "\\n"; break;
+                case '\r': out += "\\r"; break;
+                case '\t': out += "\\t"; break;
+                default:
+                    if (c < 0x20 || c == 0x7F) {
+                        out += std::format("\\u{:04x}", c);
+                    } else {
+                        out += static_cast<char>(c);
+                    }
+                }
+                ++i;
+                continue;
+            }
+            const std::size_t length = Utf8SequenceLength(text, i);
+            if (length == 0) {
+                out += "\xEF\xBF\xBD";
+                ++i;
+                continue;
+            }
+            out.append(text.substr(i, length));
+            i += length;
+        }
+        out += '"';
+    }
+
+    void AppendNumber(std::string& out, double number) {
+        if (std::isnan(number)) {
+            out += "\"NaN\"";
+            return;
+        }
+        if (std::isinf(number)) {
+            out += number > 0 ? "\"Infinity\"" : "\"-Infinity\"";
+            return;
+        }
+        std::array<char, 32> buffer{};
+        const auto [end, error] = std::to_chars(buffer.data(), buffer.data() + buffer.size(), number);
+        const std::string_view digits(buffer.data(), static_cast<std::size_t>(end - buffer.data()));
+        out += digits;
+        // A whole double keeps a fraction, so it reads back as a Number.
+        if (digits.find_first_of(".eE") == std::string_view::npos) out += ".0";
+    }
+
+    class Parser {
+    public:
+        explicit Parser(std::string_view text) : text(text) {}
+
+        Value Document() {
+            SkipWhitespace();
+            Value value = Any(0);
+            SkipWhitespace();
+            if (position != text.size()) Fail("unexpected text after the value");
+            return value;
+        }
+
+    private:
+        std::string_view text;
+        std::size_t position = 0;
+
+        [[noreturn]] void Fail(std::string_view what) const {
+            throw ParseError(position, what);
+        }
+
+        bool AtEnd() const {
+            return position >= text.size();
+        }
+
+        bool Peek(char c) const {
+            return !AtEnd() && text[position] == c;
+        }
+
+        bool Consume(char c) {
+            if (!Peek(c)) return false;
+            ++position;
+            return true;
+        }
+
+        void SkipWhitespace() {
+            while (!AtEnd() && (text[position] == ' ' || text[position] == '\t' || text[position] == '\n' || text[position] == '\r')) ++position;
+        }
+
+        void Literal(std::string_view word) {
+            if (text.substr(position, word.size()) != word) Fail("expected a value");
+            position += word.size();
+        }
+
+        Value Any(std::uint32_t depth) {
+            if (depth > maxDepth) Fail("nested too deeply");
+            if (AtEnd()) Fail("unexpected end of text");
+            switch (text[position]) {
+            case '{': return Object(depth);
+            case '[': return Array(depth);
+            case '"': return Value(String());
+            case 't': Literal("true"); return Value(true);
+            case 'f': Literal("false"); return Value(false);
+            case 'n': Literal("null"); return Value();
+            default: return Number();
+            }
+        }
+
+        Value Object(std::uint32_t depth) {
+            ++position;
+            Value object = Value::MakeObject();
+            SkipWhitespace();
+            if (Consume('}')) return object;
+            while (true) {
+                SkipWhitespace();
+                if (!Peek('"')) Fail("expected a member name in quotes");
+                const std::string key = String();
+                SkipWhitespace();
+                if (!Consume(':')) Fail("expected ':' after a member name");
+                SkipWhitespace();
+                object.Set(key, Any(depth + 1));
+                SkipWhitespace();
+                if (Consume('}')) return object;
+                if (!Consume(',')) Fail("expected ',' or '}'");
+            }
+        }
+
+        Value Array(std::uint32_t depth) {
+            ++position;
+            Value array = Value::MakeArray();
+            SkipWhitespace();
+            if (Consume(']')) return array;
+            while (true) {
+                SkipWhitespace();
+                array.Push(Any(depth + 1));
+                SkipWhitespace();
+                if (Consume(']')) return array;
+                if (!Consume(',')) Fail("expected ',' or ']'");
+            }
+        }
+
+        std::uint32_t Hex4() {
+            if (position + 4 > text.size()) Fail("truncated \\u escape");
+            std::uint32_t code = 0;
+            const auto [end, error] = std::from_chars(text.data() + position, text.data() + position + 4, code, 16);
+            if (error != std::errc() || end != text.data() + position + 4) Fail("malformed \\u escape");
+            position += 4;
+            return code;
+        }
+
+        std::string String() {
+            ++position;
+            std::string out;
+            while (true) {
+                if (AtEnd()) Fail("unterminated string");
+                const char c = text[position];
+                if (c == '"') {
+                    ++position;
+                    return out;
+                }
+                if (static_cast<std::uint8_t>(c) < 0x20) Fail("control character in a string");
+                ++position;
+                if (c != '\\') {
+                    out += c;
+                    continue;
+                }
+                if (AtEnd()) Fail("unterminated escape");
+                const char escape = text[position++];
+                switch (escape) {
+                case '"': out += '"'; break;
+                case '\\': out += '\\'; break;
+                case '/': out += '/'; break;
+                case 'b': out += '\b'; break;
+                case 'f': out += '\f'; break;
+                case 'n': out += '\n'; break;
+                case 'r': out += '\r'; break;
+                case 't': out += '\t'; break;
+                case 'u': {
+                    std::uint32_t code = Hex4();
+                    if (code >= 0xD800 && code <= 0xDBFF) {
+                        if (text.substr(position, 2) != "\\u") Fail("unpaired surrogate");
+                        position += 2;
+                        const std::uint32_t low = Hex4();
+                        if (low < 0xDC00 || low > 0xDFFF) Fail("unpaired surrogate");
+                        code = 0x10000 + ((code - 0xD800) << 10) + (low - 0xDC00);
+                    } else if (code >= 0xDC00 && code <= 0xDFFF) {
+                        Fail("unpaired surrogate");
+                    }
+                    AppendUtf8(out, code);
+                    break;
+                }
+                default:
+                    --position;
+                    Fail("unknown escape");
+                }
+            }
+        }
+
+        Value Number() {
+            const std::size_t start = position;
+            Consume('-');
+            if (AtEnd() || !IsDigit(text[position])) Fail("expected a value");
+            if (!Consume('0')) {
+                while (!AtEnd() && IsDigit(text[position])) ++position;
+            }
+            bool whole = true;
+            if (Consume('.')) {
+                whole = false;
+                if (AtEnd() || !IsDigit(text[position])) Fail("expected a digit after the decimal point");
+                while (!AtEnd() && IsDigit(text[position])) ++position;
+            }
+            if (Consume('e') || Consume('E')) {
+                whole = false;
+                if (!Consume('+')) Consume('-');
+                if (AtEnd() || !IsDigit(text[position])) Fail("expected a digit in the exponent");
+                while (!AtEnd() && IsDigit(text[position])) ++position;
+            }
+            const char* first = text.data() + start;
+            const char* last = text.data() + position;
+            if (whole) {
+                std::int64_t integer = 0;
+                const auto [end, error] = std::from_chars(first, last, integer);
+                if (error == std::errc() && end == last) return Value(integer);
+                // Beyond int64: read as a double below.
