@@ -0,0 +1,5 @@
+packs:
+  - source: https://github.com/systemfsoftware/systemfsoftware.git
+    ref: df908ddfe497e53c1c03b92519a96c913c441a58
+    path: compound-packs
+    pack: [cell-architecture, boundary-testing, schema-laws, package-topology]
@@ -0,0 +1,100 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  # The @systemfsoftware tarballs the lockfile installs from, built by this
+  # repository's flake from the pinned systemfsoftware commit. The tarballs are
+  # architecture-independent, so one build serves every job below.
+  sfs-deps:
+    name: systemfsoftware tarballs from the flake
+    runs-on: ubuntu-latest
+    timeout-minutes: 60
+    steps:
+      - uses: actions/checkout@v7
+      - uses: cachix/install-nix-action@v31
+      - name: Build the tarballs the lockfile names
+        run: cp -rL "$(nix build --no-link --print-out-paths .#sfs-deps)" .sfs-deps
+      - name: The lockfile's recorded integrities match the flake's tarballs
+        run: nix build --no-link .#checks.x86_64-linux.consumer-store
+      - uses: actions/upload-artifact@v7
+        with:
+          name: sfs-deps
+          path: .sfs-deps/
+          retention-days: 1
+          include-hidden-files: true
+          if-no-files-found: error
+
+  test:
+    name: Product tests
+    needs: sfs-deps
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - uses: actions/checkout@v7
+      - uses: actions/download-artifact@v8
+        with:
+          name: sfs-deps
+          path: .sfs-deps
+      - uses: pnpm/action-setup@v6
+      - uses: actions/setup-node@v7
+        with:
+          node-version: 24
+          cache: pnpm
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm test
+
+  host-bytes:
+    name: Output bytes on ${{ matrix.arch }}
+    needs: sfs-deps
+    strategy:
+      matrix:
+        include:
+          - arch: x86_64
+            runner: ubuntu-latest
+          - arch: arm64
+            runner: ubuntu-24.04-arm
+    runs-on: ${{ matrix.runner }}
+    timeout-minutes: 15
+    steps:
+      - uses: actions/checkout@v7
+      - uses: actions/download-artifact@v8
+        with:
+          name: sfs-deps
+          path: .sfs-deps
+      - uses: pnpm/action-setup@v6
+      - uses: actions/setup-node@v7
+        with:
+          node-version: 24
+          cache: pnpm
+      - run: pnpm install --frozen-lockfile
+      - name: Serialise a fixed-seed sample of models and verdicts
+        run: node apps/api-extractor-e2e/host-digest.ts > host-output.bin
+      - run: sha256sum host-output.bin && uname -m
+      - uses: actions/upload-artifact@v7
+        with:
+          name: host-output-${{ matrix.arch }}
+          path: host-output.bin
+
+  host-compare:
+    name: Same bytes on every host
+    needs: host-bytes
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/download-artifact@v8
+        with:
+          pattern: host-output-*
+      - run: sha256sum host-output-*/host-output.bin
+      - name: Compare x86_64 and arm64 outputs byte for byte
+        run: cmp host-output-x86_64/host-output.bin host-output-arm64/host-output.bin
@@ -0,0 +1,6 @@
+node_modules/
+dist/
+coverage/
+*.tsbuildinfo
+# systemfsoftware tarballs the lockfile installs from: `nix build .#sfs-deps`, copied here
+.sfs-deps/
@@ -0,0 +1,26 @@
+# AGENTS.md
+
+`@systemfsoftware/api-extractor`: versioned API model, diff and Effect-aware semver
+verdict. Requirements and decisions: `docs/plans/2026-10-10-0517-feat-effect-api-extractor-plan.md`.
+
+## Rules
+
+- Read `docs/solutions/` before planning or changing code here.
+- Every output goes through `canonicalJson` (`packages/api-extractor/src/canonical-json.ts`).
+  Order strings with `compareCodeUnits`; never `localeCompare`, `toLocale*` or `Intl` in `src`.
+- A change to the model schema bumps `API_MODEL_SCHEMA_VERSION`.
+- Bump semantics live in the rule corpus (`src/rules/corpus.v1.json`), not in code. A
+  corpus change bumps its `corpusVersion`.
+- A file derived from `microsoft/rushstack` or systemfsoftware PR #552 carries a header
+  naming the repository, path and commit.
+- Shared tooling (tsconfig bases, lint presets, release, mutation) comes from
+  systemfsoftware family flakes and reusable workflows. Never copy it in.
