diff --git a/AGENTS.md b/AGENTS.md
index d6be6d2..1c445f4 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -117,7 +117,7 @@ Beyond the rules:
 - There is no host-platform preprocessor split: the host is Linux. Target differences (mingw, wasm) branch on `cfg.target` strings at runtime, in the library as well as in `project.cpp` and tests.
 - A build step fails when its command exits nonzero, never because it printed. Use `RunBuildCommand` or `RunCommandChecked`; warnings go to `Progress::Diagnostic`.
 - Every output path goes through `Configuration::BinDir()`, `BuildDir()` and `PcmDir()`. Hand-rolled `path / "bin" / ...` clobbered variants before; do not add new ones.
-- Anything that changes what gets compiled or bundled belongs in `VariantId()`. It already hashes type, debug, sysroot, defines, compile flags, wasm variant flags, uninterpreted project args and the toolchain (`Toolchain::Key()`).
+- Anything that changes what gets compiled or bundled belongs in `VariantId()`. It already hashes type, debug, sysroot, defines, compile flags, the wasm baseline and variant flags, uninterpreted project args and the toolchain (`Toolchain::Key()`).
 
 ## Design rules
 
@@ -191,5 +191,7 @@ Local builds are two scripts in `project.cpp`. Run them with the native binary:
 - The bootstrap binary is compiled without `CRAFTER_BUILD_HAS_ASSET`, so `cfg.assets` only works in the self-hosted binary. Changing asset extensions means updating two maps in `Crafter.Build-Clang.cpp`: `assetOutput` and the transitive copy loop.
 - A warning is printed only when its unit actually recompiles. A build with nothing to do is silent, which is not evidence that a warning is gone.
 - A std module source only compiles against the headers of its own libc++ release, and a sysroot does not always ship one (Alpine). The Flatpak therefore carries the sources of the release next to the bundled one under `share/crafter-build/libc++-modules/<major>/`, one `libcxx-<major>-modules` manifest module each. When the bundled LLVM is bumped, keep the release it leaves behind as such a module: sysroots on the stable distro stay on it for months. The native loop has no such directory; stage one by hand under `share/crafter-build/` to test against a foreign sysroot.
+- The wasm compile flags live in one place, `WasmBaselineFlags` in `Crafter.Build-Clang.cppm`: the std module and every unit have to agree on them, and `VariantId()` and the std PCM directory hash them, so a change rebuilds instead of mixing old and new BMIs. They enable C++ exceptions, so the link takes the exception-enabled libc++ and libunwind from the sysroot's `lib/<triple>/eh/` (wasi-sdk 33 and later). The Flatpak's sysroot has them and a distro's may not; where they are missing the wasm tests skip, so run those through the Flatpak. A browser page checks the engine with the `WasmBaselineProbes` named in `variants.json`.
+- `WasiBrowserRuntimeExit` runs `wasi-runtime/runtime.js` under node and skips where there is none. The Flatpak has no node, so CI never runs it: after touching `runtime.js`, run the suite in the native loop with node on `PATH`.
 - `VariantId()` names the output directory, so CI globs `bin/crafter.build-exe-<target>-<march>-*/` rather than predicting the hash. The hash includes the compiler, so after a toolchain update the previous variant's directories stay in `bin/` and `build/` until `clean`, and such a glob matches both.
 - The self-host build and `test` keep the dependency notes section of this very file. None of Crafter.Math, Crafter.Asset and Crafter.LintRules ships an `AGENTS.consumer.md` yet. Once one does, a build here can change AGENTS.md whenever that repository changes its notes, since all three are unpinned. The tree is then dirty, so the next build's `git describe --dirty` version differs, which is a new `crafter.build-lib` variant and a full rebuild. Commit the section to settle it.
diff --git a/README.md b/README.md
index 53844fe..77514ec 100644
--- a/README.md
+++ b/README.md
@@ -187,6 +187,16 @@ For cross-arch builds, set `cfg.sysroot` on the `Configuration` to point at a sy
 
 For `--target=x86_64-w64-mingw32` (Windows from Linux), the pipeline cross-compiles as long as `mingw-w64` is installed, with libstdc++ 15 or newer for its std module. clang finds it by the `x86_64-w64-mingw32-gcc` on `PATH`; `CRAFTER_MINGW_DIR` names another prefix (the directory holding `x86_64-w64-mingw32/` and `lib/gcc/`), and inside the Flatpak the host's `/usr` is used when it has mingw-w64. Without one the build stops with a message saying where it looked. Pair it with `--runner=cmd:wine` to run the result. Only the MinGW ABI is supported, there is no MSVC target and no Windows host. `-fuse-ld=lld` is auto-injected on every link, and `-lstdc++exp` is auto-linked for the mingw target so user projects don't need to declare those flags.
 
+For `--target=wasm32-wasip1`, the WASI sysroot is the one beside the toolchain (the Flatpak bundles one) unless `CRAFTER_BUILD_WASI_SYSROOT` names another, and tests run under `wasmtime`. C++ exceptions work as on the other targets: every unit and the std module are compiled with `-fwasm-exceptions` in the standardized form of WebAssembly exception handling (exnref), against the exception-enabled libc++ in the sysroot's `lib/<triple>/eh/`, and `-lunwind` is linked automatically. A sysroot without that libc++ (wasi-sdk before 33) stops the build with one line saying so. The result runs on engines with that proposal: Chrome 137, Firefox 131 and Safari 18.4 or newer, and the wasmtime the Flatpak bundles. A page from `EnableWasiBrowserRuntime` checks first and names what an older browser lacks in the console, instead of failing to compile the module. An exception that escapes `main` leaves the program as an uncaught WebAssembly exception (wasmtime reports `thrown Wasm exception` and exits 1, a page gets a `WebAssembly.Exception`) without calling `std::terminate`, so a `std::set_terminate` handler does not run and nothing prints the exception's type or `what()`: wasm exception handling unwinds in a single phase, and nothing finds out that no handler is left until the stack is gone. Catch it in `main` to report it.
+
+A wasm build refuses three things before it starts, each with a message that says why: `-flto` in `compileFlags` or `linkFlags` (LTO code generation drops the exception handling, [WebAssembly/wasi-sdk#629](https://github.com/WebAssembly/wasi-sdk/issues/629)), `-fno-exceptions` (the std module is built with exceptions, and a unit without them cannot import it) and a shared library (the exception-enabled libc++ is static archives only). `setjmp` and `longjmp` work in C and C++ sources, with `-lsetjmp` linked automatically like `-lunwind`; a `longjmp` across a `noexcept` function ends the program, because it is a wasm exception and the function's terminate handler catches it. Outside the Flatpak, a wasmtime older than 47 has exception handling off and needs `-W exceptions=y`, and a `wasm-opt` on `PATH` from a binaryen that cannot read the standardized encoding breaks release links, since clang runs it on optimized wasm links when it finds one.
+
+In a page from `EnableWasiBrowserRuntime`, `exit` and `_Exit` end the program as they would anywhere else: none of its code runs on the way out, no destructor and no `catch (...)`, its status is in `window.crafter_wasi.exitCode`, and bridges that call into it on their own are expected to stop. A program that hands the thread to the browser for good and goes on serving calls, as Crafter.Graphics's `Window::StartSync()` does, parks instead of exiting. Parking never returns, runs nothing of the program either, and leaves `main`'s locals valid for the calls that follow:
+
+```cpp
+extern "C" [[noreturn]] __attribute__((import_module("crafter_runtime"), import_name("park"))) void Park();
+```
+
 ### Configuring runners
 
 Two layers, CLI wins over env, both override the per-test default of `Local()`:
diff --git a/examples/wasi/README.md b/examples/wasi/README.md
index 7ac0f42..8a8db56 100644
--- a/examples/wasi/README.md
+++ b/examples/wasi/README.md
@@ -4,9 +4,14 @@ Build a C++26 program for the `wasm32-wasi` target and run it in a browser.
 
 ## Prerequisites
 
-- `wasi-libc`, `wasi-libc++`, `wasi-libc++abi` (Arch packages — provide
-  `/usr/share/wasi-sysroot/`, which `crafter-build` finds automatically).
-  Other distros: install the WASI SDK and set `cfg.sysroot` in `project.cpp`.
+- A WASI sysroot with the exception-enabled libc++, `lib/wasm32-wasip1/eh/`
+  from wasi-sdk 33 or later: wasm builds compile C++ exceptions and link those
+  libraries. The Flatpak bundles one, which `crafter-build` uses as it is.
+  Elsewhere, point `CRAFTER_BUILD_WASI_SYSROOT` at a wasi-sdk sysroot, or set
+  `cfg.sysroot` in `project.cpp`. Arch's `wasi-libc++` packages have no such
+  libraries, and the build stops with a message saying so.
+- For the browser: WebAssembly exception handling, in Chrome 137, Firefox 131,
+  Safari 18.4 or newer.
 
