From b21831f483cc961322542dc037db99579b030449 Mon Sep 17 00:00:00 2001
From: Oleg S <97077423+RobotSail@users.noreply.github.com>
Date: Tue, 14 Apr 2026 19:37:55 -0400
Subject: [PATCH] fix: support fork PRs by fetching via refs/pull/N/head

For cross-repository (fork) PRs, the head branch doesn't exist on the
origin remote. The action previously tried `git fetch origin <branch>`
which fails with "couldn't find remote ref".

Now detects fork PRs via `isCrossRepository` from the GraphQL API and
fetches using `pull/<number>/head:<branch>` refspec instead, which
GitHub exposes for all PR heads regardless of source repo.

Changes:
- Add `isCrossRepository` to PR GraphQL query and type
- Use `refs/pull/N/head` refspec for fork PRs in setupBranch()

Fixes #1218
---
 src/github/api/queries/github.ts |  1 +
 src/github/operations/branch.ts  | 15 ++++++++++++++-
 src/github/types.ts              |  1 +
 test/data-formatter.test.ts      |  1 +
 4 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/github/api/queries/github.ts b/src/github/api/queries/github.ts
index 7bceb8f9d..c5257ea19 100644
--- a/src/github/api/queries/github.ts
+++ b/src/github/api/queries/github.ts
@@ -12,6 +12,7 @@ export const PR_QUERY = `
         baseRefName
         headRefName
         headRefOid
+        isCrossRepository
         createdAt
         updatedAt
         lastEditedAt
diff --git a/src/github/operations/branch.ts b/src/github/operations/branch.ts
index b2c145b7a..2f9e58226 100644
--- a/src/github/operations/branch.ts
+++ b/src/github/operations/branch.ts
@@ -168,7 +168,20 @@ export async function setupBranch(
 
       // Execute git commands to checkout PR branch (dynamic depth based on PR size)
       // Using execFileSync instead of shell template literals for security
-      execGit(["fetch", "origin", `--depth=${fetchDepth}`, branchName]);
+      // For fork PRs, the branch doesn't exist on origin - use refs/pull/N/head instead
+      if (prData.isCrossRepository) {
+        console.log(
+          `Fork PR detected, fetching via refs/pull/${entityNumber}/head`,
+        );
+        execGit([
+          "fetch",
+          "origin",
+          `--depth=${fetchDepth}`,
+          `pull/${entityNumber}/head:${branchName}`,
+        ]);
+      } else {
+        execGit(["fetch", "origin", `--depth=${fetchDepth}`, branchName]);
+      }
       execGit(["checkout", branchName, "--"]);
 
       console.log(`Successfully checked out PR branch for PR #${entityNumber}`);
diff --git a/src/github/types.ts b/src/github/types.ts
index d982620da..4c39bbe0e 100644
--- a/src/github/types.ts
+++ b/src/github/types.ts
@@ -57,6 +57,7 @@ export type GitHubPullRequest = {
   baseRefName: string;
   headRefName: string;
   headRefOid: string;
+  isCrossRepository: boolean;
   createdAt: string;
   updatedAt?: string;
   lastEditedAt?: string;
diff --git a/test/data-formatter.test.ts b/test/data-formatter.test.ts
index 4c6b150dd..ec2d78f65 100644
--- a/test/data-formatter.test.ts
+++ b/test/data-formatter.test.ts
@@ -24,6 +24,7 @@ describe("formatContext", () => {
       baseRefName: "main",
       headRefName: "feature/test",
       headRefOid: "abc123",
+      isCrossRepository: false,
       createdAt: "2023-01-01T00:00:00Z",
       additions: 50,
       deletions: 30,