+- Mutation testing runs on `main` only.
+
+## Commands
+
+```sh
+pnpm install
+pnpm test        # property tests, including the cross-locale process test
+pnpm typecheck
+```
@@ -19,3 +19,34 @@ AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
+
+------------------------------------------------------------------------------
+
+Portions of this software are derived from @microsoft/api-extractor 7.59.1
+(https://github.com/microsoft/rushstack, apps/api-extractor, commit
+bc4cd29cc4984cef267437565bf681582948285e), distributed under the following
+license:
+
+@microsoft/api-extractor
+
+Copyright (c) Microsoft Corporation. All rights reserved.
+
+MIT License
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
@@ -0,0 +1,68 @@
+# api-extractor-effect
+
+`@systemfsoftware/api-extractor` is a public-API tool for Effect-heavy TypeScript
+libraries. It reduces a package to one versioned JSON API model, diffs two models,
+and gives a semver verdict that understands Effect's `A`/`E`/`R` channels and dual
+APIs. Every output is byte-identical across locales, time zones and hosts.
+
+The design and its requirements live in
+[`docs/plans/2026-10-10-0517-feat-effect-api-extractor-plan.md`](docs/plans/2026-10-10-0517-feat-effect-api-extractor-plan.md).
+
+## Status
+
+| Part | State |
+|---|---|
+| Versioned model, canonical serialiser, diff, rule corpus and verdict | in this repository |
+| Front-ends (isolated-declarations fast path, compiler fallback) | next |
+| `.api.md` report with a differential test against `@microsoft/api-extractor` 7.59.1 | planned |
+| Benchmark, CLI, changeset-bump check | planned |
+| systemfsoftware packages from the flake (`.sfs-deps`) | in this repository |
+| Release, mutation, shared lint/build jobs, tsdown and vitest presets | waits on shared family tooling |
+
+## The verdict
+
+```ts
+import { Effect } from 'effect'
+import { Verdict } from '@systemfsoftware/api-extractor'
+
+// `before` and `after` are encoded API models (the JSON this tool writes).
+const document = await Effect.runPromise(Verdict.cell.run({ before, after }))
+process.stdout.write(document) // canonical JSON: { _tag, schemaVersion, engine, corpusVersion, declarations, bump, findings }
+```
+
+`bump` is `none`, `patch`, `minor` or `major`; each finding is
+`{ rule_id, reason_code, bump, next_action, locations }`. A model that does not decode
+fails the cell with `SurfaceRejected`; a rule corpus in which two rules match the same
+change fails it with `RuleCorpusRejected`. The decision itself, `Verdict.judge`, is a
+pure `effect-cell-types` workflow over decoded values.
+
+Rules are data: [`src/rules/corpus.v1.json`](packages/api-extractor/src/rules/corpus.v1.json)
+maps each kind of change, its position (`in`, `out`, `both`), its Effect channel and
+its direction (`widened`, `narrowed`, `changed`) to a bump. The most specific matching
+rule wins. A change no rule covers is reported as major `UNCLASSIFIED_CHANGE`.
+
+For example, a function returning `Effect<User, NotFound>` that now returns
+`Effect<User, NotFound | Timeout>` is major `E_WIDENED`; the same change to a parameter
+is minor `E_WIDENED_IN_INPUT`. Removing a tagged error from a returned error channel is
+major `E_TAG_REMOVED`, because a caller's `catchTag` for that tag stops compiling.
+
+## Development
+
+The `@systemfsoftware/*` packages install from tarballs that this repository's flake
+builds from a pinned systemfsoftware commit, never from a registry:
+
+```sh
+cp -rL "$(nix build --no-link --print-out-paths .#sfs-deps)" .sfs-deps
+pnpm install
+pnpm test
+pnpm typecheck
+```
+
+## Provenance
+
+This repository is MIT-licensed. The behaviour oracle is `@microsoft/api-extractor`
+7.59.1 (`microsoft/rushstack` `apps/api-extractor` at
+`bc4cd29cc4984cef267437565bf681582948285e`), whose MIT notice is reproduced in
+[`LICENSE`](LICENSE). A file derived from upstream, or from systemfsoftware PR #552,
+names its origin (repository, path and commit) in a header comment. No file in the
+current tree is derived from either.
