A verified Web run that asked the host for node (2026.9.12.1)

[Sunrisepeak]

Carries version 2026.9.12.1. Design record: .agents/docs/2026-09-12-a-verified-web-run-that-asked-the-host-for-node.md.

What was measured

The sandbox verification of the published 2026.9.11.4 held on 26 of 27 checks in an xlings subos use <name> --sandbox environment, which has no node on PATH. The one that did not was the Web run: mcpp build --target wasm32-emscripten succeeded, and mcpp run stopped at

/usr/bin/env: 'node': No such file or directory

An Emscripten link produces a JavaScript launcher whose first line is #!/usr/bin/env node. With nothing declared, mcpp executes the artefact, so the interpreter came from the PATH mcpp inherited. Meanwhile the node that xim:emsdk declares, and writes into its own .emscripten, sat in the store. The development host has node on PATH, which is why no earlier measurement could see the dependency.

The change

The payload names the program that runs what it produces: .mcpp-toolchain.json gains a fourth key, runner.

• One program, no arguments. The artefact path is appended, so the key cannot carry flags.
• Relative or absolute. A relative runner obeys the frontend rules. An absolute runner is honoured only inside the package store that holds the payload, compared on canonical paths, and a runner outside the store is ignored, so a payload cannot choose a host interpreter.
• Malformed values are refused by name. The path rules are now one helper shared by frontend and runner, and the frontend messages are unchanged.
• Last in precedence, run slot only. [target.<triple>] runner and a dependency's mcpp::runner(...) outrank it, and --no-runner still executes the artefact directly.

xim:emsdk writes the key: openxlings/xim-pkgindex#823, merged and published (index artifact 971571a). An engine that predates the key ignores it. A payload installed before the recipe change has no descriptor and runs as before.

Verification

• Unit: five new descriptor tests. With the store rule removed, ARunnerOutsideTheStoreIsNotHonoured fails.
• Sandbox, no node on PATH, a musl-static build of this tree:
  • with a descriptor in the sandbox's emsdk payload, the run prints 1-2-3;
  • with the descriptor removed, it fails at /usr/bin/env: 'node' again.
• CI: scan (linux-x86_64) runs a Web program with a fresh MCPP_HOME, so the payload is the one the index publishes rather than a cached one. A decoy node sits first on PATH and exits 97 with a marker. The step fails if the payload has no descriptor, if the decoy ran, or if 1-2-3 is not a line of the output.

Docs

docs/20-toolchains.md and its Chinese copy state where the interpreter comes from. Both READMEs and examples/13-platform-targets no longer imply a node from PATH. The CHANGELOG has the 2026.9.12.1 entry.