Make baseline regeneration discoverable and cover the CI guard

Drift failures are routine here: any PR that incidentally fixes an e2e or
mobile type error trips the stale-entry check. Finding the fix meant opening
the script to learn --write exists, so the failure now prints the exact
regenerate command, and both packages expose it as a script.

Resolve the env-supplied PR base against the repo root like the flag already
was. Mobile CI runs with working-directory: mobile, so a repo-relative value
would have resolved against the wrong directory.

Cover the growth guard on the check path and the env form CI actually uses;
both branches were previously exercised only through --write.

config/scripts/typecheck-diagnostic-baseline.mjs

@@ -136,6 +136,12 @@ function subtractDiagnostics(left, right) {
   })
 }
 
+/** Failures here are routine, so the message carries its own fix. */
+function regenerationHint() {
+  const argv = process.argv.slice(2).filter((value) => value !== '--write')
+  return `Regenerate with: node ${relative(repoRoot, import.meta.filename)} ${argv.join(' ')} --write`
+}
+
 function formatDiagnostic(diagnostic) {
   const location = diagnostic.file
     ? `${diagnostic.file}:${diagnostic.line}:${diagnostic.column}`
@@ -168,9 +174,9 @@ async function main() {
   const options = parseArgs(process.argv.slice(2))
   const projectPath = resolve(repoRoot, options.project)
   const baselinePath = resolve(repoRoot, options.baseline)
-  const baseBaselinePath = options.baseBaseline
-    ? resolve(repoRoot, options.baseBaseline)
-    : process.env.TYPECHECK_BASELINE_BASE_PATH
+  // Resolve the env form like the flag: mobile runs with working-directory: mobile.
+  const baseBaselineOption = options.baseBaseline ?? process.env.TYPECHECK_BASELINE_BASE_PATH
+  const baseBaselinePath = baseBaselineOption ? resolve(repoRoot, baseBaselineOption) : undefined
   const tscPath = resolve(repoRoot, options.tsc ?? 'node_modules/typescript/bin/tsc')
   const diagnostics = collectDiagnostics(projectPath, tscPath)
   const baseBaseline = baseBaselinePath ? await readBaseline(baseBaselinePath) : null
@@ -207,6 +213,7 @@ async function main() {
       console.error(formatDiagnostic(diagnostic))
     }
   }
+  console.error(regenerationHint())
   process.exitCode = 1
 }
 

config/scripts/typecheck-diagnostic-baseline.test.mjs

@@ -47,7 +47,7 @@ function runBaseline(paths, extraArgs = [], spawnOptions = {}) {
       ...extraArgs
     ],
     {
-      cwd: repoRoot,
+      cwd: spawnOptions.cwd ?? repoRoot,
       encoding: 'utf8',
       env: { ...process.env, ...spawnOptions.env }
     }
@@ -151,6 +151,80 @@ describe('typecheck diagnostic baseline', () => {
     expect(result.stdout).toContain('1 known diagnostics, no drift')
   })
 
