diff --git a/.github/workflows/test-core.yml b/.github/workflows/test-core.yml
index 47b59edab..c2cbc4d2e 100644
--- a/.github/workflows/test-core.yml
+++ b/.github/workflows/test-core.yml
@@ -25,6 +25,10 @@ jobs:
       - run: yarn --immutable --immutable-cache
       - run: yarn build
       - run: yarn lint:check
+      # Typecheck including test files. `yarn build` uses tsconfig.prod.json,
+      # which excludes **/*.test.ts, and `yarn test` runs via a type-stripping
+      # loader — so without this, type errors in test code reach no CI gate.
+      - run: yarn typecheck
       - run: yarn test:ci
       - name: Build core Docker image
         if: matrix.os == 'ubuntu-latest'
diff --git a/package.json b/package.json
index f6c51cafb..ce25101a8 100644
--- a/package.json
+++ b/package.json
@@ -188,6 +188,7 @@
     "test:auto-verify:indexing": "node --no-deprecation --import ./register.js src/tests/auto-verify/run.ts",
     "lint:check": "eslint src test",
     "lint:fix": "eslint --fix src test",
+    "typecheck": "tsc --noEmit -p tsconfig.json",
     "duplicate:check": "jscpd src",
     "duplicate:report": "jscpd src --reporters html,console",
     "duplicate:ci": "jscpd src --reporters console --exitCode 1",