+            }
+            double number = 0;
+            const auto [end, error] = std::from_chars(first, last, number);
+            if (error == std::errc::result_out_of_range) {
+                position = start;
+                Fail("number out of range");
+            }
+            if (error != std::errc() || end != last) {
+                position = start;
+                Fail("malformed number");
+            }
+            return Value(number);
+        }
+    };
+
+    std::logic_error WrongKind(ValueKind actual, std::string_view wanted) {
+        return std::logic_error(std::format("Value is {}, not {}", KindName(actual), wanted));
+    }
+}
+
+std::string_view Crafter::Debug::KindName(ValueKind kind) {
+    switch (kind) {
+    case ValueKind::Null: return "null";
+    case ValueKind::Bool: return "boolean";
+    case ValueKind::Integer: return "integer";
+    case ValueKind::Number: return "number";
+    case ValueKind::String: return "string";
+    case ValueKind::Array: return "array";
+    case ValueKind::Object: return "object";
+    }
+    return "unknown";
+}
+
+ParseError::ParseError(std::size_t offset, std::string_view what)
+    : std::runtime_error(std::format("{} at offset {}", what, offset)), offset(offset) {}
+
+Value Value::MakeArray(std::vector<Value> items) {
+    Value value;
+    value.kind = ValueKind::Array;
+    value.items = std::move(items);
+    return value;
+}
+
+Value Value::MakeObject() {
+    Value value;
+    value.kind = ValueKind::Object;
+    return value;
+}
+
+bool Value::AsBool() const {
+    if (kind != ValueKind::Bool) throw WrongKind(kind, "a boolean");
+    return boolean;
+}
+
+std::int64_t Value::AsInteger() const {
+    if (kind != ValueKind::Integer) throw WrongKind(kind, "an integer");
+    return integer;
+}
+
+double Value::AsNumber() const {
+    if (kind == ValueKind::Integer) return static_cast<double>(integer);
+    if (kind != ValueKind::Number) throw WrongKind(kind, "a number");
+    return number;
+}
+
+const std::string& Value::AsString() const {
+    if (kind != ValueKind::String) throw WrongKind(kind, "a string");
+    return text;
+}
+
+std::span<const Value> Value::Items() const {
+    if (kind != ValueKind::Array) throw WrongKind(kind, "an array");
+    return items;
+}
+
+std::span<Value> Value::Items() {
+    if (kind != ValueKind::Array) throw WrongKind(kind, "an array");
+    return items;
+}
+
+void Value::Push(Value item) {
+    if (kind == ValueKind::Null) kind = ValueKind::Array;
+    if (kind != ValueKind::Array) throw WrongKind(kind, "an array");
+    items.push_back(std::move(item));
+}
+
+const Value* Value::Find(std::string_view key) const {
+    if (kind != ValueKind::Object) return nullptr;
+    const auto found = std::ranges::find(keys, key);
+    return found == keys.end() ? nullptr : &values[static_cast<std::size_t>(found - keys.begin())];
+}
+
+Value* Value::Find(std::string_view key) {
+    return const_cast<Value*>(std::as_const(*this).Find(key));
+}
+
+Value& Value::Set(std::string_view key, Value value) {
+    if (kind == ValueKind::Null) kind = ValueKind::Object;
+    if (kind != ValueKind::Object) throw WrongKind(kind, "an object");
+    if (Value* existing = Find(key)) {
+        *existing = std::move(value);
+        return *existing;
+    }
+    keys.emplace_back(key);
+    values.push_back(std::move(value));
+    return values.back();
+}
+
+Value& Value::operator[](std::string_view key) {
+    if (Value* existing = Find(key)) return *existing;
+    return Set(key, Value());
+}
+
+bool Value::Erase(std::string_view key) {
+    if (kind != ValueKind::Object) return false;
+    const auto found = std::ranges::find(keys, key);
+    if (found == keys.end()) return false;
+    const auto index = found - keys.begin();
+    keys.erase(found);
+    values.erase(values.begin() + index);
+    return true;
+}
+
+std::size_t Value::Size() const {
+    if (kind == ValueKind::Array) return items.size();
+    if (kind == ValueKind::Object) return keys.size();
+    return 0;
+}
+
+Value Value::Parse(std::string_view json) {
+    return Parser(json).Document();
+}
+
+std::string Value::Dump() const {
+    std::string out;
+    DumpTo(out, 0, 0);
+    return out;
+}
+
+std::string Value::DumpPretty(std::uint32_t indent) const {
+    std::string out;
+    DumpTo(out, indent == 0 ? 1 : indent, 0);
+    return out;
+}
+
+void Value::DumpTo(std::string& out, std::uint32_t indent, std::uint32_t depth) const {
+    const auto newline = [&](std::uint32_t level) {
+        if (indent == 0) return;
+        out += '\n';
+        out.append(static_cast<std::size_t>(indent) * level, ' ');
+    };
+    switch (kind) {
+    case ValueKind::Null: out += "null"; return;
+    case ValueKind::Bool: out += boolean ? "true" : "false"; return;
+    case ValueKind::Integer: out += std::to_string(integer); return;
+    case ValueKind::Number: AppendNumber(out, number); return;
+    case ValueKind::String: AppendEscaped(out, text); return;
+    case ValueKind::Array:
+        if (items.empty()) {
+            out += "[]";
+            return;
+        }
+        out += '[';
+        for (std::size_t i = 0; i < items.size(); ++i) {
+            if (i > 0) out += ',';
+            newline(depth + 1);
+            items[i].DumpTo(out, indent, depth + 1);
+        }
+        newline(depth);
+        out += ']';
+        return;
+    case ValueKind::Object:
+        if (keys.empty()) {
+            out += "{}";
+            return;
+        }
+        out += '{';
+        for (std::size_t i = 0; i < keys.size(); ++i) {
+            if (i > 0) out += ',';
+            newline(depth + 1);
+            AppendEscaped(out, keys[i]);
+            out += indent == 0 ? ":" : ": ";
+            values[i].DumpTo(out, indent, depth + 1);
+        }
+        newline(depth);
+        out += '}';
+        return;
+    }
+}
+
+namespace Crafter::Debug {
+    bool operator==(const Value& a, const Value& b) {
+        if (a.IsNumber() && b.IsNumber()) {
+            if (a.kind == ValueKind::Integer && b.kind == ValueKind::Integer) return a.integer == b.integer;
+            return a.AsNumber() == b.AsNumber();
+        }
+        if (a.kind != b.kind) return false;
+        switch (a.kind) {
+        case ValueKind::Null: return true;
+        case ValueKind::Bool: return a.boolean == b.boolean;
+        case ValueKind::String: return a.text == b.text;
+        case ValueKind::Array: return a.items == b.items;
+        case ValueKind::Object:
+            if (a.keys.size() != b.keys.size()) return false;
+            for (std::size_t i = 0; i < a.keys.size(); ++i) {
+                const Value* other = b.Find(a.keys[i]);
+                if (other == nullptr || !(a.values[i] == *other)) return false;
+            }
+            return true;
+        default: return false;
+        }
+    }
+}
diff --git a/interfaces/Crafter.Debug.Core-Codec.cppm b/interfaces/Crafter.Debug.Core-Codec.cppm
new file mode 100644
index 0000000..69940fe
--- /dev/null
+++ b/interfaces/Crafter.Debug.Core-Codec.cppm
@@ -0,0 +1,512 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// How C++ values become Values and back, and how their types are described
+// to an agent. Codec<T> is the customization point: one specialization per
+// type, with three static functions,
+//
+//     static Value Encode(const T& value);
+//     static void Decode(const Value& value, T& out, DecodeContext& context);
+//     static Value Describe();
+//
+// provided here for bool, integers, floating point, std::string, enums,
+// std::optional, std::vector, std::array, string-keyed maps, Value itself,
+// Required<T> and every aggregate, whose members are found by :Reflect.
+// Specialize it for a type that is none of those.
+//
+// Decode writes into an existing object, so a field the input leaves out
+// keeps the value it had: for a command's arguments, the struct's default
+// member initializer. It reports through the DecodeContext instead of
+// throwing, and keeps going after a problem, so one reply names every
+// mistake in a call, each with its path ("target.points[2].x") and, for an
+// unknown field, the field the caller most likely meant. Describe gives a
+// JSON-Schema-like Value that `crafter-debug help` renders.
+export module Crafter.Debug.Core:Codec;
+import std;
+import :Value;
+import :Reflect;
+
+export namespace Crafter::Debug {
+    // An argument field the caller must pass. Decoding fails when it is
+    // missing, and help lists it as required instead of showing a default.
+    template<class T>
+    struct Required {
+        T value{};
+        constexpr T& operator*() noexcept { return value; }
+        constexpr const T& operator*() const noexcept { return value; }
+        constexpr T* operator->() noexcept { return &value; }
+        constexpr const T* operator->() const noexcept { return &value; }
+    };
+
+    struct DecodeIssue {
+        // Where, as "target.points[2].x"; empty for the value itself.
+        std::string path;
+        std::string message;
+    };
+
+    class DecodeContext {
+    public:
+        // Records a problem at the current path.
+        void Fail(std::string message);
+        bool Ok() const { return issues.empty(); }
+        std::span<const DecodeIssue> Issues() const { return issues; }
+        // Every problem, "path: message", separated by "; ".
+        std::string Report() const;
+
+        // Extends the path while it lives: a member, or an array item.
+        class Scope {
+        public:
+            ~Scope();
+            Scope(const Scope&) = delete;
+            Scope& operator=(const Scope&) = delete;
+
+        private:
+            friend class DecodeContext;
+            explicit Scope(DecodeContext& context) : context(context) {}
+            DecodeContext& context;
+        };
+        [[nodiscard]] Scope Field(std::string_view name);
+        [[nodiscard]] Scope Index(std::size_t index);
+
+    private:
+        std::vector<std::string> segments;
+        std::vector<DecodeIssue> issues;
+        std::string Path() const;
+    };
+
+    // The candidate a mistyped `word` most likely meant, if one is close
+    // enough to suggest: Levenshtein distance 1 for words of up to four
+    // characters, 2 for longer ones.
+    std::optional<std::string_view> Suggest(std::string_view word, std::span<const std::string_view> candidates);
+
+    template<class T>
+    struct Codec;
+
+    template<class T>
+    concept Codable = requires(const T& in, const Value& value, T& out, DecodeContext& context) {
+        { Codec<T>::Encode(in) } -> std::same_as<Value>;
+        Codec<T>::Decode(value, out, context);
+        { Codec<T>::Describe() } -> std::same_as<Value>;
+    };
+
+    template<class T>
+    Value Encode(const T& value) {
+        static_assert(Codable<T>, "Crafter::Debug has no Codec for this type: make it an aggregate, or specialize Crafter::Debug::Codec<T>");
+        return Codec<T>::Encode(value);
+    }
+
+    template<class T>
+    void Decode(const Value& value, T& out, DecodeContext& context) {
+        static_assert(Codable<T>, "Crafter::Debug has no Codec for this type: make it an aggregate, or specialize Crafter::Debug::Codec<T>");
+        Codec<T>::Decode(value, out, context);
+    }
+
+    template<class T>
+    Value Describe() {
+        static_assert(Codable<T>, "Crafter::Debug has no Codec for this type: make it an aggregate, or specialize Crafter::Debug::Codec<T>");
+        return Codec<T>::Describe();
+    }
+}
+
+// Helpers the codecs below share. Module linkage, not an anonymous
+// namespace: the exported templates that call them are instantiated in the
+// consumer's translation units.
+namespace Crafter::Debug::Detail {
+    // "expected an integer, got a string"
+    std::string Expected(std::string_view wanted, const Value& got);
+    // {"type": type}
+    Value Type(std::string_view type);
+    // The message for a field an aggregate does not have.
+    std::string UnknownField(std::string_view key, std::span<const std::string_view> fields);
+    // The message for a name a scoped enum does not have.
+    std::string UnknownName(std::string_view name, std::span<const std::string_view> names);
+
+    // The integer types that are numbers: not bool, and none of the
+    // character types, which std::in_range refuses and which read as text.
+    template<class T>
+    concept Number = std::integral<T> && !std::same_as<T, bool> && !std::same_as<T, char> && !std::same_as<T, wchar_t>
+        && !std::same_as<T, char8_t> && !std::same_as<T, char16_t> && !std::same_as<T, char32_t>;
+
+    template<class T>
+    inline constexpr bool isRequired = false;
+    template<class T>
+    inline constexpr bool isRequired<Required<T>> = true;
+
+    // Describe recursion depth, so a type that contains itself (through a
+    // vector, say) is described a few levels deep instead of forever.
+    inline thread_local std::uint32_t describeDepth = 0;
+}
+
+export namespace Crafter::Debug {
+    template<>
+    struct Codec<bool> {
+        static Value Encode(bool value) { return Value(value); }
+        static void Decode(const Value& value, bool& out, DecodeContext& context) {
+            if (!value.IsBool()) {
+                context.Fail(Detail::Expected("a boolean", value));
+                return;
+            }
+            out = value.AsBool();
+        }
+        static Value Describe() { return Detail::Type("boolean"); }
+    };
+
+    template<Detail::Number T>
+    struct Codec<T> {
+        static Value Encode(T value) { return Value(value); }
+
+        static void Decode(const Value& value, T& out, DecodeContext& context) {
+            if (value.IsInteger()) {
+                const std::int64_t integer = value.AsInteger();
+                if (std::in_range<T>(integer)) {
+                    out = static_cast<T>(integer);
+                    return;
+                }
+                context.Fail(std::format("{} is out of range ({}..{})", integer, std::numeric_limits<T>::min(), std::numeric_limits<T>::max()));
+                return;
+            }
+            if (value.IsNumber()) {
+                // A double: an unsigned 64-bit value above the int64 range
+                // arrives as one, and so does 3.0 from a lenient client.
+                const double number = value.AsNumber();
+                if (number == std::trunc(number) && number >= static_cast<double>(std::numeric_limits<T>::min()) && number <= static_cast<double>(std::numeric_limits<T>::max())) {
+                    out = static_cast<T>(number);
+                    return;
+                }
+                context.Fail(std::format("expected an integer in {}..{}, got {}", std::numeric_limits<T>::min(), std::numeric_limits<T>::max(), value.Dump()));
+                return;
+            }
+            context.Fail(Detail::Expected("an integer", value));
+        }
+
+        static Value Describe() {
+            Value description = Detail::Type("integer");
+            if constexpr (sizeof(T) < sizeof(std::int64_t)) {
+                description.Set("minimum", Value(std::numeric_limits<T>::min()));
+                description.Set("maximum", Value(std::numeric_limits<T>::max()));
+            }
+            return description;
+        }
+    };
+
+    template<std::floating_point T>
+    struct Codec<T> {
+        static Value Encode(T value) { return Value(static_cast<double>(value)); }
+
+        static void Decode(const Value& value, T& out, DecodeContext& context) {
+            if (value.IsNumber()) {
+                out = static_cast<T>(value.AsNumber());
+                return;
+            }
+            // The spellings Value::Dump uses for what JSON cannot write.
+            if (value.IsString()) {
+                const std::string& text = value.AsString();
+                if (text == "NaN") {
+                    out = std::numeric_limits<T>::quiet_NaN();
+                    return;
+                }
+                if (text == "Infinity" || text == "-Infinity") {
+                    out = text.starts_with('-') ? -std::numeric_limits<T>::infinity() : std::numeric_limits<T>::infinity();
+                    return;
+                }
+            }
+            context.Fail(Detail::Expected("a number", value));
+        }
+
+        static Value Describe() { return Detail::Type("number"); }
+    };
+
+    template<>
+    struct Codec<std::string> {
+        static Value Encode(const std::string& value) { return Value(value); }
+        static void Decode(const Value& value, std::string& out, DecodeContext& context) {
+            if (!value.IsString()) {
+                context.Fail(Detail::Expected("a string", value));
+                return;
+            }
+            out = value.AsString();
+        }
+        static Value Describe() { return Detail::Type("string"); }
+    };
+
+    // For results and watches that hand out views. Nothing decodes into
+    // one: it would point into a request that is gone by the time it is read.
+    template<>
+    struct Codec<std::string_view> {
+        static Value Encode(std::string_view value) { return Value(value); }
+        static void Decode(const Value&, std::string_view&, DecodeContext& context) {
+            context.Fail("cannot be set: a std::string_view field is read-only, use std::string");
+        }
+        static Value Describe() { return Detail::Type("string"); }
+    };
+
+    // Scoped enums travel by name, and also accept the integer; unscoped ones
+    // by value only (see EnumRange in :Reflect).
+    template<class E>
+        requires std::is_enum_v<E>
+    struct Codec<E> {
+        static Value Encode(E value) {
+            if constexpr (std::is_scoped_enum_v<E>) {
+                if (const auto name = EnumName(value)) return Value(*name);
+            }
+            return Value(std::to_underlying(value));
+        }
+
+        static void Decode(const Value& value, E& out, DecodeContext& context) {
+            if constexpr (std::is_scoped_enum_v<E>) {
+                if (value.IsString()) {
+                    if (const auto parsed = EnumFromName<E>(value.AsString())) {
+                        out = *parsed;
+                        return;
+                    }
+                    context.Fail(Detail::UnknownName(value.AsString(), Names()));
+                    return;
+                }
+            }
+            if (value.IsInteger()) {
+                const std::int64_t integer = value.AsInteger();
+                if (std::in_range<std::underlying_type_t<E>>(integer)) {
+                    out = static_cast<E>(integer);
+                    return;
+                }
+                context.Fail(std::format("{} is out of range for this enum", integer));
+                return;
+            }
+            context.Fail(Detail::Expected(std::is_scoped_enum_v<E> ? "a name or an integer" : "an integer", value));
+        }
+
+        static Value Describe() {
+            if constexpr (std::is_scoped_enum_v<E>) {
+                Value description = Detail::Type("enum");
+                Value values = Value::MakeArray();
+                for (const std::string_view name : Names()) values.Push(Value(name));
+                description.Set("values", std::move(values));
+                return description;
+            } else {
+                return Detail::Type("integer");
+            }
+        }
+
+    private:
+        static std::vector<std::string_view> Names()
+            requires std::is_scoped_enum_v<E>
+        {
+            std::vector<std::string_view> names;
+            for (const Enumerator<E>& e : enumerators<E>) names.push_back(e.name);
+            return names;
+        }
+    };
+
+    template<class T>
+    struct Codec<std::optional<T>> {
+        static Value Encode(const std::optional<T>& value) {
+            return value ? Debug::Encode(*value) : Value();
+        }
+
+        static void Decode(const Value& value, std::optional<T>& out, DecodeContext& context) {
+            if (value.IsNull()) {
+                out.reset();
+                return;
+            }
+            if (!out) out.emplace();
+            Debug::Decode(value, *out, context);
+        }
+
+        static Value Describe() {
+            Value description = Debug::Describe<T>();
+            description.Set("nullable", true);
+            return description;
+        }
+    };
+
+    template<class T, class A>
+    struct Codec<std::vector<T, A>> {
+        static Value Encode(const std::vector<T, A>& value) {
+            Value array = Value::MakeArray();
+            for (const T& item : value) array.Push(Debug::Encode(item));
+            return array;
+        }
+
+        static void Decode(const Value& value, std::vector<T, A>& out, DecodeContext& context) {
+            if (!value.IsArray()) {
+                context.Fail(Detail::Expected("an array", value));
+                return;
+            }
+            std::vector<T, A> decoded;
+            decoded.reserve(value.Size());
+            const std::span<const Value> items = value.Items();
+            for (std::size_t i = 0; i < items.size(); ++i) {
+                const auto scope = context.Index(i);
+                // Through a local, so std::vector<bool> works too.