+  it('rejects a committed baseline that grew from the PR base', () => {
+    const paths = createFixture("const first: number = 'bad'\n")
+    expect(runBaseline(paths, ['--write']).status).toBe(0)
+    // Base knows only the first error; the committed baseline was raised to cover a second.
+    const base = JSON.parse(readFileSync(paths.baseline, 'utf8'))
+    writeFileSync(paths.baseBaseline, JSON.stringify(base))
+    writeFileSync(paths.source, "const first: number = 'bad'\nconst second: boolean = 'also-bad'\n")
+    expect(runBaseline(paths, ['--write', '--base-baseline', paths.baseBaseline]).status).toBe(1)
+    writeFileSync(
+      paths.baseline,
+      JSON.stringify({
+        version: 1,
+        diagnostics: [
+          ...base.diagnostics,
+          {
+            file: 'source.ts',
+            line: 2,
+            column: 7,
+            code: 2322,
+            message: "Type 'string' is not assignable to type 'boolean'."
+          }
+        ]
+      })
+    )
+
+    const check = runBaseline(paths, ['--base-baseline', paths.baseBaseline])
+
+    expect(check.status).toBe(1)
+    expect(check.stderr).toContain('Diagnostic baseline grew (1)')
+  })
+
+  it('reads the PR base from the environment the way CI passes it', () => {
+    const paths = createFixture("const value: number = 'bad'\n")
+    expect(runBaseline(paths, ['--write']).status).toBe(0)
+    writeFileSync(paths.baseBaseline, JSON.stringify({ version: 1, diagnostics: [] }))
+
+    const result = runBaseline(paths, [], {
+      env: { TYPECHECK_BASELINE_BASE_PATH: paths.baseBaseline }
+    })
+
+    expect(result.status).toBe(1)
+    expect(result.stderr).toContain('Diagnostic baseline grew (1)')
+  })
+
+  it('resolves the env-supplied PR base against the repo, not the working directory', () => {
+    // Mobile CI runs this with working-directory: mobile, so a repo-relative base
+    // path must not resolve against the caller's cwd.
+    const result = runBaseline(
+      {
+        project: 'config/tsconfig.e2e.json',
+        baseline: 'config/typecheck-e2e-diagnostics.json'
+      },
+      [],
+      {
+        cwd: join(repoRoot, 'mobile'),
+        env: { TYPECHECK_BASELINE_BASE_PATH: 'config/typecheck-e2e-diagnostics.json' }
+      }
+    )
+
+    expect(result.stderr).not.toContain('ENOENT')
+    expect(result.status).toBe(0)
+  })
+
+  it('names the regeneration command when the baseline drifts', () => {
+    const paths = createFixture("const value: number = 'bad'\n")
+    expect(runBaseline(paths, ['--write']).status).toBe(0)
+    writeFileSync(paths.source, 'const value: number = 1\n')
+
+    const stale = runBaseline(paths)
+
+    expect(stale.status).toBe(1)
+    expect(stale.stderr).toContain('--write')
+  })
+
   it('refuses to regenerate a baseline that grows from the PR base', () => {
     const paths = createFixture("const first: number = 'bad'\n")
     expect(runBaseline(paths, ['--write']).status).toBe(0)

mobile/package.json

@@ -10,6 +10,7 @@
     "postinstall": "node scripts/build-terminal-webview-engine.mjs && node scripts/build-mermaid-webview-engine.mjs",
     "test": "vitest run",
     "typecheck": "node ../config/scripts/typecheck-diagnostic-baseline.mjs --project mobile/tsconfig.json --baseline mobile/typecheck-test-diagnostics.json --tsc mobile/node_modules/typescript/bin/tsc",
+    "typecheck:write": "node ../config/scripts/typecheck-diagnostic-baseline.mjs --project mobile/tsconfig.json --baseline mobile/typecheck-test-diagnostics.json --tsc mobile/node_modules/typescript/bin/tsc --write",
     "lint": "oxlint",
     "format": "oxfmt --write .",
     "format:check": "oxfmt --check .",

package.json

@@ -49,6 +49,7 @@
     "typecheck:cli": "tsc --noEmit -p config/tsconfig.tc.cli.json",
     "typecheck:web": "tsc --noEmit -p config/tsconfig.tc.web.json",
     "typecheck:e2e": "node config/scripts/typecheck-diagnostic-baseline.mjs --project config/tsconfig.e2e.json --baseline config/typecheck-e2e-diagnostics.json",
+    "typecheck:e2e:write": "node config/scripts/typecheck-diagnostic-baseline.mjs --project config/tsconfig.e2e.json --baseline config/typecheck-e2e-diagnostics.json --write",
     "typecheck": "node config/scripts/run-typecheck-projects-in-parallel.mjs",
     "typecheck:tsc:node": "tsc --noEmit -p config/tsconfig.node.json --composite false",
     "typecheck:tsc:cli": "tsc --noEmit -p config/tsconfig.cli.json --composite false",