diff --git a/docs/adr/012-browser-test-mocking-strategy.md b/docs/adr/012-browser-test-mocking-strategy.md
index b1e01ead..0e33b309 100644
--- a/docs/adr/012-browser-test-mocking-strategy.md
+++ b/docs/adr/012-browser-test-mocking-strategy.md
@@ -88,5 +88,5 @@ These modules are resolved at static import time (before any test runs). Their `
 - New browser-mode specs that need to stub an external library **must not** use `vi.mock(externalLib, factory)`. Add a loader/factory parameter to the underlying hook or service instead.
 - The CI `unit-tests` job includes a permanent grep guard that fails the build if `rpc is closed` appears in any coverage run log. This catches regressions before they reach the acceptance criterion.
 - Acceptance criterion for #535: 60 consecutive green `workflow_dispatch` CI runs against `main` after the fix is merged, with zero `rpc is closed` lines in any log.
-- **Enforcement:** No automated lint rule is planned; the CI coverage guard is the regression backstop. If a lint rule is added later (e.g. an ESLint rule flagging `vi.mock` of non-virtual modules in browser-mode spec files), update this ADR.
+- **Enforcement:** An ESLint `no-restricted-syntax` rule in `eslint.config.js` (scoped to `**/*.spec.ts` and `**/*.test.ts`) flags any `vi.mock('pdfjs-dist', ...)` call with a message referencing this ADR. This turns a ~2-minute CI wait into an immediate lint error on save. The CI birpc grep guard remains as belt-and-suspenders for the coverage run. A CI static grep step (added in issue #546) also catches the pattern before the test suite launches.
 - **When to revisit the LibLoader home:** If three or more components adopt this pattern, consider extracting a shared `$lib/types/lib-loader.ts` or a generic `DynamicImportLoader<T>` type to avoid parallel type definitions across modules.
diff --git a/frontend/eslint.config.js b/frontend/eslint.config.js
index a22043f6..a56c710b 100644
--- a/frontend/eslint.config.js
+++ b/frontend/eslint.config.js
@@ -72,6 +72,20 @@ export default defineConfig(
 			]
 		}
 	},
+	{
+		files: ['**/*.spec.ts', '**/*.test.ts'],
+		rules: {
+			'no-restricted-syntax': [
+				'error',
+				{
+					selector:
+						"CallExpression[callee.object.name='vi'][callee.property.name='mock'] > Literal[value=/^pdfjs-dist/]",
+					message:
+						"Banned: vi.mock('pdfjs-dist', factory) causes a birpc teardown race in browser-mode specs — see ADR 012. Use the libLoader prop injection pattern instead."
+				}
+			]
+		}
+	},
 	{
 		plugins: { boundaries },
 		settings: {