 ## Build
 
diff --git a/implementations/Crafter.Build-Clang.cpp b/implementations/Crafter.Build-Clang.cpp
index 35b64d7..e0d8329 100644
--- a/implementations/Crafter.Build-Clang.cpp
+++ b/implementations/Crafter.Build-Clang.cpp
@@ -256,6 +256,15 @@ fs::path Crafter::StdPcmDir(const Configuration& config) {
     // the consuming TUs see a target-feature mismatch. Suffix the cache dir
     // with the variant flags when present.
     std::string stdPcmKey = std::format("{}-{}-{}", config.target, config.march, GetToolchain().Key());
+    // The flags every wasm unit gets are compiled into the std PCM as well,
+    // and the importing units have to match them. Two releases that share a
+    // cache and a compiler, as the Flatpak's branches do, would otherwise
+    // take turns rebuilding this one std PCM, each time underneath the PCMs
+    // built against the previous one, and clang crashes on those rather than
+    // rejecting them.
+    if (config.target.starts_with("wasm32")) {
+        stdPcmKey += std::format("-{:08x}", static_cast<std::uint32_t>(std::hash<std::string_view>{}(WasmBaselineFlags)));
+    }
     for (const std::string& f : config.wasmVariantFlags) {
         stdPcmKey += "+";
         for (char c : f) stdPcmKey += (c == '/' || c == '\\') ? '_' : c;
@@ -263,6 +272,12 @@ fs::path Crafter::StdPcmDir(const Configuration& config) {
     return GetCacheDir()/stdPcmKey;
 }
 
+std::string Crafter::WasmCompileFlags(const Configuration& config) {
+    std::string flags = std::format(" {}", WasmBaselineFlags);
+    for (const std::string& f : config.wasmVariantFlags) flags += std::format(" {}", f);
+    return flags;
+}
+
 fs::path Crafter::ExpectedOutput(const Configuration& config) {
     const fs::path dir = config.BinDir();
     const bool mingwTarget = config.target == "x86_64-w64-mingw32";
@@ -317,15 +332,11 @@ CompileCommand Crafter::GetCompileCommand(const Configuration& config) {
     if (isWasm) {
         // -mllvm is consumed by codegen but not the link driver, which is the
         // same command line; quiet the unused-flag warning rather than split
-        // compile and link commands.
-        out.command += " -fno-exceptions -msimd128 -fno-c++-static-destructors -mllvm -wasm-enable-sjlj -D_WASI_EMULATED_SIGNAL -Wno-unused-command-line-argument";
-        // Active variant pass (see the variant driver at the end of Build):
-        // extra codegen flags (e.g. -mrelaxed-simd) applied to every TU. Empty
-        // for the baseline build. Part of VariantId, so these objects/PCMs
-        // land in their own dir.
-        for (const std::string& f : config.wasmVariantFlags) {
-            out.command += std::format(" {}", f);
-        }
+        // compile and link commands. An active variant pass (see the variant
+        // driver at the end of Build) adds its codegen flags (e.g.
+        // -mrelaxed-simd) to every TU; they are part of VariantId, so those
+        // objects/PCMs land in their own dir.
+        out.command += std::format("{} -Wno-unused-command-line-argument", WasmCompileFlags(config));
     }
     if (config.target == "x86_64-w64-mingw32") {
         // mingw libstdc++ defines TLS via __emutls_v.* (emulated TLS); without
@@ -405,12 +416,14 @@ CompileCommand Crafter::GetCompileCommand(const Configuration& config) {
     // (cross-TU inlining) plus dead-section GC and safe identical-code folding.
     // Default-on in Release, never Debug (keeps builds fast and debuggable).
     // Excluded for wasm32 — its -mllvm/sjlj codegen flags don't compose with
-    // LTO here — and for nvcc .cu objects below (they can't emit LLVM bitcode,
-    // so they link in as ordinary objects alongside the bitcode ones). ThinLTO
-    // rather than monolithic -flto so link time and memory scale with arbitrary
-    // user project sizes. Compile-side flags (-flto, -ffunction/-fdata-sections)
-    // ride on `command`, which is reused as the link base; the link-only -Wl
-    // flags go on linkExtras so they don't warn on each -c compile.
+    // LTO here, and wasi-sdk lists exceptions under LTO as broken
+    // (WebAssembly/wasi-sdk#629) — and for nvcc .cu objects below (they can't
+    // emit LLVM bitcode, so they link in as ordinary objects alongside the
+    // bitcode ones). ThinLTO rather than monolithic -flto so link time and
+    // memory scale with arbitrary user project sizes. Compile-side flags
+    // (-flto, -ffunction/-fdata-sections) ride on `command`, which is reused
+    // as the link base; the link-only -Wl flags go on linkExtras so they
+    // don't warn on each -c compile.
     out.useLto = !config.debug && !isWasm;
     out.ltoCompileFlags = out.useLto ? " -flto=thin -ffunction-sections -fdata-sections" : "";
     out.ltoLinkFlags = out.useLto ? " -flto=thin -Wl,--gc-sections -Wl,--icf=safe" : "";
@@ -480,6 +493,54 @@ namespace {
         walk(config, config.name);
         return error;
     }
+
+    // What a wasm32 configuration cannot be built with now that every unit
+    // has C++ exceptions, and what its sysroot has to have for that; empty
+    // for any other target and when nothing is wrong. Each would otherwise
+    // fail at the first compile or link with a message that does not say
+    // why, or not fail at all.
+    std::string WasmBuildError(const Configuration& config) {
+        if (!config.target.starts_with("wasm32")) return "";
+        for (const std::vector<std::string>* flags : { &config.compileFlags, &config.linkFlags }) {
+            for (const std::string& flag : *flags) {
+                // The compile has the backend options that keep exception
+                // handling; LTO code generation at the link does not, so it
+                // drops every landing pad and the build succeeds with throws
+                // that escape their catch blocks (WebAssembly/wasi-sdk#629).
+                if (flag.starts_with("-flto")) {
+                    return std::format("{}: {} is not supported on wasm32: LTO code generation drops C++ exception handling (WebAssembly/wasi-sdk#629), so every throw would escape its catch", config.name, flag);
+                }
+                // The std module is built with exceptions, and a unit built
+                // without them cannot import it.
+                if (flag == "-fno-exceptions" || flag == "-fno-cxx-exceptions") {
+                    return std::format("{}: {} is not supported on wasm32: wasm builds have C++ exceptions, the std module included, and a unit compiled without them cannot import std", config.name, flag);
+                }
+            }
+        }
+        // wasi-sdk's exception support does not cover shared libraries: the
+        // libc++ under lib/<triple>/eh is static archives built without -fPIC.
+        if (config.type == ConfigurationType::LibraryDynamic) {
+            return std::format("{}: wasm32 shared libraries are not supported: the exception-enabled libc++ they would link has static archives only", config.name);
+        }
+        const fs::path sysroot = config.sysroot;
+        if (!fs::is_directory(sysroot)) {
+            return std::format("no WASI sysroot at {}: the Flatpak bundles one, and CRAFTER_BUILD_WASI_SYSROOT selects another", sysroot.string());
+        }
+        const fs::path libDir = sysroot / "lib" / NormalizeTarget(config.target);
+        if (!fs::is_directory(libDir)) {
+            return std::format("the WASI sysroot at {} has nothing for {} (no {})", sysroot.string(), config.target, libDir.string());
+        }
+        // Only a link takes the exception-enabled libraries: a static
+        // library's objects compile against the headers alone. The driver
+        // looks in eh/ first and then in the triple's own directory, where a
+        // sysroot built with exceptions only (WASI_SDK_EXCEPTIONS=ON) has them.
+        // Without them the first link fails with "unable to find library
+        // -lunwind", which says nothing about why it was asked for.
+        if (config.type == ConfigurationType::Executable && !fs::exists(libDir / "eh" / "libunwind.a") && !fs::exists(libDir / "libunwind.a")) {
+            return std::format("the WASI sysroot at {} has no C++ exception support (no {}): wasm builds use WebAssembly exception handling and need the exception-enabled libc++ and libunwind that wasi-sdk 33 and later ship. The Flatpak bundles such a sysroot; CRAFTER_BUILD_WASI_SYSROOT selects another", sysroot.string(), (libDir / "eh" / "libunwind.a").string());
+        }
+        return "";
+    }
 }
 
 BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, std::shared_future<BuildResult>>& depResults, std::mutex& depMutex) {
@@ -540,6 +601,11 @@ BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, s
     if (config.sysroot.empty() && config.target.starts_with("wasm32")) {
         config.sysroot = GetToolchain().wasiSysroot.string();
     }
+    // As early as the duplicate-module check, so a refused build leaves no
+    // directory behind and starts no thread, but after the sysroot is known.
+    if (std::string refused = WasmBuildError(config); !refused.empty()) {
+        return {std::move(refused), false, {}};
+    }
 
     fs::path buildDir = config.BuildDir();
     fs::path outputDir = config.BinDir();
@@ -857,8 +923,7 @@ BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, s
     // Same target-aware setup as the C++ compile path (line 459-): wasm32
     // rejects -march, silently ignores -mtune, and needs --sysroot to find
     // wasi-libc headers. Build the prefix once so all C compiles share it.
-    const bool cIsWasm = config.target.starts_with("wasm32");
-    std::string cArchFlags = cIsWasm
+    std::string cArchFlags = isWasm
         ? std::string()
         : std::format(" -march={} -mtune={}", config.march, config.mtune);
     if (!config.sysroot.empty()) {
@@ -867,11 +932,13 @@ BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, s
         // The C++ side gets it through GetBaseCommand.
         cArchFlags += std::format(" --sysroot={}", GetToolchain().mingwSysroot.string());
     }
-    if (cIsWasm) {
-        // Matches the C++ path's wasi flag set so any libc shim defines
-        // (e.g. _WASI_EMULATED_SIGNAL → signal.h shims) are visible to
-        // C dependencies that drag in signal.h transitively.
-        cArchFlags += " -D_WASI_EMULATED_SIGNAL";
+    if (isWasm) {
+        // The C++ path's wasm flags, all of them, a variant pass's included.
+        // With only the signal.h define, wasi-libc's setjmp.h refused to
+        // compile in C, and a C object given -wasm-enable-sjlj through
+        // compileFlags used LLVM's legacy exception encoding next to C++
+        // objects in the standard one, a mix engines refuse to load.
+        cArchFlags += WasmCompileFlags(config);
     }
     for (const fs::path& cFile : config.cFiles) {
         files += std::format(" {}_source.o ", (buildDir / cFile.filename()).string());
@@ -1139,6 +1206,17 @@ BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, s
         // compile so __once_callable et al resolve in static libstdc++.
         linkExtras += " -lstdc++exp -static-libstdc++ -static-libgcc -Wl,-Bstatic -lpthread -Wl,-Bdynamic";
     }
+    // Code built with -fwasm-exceptions calls into libunwind (the
+    // __cpp_exception tag, the landing-pad context, _Unwind_RaiseException),
+    // and -wasm-enable-sjlj turns setjmp and longjmp into calls into
+    // wasi-libc's libsetjmp (__wasm_setjmp, __wasm_longjmp, the __c_longjmp
+    // tag). The driver adds neither to the link the way it adds libc++. A
+    // static archive contributes only what something references, so a
+    // program without setjmp gets nothing from libsetjmp, and every sysroot
+    // that passes WasmBuildError ships both.
+    if (isWasm) {
+        linkExtras += " -lunwind -lsetjmp";
+    }
 