+                T item{};
+                Debug::Decode(items[i], item, context);
+                decoded.push_back(std::move(item));
+            }
+            out = std::move(decoded);
+        }
+
+        static Value Describe() {
+            Value description = Detail::Type("array");
+            description.Set("items", Debug::Describe<T>());
+            return description;
+        }
+    };
+
+    template<class T, std::size_t N>
+    struct Codec<std::array<T, N>> {
+        static Value Encode(const std::array<T, N>& value) {
+            Value array = Value::MakeArray();
+            for (const T& item : value) array.Push(Debug::Encode(item));
+            return array;
+        }
+
+        static void Decode(const Value& value, std::array<T, N>& out, DecodeContext& context) {
+            if (!value.IsArray()) {
+                context.Fail(Detail::Expected("an array", value));
+                return;
+            }
+            if (value.Size() != N) {
+                context.Fail(std::format("expected {} items, got {}", N, value.Size()));
+                return;
+            }
+            const std::span<const Value> items = value.Items();
+            for (std::size_t i = 0; i < N; ++i) {
+                const auto scope = context.Index(i);
+                Debug::Decode(items[i], out[i], context);
+            }
+        }
+
+        static Value Describe() {
+            Value description = Detail::Type("array");
+            description.Set("items", Debug::Describe<T>());
+            description.Set("length", Value(N));
+            return description;
+        }
+    };
+
+    // Maps keyed by string: std::map and std::unordered_map alike.
+    template<class M>
+    struct MapCodec {
+        using Mapped = typename M::mapped_type;
+
+        static Value Encode(const M& value) {
+            Value object = Value::MakeObject();
+            for (const auto& [key, item] : value) object.Set(key, Debug::Encode(item));
+            return object;
+        }
+
+        static void Decode(const Value& value, M& out, DecodeContext& context) {
+            if (!value.IsObject()) {
+                context.Fail(Detail::Expected("an object", value));
+                return;
+            }
+            M decoded;
+            for (const auto& [key, item] : value.Members()) {
+                const auto scope = context.Field(key);
+                Mapped mapped{};
+                Debug::Decode(item, mapped, context);
+                decoded.emplace(key, std::move(mapped));
+            }
+            out = std::move(decoded);
+        }
+
+        static Value Describe() {
+            Value description = Detail::Type("object");
+            description.Set("values", Debug::Describe<Mapped>());
+            return description;
+        }
+    };
+
+    template<class T, class C, class A>
+    struct Codec<std::map<std::string, T, C, A>> : MapCodec<std::map<std::string, T, C, A>> {};
+
+    template<class T, class H, class E, class A>
+    struct Codec<std::unordered_map<std::string, T, H, E, A>> : MapCodec<std::unordered_map<std::string, T, H, E, A>> {};
+
+    template<>
+    struct Codec<Value> {
+        static Value Encode(const Value& value) { return value; }
+        static void Decode(const Value& value, Value& out, DecodeContext&) { out = value; }
+        static Value Describe() { return Detail::Type("any"); }
+    };
+
+    template<class T>
+    struct Codec<Required<T>> {
+        static Value Encode(const Required<T>& value) { return Debug::Encode(value.value); }
+        static void Decode(const Value& value, Required<T>& out, DecodeContext& context) { Debug::Decode(value, out.value, context); }
+        static Value Describe() { return Debug::Describe<T>(); }
+    };
+
+    // Every aggregate: an object with one member per field, by name.
+    template<class T>
+        requires Aggregate<T>
+    struct Codec<T> {
+        static Value Encode(const T& value) {
+            Value object = Value::MakeObject();
+            [&]<std::size_t... I>(std::index_sequence<I...>) {
+                (object.Set(MemberName<I, T>(), Debug::Encode(Member<I>(value))), ...);
+            }(std::make_index_sequence<memberCount<T>>{});
+            return object;
+        }
+
+        static void Decode(const Value& value, T& out, DecodeContext& context) {
+            if (!value.IsObject()) {
+                context.Fail(Detail::Expected("an object", value));
+                return;
+            }
+            [&]<std::size_t... I>(std::index_sequence<I...>) {
+                ([&] {
+                    constexpr std::string_view name = MemberName<I, T>();
+                    const auto scope = context.Field(name);
+                    if (const Value* field = value.Find(name)) {
+                        Debug::Decode(*field, Member<I>(out), context);
+                    } else if constexpr (Detail::isRequired<MemberType<I, T>>) {
+                        context.Fail("missing, and this field is required");
+                    }
+                }(), ...);
+            }(std::make_index_sequence<memberCount<T>>{});
+            static constexpr std::array<std::string_view, memberCount<T>> names = Names();
+            for (const auto& [key, item] : value.Members()) {
+                if (std::ranges::find(names, std::string_view(key)) != names.end()) continue;
+                const auto scope = context.Field(key);
+                context.Fail(Detail::UnknownField(key, names));
+            }
+        }
+
+        static Value Describe() {
+            Value description = Detail::Type("object");
+            description.Set("name", Value(TypeName<T>()));
+            if (Detail::describeDepth >= 8) {
+                description.Set("truncated", true);
+                return description;
+            }
+            ++Detail::describeDepth;
+            Value fields = Value::MakeArray();
+            const T defaults{};
+            [&]<std::size_t... I>(std::index_sequence<I...>) {
+                ([&] {
+                    using M = MemberType<I, T>;
+                    Value field = Value::MakeObject();
+                    field.Set("name", Value(MemberName<I, T>()));
+                    field.Set("type", Debug::Describe<M>());
+                    if constexpr (Detail::isRequired<M>) {
+                        field.Set("required", true);
+                    } else {
+                        field.Set("default", Debug::Encode(Member<I>(defaults)));
+                    }
+                    fields.Push(std::move(field));
+                }(), ...);
+            }(std::make_index_sequence<memberCount<T>>{});
+            --Detail::describeDepth;
+            description.Set("fields", std::move(fields));
+            return description;
+        }
+
+    private:
+        static consteval std::array<std::string_view, memberCount<T>> Names() {
+            return [&]<std::size_t... I>(std::index_sequence<I...>) {
+                return std::array<std::string_view, memberCount<T>>{ MemberName<I, T>()... };
+            }(std::make_index_sequence<memberCount<T>>{});
+        }
+    };
+}
diff --git a/interfaces/Crafter.Debug.Core-Reflect.cppm b/interfaces/Crafter.Debug.Core-Reflect.cppm
new file mode 100644
index 0000000..bad95f1
--- /dev/null
+++ b/interfaces/Crafter.Debug.Core-Reflect.cppm
@@ -0,0 +1,205 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// Compile-time introspection without P2996 (clang 23 has no <meta>), taken
+// from Crafter.DB:Reflect. For aggregates: structured-binding packs give the
+// member count and per-member access, and std::source_location's
+// function_name() spells a member's name when a pointer to a member of an
+// extern object is a template argument. For scoped enums: function_name()
+// spells a template argument that is an enumerator by its name and any other
+// value as a cast, so probing a range of values finds every enumerator.
+// When a clang ships <meta>, this partition is the only thing to replace.
+export module Crafter.Debug.Core:Reflect;
+import std;
+
+namespace Crafter::Debug::Reflect {
+    template<class T>
+    struct CountHelper {
+        template<class U>
+        static consteval auto Count(U& u) {
+            auto&& [...m] = u;
+            return std::integral_constant<std::size_t, sizeof...(m)>{};
+        }
+        static constexpr std::size_t value = decltype(Count(std::declval<T&>()))::value;
+    };
+
+    // An instance whose member addresses are turned into text. Only its
+    // address is used, in constant expressions; no_destroy keeps the compiler
+    // from registering a destructor for it.
+    template<class T>
+    [[clang::no_destroy]] inline const T external{};
+
+    template<auto Probe>
+    consteval std::string_view RawName() {
+        return std::source_location::current().function_name();
+    }
+
+    template<class T>
+    consteval std::string_view RawTypeName() {
+        return std::source_location::current().function_name();
+    }
+
+    // The part of RawName<P>()'s text that spells P: what follows "Probe = "
+    // up to the closing bracket.
+    consteval std::string_view ProbeText(std::string_view raw) {
+        const std::size_t end = raw.rfind(']');
+        const std::size_t start = raw.rfind("Probe = ", end);
+        return raw.substr(start + 8, end - start - 8);
+    }
+
+    // The enumerator name for V, or "" when V is not an enumerator: clang
+    // spells those as a cast, "(Ns::Color)5".
+    template<auto V>
+    consteval std::string_view EnumeratorName() {
+        const std::string_view text = ProbeText(RawName<V>());
+        if (text.starts_with('(')) return {};
+        const std::size_t colon = text.rfind("::");
+        return colon == std::string_view::npos ? text : text.substr(colon + 2);
+    }
+}
+
+export namespace Crafter::Debug {
+    // True for class types whose non-static data members a structured
+    // binding can bind: public members, no bases carrying members, no
+    // user-declared constructors.
+    template<class T>
+    concept Aggregate = std::is_class_v<T> && std::is_aggregate_v<T> && requires { Reflect::CountHelper<T>::value; };
+
+    template<Aggregate T>
+    inline constexpr std::size_t memberCount = Reflect::CountHelper<T>::value;
+
+    // The I-th member of an aggregate lvalue.
+    template<std::size_t I, class T>
+    constexpr decltype(auto) Member(T&& t) noexcept {
+        auto&& [...m] = t;
+        return (m...[I]);
+    }
+
+    template<std::size_t I, class T>
+    using MemberType = std::remove_cvref_t<decltype(Member<I>(std::declval<T&>()))>;
+
+    // The declared name of the I-th member: "yaw". Parsed out of
+    // "std::string_view ...::RawName() [Probe = &external.yaw]".
+    template<std::size_t I, Aggregate T>
+    consteval std::string_view MemberName() {
+        constexpr std::string_view text = Reflect::ProbeText(Reflect::RawName<&Member<I>(Reflect::external<T>)>());
+        return text.substr(text.rfind('.') + 1);
+    }
+
+    // The type's name as the compiler spells it ("Game::AimArgs"), for help
+    // and errors; the spelling is not load-bearing.
+    template<class T>
+    consteval std::string_view TypeName() {
+        constexpr std::string_view raw = Reflect::RawTypeName<T>();
+        const std::size_t end = raw.rfind(']');
+        const std::size_t start = raw.rfind("T = ", end);
+        return raw.substr(start + 4, end - start - 4);
+    }
+
+    // Calls f.template operator()<I>(member) for every member, in order.
+    template<Aggregate T, class F>
+    constexpr void ForEachMember(T& t, F&& f) {
+        [&]<std::size_t... I>(std::index_sequence<I...>) {
+            (f.template operator()<I>(Member<I>(t)), ...);
+        }(std::make_index_sequence<memberCount<T>>{});
+    }
+
+    // The values probed for enumerator names, a half-open range clamped to
+    // the enum's underlying type. Specialize for an enum with values outside
+    // the default range; every probe is one template instantiation.
+    template<class E>
+    struct EnumRange {
+        static constexpr std::int64_t min = -128;
+        static constexpr std::int64_t max = 128;
+    };
+
+    template<class E>
+    struct Enumerator {
+        E value;
+        std::string_view name;
+    };
+}
+
+namespace Crafter::Debug::Reflect {
+    template<class E>
+    consteval std::int64_t ProbeMin() {
+        using U = std::underlying_type_t<E>;
+        return std::max<std::int64_t>(EnumRange<E>::min, static_cast<std::int64_t>(std::numeric_limits<U>::min()));
+    }
+
+    template<class E>
+    consteval std::int64_t ProbeMax() {
+        using U = std::underlying_type_t<E>;
+        constexpr std::int64_t limit = static_cast<std::uint64_t>(std::numeric_limits<U>::max()) >= static_cast<std::uint64_t>(std::numeric_limits<std::int64_t>::max())
+            ? std::numeric_limits<std::int64_t>::max()
+            : static_cast<std::int64_t>(std::numeric_limits<U>::max()) + 1;
+        return std::min<std::int64_t>(EnumRange<E>::max, limit);
+    }
+
+    template<class E, std::size_t... I>
+    consteval auto EnumTable(std::index_sequence<I...>) {
+        constexpr std::int64_t first = ProbeMin<E>();
+        constexpr std::array<std::string_view, sizeof...(I)> names = {
+            EnumeratorName<static_cast<E>(first + static_cast<std::int64_t>(I))>()...
+        };
+        constexpr std::size_t count = static_cast<std::size_t>(std::ranges::count_if(names, [](std::string_view name) { return !name.empty(); }));
+        std::array<Enumerator<E>, count> table{};
+        std::size_t next = 0;
+        for (std::size_t i = 0; i < names.size(); ++i) {
+            if (!names[i].empty()) table[next++] = { static_cast<E>(first + static_cast<std::int64_t>(i)), names[i] };
+        }
+        return table;
+    }
+}
+
+export namespace Crafter::Debug {
+    // Every named value of a scoped enum within EnumRange, in value order.
+    // Scoped only: clang rejects casting an out-of-range value to an unscoped
+    // enum without a fixed underlying type, even in a template argument.
+    template<class E>
+        requires std::is_scoped_enum_v<E>
+    inline constexpr auto enumerators = Reflect::EnumTable<E>(std::make_index_sequence<static_cast<std::size_t>(Reflect::ProbeMax<E>() - Reflect::ProbeMin<E>())>{});
+
+    template<class E>
+        requires std::is_scoped_enum_v<E>
+    constexpr std::optional<std::string_view> EnumName(E value) {
+        for (const Enumerator<E>& e : enumerators<E>) {
+            if (e.value == value) return e.name;
+        }
+        return std::nullopt;
+    }
+
+    template<class E>
+        requires std::is_scoped_enum_v<E>
+    constexpr std::optional<E> EnumFromName(std::string_view name) {
+        for (const Enumerator<E>& e : enumerators<E>) {
+            if (e.name == name) return e.value;
+        }
+        return std::nullopt;
+    }
+}
+
+// Self-tests: a compiler whose function_name() text changes shape fails
+// here, at the library's build, instead of producing garbage names.
+namespace Crafter::Debug::Reflect {
+    struct ProbeStruct {
+        std::int32_t alpha;
+        double beta;
+        std::string gamma;
+    };
+    enum class ProbeEnum : std::uint8_t { Zero, One, Seven = 7 };
+    enum class ProbeSigned : std::int16_t { Minus = -3, Big = 100 };
+
+    static_assert(Aggregate<ProbeStruct>);
+    static_assert(memberCount<ProbeStruct> == 3);
+    static_assert(MemberName<0, ProbeStruct>() == "alpha");
+    static_assert(MemberName<2, ProbeStruct>() == "gamma");
+    static_assert(std::is_same_v<MemberType<1, ProbeStruct>, double>);
+    static_assert(TypeName<ProbeStruct>().ends_with("ProbeStruct"));
+    static_assert(enumerators<ProbeEnum>.size() == 3);
+    static_assert(EnumName(ProbeEnum::Seven) == "Seven");
+    static_assert(!EnumName(static_cast<ProbeEnum>(5)).has_value());
+    static_assert(EnumFromName<ProbeEnum>("One") == ProbeEnum::One);
+    static_assert(EnumName(ProbeSigned::Minus) == "Minus");
+    static_assert(enumerators<ProbeSigned>.size() == 2);
+}
diff --git a/interfaces/Crafter.Debug.Core-Result.cppm b/interfaces/Crafter.Debug.Core-Result.cppm
new file mode 100644
index 0000000..813b22c
--- /dev/null
+++ b/interfaces/Crafter.Debug.Core-Result.cppm
@@ -0,0 +1,93 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// What a command handler returns besides a plain value: an Error with a
+// stable code and a message written for the agent reading it, a Result that
+// is either, and an Async for a result that takes frames to produce.
+export module Crafter.Debug.Core:Result;
+import std;
+
+export namespace Crafter::Debug {
+    struct Error {
+        // Stable and machine-readable, "InvalidScene": scripts branch on it.
+        std::string code;
+        // For whoever reads the reply: what went wrong, and the next step
+        // ("not in a game; run Lobby_Start first").
+        std::string message;
+    };
+
+    template<class T = void>
+    class Result {
+    public:
+        // Anything T can be made from, so `return "text";` works for a
+        // Result<std::string>.
+        template<class U = T>
+            requires (std::constructible_from<T, U &&> && !std::same_as<std::remove_cvref_t<U>, Error> && !std::same_as<std::remove_cvref_t<U>, Result>)
+        Result(U&& value) : state(std::in_place_index<0>, std::forward<U>(value)) {}
+        Result(Error error) : state(std::in_place_index<1>, std::move(error)) {}
+
+        bool Ok() const { return state.index() == 0; }
+        explicit operator bool() const { return Ok(); }
+        T& Get() { return std::get<0>(state); }
+        const T& Get() const { return std::get<0>(state); }
+        const Error& GetError() const { return std::get<1>(state); }
+
+    private:
+        std::variant<T, Error> state;
+    };
+
+    template<>
+    class Result<void> {
+    public:
+        Result() = default;
+        Result(Error error) : error(std::move(error)) {}
+
+        bool Ok() const { return !error.has_value(); }
+        explicit operator bool() const { return Ok(); }
+        const Error& GetError() const { return *error; }
+
+    private:
+        std::optional<Error> error;
+    };
+
+    // A result that takes frames to arrive, such as a GPU readback. The
+    // session polls it once per Pump, on the app's thread, until it yields;
+    // the caller simply waits, with no ready:false loop of its own. The
+    // poller returns std::optional<T> (empty while not ready), or
+    // std::optional<Result<T>> to fail; for Async<void>, a bool or a
+    // std::optional<Result<>>.
+    template<class T = void>
+    class Async {
+    public:
+        template<class F>
+            requires std::invocable<F&>
+        Async(F poller) : poll(Wrap(std::move(poller))) {}
+
+        // Empty while not ready.
+        std::optional<Result<T>> Poll() { return poll(); }
+
+    private:
+        std::function<std::optional<Result<T>>()> poll;
+
+        template<class F>
+        static std::function<std::optional<Result<T>>()> Wrap(F poller) {
+            using R = std::invoke_result_t<F&>;
+            if constexpr (std::is_same_v<R, std::optional<Result<T>>>) {
+                return poller;
+            } else if constexpr (std::is_void_v<T>) {
+                static_assert(std::is_same_v<R, bool>, "an Async<void> poller returns bool or std::optional<Result<>>");
+                return [poller = std::move(poller)]() mutable -> std::optional<Result<T>> {
+                    if (!poller()) return std::nullopt;
+                    return Result<T>();
+                };
+            } else {
+                static_assert(std::is_same_v<R, std::optional<T>>, "an Async<T> poller returns std::optional<T> or std::optional<Result<T>>");
+                return [poller = std::move(poller)]() mutable -> std::optional<Result<T>> {
+                    std::optional<T> value = poller();
+                    if (!value) return std::nullopt;
+                    return Result<T>(std::move(*value));
+                };
+            }
+        }
+    };
+}
diff --git a/interfaces/Crafter.Debug.Core-Value.cppm b/interfaces/Crafter.Debug.Core-Value.cppm
new file mode 100644
index 0000000..fec1673
--- /dev/null
+++ b/interfaces/Crafter.Debug.Core-Value.cppm
@@ -0,0 +1,130 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// A JSON value: what travels between an app and crafter-debug, and what
+// command arguments, results, watches and events are encoded to. Its own type
+// rather than a JSON library's, so no consumer interface ever sees one, and
+// small enough to read in one sitting. Objects keep their members in the
+// order they were set, which is the order an agent reads them in.
