diff --git a/.github/workflows/mobile.yml b/.github/workflows/mobile.yml
index d476360b882..6dad79cd502 100644
--- a/.github/workflows/mobile.yml
+++ b/.github/workflows/mobile.yml
@@ -34,6 +34,20 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v6
+        with:
+          fetch-depth: 2
+
+      - name: Load base mobile diagnostic baseline
+        env:
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+        run: |
+          set -euo pipefail
+          git cat-file -e "$BASE_SHA^{commit}"
+          if git cat-file -e "$BASE_SHA:mobile/typecheck-test-diagnostics.json" 2>/dev/null; then
+            base_baseline="$RUNNER_TEMP/typecheck-mobile-base.json"
+            git show "$BASE_SHA:mobile/typecheck-test-diagnostics.json" > "$base_baseline"
+            echo "TYPECHECK_BASELINE_BASE_PATH=$base_baseline" >> "$GITHUB_ENV"
+          fi
 
       - name: Setup Node.js
         uses: actions/setup-node@v6
diff --git a/.github/workflows/pr.yml b/.github/workflows/pr.yml
index 338bcda7dbd..f4b9adc5919 100644
--- a/.github/workflows/pr.yml
+++ b/.github/workflows/pr.yml
@@ -209,8 +209,21 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v6
         with:
+          fetch-depth: 2
           persist-credentials: false
 
+      - name: Load base E2E diagnostic baseline
+        env:
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+        run: |
+          set -euo pipefail
+          git cat-file -e "$BASE_SHA^{commit}"
+          if git cat-file -e "$BASE_SHA:config/typecheck-e2e-diagnostics.json" 2>/dev/null; then
+            base_baseline="$RUNNER_TEMP/typecheck-e2e-base.json"
+            git show "$BASE_SHA:config/typecheck-e2e-diagnostics.json" > "$base_baseline"
+            echo "TYPECHECK_BASELINE_BASE_PATH=$base_baseline" >> "$GITHUB_ENV"
+          fi
+
       - uses: ./.github/actions/install-node-dependencies
 
       # Why: every project is `composite`, so tsc already writes a .tsbuildinfo that lets
diff --git a/config/scripts/native-chat-live-session-benchmark.ts b/config/scripts/native-chat-live-session-benchmark.ts
index 16f3cb6f228..4fb05c48759 100644
--- a/config/scripts/native-chat-live-session-benchmark.ts
+++ b/config/scripts/native-chat-live-session-benchmark.ts
@@ -33,12 +33,13 @@ let checksum = 0
 let expectedChecksum = 0
 let validatedCases = 0
 let cappedCalibrations = 0
+let sessionSink: NativeChatSession | null = null
 let messageArraySink: NativeChatMessage[] | null = null
 let contentSink = ''
 let pendingMatchSink: Set<number> | null = null
 let userRowSink: readonly NativeChatUserRow[] | null = null
 const benchmarkStartedAt = performance.now()
-const getPendingSink = (): Set<number> | null => pendingMatchSink
+const getSinks = () => [sessionSink, pendingMatchSink] as const
 
 function proseFixture(count: number, bytes: number, withTurnId: boolean): NativeChatMessage[] {
   const payload = 'Ab Cd  '.repeat(Math.ceil(bytes / 7)).slice(0, bytes)
@@ -127,6 +128,7 @@ function messageWeight(message: NativeChatMessage, content: string): number {
 }
 
 function consumeSession(session: NativeChatSession, index: number): number {
+  sessionSink = session
   messageArraySink = session.messages
   const message = session.messages[index % session.messages.length]
   if (!message) {
@@ -325,9 +327,10 @@ benchmark(
 )
 