     // Force a relink if the expected output is missing or older than any dep
     // artifact. Missing covers: previous build produced a different outputName,
@@ -1482,10 +1560,13 @@ void Crafter::EnableWasiBrowserRuntime(Configuration& cfg) {
     // Variant manifest. runtime.js fetches this, runs each entry's probes in
     // order, and instantiates the first whose probes all pass — so the
     // preferred (most feature-rich) variants come first and the baseline last
-    // as the universal fallback (empty probes always pass). A variant with a
-    // non-empty label maps to outputName.<label>.wasm (produced by Build's
-    // variant driver); the baseline maps to outputName.wasm. Even with no
-    // declared variants we still emit a one-entry manifest so the runtime has a
+    // as the fallback. Every entry carries WasmBaselineProbes besides its own:
+    // every file is built with the baseline flags, so an engine without
+    // exception handling can run none of them, and runtime.js tells it so
+    // instead of failing to compile the module. A variant with a non-empty
+    // label maps to outputName.<label>.wasm (produced by Build's variant
+    // driver); the baseline maps to outputName.wasm. Even with no declared
+    // variants we still emit a one-entry manifest so the runtime has a
     // single, uniform code path.
     auto jsonEscape = [](std::string_view s) {
         std::string out;
@@ -1500,12 +1581,14 @@ void Crafter::EnableWasiBrowserRuntime(Configuration& cfg) {
         std::ofstream v(variantsPath);
         v << "[";
         bool first = true;
-        auto emit = [&](const std::string& label, const std::vector<std::string>& probes) {
+        auto emit = [&](const std::string& label, const std::vector<std::string>& ownProbes) {
             if (!first) v << ",";
             first = false;
             std::string url = label.empty()
                 ? std::format("{}.wasm", cfg.outputName)
                 : std::format("{}.{}.wasm", cfg.outputName, label);
+            std::vector<std::string> probes = ownProbes;
+            probes.insert(probes.end(), WasmBaselineProbes.begin(), WasmBaselineProbes.end());
             v << "{\"label\":\"" << jsonEscape(label) << "\""
               << ",\"url\":\"" << jsonEscape(url) << "\""
               << ",\"probes\":[";
diff --git a/implementations/Crafter.Build-Platform.cpp b/implementations/Crafter.Build-Platform.cpp
index 5f7460c..ad88ab6 100644
--- a/implementations/Crafter.Build-Platform.cpp
+++ b/implementations/Crafter.Build-Platform.cpp
@@ -818,19 +818,12 @@ std::string Crafter::BuildStdPcm(const Configuration& config, fs::path stdPcm) {
         std::string sysrootFlag = config.sysroot.empty()
             ? std::string()
             : std::format(" --sysroot={}", config.sysroot);
-        // wasm32 rejects -march. wasi-libc++ headers require these baselines
-        // to compile: setjmp.h needs -mllvm -wasm-enable-sjlj (lowered to wasm
-        // EH); signal.h requires the emulation define; and EH itself isn't
-        // wired up so -fno-exceptions stays.
+        // wasm32 rejects -march, and the std module has to be compiled the way
+        // the units importing it are, a variant pass's flags (e.g.
+        // -mrelaxed-simd) included, or the BMI is not feature-compatible.
         std::string archFlags = isWasm
-            ? std::string(" -fno-exceptions -msimd128 -fno-c++-static-destructors -mllvm -wasm-enable-sjlj -D_WASI_EMULATED_SIGNAL")
+            ? WasmCompileFlags(config)
             : std::format(" -march={} -mtune={}", config.march, config.mtune);
-        // Match the variant codegen flags the consuming TUs are built with
-        // (e.g. -mrelaxed-simd) so the std BMI is feature-compatible. wasm only;
-        // wasmVariantFlags is empty on every other path.
-        if (isWasm) {
-            for (const std::string& f : config.wasmVariantFlags) archFlags += std::format(" {}", f);
-        }
         // non-wasm cross builds: the driver ranks the host toolchain's own
         // libc++ headers (<install>/../include/c++/v1) above --sysroot, so
         // the PCM would mix host libc++ with the foreign libc — force the
diff --git a/interfaces/Crafter.Build-Clang.cppm b/interfaces/Crafter.Build-Clang.cppm
index 23b53b5..0797e88 100644
--- a/interfaces/Crafter.Build-Clang.cppm
+++ b/interfaces/Crafter.Build-Clang.cppm
@@ -11,6 +11,35 @@ import :Toolchain;
 import :Agents;
 namespace fs = std::filesystem;
 
+namespace Crafter {
+    // What every wasm32 translation unit is compiled with, C or C++, and the
+    // std module the C++ ones import. A BMI cannot be loaded by a unit whose
+    // options differ from the ones that wrote it, so GetCompileCommand and
+    // BuildStdPcm both take the set from here (through WasmCompileFlags), and
+    // VariantId and StdPcmDir hash it so that a crafter-build which changes
+    // it never reuses what the old set compiled.
+    //
+    // -fwasm-exceptions compiles C++ exceptions to WebAssembly exception
+    // handling. It also selects the sysroot's include/<triple>/eh and
+    // lib/<triple>/eh, the libc++ built with exceptions that wasi-sdk 33 and
+    // later ship next to the noeh/ one, so it has to reach the link too; the
+    // link reuses this command line, so it does. Those libraries use the
+    // standardized form of the proposal (try_table, exnref), while LLVM 23
+    // still defaults to the legacy one: wasmtime refuses to load that, and it
+    // would mix two encodings in one binary. -wasm-use-legacy-eh=false asks
+    // for the standard form, for setjmp as well. wasi-libc's setjmp.h does
+    // not compile without -wasm-enable-sjlj, which lowers setjmp to the same
+    // exception handling, and signal.h needs the emulation define.
+    inline constexpr std::string_view WasmBaselineFlags = "-fwasm-exceptions -mllvm -wasm-use-legacy-eh=false -msimd128 -fno-c++-static-destructors -mllvm -wasm-enable-sjlj -D_WASI_EMULATED_SIGNAL";
+    // The wasi-runtime/runtime.js probes an engine has to pass to run anything
+    // built with WasmBaselineFlags; EnableWasiBrowserRuntime puts them on every
+    // entry of variants.json. Exception handling with exnref is the newest
+    // feature those flags need (Chrome 137, Firefox 131, Safari 18.4). SIMD128
+    // is older, but V8 and SpiderMonkey switch it off on x86 CPUs without
+    // SSE4.1, which virtual machines often present.
+    inline constexpr auto WasmBaselineProbes = std::to_array<std::string_view>({"simd", "exception-handling"});
+}
+
 export namespace Crafter {
     struct BuildResult {
         std::string result;
@@ -38,8 +67,10 @@ export namespace Crafter {
     // `label` emits an additional `outputName.<label>.wasm` alongside the
     // baseline `outputName.wasm`; `probes` names the runtime feature-detect
     // checks (see wasi-runtime/runtime.js's probe registry) that must all pass
-    // for a browser to be served this variant. The first variant whose probes
-    // pass wins; the baseline is the universal fallback.
+    // for a browser to be served this variant, on top of WasmBaselineProbes,
+    // which every build needs. The first variant whose probes pass wins; the
+    // baseline is the fallback, and an engine that fails even its probes gets
+    // an error naming them.
     struct WasmVariant {
         std::string label;
         std::vector<std::string> flags;
@@ -586,6 +617,16 @@ export namespace Crafter {
                 compileKey += "|W:";
                 compileKey += f;
             }
+            // So does the set every wasm unit gets before those. Unkeyed, the
+            // release that turned exceptions on would have kept the objects
+            // and PCMs compiled without them: the first unit recompiled after
+            // the update would import BMIs built without exceptions, which
+            // clang rejects, and objects nothing recompiled would let
+            // exceptions pass through without running their destructors.
+            if (target.starts_with("wasm32")) {
+                compileKey += "|B:";
+                compileKey += WasmBaselineFlags;
+            }
             // Sorted by ApplyStandardArgs, so flag order on the command line
             // doesn't split the cache.
             for (const std::string& a : projectArgs) {
@@ -622,7 +663,8 @@ export namespace Crafter {
 
     // Directory holding the std module PCM for this Configuration. Keyed on
     // target+march because a BMI is only loadable by a TU with matching target
-    // features, and suffixed with the wasm variant flags for the same reason.
+    // features, and suffixed with the wasm flags (WasmBaselineFlags, then the
+    // variant's) for the same reason.
     fs::path StdPcmDir(const Configuration& config);
 