+//
+// Numbers keep the distinction JSON text does not: an Integer is a whole
+// number that fits an int64, anything else is a Number (a double). Integer 1
+// and Number 1.0 still compare equal. Non-finite doubles have no JSON
+// spelling, and a debugger must not hide a NaN, so Dump writes them as the
+// strings "NaN", "Infinity" and "-Infinity", which Codec reads back.
+export module Crafter.Debug.Core:Value;
+import std;
+
+export namespace Crafter::Debug {
+    enum class ValueKind : std::uint8_t {
+        Null,
+        Bool,
+        Integer,
+        Number,
+        String,
+        Array,
+        Object,
+    };
+
+    // "null", "boolean", "integer", "number", "string", "array" or "object":
+    // how errors name a kind.
+    std::string_view KindName(ValueKind kind);
+
+    // Thrown by Value::Parse: what is wrong, and the byte offset in the text.
+    class ParseError : public std::runtime_error {
+    public:
+        std::size_t offset;
+        ParseError(std::size_t offset, std::string_view what);
+    };
+
+    class Value {
+    public:
+        Value() = default;
+        Value(std::nullptr_t) {}
+        Value(bool value) : kind(ValueKind::Bool), boolean(value) {}
+        // Any integer type but bool and char. An unsigned value above the
+        // int64 range becomes a Number, which is exact up to 2^53.
+        template<std::integral T>
+            requires (!std::same_as<T, bool> && !std::same_as<T, char>)
+        Value(T value) {
+            if constexpr (std::is_unsigned_v<T> && sizeof(T) >= sizeof(std::int64_t)) {
+                if (value > static_cast<std::uint64_t>(std::numeric_limits<std::int64_t>::max())) {
+                    kind = ValueKind::Number;
+                    number = static_cast<double>(value);
+                    return;
+                }
+            }
+            kind = ValueKind::Integer;
+            integer = static_cast<std::int64_t>(value);
+        }
+        Value(double value) : kind(ValueKind::Number), number(value) {}
+        Value(float value) : Value(static_cast<double>(value)) {}
+        Value(std::string value) : kind(ValueKind::String), text(std::move(value)) {}
+        Value(std::string_view value) : Value(std::string(value)) {}
+        // String literals; without this, `Value("x")` would pick the bool
+        // constructor, a standard conversion beating a user-defined one.
+        Value(const char* value) : Value(std::string(value)) {}
+
+        static Value MakeArray(std::vector<Value> items = {});
+        static Value MakeObject();
+
+        ValueKind Kind() const { return kind; }
+        bool IsNull() const { return kind == ValueKind::Null; }
+        bool IsBool() const { return kind == ValueKind::Bool; }
+        bool IsInteger() const { return kind == ValueKind::Integer; }
+        // Integer or Number.
+        bool IsNumber() const { return kind == ValueKind::Integer || kind == ValueKind::Number; }
+        bool IsString() const { return kind == ValueKind::String; }
+        bool IsArray() const { return kind == ValueKind::Array; }
+        bool IsObject() const { return kind == ValueKind::Object; }
+
+        // Each throws std::logic_error for a value of another kind; check the
+        // kind first. AsNumber accepts an Integer too.
+        bool AsBool() const;
+        std::int64_t AsInteger() const;
+        double AsNumber() const;
+        const std::string& AsString() const;
+
+        // Array items. Push turns a null value into an array first.
+        std::span<const Value> Items() const;
+        std::span<Value> Items();
+        void Push(Value item);
+
+        // Object members, in the order they were first set. Set replaces the
+        // value of an existing key in place, and turns a null value into an
+        // object first, as operator[] does when the key is missing.
+        const Value* Find(std::string_view key) const;
+        Value* Find(std::string_view key);
+        Value& Set(std::string_view key, Value value);
+        Value& operator[](std::string_view key);
+        bool Erase(std::string_view key);
+        auto Members() const { return std::views::zip(keys, values); }
+
+        // Array items or object members; 0 for anything else.
+        std::size_t Size() const;
+
+        // JSON text. Parse throws ParseError; a duplicate key keeps the last
+        // value. Dump writes compact text, DumpPretty indents.
+        static Value Parse(std::string_view json);
+        std::string Dump() const;
+        std::string DumpPretty(std::uint32_t indent = 2) const;
+
+        // Numbers compare by value across Integer and Number; objects compare
+        // their members whatever their order.
+        friend bool operator==(const Value& a, const Value& b);
+
+    private:
+        ValueKind kind = ValueKind::Null;
+        bool boolean = false;
+        std::int64_t integer = 0;
+        double number = 0;
+        std::string text;
+        std::vector<Value> items;
+        // Object members as two parallel lists: a member type holding a Value
+        // could not be complete inside Value's own definition.
+        std::vector<std::string> keys;
+        std::vector<Value> values;
+
+        void DumpTo(std::string& out, std::uint32_t indent, std::uint32_t depth) const;
+    };
+}
diff --git a/interfaces/Crafter.Debug.Core.cppm b/interfaces/Crafter.Debug.Core.cppm
new file mode 100644
index 0000000..33eb239
--- /dev/null
+++ b/interfaces/Crafter.Debug.Core.cppm
@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// What the app side and the crafter-debug client share: Value and its text
+// form, reflection, the codecs, and the result types. The app imports it
+// through Crafter.Debug; the client imports it directly, without the session.
+export module Crafter.Debug.Core;
+export import :Value;
+export import :Reflect;
+export import :Codec;
+export import :Result;
diff --git a/interfaces/Crafter.Debug.cppm b/interfaces/Crafter.Debug.cppm
index a17cd1f..b98a747 100644
--- a/interfaces/Crafter.Debug.cppm
+++ b/interfaces/Crafter.Debug.cppm
@@ -1,12 +1,15 @@
 // SPDX-License-Identifier: LGPL-3.0-only
 // SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
 
-// Spike placeholder: the real API replaces this in Phase 1.
+// Runtime debugging for AI agents: what an app imports. The session, the
+// registry of commands and watches, the timeline and pause control follow in
+// later phases; for now this re-exports the shared core.
 export module Crafter.Debug;
+export import Crafter.Debug.Core;
 import std;
 
 export namespace Crafter::Debug {
     std::string_view Version() {
-        return "0.0.0-spike";
+        return "0.0.0-dev";
     }
 }
diff --git a/project.cpp b/project.cpp
index fdc129c..24b0d02 100644
--- a/project.cpp
+++ b/project.cpp
@@ -1,6 +1,14 @@
 // SPDX-License-Identifier: LGPL-3.0-only
 // SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
 
+// Three configurations:
+//   Crafter.Debug.Core  what the app and the client share (values, codecs)
+//   crafter.debug-cli   the client, crafter-debug
+//   Crafter.Debug       what an app links (returned), depending on both
+// The client is one of the library's dependencies only so that a consumer's
+// debug build compiles it in the same pass as the app; an executable adds no
+// link flags to what depends on it. Consumers load this file through
+// Debug::Use (tooling/), which also writes their launcher.
 import std;
 import Crafter.Build;
 namespace fs = std::filesystem;
@@ -18,6 +26,13 @@ namespace {
     fs::path Root() {
         return fs::path(std::source_location::current().file_name()).parent_path();
     }
+
+    constexpr std::array<std::string_view, 4> tests = {
+        "ShouldRoundTripJson",
+        "ShouldRoundTripAggregates",
+        "ShouldReportDecodeErrorsWithPaths",
+        "ShouldDescribeDefaultsAndRequired",
+    };
 }
 
 extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> args) {
@@ -27,17 +42,30 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
     cfg.outputName = "Crafter.Debug";
     cfg.type = ConfigurationType::LibraryStatic;
     ApplyStandardArgs(cfg, args);
+
+    Configuration& core = owned.emplace_back();
+    core.path = Root();
+    core.name = "Crafter.Debug.Core";
+    core.outputName = "Crafter.Debug.Core";
+    core.type = ConfigurationType::LibraryStatic;
+    ApplyStandardArgs(core, args);
     {
-        std::array<fs::path, 1> interfaces = { "interfaces/Crafter.Debug" };
-        std::array<fs::path, 0> implementations = {};
-        cfg.GetInterfacesAndImplementations(interfaces, implementations);
+        std::array<fs::path, 5> interfaces = {
+            "interfaces/Crafter.Debug.Core-Value",
+            "interfaces/Crafter.Debug.Core-Reflect",
+            "interfaces/Crafter.Debug.Core-Codec",
+            "interfaces/Crafter.Debug.Core-Result",
+            "interfaces/Crafter.Debug.Core",
+        };
+        std::array<fs::path, 2> implementations = {
+            "implementations/Crafter.Debug.Core-Value",
+            "implementations/Crafter.Debug.Core-Codec",
+        };
+        core.GetInterfacesAndImplementations(interfaces, implementations);
     }
 
-    // The client, crafter-debug. One of the library's dependencies, so a
-    // consumer's debug build compiles it in the same pass as the app; an
-    // executable adds no link flags to what depends on it. Built for the same
-    // target as the library, and not through ApplyStandardArgs, which would
-    // turn it into a library when a consumer passes --lib.