 strictEqual(checksum, expectedChecksum, 'benchmark checksum accounting drifted')
+strictEqual(getSinks()[0]?.sessionId, 'benchmark', 'session outputs did not escape')
 strictEqual(Array.isArray(messageArraySink), true, 'message arrays did not escape')
 strictEqual(contentSink.length > 0, true, 'message content did not escape')
-strictEqual(getPendingSink() instanceof Set, true, 'pending baseline scan did not escape')
+strictEqual(getSinks()[1] instanceof Set, true, 'pending baseline scan did not escape')
 strictEqual(Array.isArray(userRowSink), true, 'user row scan did not escape')
 console.log(
   `validated=${validatedCases} cases, checksum=${checksum}, capped calibrations=${cappedCalibrations}, runtime=${(
diff --git a/config/scripts/typecheck-diagnostic-baseline.mjs b/config/scripts/typecheck-diagnostic-baseline.mjs
index 024f4a0810f..ae7cc1bcdd3 100644
--- a/config/scripts/typecheck-diagnostic-baseline.mjs
+++ b/config/scripts/typecheck-diagnostic-baseline.mjs
@@ -21,6 +21,8 @@ function parseArgs(argv) {
       options.project = next
     } else if (value === '--baseline') {
       options.baseline = next
+    } else if (value === '--base-baseline') {
+      options.baseBaseline = next
     } else if (value === '--tsc') {
       options.tsc = next
     } else {
@@ -29,7 +31,9 @@ function parseArgs(argv) {
     index += 1
   }
   if (!options.project || !options.baseline) {
-    throw new Error('Usage: --project <tsconfig> --baseline <json> [--tsc <binary>] [--write]')
+    throw new Error(
+      'Usage: --project <tsconfig> --baseline <json> [--base-baseline <json>] [--tsc <binary>] [--write]'
+    )
   }
   return options
 }
@@ -102,7 +106,11 @@ function collectDiagnostics(projectPath, tscPath) {
 function diagnosticCounts(diagnostics) {
   const counts = new Map()
   for (const diagnostic of diagnostics) {
-    const key = JSON.stringify(diagnostic)
+    const key = JSON.stringify({
+      file: diagnostic.file,
+      code: diagnostic.code,
+      message: diagnostic.message
+    })
     counts.set(key, (counts.get(key) ?? 0) + 1)
   }
   return counts
@@ -111,7 +119,11 @@ function diagnosticCounts(diagnostics) {
 function subtractDiagnostics(left, right) {
   const remaining = diagnosticCounts(right)
   return left.filter((diagnostic) => {
-    const key = JSON.stringify(diagnostic)
+    const key = JSON.stringify({
+      file: diagnostic.file,
+      code: diagnostic.code,
+      message: diagnostic.message
+    })
     const count = remaining.get(key) ?? 0
     if (count === 0) {
       return true
@@ -128,23 +140,51 @@ function formatDiagnostic(diagnostic) {
   return `${location} TS${diagnostic.code}: ${diagnostic.message}`
 }
 
+async function readBaseline(baselinePath) {
+  const baseline = JSON.parse(await readFile(baselinePath, 'utf8'))
+  if (baseline.version !== 1 || !Array.isArray(baseline.diagnostics)) {
+    throw new Error(`Invalid diagnostic baseline: ${relative(repoRoot, baselinePath)}`)
+  }
+  return baseline
+}
+
+function reportBaselineGrowth(diagnostics, baseDiagnostics) {
+  const added = subtractDiagnostics(diagnostics, baseDiagnostics)
+  if (added.length === 0) {
+    return false
+  }
+  console.error(`Diagnostic baseline grew (${added.length}); only removals are allowed:`)
+  for (const diagnostic of added) {
+    console.error(formatDiagnostic(diagnostic))
+  }
+  process.exitCode = 1
+  return true
+}
+
 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
   const tscPath = resolve(repoRoot, options.tsc ?? 'node_modules/typescript/bin/tsc')
   const diagnostics = collectDiagnostics(projectPath, tscPath)
+  const baseBaseline = baseBaselinePath ? await readBaseline(baseBaselinePath) : null
 
   if (options.write) {
+    if (baseBaseline && reportBaselineGrowth(diagnostics, baseBaseline.diagnostics)) {
+      return
+    }
     const baseline = { version: 1, diagnostics }
     await writeFile(baselinePath, `${JSON.stringify(baseline, null, 2)}\n`)
     console.log(`Wrote ${diagnostics.length} diagnostics to ${relative(repoRoot, baselinePath)}`)
     return
   }
 
-  const baseline = JSON.parse(await readFile(baselinePath, 'utf8'))
-  if (baseline.version !== 1 || !Array.isArray(baseline.diagnostics)) {
-    throw new Error(`Invalid diagnostic baseline: ${relative(repoRoot, baselinePath)}`)
+  const baseline = await readBaseline(baselinePath)
+  if (baseBaseline && reportBaselineGrowth(baseline.diagnostics, baseBaseline.diagnostics)) {
+    return
   }
   const unexpected = subtractDiagnostics(diagnostics, baseline.diagnostics)
   const stale = subtractDiagnostics(baseline.diagnostics, diagnostics)
diff --git a/config/scripts/typecheck-diagnostic-baseline.test.mjs b/config/scripts/typecheck-diagnostic-baseline.test.mjs
index c5aa3be3a97..fc4dc426ce0 100644
--- a/config/scripts/typecheck-diagnostic-baseline.test.mjs
+++ b/config/scripts/typecheck-diagnostic-baseline.test.mjs
@@ -26,6 +26,7 @@ function createFixture(source) {
   writeFileSync(join(fixtureDir, 'source.ts'), source)
   return {
     baseline: join(fixtureDir, 'baseline.json'),
+    baseBaseline: join(fixtureDir, 'base-baseline.json'),
     project: join(fixtureDir, 'tsconfig.json'),
     source: join(fixtureDir, 'source.ts')
   }
@@ -40,7 +41,17 @@ function runBaseline(paths, extraArgs = []) {
 }
 