     // Where Build leaves this Configuration's linked artifact: the executable
@@ -696,7 +738,11 @@ export namespace Crafter {
     // open it). Opt-in: WASI builds destined for wasmtime/wasmer don't need
     // this and shouldn't carry the extra files. Call from project.cpp after
     // outputName is set; index.html is generated against the current
-    // outputName so renaming the binary later requires another call.
+    // outputName so renaming the binary later requires another call. In that
+    // runtime exit and _Exit end the program without running any more of it;
+    // a program that hands the thread to the browser and keeps serving calls
+    // parks instead, through the crafter_runtime.park import (runtime.js,
+    // PROGRAM_END).
     void EnableWasiBrowserRuntime(Configuration& cfg);
 
     // Register the relaxed-SIMD codegen variant on cfg. Builds an additional
@@ -705,7 +751,8 @@ export namespace Crafter {
     // it only to engines that report relaxed-SIMD support (Chrome 114+, Firefox
     // 120+) and falls back to the baseline outputName.wasm everywhere else
     // (e.g. Safari, which still gates relaxed SIMD behind a flag as of mid
-    // 2026). Idempotent. Call before EnableWasiBrowserRuntime so the emitted
+    // 2026). Both need what WasmBaselineProbes names, so an engine without
+    // exception handling gets neither but an error saying so. Idempotent. Call before EnableWasiBrowserRuntime so the emitted
     // variants.json manifest includes it. A no-op for non-wasm targets at build
     // time (no wasm is produced), but harmless to declare unconditionally.
     void EnableWasiRelaxedSimdVariant(Configuration& cfg);
@@ -739,3 +786,11 @@ export namespace Crafter {
     // for-arg-in-args loop.
     ArgQuery ApplyStandardArgs(Configuration& cfg, std::span<const std::string_view> args);
 }
+
+namespace Crafter {
+    // WasmBaselineFlags followed by the active variant pass's
+    // wasmVariantFlags, each with a leading space: the wasm part of every
+    // compile command and of the std module's, which have to agree flag for
+    // flag.
+    std::string WasmCompileFlags(const Configuration& config);
+}
diff --git a/packaging/flatpak/net.catcrafts.CrafterBuild.yaml b/packaging/flatpak/net.catcrafts.CrafterBuild.yaml
index 3584063..f812261 100644
--- a/packaging/flatpak/net.catcrafts.CrafterBuild.yaml
+++ b/packaging/flatpak/net.catcrafts.CrafterBuild.yaml
@@ -135,11 +135,22 @@ modules:
       - -DWASI_SDK_INSTALL_TO_CLANG_RESOURCE_DIR=ON
       - -DWASI_SDK_TARGETS=wasm32-wasip1;wasm32-wasip1-threads
       - -DWASI_SDK_INCLUDE_TESTS=OFF
+      # crafter-build compiles every wasm unit with C++ exceptions and links
+      # the libc++ and libunwind built with them, from lib/<triple>/eh. The
+      # default is DUAL only when the driving clang is 23 or newer and OFF
+      # below that; said outright, an LLVM downgrade cannot quietly ship a
+      # sysroot that no wasm build links against.
+      - -DWASI_SDK_EXCEPTIONS=DUAL
       - -DCMAKE_INSTALL_PREFIX=/app
     post-install:
       # wasi-sdk installs into <prefix>/share/wasi-sysroot; crafter-build
       # looks there (Crafter.Build:Toolchain), so nothing to move.
       - test -f /app/share/wasi-sysroot/share/libc++/v1/std.cppm
+      # The wasm tests skip when these are missing, as on a distro sysroot,
+      # so the Flatpak checks for them itself: without them its CI would
+      # pass while every wasm build failed.
+      - test -f /app/share/wasi-sysroot/lib/wasm32-wasip1/eh/libunwind.a
+      - test -f /app/share/wasi-sysroot/lib/wasm32-wasip1-threads/eh/libunwind.a
     sources:
       - type: git
         url: https://github.com/WebAssembly/wasi-sdk.git
diff --git a/project.cpp b/project.cpp
index 8a68830..83f70d6 100644
--- a/project.cpp
+++ b/project.cpp
@@ -266,8 +266,12 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
         cfg.AddTest("SysrootCompileFlags").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("VersionFlag").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("WasiBrowserRuntime").Dependencies({ CrafterBuildLib.get() });
+        cfg.AddTest("WasiBrowserRuntimeExit").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("WasmVariants").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("WasmHelloRuns").Dependencies({ CrafterBuildLib.get() });
+        cfg.AddTest("WasmExceptions").Dependencies({ CrafterBuildLib.get() });
+        cfg.AddTest("WasiSysrootWithoutExceptions").Dependencies({ CrafterBuildLib.get() });
+        cfg.AddTest("WasmBuildRefusals").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("RunSingleTestExit").Dependencies({ CrafterBuildLib.get() });
         // LoadProject dlopens the synthesized project.so, which references
         // Crafter:: symbols (HostTarget, Configuration ctors) that have to be
diff --git a/tests/WasiBrowserRuntime/main.cpp b/tests/WasiBrowserRuntime/main.cpp
index 103de24..9f0fe27 100644
--- a/tests/WasiBrowserRuntime/main.cpp
+++ b/tests/WasiBrowserRuntime/main.cpp
@@ -87,7 +87,10 @@ int main() {
     }
 
