diff --git a/.github/workflows/code-review.yml b/.github/workflows/code-review.yml
index cdeebc1..d219b17 100644
--- a/.github/workflows/code-review.yml
+++ b/.github/workflows/code-review.yml
@@ -38,6 +38,13 @@ on:
         description: Run the Codex reviewer.
         type: boolean
         default: true
+      block_on_severity:
+        description: >-
+          Fail the review job when a reviewer reports a [CRITICAL] or [HIGH]
+          finding, turning review into a merge gate. Off by default: reviews
+          are advisory unless a repo opts in.
+        type: boolean
+        default: false
     secrets:
       CLAUDE_CODE_OAUTH_TOKEN:
         required: false
@@ -267,6 +274,21 @@ jobs:
             gh pr comment "$PR_NUMBER" --repo "$REPO" --body-file /tmp/review-formatted.txt > /dev/null
           fi
 
+      # Opt-in merge gate. The review comment is upserted by the step above
+      # regardless, so a blocking finding still gets posted before the job
+      # goes red. Runs on always() so it also fires when an earlier step
+      # failed — a review that produced no output cannot match, so it will
+      # not block.
+      - name: Fail on blocking findings
+        if: always() && inputs.block_on_severity
+        run: |
+          if [ -s /tmp/review.txt ] && grep -qE '\[(CRITICAL|HIGH)\]' /tmp/review.txt; then
+            echo "::error::Review reported CRITICAL or HIGH severity findings"
+            grep -nE '\[(CRITICAL|HIGH)\]' /tmp/review.txt
+            exit 1
+          fi
+          echo "No blocking findings"
+
   codex-review:
     needs: changes
     # Skip on forks — secrets aren't available to fork PRs.
@@ -454,3 +476,18 @@ jobs:
       - name: Fail on Codex runtime error
         if: steps.codex-review.outputs.exit_code != '0'
         run: exit 1
+
+      # Opt-in merge gate. The review comment is upserted by the step above
+      # regardless, so a blocking finding still gets posted before the job
+      # goes red. Runs on always() so it also fires when an earlier step
+      # failed — a review that produced no output cannot match, so it will
+      # not block.
+      - name: Fail on blocking findings
+        if: always() && inputs.block_on_severity
+        run: |
+          if [ -s /tmp/review.txt ] && grep -qE '\[(CRITICAL|HIGH)\]' /tmp/review.txt; then
+            echo "::error::Review reported CRITICAL or HIGH severity findings"
+            grep -nE '\[(CRITICAL|HIGH)\]' /tmp/review.txt
+            exit 1
+          fi
+          echo "No blocking findings"
diff --git a/README.md b/README.md
index cb57f83..78fb579 100644
--- a/README.md
+++ b/README.md
@@ -54,7 +54,25 @@ review config and prompts.
    ```
 
    Optional inputs: `config_path` (default `.github/review-agents.json`),
-   `enable_claude` / `enable_codex` (default `true`).
+   `enable_claude` / `enable_codex` (default `true`), and
+   `block_on_severity` (default `false`).
+
+   **`block_on_severity`** turns review into a merge gate: the review job
+   fails when a reviewer reports a `[CRITICAL]` or `[HIGH]` finding. Off by
+   default — reviews are advisory unless a repo opts in:
+
+   ```yaml
+   jobs:
+     review:
+       uses: Provable-Games/.github/.github/workflows/code-review.yml@v1
+       with:
+         block_on_severity: true
+       secrets: inherit
+   ```
+
+   The review comment is posted either way, so a blocking finding is always
+   visible before the job goes red. A review that errored or produced no
+   output cannot match, so it never blocks on its own.
 
 2. Add `.github/review-agents.json` describing what to review: