diff --git a/.changes/fork-pr-reviewer-router-identity.md b/.changes/fork-pr-reviewer-router-identity.md
new file mode 100644
index 00000000..ae91d481
--- /dev/null
+++ b/.changes/fork-pr-reviewer-router-identity.md
@@ -0,0 +1,5 @@
+---
+category: Fixed
+---
+
+- **Fork pull-request admission** — keeps the read-only Reviewer Router identity check fail-closed while allowing external contributors' CI to use the reviewed public App slug when GitHub withholds repository variables.
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 31c4a2ee..f9216835 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -938,7 +938,15 @@ jobs:
             const pullNumber = context.payload.pull_request.number;
             const eventHeadSha = context.payload.pull_request.head.sha;
             const eventBaseSha = context.payload.pull_request.base.sha;
-            const appSlug = process.env.REVIEWER_ROUTER_APP_SLUG?.trim();
+            const configuredAppSlug = process.env.REVIEWER_ROUTER_APP_SLUG?.trim();
+            const reviewedForkAppSlug = 'dingtalk-dws-reviewer-router';
+            const pullHeadRepository =
+              context.payload.pull_request.head.repo.full_name?.toLowerCase();
+            const baseRepository = `${owner}/${repo}`.toLowerCase();
+            const isForkPull =
+              Boolean(pullHeadRepository) && pullHeadRepository !== baseRepository;
+            const appSlug =
+              configuredAppSlug || (isForkPull ? reviewedForkAppSlug : '');
             if (
               !appSlug ||
               appSlug !== appSlug.toLowerCase() ||
@@ -949,6 +957,11 @@ jobs:
               );
               return;
             }
+            if (!configuredAppSlug) {
+              core.info(
+                `Fork pull request cannot read the repository App slug variable; using the reviewed public slug ${reviewedForkAppSlug}.`,
+              );
+            }
             const expectedAppOwner = `${appSlug}[bot]`;
             const writerRulesetName = 'main-merge-writers';
             const skipWorkflowPattern =
diff --git a/docs/ci-pr-gates.md b/docs/ci-pr-gates.md
index 2730c99e..20500e12 100644
--- a/docs/ci-pr-gates.md
+++ b/docs/ci-pr-gates.md
@@ -184,8 +184,16 @@ ruleset before it can enable, reconcile, or synchronously merge. The read-only
 `Test`
 token may receive a repository projection with both merge-default properties
 omitted; it accepts only that complete omission or exact `MERGE_MESSAGE` plus
-`PR_TITLE`/`BLANK`, while partial or malformed projections fail closed. The
-minted App's `Contents: write` token must observe the exact reviewed defaults
+`PR_TITLE`/`BLANK`, while partial or malformed projections fail closed.
+The same unprivileged `pull_request` job may receive an empty repository-variable
+projection for an external fork. Only when the event head repository differs
+from the base repository does it substitute the exact reviewed public slug
+`dingtalk-dws-reviewer-router` for identity comparison. An empty variable on a
+same-repository PR and every malformed non-empty value still fail closed. This
+fallback neither mints a token nor grants merge authority; the base-owned
+Router continues to require its minted App slug to equal the repository
+variable before any mutation. The minted App's `Contents: write` token must
+observe the exact reviewed defaults
 before either mutation path proceeds. GitHub hides the complete
 `bypass_actors` list from low-privilege callers, so the rollout audit must still
 keep the writer list at exactly the Reviewer App, `haofeng0705` (ID
diff --git a/test/scripts/reviewer_router_workflow_test.go b/test/scripts/reviewer_router_workflow_test.go
index 6a47d788..c2918544 100644
--- a/test/scripts/reviewer_router_workflow_test.go
+++ b/test/scripts/reviewer_router_workflow_test.go
@@ -724,7 +724,13 @@ func TestCodeAdmissionEnforcesReviewerRouterWriterBoundary(t *testing.T) {
 		"name: Verify auto-merge identity",
 		"if: github.event_name == 'pull_request' && github.event.pull_request.draft == false",
 		"REVIEWER_ROUTER_APP_SLUG: ${{ vars.REVIEWER_ROUTER_APP_SLUG }}",
-		"process.env.REVIEWER_ROUTER_APP_SLUG?.trim()",
+		"const configuredAppSlug = process.env.REVIEWER_ROUTER_APP_SLUG?.trim()",
+		"const reviewedForkAppSlug = 'dingtalk-dws-reviewer-router'",
+		"context.payload.pull_request.head.repo.full_name?.toLowerCase()",
+		"const baseRepository = `${owner}/${repo}`.toLowerCase()",
+		"Boolean(pullHeadRepository) && pullHeadRepository !== baseRepository",
+		"configuredAppSlug || (isForkPull ? reviewedForkAppSlug : '')",
+		"Fork pull request cannot read the repository App slug variable; using the reviewed public slug",
 		"const expectedAppOwner = `${appSlug}[bot]`",
 		"github.rest.pulls.get",
 		"github.rest.repos.get",