     // With no declared variants, variants.json is a single baseline entry
-    // pointing at outputName.wasm with no probes.
+    // pointing at outputName.wasm, with only the probes for what every wasm
+    // build needs, SIMD128 and exception handling, so that runtime.js can
+    // turn an engine without them away with a message rather than a
+    // CompileError.
     {
         std::ifstream vin(*variants);
         std::string vjson{std::istreambuf_iterator<char>(vin), std::istreambuf_iterator<char>()};
@@ -95,6 +98,10 @@ int main() {
             std::println(std::cerr, "variants.json missing baseline url: {}", vjson);
             return 1;
         }
+        if (vjson.find("\"probes\":[\"simd\",\"exception-handling\"]") == std::string::npos) {
+            std::println(std::cerr, "variants.json baseline does not probe for SIMD and exception handling: {}", vjson);
+            return 1;
+        }
         if (vjson.find("relaxed-simd") != std::string::npos) {
             std::println(std::cerr, "variants.json unexpectedly mentions relaxed-simd: {}", vjson);
             return 1;
diff --git a/tests/WasiBrowserRuntimeExit/fixture/harness.mjs b/tests/WasiBrowserRuntimeExit/fixture/harness.mjs
new file mode 100644
index 0000000..4e1cde9
--- /dev/null
+++ b/tests/WasiBrowserRuntimeExit/fixture/harness.mjs
@@ -0,0 +1,167 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// Runs wasi-runtime/runtime.js (argv[2], copied to a .mjs so that node loads
+// it as a module) once per scenario against the module assembled below, and
+// prints one line per scenario for the test to check. Between _start and the
+// call that ends it, the module has the frames C++ built with
+// -fwasm-exceptions puts there: a destructor (catch_all_ref, then
+// throw_ref), a catch (...) (catch_all) and a noexcept function (catch_all,
+// then std::terminate). Each reports through env.trace when it runs, and on
+// the way out of an exit or a park none of them may.
+import { pathToFileURL } from "node:url";
+
+const uleb = (n) => {
+    const out = [];
+    do {
+        const byte = n & 0x7f;
+        n >>>= 7;
+        out.push(n ? byte | 0x80 : byte);
+    } while (n);
+    return out;
+};
+const sleb = (n) => {
+    const out = [];
+    for (;;) {
+        const byte = n & 0x7f;
+        n >>= 7;
+        const done = (n === 0 && !(byte & 0x40)) || (n === -1 && (byte & 0x40));
+        out.push(done ? byte : byte | 0x80);
+        if (done) return out;
+    }
+};
+const vec = (items) => [...uleb(items.length), ...items.flat()];
+const name = (text) => vec([...text].map((c) => [c.charCodeAt(0)]));
+const section = (id, items) => {
+    const bytes = vec(items);
+    return [id, ...uleb(bytes.length), ...bytes];
+};
+const UNREACHABLE = 0x00, BLOCK = 0x02, IF = 0x04, ELSE = 0x05, THROW = 0x08, THROW_REF = 0x0a, END = 0x0b;
+const RETURN = 0x0f, CALL = 0x10, TRY_TABLE = 0x1f, LOCAL_GET = 0x20, LOCAL_SET = 0x21, GLOBAL_GET = 0x23;
+const GLOBAL_SET = 0x24, I32_CONST = 0x41, I32_EQ = 0x46;
+const I32 = 0x7f, EXNREF = 0x69, NO_RESULT = 0x40, FUNC_TYPE = 0x60, CATCH_ALL = 0x02, CATCH_ALL_REF = 0x03;
+const code = (locals, instructions) => {
+    const bytes = [...vec(locals), ...instructions, END];
+    return [...uleb(bytes.length), ...bytes];
+};
+
+// Types, imported functions, then the module's own.
+const TAKES_I32 = 0, NOTHING = 1, GIVES_I32 = 2;
+const PROC_EXIT = 0, PARK = 1, SCENARIO = 2, TRACE = 3;
+const END_WITH = 4, NOEXCEPT = 5, CATCHES_ALL = 6, CLEANS_UP = 7, START = 8, FRAME = 9, QUIT = 10;
+// What env.trace reports.
+const DESTRUCTOR = 1, CATCH = 2, TERMINATE = 3, AFTER_PARK = 4;
+// What env.scenario selects in _start.
+const EXIT = 1, PARK_MAIN = 2, THROW_UNCAUGHT = 4, CRASH = 5;
+const PARK_CODE = -1;
+const ifScenario = (scenario, instructions) => [LOCAL_GET, 0, I32_CONST, ...sleb(scenario), I32_EQ, IF, NO_RESULT, ...instructions, END];
+
+const module = new Uint8Array([
+    0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00,
+    ...section(1, [
+        [FUNC_TYPE, ...vec([[I32]]), ...vec([])],
+        [FUNC_TYPE, ...vec([]), ...vec([])],
+        [FUNC_TYPE, ...vec([]), ...vec([[I32]])],
+    ]),
+    ...section(2, [
+        [...name("wasi_snapshot_preview1"), ...name("proc_exit"), 0x00, TAKES_I32],
+        [...name("crafter_runtime"), ...name("park"), 0x00, NOTHING],
+        [...name("env"), ...name("scenario"), 0x00, GIVES_I32],
+        [...name("env"), ...name("trace"), 0x00, TAKES_I32],
+    ]),
+    ...section(3, [[TAKES_I32], [TAKES_I32], [TAKES_I32], [TAKES_I32], [NOTHING], [GIVES_I32], [TAKES_I32]]),
+    ...section(5, [[0x00, 0x01]]),
+    // One tag, for the exception nothing catches.
+    ...section(13, [[0x00, TAKES_I32]]),
+    ...section(6, [[I32, 0x01, I32_CONST, 0x00, END]]),
+    ...section(7, [
+        [...name("memory"), 0x02, 0],
+        [...name("_start"), 0x00, START],
+        [...name("frame"), 0x00, FRAME],
+        [...name("quit"), 0x00, QUIT],
+    ]),
+    ...section(10, [
+        // END_WITH(code): park for PARK_CODE, else proc_exit(code).
+        code([], [LOCAL_GET, 0, I32_CONST, ...sleb(PARK_CODE), I32_EQ, IF, NO_RESULT, CALL, PARK, ELSE, LOCAL_GET, 0, CALL, PROC_EXIT, END]),
+        // NOEXCEPT(code): anything thrown through it is std::terminate.
+        code([], [
+            BLOCK, NO_RESULT, TRY_TABLE, NO_RESULT, ...vec([[CATCH_ALL, 0]]), LOCAL_GET, 0, CALL, END_WITH, END, RETURN, END,
+            I32_CONST, TERMINATE, CALL, TRACE, UNREACHABLE,
+        ]),
+        // CATCHES_ALL(code): try { NOEXCEPT(code); } catch (...) {}
+        code([], [
+            BLOCK, NO_RESULT, TRY_TABLE, NO_RESULT, ...vec([[CATCH_ALL, 0]]), LOCAL_GET, 0, CALL, NOEXCEPT, END, RETURN, END,
+            I32_CONST, CATCH, CALL, TRACE,
+        ]),
+        // CLEANS_UP(code): a frame with a destructor around CATCHES_ALL(code).
+        code([[...uleb(1), EXNREF]], [
+            BLOCK, EXNREF, TRY_TABLE, NO_RESULT, ...vec([[CATCH_ALL_REF, 0]]), LOCAL_GET, 0, CALL, CATCHES_ALL, END, RETURN, END,
+            LOCAL_SET, 1, I32_CONST, DESTRUCTOR, CALL, TRACE, LOCAL_GET, 1, THROW_REF,
+        ]),
+        // _start: what env.scenario says; anything else returns from main.
+        code([[...uleb(1), I32]], [
+            CALL, SCENARIO, LOCAL_SET, 0,
+            ...ifScenario(EXIT, [I32_CONST, 3, CALL, CLEANS_UP]),
+            ...ifScenario(PARK_MAIN, [I32_CONST, 42, GLOBAL_SET, 0, I32_CONST, ...sleb(PARK_CODE), CALL, CLEANS_UP, I32_CONST, AFTER_PARK, CALL, TRACE]),
+            ...ifScenario(THROW_UNCAUGHT, [I32_CONST, 9, THROW, 0]),
+            ...ifScenario(CRASH, [UNREACHABLE]),
+        ]),
+        // frame(): the state _start left, as a later frame callback reads it.
+        code([], [GLOBAL_GET, 0]),
+        // quit(code): an exit from a later call, through the same frames.
+        code([], [LOCAL_GET, 0, CALL, CLEANS_UP]),
+    ]),
+]);
+
+// The standardized encoding (try_table) is what the module and runtime.js
+// both depend on; an older node has only the legacy one.
+const exnrefProbe = new Uint8Array([0, 97, 115, 109, 1, 0, 0, 0, 1, 4, 1, 96, 0, 0, 3, 2, 1, 0, 10, 8, 1, 6, 0, 31, 64, 0, 11, 11]);
+if (!WebAssembly.validate(exnrefProbe)) {
+    console.log("SKIP: this node has no WebAssembly exception handling");
+    process.exit(0);
+}
+if (!WebAssembly.validate(module)) {
+    console.log("FAIL: the fixture module does not validate");
+    process.exit(1);
+}
+
+// What runtime.js expects of a page: no variants.json or files.json, so it
+// loads CRAFTER_WASM_URL.
+globalThis.window = globalThis;
+globalThis.addEventListener = () => {};
+window.CRAFTER_WASM_URL = "fixture.wasm";
+globalThis.fetch = async (url) => {
+    if (String(url).split("?")[0] === "fixture.wasm") return new Response(module, { headers: { "Content-Type": "application/wasm" } });
+    return new Response(null, { status: 404 });
+};
+
+const runtime = pathToFileURL(process.argv[2]).href;
+const run = async (label, scenario, after = () => "") => {
+    const trace = [];
+    window.crafter_webbuild_env = { scenario: () => scenario, trace: (what) => { trace.push(what); } };
+    delete window.crafter_wasi;
+    let outcome = "ended";
+    try {
+        // A query makes it a module of its own, run afresh.
+        await import(`${runtime}?scenario=${scenario}`);
+    } catch (e) {
+        outcome = `rejected with ${e?.constructor?.name}`;
+    }
+    const wasi = window.crafter_wasi;
+    console.log(`${label}: ${outcome}, exitCode=${wasi?.exitCode} parked=${wasi?.parked} trace=[${trace}]${after(wasi, trace)}`);
+};
+
+await run("exit", EXIT);
+await run("park", PARK_MAIN, (wasi, trace) => {
+    const frame = wasi.instance.exports.frame();
+    let quit = "returned";
+    try {
+        wasi.instance.exports.quit(7);
+    } catch (e) {
+        quit = `threw ${e.constructor.name}`;
+    }
+    return `; frame()=${frame}; quit(7) ${quit}, exitCode=${wasi.exitCode} trace=[${trace}]`;
+});
+await run("return", 3);
+await run("uncaught exception", THROW_UNCAUGHT);
+await run("crash", CRASH);
diff --git a/tests/WasiBrowserRuntimeExit/main.cpp b/tests/WasiBrowserRuntimeExit/main.cpp
new file mode 100644
index 0000000..4af52c8
--- /dev/null
+++ b/tests/WasiBrowserRuntimeExit/main.cpp
@@ -0,0 +1,75 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// The browser runtime has to end a call into the program without running any
+// more of it: when the program exits (exit, _Exit, a nonzero return from
+// main), and when it parks to hand the thread to the browser for good, which
+// Crafter::Window::StartSync does on DOM. runtime.js used to throw a
+// JavaScript error out of proc_exit, and a program built with
+// -fwasm-exceptions catches that. An exit then ran the destructors of every
+// frame it left, a catch (...) swallowed it and carried on, and a noexcept
+// function turned it into std::terminate, while the page went on calling
+// into a program whose state those destructors had torn down.
+//
+// fixture/harness.mjs runs the real runtime.js under node against a module
+// with the frames that catch between _start and the call that ends it: a
+// destructor, a catch (...) and a noexcept function, each reporting when it
+// runs. It checks that an exit runs none of them and records its status;
+// that a park runs none of them either, nor the code after it, and leaves the
+// program callable with its state intact; that an exit from a later call
+// ends the program too; that returning from main ends it with status 0; and
+// that a crash or an exception nothing caught still surfaces as an error.
+// Skips without node, or with a node whose engine lacks exception handling.
+// The Flatpak has no node, so this one runs in the native loop.
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+}
+
+int main() {
+    if (std::system("which node > /dev/null 2>&1") != 0) {
+        std::println(std::cerr, "SKIP: node not on PATH");
+        return 77;
+    }
+
+    const fs::path scratch = fs::temp_directory_path() / "crafter-build-wasi-browser-runtime-exit";
+    fs::remove_all(scratch);
+    fs::create_directories(scratch);
+    // node runs a .js file as a CommonJS script, which cannot await at its
+    // top level the way runtime.js does.
+    const fs::path runtime = scratch / "runtime.mjs";
+    fs::copy_file(fs::current_path() / "wasi-runtime" / "runtime.js", runtime);
+    const fs::path harness = fs::current_path() / "tests" / "WasiBrowserRuntimeExit" / "fixture" / "harness.mjs";
+
+    const auto run = RunCommandWithTimeout(std::format("node {} {}", ShellQuote(harness.string()), ShellQuote(runtime.string())), std::chrono::seconds(60));
+    fs::remove_all(scratch);
+    if (run.output.contains("SKIP:")) {
+        std::println(std::cerr, "{}", run.output);
+        return 77;
+    }
+
+    Check(run.exitCode == 0, std::format("the harness ran to the end (exit {})", run.exitCode));
+    Check(run.output.contains("exit: ended, exitCode=3 parked=false trace=[]"), "an exit runs no destructor, catch (...) or terminate, and its status is recorded");
+    Check(run.output.contains("park: ended, exitCode=null parked=true trace=[]; frame()=42"), "a park runs none of them nor the code after it, and the program stays callable with its state");
+    Check(run.output.contains("quit(7) threw RuntimeError, exitCode=7 trace=[]"), "an exit from a later call ends the program the same way");
+    Check(run.output.contains("return: ended, exitCode=0 parked=false trace=[]"), "returning from main ends the program with status 0");
+    Check(run.output.contains("uncaught exception: rejected with Exception, exitCode=null"), "an exception nothing caught is not taken for an exit");
+    Check(run.output.contains("crash: rejected with RuntimeError, exitCode=null"), "a crash is not taken for an exit");
+
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed; harness output:\n{}", failures, run.output);
+        return 1;
+    }
+    return 0;
+}
diff --git a/tests/WasiSysrootWithoutExceptions/fixture/main.cpp b/tests/WasiSysrootWithoutExceptions/fixture/main.cpp
new file mode 100644
index 0000000..97eb9af
--- /dev/null
+++ b/tests/WasiSysrootWithoutExceptions/fixture/main.cpp
@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+int main() {
+    return 0;
+}
diff --git a/tests/WasiSysrootWithoutExceptions/main.cpp b/tests/WasiSysrootWithoutExceptions/main.cpp
new file mode 100644
index 0000000..b74fac2
--- /dev/null
+++ b/tests/WasiSysrootWithoutExceptions/main.cpp
@@ -0,0 +1,110 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// A WASI sysroot without exception support has to stop a wasm build with one
+// line that says so. wasm builds compile with -fwasm-exceptions and link the
+// libc++ and libunwind that wasi-sdk 33 and later ship under lib/<triple>/eh;
+// a sysroot that lacks them (an older wasi-sdk, one built without its
+// exceptions option) otherwise compiles and then fails the first link with
+// "unable to find library -lunwind", which does not say why it was asked
+// for. A sysroot that is not there at all, or has nothing for the target,
+// gets a message of its own rather than that one. The checks run before
+// anything is made, so a refused build leaves no bin/ behind, and they need
+// no working wasm toolchain.
+//
+// A scratch sysroot is taken through these states: missing; without
+// lib/wasm32-wasip1; without libunwind, for an executable and for a static
+// library, which links nothing and passes; with libunwind under eh/; with
+// libunwind in the triple's own directory, which the driver searches after
+// eh/. The cases that pass the check fail later on the sysroot's empty std
+// module source, which is not what is under test. The cache is redirected
+// into the scratch directory as well: the std PCM cache is not keyed on the
+// sysroot, and a PCM built from the fake source must not land where the real
+// wasm tests read theirs.
+
+extern "C" int setenv(const char* name, const char* value, int overwrite);
+
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+
+    BuildResult BuildAgainst(const fs::path& project, const fs::path& sysroot, ConfigurationType type = ConfigurationType::Executable) {
+        Configuration app;
+        app.path = project;
+        app.name = "hello";
+        app.outputName = "hello";
+        app.target = "wasm32-wasip1";
+        app.sysroot = sysroot.string();
+        app.type = type;
+        std::array<fs::path, 0> ifaces = {};
+        std::array<fs::path, 1> impls = { "main" };
+        app.GetInterfacesAndImplementations(ifaces, impls);
+        std::unordered_map<fs::path, std::shared_future<BuildResult>> depResults;
+        std::mutex depMutex;
+        return Build(app, depResults, depMutex);
+    }
+}
+
+int main() {