@@ -0,0 +1,7 @@
+import { ApiModel } from '@systemfsoftware/api-extractor'
+import { Arbitrary, Effect, Schema } from 'effect'
+import { outputsOf } from './outputs.ts'
+
+const Models = Schema.Array(ApiModel.ApiModel).check(Schema.isBetweenLength(2, 4))
+const samples = Effect.runSync(Arbitrary.sampleEffect(Arbitrary.schema(Models), { count: 200, seed: 20261010 }))
+process.stdout.write(samples.flatMap(outputsOf).join(''))
@@ -0,0 +1,60 @@
+import { ApiModel } from '@systemfsoftware/api-extractor'
+import { describe } from '@systemfsoftware/vitest'
+import { Schema } from 'effect'
+import { spawnSync } from 'node:child_process'
+import { mkdtempSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { outputsOf } from './outputs.ts'
+
+const probe = fileURLToPath(new URL('./probe.ts', import.meta.url))
+const elsewhere = mkdtempSync(join(tmpdir(), 'api-extractor-locale-'))
+
+const environments = [
+  { LC_ALL: 'C', TZ: 'UTC', cwd: process.cwd(), expectLocale: undefined },
+  { LC_ALL: 'tr_TR.UTF-8', TZ: 'Europe/Istanbul', cwd: process.cwd(), expectLocale: 'tr' },
+  { LC_ALL: 'de_DE.UTF-8', TZ: 'America/Los_Angeles', cwd: elsewhere, expectLocale: 'de' },
+] as const
+type Environment = (typeof environments)[number]
+
+const inherited = Object.fromEntries(Object.entries(process.env).filter(([key]) => !/^(LC_|LANG|LANGUAGE$|TZ$)/.test(key)))
+
+interface ProbeResult {
+  readonly locale: string
+  readonly timeZone: string
+  readonly cwd: string
+  readonly outputs: ReadonlyArray<string>
+}
+
+const run = (environment: Environment, input: string): ProbeResult => {
+  const child = spawnSync(process.execPath, [probe], {
+    input,
+    cwd: environment.cwd,
+    env: { ...inherited, LC_ALL: environment.LC_ALL, TZ: environment.TZ },
+    encoding: 'utf8',
+  })
+  if (child.status !== 0) throw new Error(`probe failed under ${environment.LC_ALL}: ${child.stderr}`)
+  return JSON.parse(child.stdout) as ProbeResult
+}
+
+const runEverywhere = (models: ReadonlyArray<ApiModel.ApiModel>): ReadonlyArray<ProbeResult> => {
+  const input = JSON.stringify(models.map(ApiModel.encode))
+  return environments.map((environment) => run(environment, input))
+}
+
+const resolved = (environment: Environment, result: ProbeResult): boolean =>
+  (environment.expectLocale === undefined || result.locale.startsWith(environment.expectLocale)) &&
+  result.timeZone === environment.TZ &&
+  result.cwd === environment.cwd
+
+describe('output bytes across locales, time zones and working directories', (it) => {
+  it.prop(
+    'AE5: every model and verdict serialises, in every environment, to the bytes this process writes',
+    { of: [Schema.Array(ApiModel.ApiModel).check(Schema.isBetweenLength(2, 4))], subject: runEverywhere, runs: 4 },
+    (runEverywhere, [models]) => {
+      const expected = JSON.stringify(outputsOf(models))
+      return runEverywhere(models).every((result, index) => resolved(environments[index]!, result) && JSON.stringify(result.outputs) === expected)
+    },
+  )
+})
@@ -0,0 +1,8 @@
+import { ApiModel, Verdict } from '@systemfsoftware/api-extractor'
+import { Effect } from 'effect'
+
+/** The bytes the determinism checks compare: every model, then the verdict between each consecutive pair. */
+export const outputsOf = (models: ReadonlyArray<ApiModel.ApiModel>): ReadonlyArray<string> => [
+  ...models.map(ApiModel.serialise),
+  ...models.slice(1).map((after, index) => Effect.runSync(Verdict.cell.run({ before: models[index]!, after }))),
+]
@@ -0,0 +1,21 @@
+{
+  "name": "api-extractor-e2e",
+  "private": true,
+  "type": "module",
+  "engines": {
+    "node": ">=24"
+  },
+  "scripts": {
+    "test": "vitest run",
+    "typecheck": "tsc --noEmit"
+  },
+  "devDependencies": {
+    "@systemfsoftware/api-extractor": "workspace:*",
+    "@systemfsoftware/tsconfig": "^2.0.1",
+    "@systemfsoftware/vitest": "^2.0.0",
+    "@types/node": "catalog:",
+    "effect": "catalog:",
+    "typescript": "catalog:",
+    "vitest": "catalog:"
+  }
+}