+    // The client, built for the library's target. Not through
+    // ApplyStandardArgs: a consumer's --lib would make it a library.
     Configuration& client = owned.emplace_back();
     client.path = Root();
     client.name = "crafter.debug-cli";
@@ -48,11 +76,24 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
     client.mtune = cfg.mtune;
     client.sysroot = cfg.sysroot;
     client.debug = cfg.debug;
+    client.dependencies = { &core };
     {
         std::array<fs::path, 0> interfaces = {};
         std::array<fs::path, 1> implementations = { "implementations/main" };
         client.GetInterfacesAndImplementations(interfaces, implementations);
     }
-    cfg.dependencies.push_back(&client);
+
+    cfg.dependencies = { &core, &client };
+    {
+        std::array<fs::path, 1> interfaces = { "interfaces/Crafter.Debug" };
+        std::array<fs::path, 0> implementations = {};
+        cfg.GetInterfacesAndImplementations(interfaces, implementations);
+    }
+
+    // Tests only when this is the project being built: a consumer's load of
+    // this file has no use for them.
+    if (IsTopLevelProject()) {
+        for (const std::string_view test : tests) cfg.AddTest(test).Dependencies({ &core });
+    }
     return cfg;
 }
diff --git a/tests/ShouldDescribeDefaultsAndRequired/main.cpp b/tests/ShouldDescribeDefaultsAndRequired/main.cpp
new file mode 100644
index 0000000..538735e
--- /dev/null
+++ b/tests/ShouldDescribeDefaultsAndRequired/main.cpp
@@ -0,0 +1,67 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// Describe gives `crafter-debug help` what an agent needs to call a command
+// right the first time: each field's name and type, its default taken from
+// the member initializer, Required fields marked with no default, a narrow
+// integer's range, an enum's names, a nested aggregate's own fields. A type
+// that contains itself is described a few levels deep instead of forever.
+import std;
+import Crafter.Debug.Core;
+using namespace Crafter::Debug;
+
+namespace Game {
+    enum class Mode : std::uint8_t { Idle, Build };
+
+    struct Vec3 {
+        float x = 0;
+        float y = 1;
+        float z = 0;
+    };
+
+    struct Aim {
+        Required<std::uint8_t> slot;
+        float yaw = 45;
+        Vec3 at;
+        std::optional<std::string> label;
+        Mode mode = Mode::Build;
+        std::vector<std::int32_t> ids;
+    };
+
+    struct Node {
+        std::vector<Node> children;
+    };
+}
+
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool ok, std::string_view what) {
+        std::println("{} {}", ok ? "PASS" : "FAIL", what);
+        if (!ok) ++failures;
+    }
+}
+
+int main() {
+    const Value description = Describe<Game::Aim>();
+    std::println("{}", description.DumpPretty());
+    Check(description.Find("type")->AsString() == "object" && description.Find("name")->AsString().ends_with("Aim"), "an aggregate describes as a named object");
+    const std::span<const Value> fields = description.Find("fields")->Items();
+    Check(fields.size() == 6, "one entry per field");
+    const auto field = [&](std::string_view name) -> const Value& {
+        for (const Value& f : fields) {
+            if (f.Find("name")->AsString() == name) return f;
+        }
+        throw std::runtime_error(std::format("no field {}", name));
+    };
+    Check(field("slot").Find("required") != nullptr && field("slot").Find("default") == nullptr, "a Required field is marked required, with no default");
+    Check(field("slot").Find("type")->Find("maximum")->AsInteger() == 255, "a narrow integer carries its range");
+    Check(field("yaw").Find("default")->AsNumber() == 45, "a default comes from the member initializer");
+    Check(field("at").Find("type")->Find("fields")->Size() == 3 && field("at").Find("default")->Find("y")->AsNumber() == 1, "a nested aggregate describes its fields and default");
+    Check(field("label").Find("type")->Find("nullable") != nullptr && field("label").Find("default")->IsNull(), "an optional is nullable and defaults to null");
+    Check(field("mode").Find("type")->Find("values")->Size() == 2 && field("mode").Find("default")->AsString() == "Build", "an enum lists its names");
+    Check(field("ids").Find("type")->Find("items")->Find("type")->AsString() == "integer", "a vector describes its items");
+    Check(!Describe<Game::Node>().Dump().empty(), "a type that contains itself is described without recursing forever");
+
+    return failures == 0 ? 0 : 1;
+}
diff --git a/tests/ShouldReportDecodeErrorsWithPaths/main.cpp b/tests/ShouldReportDecodeErrorsWithPaths/main.cpp
new file mode 100644
index 0000000..6060b04
--- /dev/null
+++ b/tests/ShouldReportDecodeErrorsWithPaths/main.cpp
@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// A command call with several mistakes gets one reply that names each of
+// them: a missing Required field, a wrong type naming both kinds, a nested
+// path ("path[1].x"), an out-of-range integer with the range, and for an
+// unknown field or enum name the one the caller most likely meant. Decoding
+// never throws, so a bad call cannot take the app down. Guards the replies an
+// agent corrects its calls from.
+import std;
+import Crafter.Debug.Core;
+using namespace Crafter::Debug;
+
+namespace Game {
+    enum class Mode : std::uint8_t { Idle, Build, Fight };
+
+    struct Vec3 {
+        float x = 0;
+        float y = 0;
+        float z = 0;
+    };
+
+    struct Aim {
+        Required<std::uint8_t> slot;
+        float yaw = 0;
+        float pitch = 0;
+        std::vector<Vec3> path;
+        Mode mode = Mode::Idle;
+    };
+}
+
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool ok, std::string_view what) {
+        std::println("{} {}", ok ? "PASS" : "FAIL", what);
+        if (!ok) ++failures;
+    }
+
+    bool Has(const DecodeContext& context, std::string_view path, std::string_view text) {
+        return std::ranges::any_of(context.Issues(), [&](const DecodeIssue& issue) {
+            return issue.path == path && issue.message.find(text) != std::string::npos;
+        });
+    }
+}
+
+int main() {
+    Game::Aim aim;
+    DecodeContext context;
+    Decode(Value::Parse(R"({"yw": 1, "pitch": "up", "path": [{"x": 1}, {"x": "no"}], "mode": "Fihgt"})"), aim, context);
+    std::println("report: {}", context.Report());
+    Check(context.Issues().size() == 5, std::format("five mistakes, five issues ({})", context.Issues().size()));
+    Check(Has(context, "slot", "required"), "a missing Required field is reported");
+    Check(Has(context, "pitch", "expected a number, got a string"), "a wrong type names both kinds");
+    Check(Has(context, "path[1].x", "expected a number"), "a nested path names the item and the member");
+    Check(Has(context, "mode", "did you mean 'Fight'"), "a mistyped enum name gets a suggestion");
+    Check(Has(context, "yw", "did you mean 'yaw'"), "an unknown field gets a suggestion");
+    Check(context.Report().find("slot: missing") != std::string::npos, "the report prefixes each issue with its path");
+
+    Game::Aim outOfRange;
+    DecodeContext rangeContext;
+    Decode(Value::Parse(R"({"slot": 300})"), outOfRange, rangeContext);
+    Check(Has(rangeContext, "slot", "300 is out of range (0..255)"), "an out-of-range integer names the range");
+
+    Game::Aim notAnObject;
+    DecodeContext objectContext;
+    Decode(Value::Parse("[1]"), notAnObject, objectContext);
+    Check(objectContext.Issues().size() == 1 && objectContext.Issues()[0].path.empty()
+              && objectContext.Issues()[0].message == "expected an object, got an array",
+          "a non-object is one issue at the root");
+
+    Game::Aim farOff;
+    DecodeContext farContext;
+    Decode(Value::Parse(R"({"slot": 1, "zzzzzz": 1})"), farOff, farContext);
+    Check(Has(farContext, "zzzzzz", "unknown field (fields: slot, yaw, pitch, path, mode)"), "an unknown field with no close match lists the fields");
+
+    return failures == 0 ? 0 : 1;
+}
diff --git a/tests/ShouldRoundTripAggregates/main.cpp b/tests/ShouldRoundTripAggregates/main.cpp
new file mode 100644
index 0000000..be30d8e
--- /dev/null
+++ b/tests/ShouldRoundTripAggregates/main.cpp
@@ -0,0 +1,94 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// Codec round-trips an aggregate built from every supported field type
+// (strings, nested aggregates in a vector, optional, std::array, string-keyed
+// maps, a scoped enum, Required) through Value and its text, with no
+// hand-written serialization. Fields the input leaves out keep their default
+// member initializers, null clears an optional, and an enum also accepts its
+// integer. Guards the reflection that command arguments and results rely on.