+    // Build() reports a missing compiler before it gets to the sysroot.
+    if (const Toolchain& tc = GetToolchain(); !tc.error.empty()) {
+        std::println(std::cerr, "SKIP: {}", tc.error);
+        return 77;
+    }
+
+    const fs::path scratch = fs::temp_directory_path() / "crafter-build-wasi-sysroot-without-exceptions";
+    fs::remove_all(scratch);
+    fs::create_directories(scratch);
+    setenv("XDG_CACHE_HOME", (scratch / "cache").c_str(), 1);
+    const fs::path project = scratch / "project";
+    fs::copy(fs::current_path() / "tests" / "WasiSysrootWithoutExceptions" / "fixture", project, fs::copy_options::recursive);
+
+    const fs::path sysroot = scratch / "sysroot";
+    const fs::path libDir = sysroot / "lib" / "wasm32-wasip1";
+    {
+        const BuildResult r = BuildAgainst(project, sysroot);
+        Check(r.result.contains(std::format("no WASI sysroot at {}", sysroot.string())), std::format("a missing sysroot is named as missing: {}", r.result));
+    }
+
+    fs::create_directories(sysroot / "share" / "libc++" / "v1");
+    std::ofstream(sysroot / "share" / "libc++" / "v1" / "std.cppm");
+    {
+        const BuildResult r = BuildAgainst(project, sysroot);
+        Check(r.result.contains(std::format("has nothing for wasm32-wasip1 (no {})", libDir.string())), std::format("a sysroot without the target's directory says so: {}", r.result));
+    }
+
+    fs::create_directories(libDir / "eh");
+    constexpr std::string_view missing = "has no C++ exception support";
+    {
+        const BuildResult r = BuildAgainst(project, sysroot);
+        Check(r.result.contains(missing), std::format("without libunwind the build stops on the sysroot: {}", r.result));
+        Check(r.result.contains((libDir / "eh" / "libunwind.a").string()), std::format("the message names the library it looked for: {}", r.result));
+        Check(!fs::exists(project / "bin") && !fs::exists(project / "build"), "a refused build leaves no bin/ or build/ behind");
+    }
+    Check(!BuildAgainst(project, sysroot, ConfigurationType::LibraryStatic).result.contains(missing), "a static library links nothing and passes without libunwind");
+
+    std::ofstream(libDir / "eh" / "libunwind.a");
+    Check(!BuildAgainst(project, sysroot).result.contains(missing), "libunwind under eh/ passes the check");
+
+    fs::remove(libDir / "eh" / "libunwind.a");
+    std::ofstream(libDir / "libunwind.a");
+    Check(!BuildAgainst(project, sysroot).result.contains(missing), "libunwind in the triple's directory passes the check");
+
+    fs::remove_all(scratch);
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}
diff --git a/tests/WasmBuildRefusals/fixture/main.cpp b/tests/WasmBuildRefusals/fixture/main.cpp
new file mode 100644
index 0000000..97eb9af
--- /dev/null
+++ b/tests/WasmBuildRefusals/fixture/main.cpp
@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+int main() {
+    return 0;
+}
diff --git a/tests/WasmBuildRefusals/main.cpp b/tests/WasmBuildRefusals/main.cpp
new file mode 100644
index 0000000..42e8fee
--- /dev/null
+++ b/tests/WasmBuildRefusals/main.cpp
@@ -0,0 +1,85 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// What a wasm32 build refuses before it starts, now that every wasm unit has
+// C++ exceptions, each with a message naming the cause:
+//   - -flto in compileFlags or linkFlags. LTO code generation drops the
+//     exception handling, and the build used to succeed with every throw
+//     escaping its catch (WebAssembly/wasi-sdk#629).
+//   - -fno-exceptions. The std module is built with exceptions, and every
+//     `import std;` failed on a BMI configuration mismatch.
+//   - A shared library. The exception-enabled libc++ is static archives
+//     only, and the link failed on a relocation that wants -fPIC.
+// None of them leaves bin/ or build/ behind, and none needs a working wasm
+// toolchain. The sysroot given does not exist, so a refusal that did not
+// fire would show up as the missing sysroot instead.
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+
+    struct Refusal {
+        std::string_view what;
+        std::vector<std::string> compileFlags;
+        std::vector<std::string> linkFlags;
+        ConfigurationType type;
+        std::string_view expected;
+    };
+}
+
+int main() {
+    // Build() reports a missing compiler before it gets to any of this.
+    if (const Toolchain& tc = GetToolchain(); !tc.error.empty()) {
+        std::println(std::cerr, "SKIP: {}", tc.error);
+        return 77;
+    }
+
+    const fs::path scratch = fs::temp_directory_path() / "crafter-build-wasm-build-refusals";
+    fs::remove_all(scratch);
+    fs::create_directories(scratch);
+    const fs::path project = scratch / "project";
+    fs::copy(fs::current_path() / "tests" / "WasmBuildRefusals" / "fixture", project, fs::copy_options::recursive);
+
+    const std::array refusals = {
+        Refusal{ "-flto in compileFlags", { "-flto=thin" }, {}, ConfigurationType::Executable, "-flto=thin is not supported on wasm32: LTO code generation drops C++ exception handling" },
+        Refusal{ "-flto in linkFlags", {}, { "-flto" }, ConfigurationType::Executable, "-flto is not supported on wasm32" },
+        Refusal{ "-fno-exceptions", { "-fno-exceptions" }, {}, ConfigurationType::Executable, "-fno-exceptions is not supported on wasm32" },
+        Refusal{ "a shared library", {}, {}, ConfigurationType::LibraryDynamic, "wasm32 shared libraries are not supported" },
+    };
+    for (const Refusal& refusal : refusals) {
+        Configuration app;
+        app.path = project;
+        app.name = "hello";
+        app.outputName = "hello";
+        app.target = "wasm32-wasip1";
+        app.sysroot = (scratch / "no-sysroot").string();
+        app.type = refusal.type;
+        app.compileFlags = refusal.compileFlags;
+        app.linkFlags = refusal.linkFlags;
+        std::array<fs::path, 0> ifaces = {};
+        std::array<fs::path, 1> impls = { "main" };
+        app.GetInterfacesAndImplementations(ifaces, impls);
+        std::unordered_map<fs::path, std::shared_future<BuildResult>> depResults;
+        std::mutex depMutex;
+        const BuildResult r = Build(app, depResults, depMutex);
+        Check(r.result.contains(refusal.expected), std::format("{} is refused with its cause: {}", refusal.what, r.result));
+        Check(!fs::exists(project / "bin") && !fs::exists(project / "build"), std::format("refusing {} leaves no bin/ or build/ behind", refusal.what));
+    }
+
+    fs::remove_all(scratch);
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}
diff --git a/tests/WasmExceptions/fixture/jump.c b/tests/WasmExceptions/fixture/jump.c
new file mode 100644
index 0000000..76e3146
--- /dev/null
+++ b/tests/WasmExceptions/fixture/jump.c
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+#include <setjmp.h>
+#include <stdint.h>
+
+static jmp_buf target;
+
+static void JumpBack(int32_t value) {
+    longjmp(target, value);
+}
+
+// setjmp's value may only be compared or switched on, never stored.
+int32_t JumpRoundTrip(void) {
+    switch (setjmp(target)) {
+    case 0:
+        JumpBack(7);
+        return 0;
+    case 7:
+        return 7;
+    default:
+        return -1;
+    }
+}
diff --git a/tests/WasmExceptions/fixture/main.cpp b/tests/WasmExceptions/fixture/main.cpp
new file mode 100644
index 0000000..e068a18
--- /dev/null
+++ b/tests/WasmExceptions/fixture/main.cpp
@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+
+// jump.c, compiled as C.
+extern "C" std::int32_t JumpRoundTrip();
+
+namespace {
+    std::int32_t unwound = 0;
+
+    struct Guard {
+        ~Guard() { ++unwound; }
+    };
+
+    void Throw(std::int32_t depth) {
+        Guard guard;
+        if (depth == 0) throw std::runtime_error("thrown in wasm");
+        Throw(depth - 1);
+    }
+}
+
+int main(int argc, char** argv) {
+    // With an argument, nothing catches: the run has to fail.
+    if (argc > 1) Throw(0);
+    try {
+        Throw(3);
+    } catch (const std::runtime_error& e) {
+        std::println("caught: {} after {} destructors", e.what(), unwound);
+    }
+    try {
+        const std::vector<std::int32_t> empty;
+        std::println("unreachable: {}", empty.at(1));
+    } catch (const std::out_of_range&) {
+        std::println("caught: out_of_range from libc++");
+    }
+    // vector::at throws from a header, compiled into this unit; stoi throws
+    // inside libc++.a, which therefore has to be the exception-enabled one.
+    try {
+        std::println("unreachable: {}", std::stoi("not a number"));
+    } catch (const std::invalid_argument&) {
+        std::println("caught: invalid_argument from inside libc++");
+    }
+    std::println("longjmp came back with {}", JumpRoundTrip());
+    return 0;
+}
diff --git a/tests/WasmExceptions/main.cpp b/tests/WasmExceptions/main.cpp
new file mode 100644
index 0000000..a059a56
--- /dev/null
+++ b/tests/WasmExceptions/main.cpp
@@ -0,0 +1,84 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// C++ exceptions in a wasm build, run under wasmtime: the fixture throws
+// through frames whose destructors have to run, catches the exception,
+// catches one from a libc++ header (vector::at) and one thrown inside
+// libc++.a itself (stoi), and with an argument lets one escape main, which
+// has to end the run as an uncaught exception rather than exit 0, trap or
+// hang. Its C source uses setjmp, which compiles and links next to the C++
+// objects only when C gets the same wasm flags, and links at all only
+// because the build adds libsetjmp: the configuration asks for nothing.
+//
+// wasm builds used to be compiled with -fno-exceptions, so a throw did not
+// compile and libc++'s own throws aborted. Each part of turning them on fails
+// in its own way, and this is the test that sees it: a std module or a unit
+// built without exceptions (clang rejects the BMI), a link without libunwind
+// (undefined __cpp_exception), the libc++ built without exceptions (stoi
+// aborts), or LLVM's default legacy encoding, which wasmtime refuses to load.
+// Skips like WasmHelloRuns when there is no exception-enabled WASI sysroot or
+// no wasmtime; inside the Flatpak both are bundled.
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+}
+
+int main() {
+    const Toolchain& tc = GetToolchain();
+    const fs::path libDir = tc.wasiSysroot / "lib" / "wasm32-wasip1";
+    if (!fs::exists(libDir / "eh" / "libunwind.a") && !fs::exists(libDir / "libunwind.a")) {
+        std::println(std::cerr, "SKIP: no exception-enabled WASI sysroot at {}", tc.wasiSysroot.string());
+        return 77;
+    }
+    if (std::system("which wasmtime > /dev/null 2>&1") != 0) {
+        std::println(std::cerr, "SKIP: wasmtime not on PATH");
+        return 77;
+    }
+
+    Configuration cfg;
+    cfg.path = fs::current_path() / "tests" / "WasmExceptions" / "fixture";
+    cfg.name = "wasm-exceptions";
+    cfg.outputName = "wasm-exceptions";
+    cfg.target = "wasm32-wasip1";
+    cfg.type = ConfigurationType::Executable;
+    std::array<fs::path, 0> ifaces = {};
+    std::array<fs::path, 1> impls = { "main" };
+    cfg.GetInterfacesAndImplementations(ifaces, impls);
+    cfg.cFiles = { cfg.path / "jump" };
+
+    std::unordered_map<fs::path, std::shared_future<BuildResult>> depResults;
+    std::mutex depMutex;
+    const BuildResult r = Build(cfg, depResults, depMutex);
+    if (!r.result.empty()) {
+        std::println(std::cerr, "build failed: {}", r.result);
+        return 1;
+    }
+    const fs::path wasm = ExpectedOutput(cfg);
+
+    const auto caught = RunCommandWithTimeout(std::format("wasmtime {}", wasm.string()), std::chrono::seconds(60));
+    Check(caught.exitCode == 0, std::format("the run that catches everything exits 0 (exit {}): {}", caught.exitCode, caught.output));
+    Check(caught.output.contains("caught: thrown in wasm after 4 destructors"), std::format("the exception is caught after unwinding all four frames: {}", caught.output));
+    Check(caught.output.contains("caught: out_of_range from libc++"), std::format("an exception from a libc++ header is catchable: {}", caught.output));
+    Check(caught.output.contains("caught: invalid_argument from inside libc++"), std::format("an exception from inside libc++.a is catchable: {}", caught.output));
+    Check(caught.output.contains("longjmp came back with 7"), std::format("setjmp in a C source works next to the C++ objects: {}", caught.output));
+
+    const auto escaped = RunCommandWithTimeout(std::format("wasmtime {} uncaught", wasm.string()), std::chrono::seconds(60));
+    Check(escaped.exitCode != 0 && !escaped.timedOut && !escaped.crashed && escaped.output.contains("thrown Wasm exception"), std::format("an exception escaping main ends the run as an uncaught exception (exit {}): {}", escaped.exitCode, escaped.output));
+
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}
diff --git a/tests/WasmHelloRuns/main.cpp b/tests/WasmHelloRuns/main.cpp
index 58a910f..a3c660f 100644
--- a/tests/WasmHelloRuns/main.cpp
+++ b/tests/WasmHelloRuns/main.cpp
@@ -11,12 +11,15 @@ using namespace Crafter;
 // wasmtime. The other wasm tests only inspect generated files; this is the
 // one that proves the sysroot, the std module for wasm and wasm-ld all work
 // together. It skips when the sysroot or wasmtime is missing, so a
-// development box without them is not a failure. Inside the Flatpak both are
-// bundled, and there the skip would be the failure to look for.
+// development box without them is not a failure; a sysroot without the
+// exception-enabled libraries counts as missing, since wasm builds link them
+// (WasiSysrootWithoutExceptions covers that message). Inside the Flatpak both
+// are bundled, and there the skip would be the failure to look for.
 int main() {
     const Toolchain& tc = GetToolchain();
-    if (!fs::exists(tc.wasiSysroot / "include")) {
-        std::println(std::cerr, "SKIP: no WASI sysroot at {}", tc.wasiSysroot.string());
+    const fs::path libDir = tc.wasiSysroot / "lib" / "wasm32-wasip1";
+    if (!fs::exists(libDir / "eh" / "libunwind.a") && !fs::exists(libDir / "libunwind.a")) {
+        std::println(std::cerr, "SKIP: no exception-enabled WASI sysroot at {}", tc.wasiSysroot.string());
         return 77;
     }
     if (std::system("which wasmtime > /dev/null 2>&1") != 0) {
diff --git a/tests/WasmVariants/main.cpp b/tests/WasmVariants/main.cpp
index 06f291f..af26a9b 100644
--- a/tests/WasmVariants/main.cpp
+++ b/tests/WasmVariants/main.cpp
@@ -97,6 +97,11 @@ int main() {
             Check(j.find("\"label\":\"relaxed-simd\"") != std::string::npos, "manifest has relaxed-simd label");
             Check(j.find("\"wvhelper.relaxed-simd.wasm\"") != std::string::npos, "manifest has labelled url");
             Check(j.find("\"relaxed-simd\"") != std::string::npos, "manifest has relaxed-simd probe");
+            // Every file is built with SIMD128 and exceptions, so every entry
+            // needs them besides its own features; the baseline needs
+            // nothing else.
+            Check(j.find("\"probes\":[\"relaxed-simd\",\"simd\",\"exception-handling\"]") != std::string::npos, "relaxed-simd entry also probes for the baseline's features");
+            Check(j.find("\"url\":\"wvhelper.wasm\",\"probes\":[\"simd\",\"exception-handling\"]") != std::string::npos, "baseline entry probes for its own features only");
             // baseline fallback entry.
             Check(j.find("\"wvhelper.wasm\"") != std::string::npos, "manifest has baseline url");
             // The relaxed-simd variant must precede the baseline so the runtime
diff --git a/wasi-runtime/runtime.js b/wasi-runtime/runtime.js
index 2f43876..75b9f04 100644
--- a/wasi-runtime/runtime.js
+++ b/wasi-runtime/runtime.js
@@ -176,11 +176,53 @@ function markDirty(path) {
     scheduleFlush();
 }
 
