[ssalbdivad]

Adds a checker: "tsjs" | "tsgo" | "tsrs" setting (--checker tsrs, ATTEST_checker='"tsrs"'). tsrs, the Rust port of TypeScript 7, serves the same native API as tsgo, so it only changes which package attest loads its API client, AST and compiler binary from.

Unset, attest uses tsrs when its binary resolves alongside TypeScript 7, else tsgo on TypeScript 7, else tsjs. An unknown checker, or one that doesn't match the installed TypeScript major, throws. tsrs is an optional peer dependency.

CI: this repo's typed tests now default to tsrs. A new attest@tsgo task in testTsVersions reruns them on tsgo, and the repo typecheck stays on tsc.

Measured locally (Linux x64, tsrs 0.18.0 vs tsgo 7.1.0-dev.20261006.1):

	tsgo	tsrs
ark/attest (63 passing)	62s	28s
ark/type (890 passing)	95s	61s
repo check time	30.8s	8.3s
repo instantiations	5,172,042	5,142,738

Locally, prChecks passed everything except buildDocs: docs pages exceeded Next.js's 60s render timeout on WSL, and docs pages don't import attest.

🤖 Generated with Claude Code

[pullfrog]

Important

The new checker option is silently ignored when set through setup() — only --checker/ATTEST_checker take effect. Details inline.

Reviewed changes

Adds a checker: "tsjs" | "tsgo" | "tsrs" attest option selecting which TypeScript-compatible API and compiler binary attest analyzes with, defaulting to tsrs where its binary is installed, then tsgo on TypeScript 7, then tsjs.

• checker config (ark/attest/config.ts) — new Checker type and checker: undefined default.
• Checker selection (ark/attest/ts7/server.ts) — getChecker/hasTsrsBinary choose the compiler package that ast, API, createFileSystemLayer and tscPath load from, throwing on an unknown or version-mismatched checker.
• Packaging (ark/attest/package.json, pnpm-lock.yaml) — tsrs devDependency plus an optional peer, with per-platform optional binaries.
• CI (ark/repo/testTsVersions.ts) — typed tests default to tsrs, with a new attest@tsgo task rerunning them on tsgo.

ℹ️ New checker option isn't on the options page

ark/docs/content/docs/attest/options.mdx states it lists "the current defaults for all available options" and mirrors getDefaultAttestConfig, but the new checker: undefined default isn't included, so the --checker/ATTEST_checker surface ships undocumented.

Technical details

# checker missing from options.mdx

## Affected sites
- ark/docs/content/docs/attest/options.mdx:25-41 — defaults snippet omits `checker`
- ark/attest/config.ts:48-65 — `getDefaultAttestConfig` now includes `checker: undefined`

## Required outcome
- The options page reflects the new option. A short subsection like the existing `skipTypes`/`typeToStringFormat` ones would fit, since the default-selection behavior (tsrs → tsgo → tsjs) is not obvious from the name alone.

  ^{｜ Fix all ➔ ｜ Fix 👍s ➔ ｜ View workflow run ｜ Using deepseek-v4.1-flash (free via Pullfrog for OSS) ｜ 𝕏}

[pullfrog]

checker is resolved eagerly at module load, so setup({ checker: "tsgo" }) is silently ignored: @ark/attest's entry graph (index.ts → fixtures.ts → ts7/analyze.ts → here) evaluates before setup() writes ATTEST_CONFIG, and the derived ast/API/tscPath constants can't change afterward. Only --checker/ATTEST_checker work.

Technical details

# setup({ checker }) has no effect

## Affected sites
- ark/attest/ts7/server.ts:59 — `const checker = getChecker()` runs during module evaluation
- ark/attest/ts7/server.ts:62-80 — `compiler`, `ast`, `API`, `createFileSystemLayer`, `tscPath` are all module-level consts derived from `checker`
- ark/attest/fixtures.ts:9-12 — `setup()` only writes `process.env.ATTEST_CONFIG` when it is called
- ark/attest/config.ts:111-117 — `addEnvConfig` is what reads that env var
- ark/repo/mocha.globalSetup.ts:1-10 — the repo's own global setup imports `@ark/attest` (line 1) before calling `setup()` (line 6), so it can't set `checker` either

## Required outcome
- `setup({ checker })` must select the checker, or `checker` must not be offered on the setup-facing `AttestConfig` (with CLI/env-only usage made explicit). As written, the type advertises the option but the runtime ignores it, and a user following the docs gets the default checker with no warning.

## Suggested approach (optional)
- Resolve the checker lazily on first `TsgoServer.instance` access (turn `ast`/`API`/`createFileSystemLayer`/`tscPath` into getters), since `ATTEST_CONFIG` cannot exist before the module graph is evaluated.

[pullfrog]

Nit: the two-line comment just above (type snapshots ... differ) originally explained the attest@6.0.3 task below it; inserting the new comment between them now leaves it reading as if it describes attest@tsgo. Move the new comment/task below it so each comment stays attached to the task it explains.