+import std;
+import Crafter.Debug.Core;
+using namespace Crafter::Debug;
+
+namespace Game {
+    enum class Mode : std::uint8_t { Idle, Build, Fight = 5 };
+
+    struct Vec3 {
+        float x = 0;
+        float y = 0;
+        float z = 0;
+    };
+
+    struct Loadout {
+        std::string name = "default";
+        std::vector<Vec3> points;
+        std::optional<std::int32_t> target;
+        std::array<bool, 3> flags{};
+        std::map<std::string, double> weights;
+        Mode mode = Mode::Idle;
+        Required<std::uint16_t> slot;
+    };
+}
+
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool ok, std::string_view what) {
+        std::println("{} {}", ok ? "PASS" : "FAIL", what);
+        if (!ok) ++failures;
+    }
+}
+
+int main() {
+    Game::Loadout in;
+    in.name = "fort \"one\"";
+    in.points = { { 1, 2, 3 }, { -0.5F, 0, 1e10F } };
+    in.target = -7;
+    in.flags = { true, false, true };
+    in.weights = { { "cannon", 0.25 }, { "laser", 1 } };
+    in.mode = Game::Mode::Fight;
+    *in.slot = 3;
+
+    const Value encoded = Encode(in);
+    Check(encoded.IsObject() && encoded.Size() == 7, "an aggregate encodes as an object with one member per field");
+    Check(encoded.Find("mode")->AsString() == "Fight", "a scoped enum encodes by name");
+    Check(encoded.Find("slot")->AsInteger() == 3, "Required encodes as its value");
+    Check(encoded.Find("target")->AsInteger() == -7, "an optional with a value encodes as the value");
+    Check(encoded.Find("points")->Items()[1].Find("z")->AsNumber() == 1e10F, "nested aggregates encode inside a vector");
+
+    Game::Loadout out;
+    DecodeContext context;
+    Decode(Value::Parse(encoded.Dump()), out, context);
+    Check(context.Ok(), std::format("decodes what it encoded ({})", context.Report()));
+    Check(out.name == in.name && out.points.size() == 2 && out.points[1].z == 1e10F && out.target == -7 && out.flags[2]
+              && out.weights.at("laser") == 1 && out.mode == Game::Mode::Fight && *out.slot == 3,
+          "every field survives the round trip");
+
+    Game::Loadout partial;
+    partial.target = 9;
+    DecodeContext partialContext;
+    Decode(Value::Parse(R"({"slot": 1, "target": null, "mode": 1})"), partial, partialContext);
+    Check(partialContext.Ok(), std::format("a partial object decodes ({})", partialContext.Report()));
+    Check(partial.name == "default" && partial.points.empty(), "fields left out keep their defaults");
+    Check(!partial.target.has_value(), "null clears an optional");
+    Check(partial.mode == Game::Mode::Build, "an enum accepts its integer");
+
+    float nan = 0;
+    DecodeContext nanContext;
+    Decode(Encode(std::numeric_limits<float>::quiet_NaN()), nan, nanContext);
+    Check(nanContext.Ok() && std::isnan(nan), "NaN survives the round trip");
+
+    std::vector<bool> bits;
+    DecodeContext bitsContext;
+    Decode(Value::Parse("[true,false,true]"), bits, bitsContext);
+    Check(bitsContext.Ok() && bits.size() == 3 && bits[2] && !bits[1], "std::vector<bool> decodes");
+
+    const std::uint64_t large = (std::uint64_t{ 1 } << 63) + 2048;
+    std::uint64_t decoded = 0;
+    DecodeContext largeContext;
+    Decode(Value::Parse(Encode(large).Dump()), decoded, largeContext);
+    Check(largeContext.Ok() && decoded == large, "a uint64 above the int64 range survives");
+
+    return failures == 0 ? 0 : 1;
+}
diff --git a/tests/ShouldRoundTripJson/main.cpp b/tests/ShouldRoundTripJson/main.cpp
new file mode 100644
index 0000000..e04a1e9
--- /dev/null
+++ b/tests/ShouldRoundTripJson/main.cpp
@@ -0,0 +1,81 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// Value's JSON text, the format the app and crafter-debug exchange. Parse
+// and Dump round-trip every kind, escapes and \u sequences (surrogate pairs
+// included) decode to UTF-8, integers stay integers and whole doubles stay
+// doubles, non-finite doubles are written as the strings NaN and Infinity,
+// invalid UTF-8 in a string is replaced so the output always parses, and
+// malformed text is refused with the offset of the problem.
+import std;
+import Crafter.Debug.Core;
+using namespace Crafter::Debug;
+
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool ok, std::string_view what) {
+        std::println("{} {}", ok ? "PASS" : "FAIL", what);
+        if (!ok) ++failures;
+    }
+
+    bool Refused(std::string_view text) {
+        try {
+            (void)Value::Parse(text);
+            return false;
+        } catch (const ParseError&) {
+            return true;
+        }
+    }
+}
+
+int main() {
+    const Value parsed = Value::Parse(R"({"a":1,"b":[true,false,null],"c":"x\"y\\z\n","d":-2.5,"e":{"f":"éé😀"}})");
+    Check(parsed.IsObject() && parsed.Size() == 5, "parses an object with five members");
+    Check(parsed.Find("a") != nullptr && parsed.Find("a")->IsInteger() && parsed.Find("a")->AsInteger() == 1, "an integer stays an integer");
+    Check(parsed.Find("d")->Kind() == ValueKind::Number && parsed.Find("d")->AsNumber() == -2.5, "a fraction is a number");
+    Check(parsed.Find("c")->AsString() == "x\"y\\z\n", "escapes decode");
+    Check(parsed.Find("e")->Find("f")->AsString() == "\xC3\xA9\xC3\xA9\xF0\x9F\x98\x80", "\\u escapes and surrogate pairs decode to UTF-8");
+    Check(Value::Parse(parsed.Dump()) == parsed, "compact text round-trips");
+    Check(Value::Parse(parsed.DumpPretty()) == parsed, "pretty text round-trips");
+    Check(parsed.Dump().starts_with(R"({"a":1,"b":[true,false,null],)"), "members keep their order");
+
+    Check(Value(1.0).Dump() == "1.0", "a whole double keeps a fraction");
+    Check(Value::Parse("1.0").Kind() == ValueKind::Number, "1.0 reads back as a number");
+    Check(Value(1.0) == Value(1), "Integer 1 equals Number 1.0");
+    Check(Value(std::numeric_limits<double>::quiet_NaN()).Dump() == "\"NaN\"", "NaN is written as a string");
+    Check(Value(-std::numeric_limits<double>::infinity()).Dump() == "\"-Infinity\"", "-Infinity is written as a string");
+    Check(Value(std::numeric_limits<std::uint64_t>::max()).Kind() == ValueKind::Number, "a uint64 above the int64 range becomes a number");
+    Check(Value::Parse("9223372036854775807").AsInteger() == std::numeric_limits<std::int64_t>::max(), "int64 max parses as an integer");
+    Check(Value::Parse("9223372036854775808").Kind() == ValueKind::Number, "beyond int64 parses as a number");
+
+    Check(Value(std::string("ok\xff\xc3")).Dump() == "\"ok\xEF\xBF\xBD\xEF\xBF\xBD\"", "invalid UTF-8 becomes U+FFFD");
+    Check(Value(std::string("\x01\x7f")).Dump() == "\"\\u0001\\u007f\"", "control characters are escaped");
+    Check(Value::Parse(Value(std::string("tab\tquote\"")).Dump()).AsString() == "tab\tquote\"", "escaped text reads back");
+
+    Check(Refused(R"({"a":1,})"), "a trailing comma is refused");
+    Check(Refused("[1 2]"), "a missing comma is refused");
+    Check(Refused("01"), "a leading zero is refused");
+    Check(Refused(R"("\ud800")"), "an unpaired surrogate is refused");
+    Check(Refused("\"a\nb\""), "a raw newline in a string is refused");
+    Check(Refused("1e999"), "an out-of-range number is refused");
+    Check(Refused(std::string(300, '[') + std::string(300, ']')), "nesting beyond the limit is refused");
+    Check(Refused(""), "empty text is refused");
+    try {
+        (void)Value::Parse("[1, x]");
+        Check(false, "a parse error carries its offset");
+    } catch (const ParseError& error) {
+        Check(error.offset == 4, std::format("a parse error carries its offset ({})", error.offset));
+    }
+
+    Value object;
+    object["z"] = Value(1);
+    object["a"] = Value("two");
+    object.Set("z", Value(3));
+    Check(object.Dump() == R"({"z":3,"a":"two"})", "Set replaces in place, keeping the order");
+    Check(object.Erase("z") && object.Dump() == R"({"a":"two"})", "Erase removes a member");
+    Check(Value::Parse(R"({"a":1,"b":2})") == Value::Parse(R"({"b":2,"a":1})"), "member order does not affect equality");
+    Check(!(Value::Parse("[1,2]") == Value::Parse("[2,1]")), "item order does affect equality");
+
+    return failures == 0 ? 0 : 1;
+}