+// ─── leaving the program ───────────────────────────────────────────────
+// exit and _Exit (proc_exit) end the program. park hands the thread to the
+// browser for good while the program goes on serving the calls bridges make
+// into it (Crafter::Window::StartSync on DOM): the import
+// crafter_runtime.park, `void park(void)`, which never returns. Either way
+// the current call into the program has to end without running any more of
+// its code: not the destructors of the frames it leaves, not a catch (...),
+// not the std::terminate of a noexcept function. Built with
+// -fwasm-exceptions, every one of those catches a JavaScript exception
+// thrown through it, so the way out cannot be a throw from JS. A trap is
+// the one thing wasm code cannot catch, and this module raises it in wasm
+// itself, bound directly as the program's import; a trap raised from JS
+// would depend on each engine keeping it uncatchable when it passes back
+// into wasm. It reports first, through host.ended, which has returned by
+// the time it traps:
+//   (import "host" "ended" (func $ended (param $how i32) (param $code i32)))
+//   (func (export "exit") (param $code i32)
+//     i32.const 1  local.get $code  call $ended  unreachable)
+//   (func (export "park")
+//     i32.const 2  i32.const 0  call $ended  unreachable)
+// The program's own state is left as it is, its shadow stack included, so
+// the locals of a parked main stay valid for the calls that follow.
+const PROGRAM_END = new Uint8Array([
+    0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00,
+    0x01, 0x0d, 0x03, 0x60, 0x02, 0x7f, 0x7f, 0x00, 0x60, 0x01, 0x7f, 0x00, 0x60, 0x00, 0x00,
+    0x02, 0x0e, 0x01, 0x04, 0x68, 0x6f, 0x73, 0x74, 0x05, 0x65, 0x6e, 0x64, 0x65, 0x64, 0x00, 0x00,
+    0x03, 0x03, 0x02, 0x01, 0x02,
+    0x07, 0x0f, 0x02, 0x04, 0x65, 0x78, 0x69, 0x74, 0x00, 0x01, 0x04, 0x70, 0x61, 0x72, 0x6b, 0x00, 0x02,
+    0x0a, 0x15, 0x02,
+    0x09, 0x00, 0x41, 0x01, 0x20, 0x00, 0x10, 0x00, 0x00, 0x0b,
+    0x09, 0x00, 0x41, 0x02, 0x41, 0x00, 0x10, 0x00, 0x00, 0x0b,
+]);
+const ENDED_BY_EXIT = 1;
+const ENDED_BY_PARK = 2;
+
 class Wasi {
     #encodedStdin;
     #envEncodedStrings;
     #argEncodedStrings;
     instance;
+    // The program's exit status once it has ended, through exit or _Exit or
+    // by returning from main; null while it runs or is parked. A bridge that
+    // calls into the program on its own, such as a frame loop, stops once
+    // this is set: nothing of an ended program may run.
+    exitCode = null;
+    // True once the program has parked (see PROGRAM_END).
+    parked = false;
     #fdTable;
     #nextFd;
     #decoder;
@@ -194,6 +236,27 @@ class Wasi {
         this.#nextFd = 100;            // stay clear of stdio + preopens
         this.#decoder = new TextDecoder();
         this.bind();
+        const end = new WebAssembly.Instance(new WebAssembly.Module(PROGRAM_END), {
+            host: { ended: (how, code) => this.ended(how, code) },
+        }).exports;
+        // Own properties, so instantiation takes these wasm functions as the
+        // program's imports rather than a JS method.
+        this.proc_exit = end.exit;
+        this.park = end.park;
+    }
+
+    // Called by PROGRAM_END just before its trap ends the call into the
+    // program. An exit after a park ends the program all the same.
+    ended(how, code) {
+        if (how === ENDED_BY_PARK) {
+            this.parked = true;
+        } else if (how === ENDED_BY_EXIT) {
+            console.log(`[wasi] proc_exit(${code})`);
+            this.exitCode = code;
+            // The last chance for anything written this session to reach
+            // OPFS before the page is torn down.
+            opfsFlushAll();
+        }
     }
 