 describe('typecheck diagnostic baseline', () => {
-  it('accepts only the exact recorded diagnostics', () => {
+  it('loads PR-base baselines in desktop and mobile CI', () => {
+    const desktopWorkflow = readFileSync(join(repoRoot, '.github/workflows/pr.yml'), 'utf8')
+    const mobileWorkflow = readFileSync(join(repoRoot, '.github/workflows/mobile.yml'), 'utf8')
+
+    expect(desktopWorkflow).toContain('$BASE_SHA:config/typecheck-e2e-diagnostics.json')
+    expect(mobileWorkflow).toContain('$BASE_SHA:mobile/typecheck-test-diagnostics.json')
+    expect(desktopWorkflow).toContain('TYPECHECK_BASELINE_BASE_PATH=')
+    expect(mobileWorkflow).toContain('TYPECHECK_BASELINE_BASE_PATH=')
+  })
+
+  it('requires recorded diagnostics to remain present', () => {
     const paths = createFixture("const value: number = 'bad'\n")
     expect(runBaseline(paths, ['--write']).status).toBe(0)
     expect(JSON.parse(readFileSync(paths.baseline, 'utf8')).diagnostics).toHaveLength(1)
@@ -61,4 +72,34 @@ describe('typecheck diagnostic baseline', () => {
     expect(unexpected.status).toBe(1)
     expect(unexpected.stderr).toContain('Unexpected diagnostics (1)')
   })
+
+  it('rejects another occurrence of an allowlisted diagnostic', () => {
+    const paths = createFixture("const first: number = 'bad'\n")
+    expect(runBaseline(paths, ['--write']).status).toBe(0)
+
+    writeFileSync(paths.source, "const first: number = 'bad'\nconst second: number = 'also-bad'\n")
+    const unexpected = runBaseline(paths)
+    expect(unexpected.status).toBe(1)
+    expect(unexpected.stderr).toContain('Unexpected diagnostics (1)')
+  })
+
+  it('does not drift when only diagnostic positions move', () => {
+    const paths = createFixture("const value: number = 'bad'\n")
+    expect(runBaseline(paths, ['--write']).status).toBe(0)
+
+    writeFileSync(paths.source, "\nconst value: number = 'bad'\n")
+    expect(runBaseline(paths).status).toBe(0)
+  })
+
+  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)
+    writeFileSync(paths.baseBaseline, readFileSync(paths.baseline, 'utf8'))
+
+    writeFileSync(paths.source, "const first: number = 'bad'\nconst second: boolean = 'also-bad'\n")
+    const growth = runBaseline(paths, ['--write', '--base-baseline', paths.baseBaseline])
+    expect(growth.status).toBe(1)
+    expect(growth.stderr).toContain('Diagnostic baseline grew (1)')
+    expect(JSON.parse(readFileSync(paths.baseline, 'utf8')).diagnostics).toHaveLength(1)
+  })
 })
diff --git a/config/tsconfig.e2e.json b/config/tsconfig.e2e.json
index 32df2cee220..7c2535b8338 100644
--- a/config/tsconfig.e2e.json
+++ b/config/tsconfig.e2e.json
@@ -1,6 +1,6 @@
 {
-  // Enforced by `pnpm typecheck`; the exact pre-existing diagnostics are ratcheted in
-  // typecheck-e2e-diagnostics.json so new errors fail while the backlog shrinks.
+  // Enforced by `pnpm typecheck`; pre-existing diagnostic identities and counts are ratcheted
+  // in typecheck-e2e-diagnostics.json so new errors fail while the backlog shrinks.
   "extends": "@electron-toolkit/tsconfig/tsconfig.node.json",
   // Why: specs import renderer and main modules directly, so the ambient declarations those
   // modules rely on (vite client types, build-time defines) have to be in the program too.