     bind() {
@@ -805,22 +868,6 @@ class Wasi {
         sizePointerBuffer[0] = envByteLength;
         return 0;
     }
-    proc_exit(code) {
-        // Throw a sentinel so the wasm stack unwinds back to runtime.js
-        // WITHOUT executing the `unreachable` instruction wasi-libc emits
-        // after __wasi_proc_exit (which is declared noreturn). Trapping
-        // there would mark the wasm call as crashed and may interrupt any
-        // browser-side render loop that called _Exit on purpose (e.g.
-        // Crafter::Window::StartSync on DOM, which hands the loop to rAF
-        // and then exits main without running static destructors).
-        console.log(`[wasi] proc_exit(${code})`);
-        // Trigger a final flush so anything written this session has the
-        // best chance of reaching OPFS before the page is torn down.
-        opfsFlushAll();
-        const e = new Error(`wasi proc_exit(${code})`);
-        e.crafterWasiExit = code;
-        throw e;
-    }
 }
 
 // ─── wasm feature-detect probes ────────────────────────────────────────
@@ -829,21 +876,28 @@ class Wasi {
 // GoogleChromeLabs/wasm-feature-detect, inlined so we ship no dependency. A
 // variant in variants.json lists the probe names that must all pass for that
 // variant to be served; an unknown name is treated as unsupported (we fall
-// through to a less demanding variant, ultimately the baseline). Keep the keys
-// in sync with the `probes` a WasmVariant declares in Crafter.Build.
+// through to a less demanding variant, ultimately the baseline, and to an
+// error naming what is missing when not even the baseline fits). Keep the
+// keys in sync with the `probes` a WasmVariant declares in Crafter.Build, and
+// with WasmBaselineProbes, which every entry carries.
 const WASM_PROBES = {
     // relaxed SIMD: i8x16.relaxed_swizzle (Chrome 114+, Firefox 120+; Safari
     // still flag-gated as of mid-2026).
     "relaxed-simd": () => WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,5,1,96,0,1,123,3,2,1,0,10,15,1,13,0,65,1,253,15,65,2,253,15,253,128,2,11])),
-    // fixed-width SIMD128 (v128.const + i8x16.popcnt) — the current baseline,
-    // here for completeness / future baseline shifts.
+    // fixed-width SIMD128 (v128.const + i8x16.popcnt), which every build
+    // uses (-msimd128). V8 and SpiderMonkey switch it off on x86 CPUs without
+    // SSE4.1, so it is a baseline probe even though every engine with
+    // exception handling has it.
     "simd": () => WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,5,1,96,0,1,123,3,2,1,0,10,10,1,8,0,65,0,253,15,253,98,11])),
     // tail calls (return_call).
     "tail-call": () => WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,4,1,96,0,0,3,2,1,0,10,6,1,4,0,18,0,11])),
     // bulk memory ops (memory.init).
     "bulk-memory": () => WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,4,1,96,0,0,3,2,1,0,5,3,1,0,1,10,14,1,12,0,65,0,65,0,65,0,252,10,0,0,11])),
-    // legacy exception handling (try/catch).
-    "exception-handling": () => WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,4,1,96,0,0,3,2,1,0,10,8,1,6,0,6,64,25,11,11])),
+    // exception handling in its standardized form (try_table, exnref; Chrome
+    // 137, Firefox 131, Safari 18.4), which every crafter-build wasm uses for
+    // C++ exceptions. Probing the legacy try/catch encoding instead says
+    // nothing about this one: V8 accepts it with exnref switched off.
+    "exception-handling": () => WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,4,1,96,0,0,3,2,1,0,10,8,1,6,0,31,64,0,11,11])),
     // threads/atomics — also requires SharedArrayBuffer (cross-origin isolated).
     "threads": () => {
         try {
@@ -866,6 +920,9 @@ function variantSupported(probes) {
 
 // Pick the wasm URL: prefer the variants.json manifest (first entry whose
 // probes all pass), else fall back to the single baked window.CRAFTER_WASM_URL.
+// When the manifest has no entry this engine can run, not even the baseline,
+// say which features it lacks: loading the baseline anyway would only fail
+// with a CompileError about an unknown opcode.
 async function pickWasmUrl() {
     let variants = null;
     try {
@@ -881,6 +938,9 @@ async function pickWasmUrl() {
                 return v.url;
             }
         }
+        const baseline = variants[variants.length - 1];
+        const missing = (baseline.probes || []).filter((name) => !variantSupported([name]));
+        throw new Error(`[wasi] ${baseline.url} needs WebAssembly ${missing.join(", ")}, which this browser does not support; a current Chrome, Firefox or Safari does`);
     }
     return window.CRAFTER_WASM_URL;
 }
@@ -970,6 +1030,7 @@ if (window.crafter_webbuild_env_ready) {
 const { instance } = await WebAssembly.instantiateStreaming(fetch(wasmUrl), {
     wasi_snapshot_preview1: wasi,
     env: window.crafter_webbuild_env,
+    crafter_runtime: { park: wasi.park },
 });
 wasi.instance = instance;
 // Alias under both names — `crafter_wasi` is the canonical, but the older
@@ -980,12 +1041,12 @@ window.crafter_webbuild_wasi = wasi;
 
 try {
     instance.exports._start();
+    // main returned 0; any other status reaches proc_exit.
+    wasi.exitCode = 0;
+    opfsFlushAll();
 } catch (e) {
-    if (e && typeof e.crafterWasiExit === "number") {
-        // Clean exit from std::_Exit / __wasi_proc_exit. The wasm instance
-        // remains valid; any rAF callback registered before exit keeps
-        // running. (See Crafter::Window::StartSync DOM impl.)
-    } else {
-        throw e;
-    }
+    // proc_exit and park end _start with PROGRAM_END's trap after saying
+    // which they were. Any other error, a crash or a C++ exception that
+    // nothing caught, is the program failing.
+    if (wasi.exitCode === null && !wasi.parked) throw e;
 }