fix: fall back to existing remote tracking ref when incremental patch fetch fails

actions/setup/js/generate_git_patch.cjs

@@ -165,7 +165,9 @@ async function generateGitPatch(branchName, baseBranch, options = {}) {
           // INCREMENTAL MODE (for push_to_pull_request_branch):
           // Only include commits that are new since origin/branchName.
           // This prevents including commits that already exist on the PR branch.
-          // We must explicitly fetch origin/branchName and fail if it doesn't exist.
+          // Prefer a fresh fetch of origin/branchName; fall back to the existing
+          // remote tracking ref (set up by the initial shallow checkout) when the
+          // fetch fails (e.g. due to shallow clone limitations or missing credentials).
 
           debugLog(`Strategy 1 (incremental): Fetching origin/${branchName}`);
           // Configure git authentication via GIT_CONFIG_* environment variables.
@@ -183,15 +185,29 @@ async function generateGitPatch(branchName, baseBranch, options = {}) {
             baseRef = `origin/${branchName}`;
             debugLog(`Strategy 1 (incremental): Successfully fetched, baseRef=${baseRef}`);
           } catch (fetchError) {
-            // In incremental mode, we MUST have origin/branchName - no fallback
-            debugLog(`Strategy 1 (incremental): Fetch failed - ${getErrorMessage(fetchError)}`);
-            errorMessage = `Cannot generate incremental patch: failed to fetch origin/${branchName}. This typically happens when the remote branch doesn't exist yet or was force-pushed. Error: ${getErrorMessage(fetchError)}`;
-            // Don't try other strategies in incremental mode
-            return {
-              success: false,
-              error: errorMessage,
-              patchPath: patchPath,
-            };
+            // Fetch failed. Check if origin/branchName already exists from the initial shallow checkout.
+            // This handles cases where git fetch fails due to shallow clone limitations or when
+            // GITHUB_TOKEN is unavailable in the MCP server process (e.g. after clean_git_credentials.sh).
+            // Using the existing remote tracking ref as a fallback is safe: it represents the state
+            // of the branch at checkout time, so the incremental patch will include all commits
+            // made by the agent since then.
+            debugLog(`Strategy 1 (incremental): Fetch failed - ${getErrorMessage(fetchError)}, checking for existing remote tracking ref`);
+            try {
+              execGitSync(["show-ref", "--verify", "--quiet", `refs/remotes/origin/${branchName}`], { cwd });
+              // Remote tracking ref exists from initial shallow checkout — use it as base
+              baseRef = `origin/${branchName}`;
+              debugLog(`Strategy 1 (incremental): Using existing remote tracking ref as fallback, baseRef=${baseRef}`);
+            } catch (refCheckError) {
+              // No remote tracking ref at all — cannot safely generate an incremental patch.
+              // Report both errors: the original fetch failure and the missing ref.
+              debugLog(`Strategy 1 (incremental): No existing remote tracking ref found (${getErrorMessage(refCheckError)}), failing`);
+              errorMessage = `Cannot generate incremental patch: failed to fetch origin/${branchName} and no existing remote tracking ref found. This typically happens when the remote branch doesn't exist yet or was force-pushed. Fetch error: ${getErrorMessage(fetchError)}`;
+              return {
+                success: false,
+                error: errorMessage,
+                patchPath: patchPath,
+              };
+            }
           }
         } else {
           // FULL MODE (for create_pull_request):

actions/setup/js/git_patch_integration.test.cjs

@@ -739,6 +739,51 @@ describe("git patch integration tests", () => {
       }
     });
 
+    it("should fall back to existing remote tracking ref when fetch fails in incremental mode", async () => {
+      // Simulate a shallow checkout scenario:
+      // 1. feature-branch is created, first commit pushed to origin (origin/feature-branch exists)
+      // 2. Agent adds a new commit locally
+      // 3. Remote URL is broken so git fetch fails
+      // 4. We expect patch generation to succeed using the existing origin/feature-branch ref
+      execGit(["checkout", "-b", "shallow-fetch-fail"], { cwd: workingRepo });
+      fs.writeFileSync(path.join(workingRepo, "base.txt"), "Base content\n");
+      execGit(["add", "base.txt"], { cwd: workingRepo });
+      execGit(["commit", "-m", "Base commit (already on remote)"], { cwd: workingRepo });
+
+      // Push so origin/shallow-fetch-fail tracking ref is set up (simulating shallow checkout)
+      execGit(["push", "-u", "origin", "shallow-fetch-fail"], { cwd: workingRepo });
+
+      // Add a new commit (the agent's work)
+      fs.writeFileSync(path.join(workingRepo, "agent-change.txt"), "Agent change\n");
+      execGit(["add", "agent-change.txt"], { cwd: workingRepo });
+      execGit(["commit", "-m", "Agent commit - should appear in patch"], { cwd: workingRepo });
+
+      // Break the remote URL to simulate fetch failure (e.g. missing credentials or network issue)
+      execGit(["remote", "set-url", "origin", "https://invalid.example.invalid/nonexistent-repo.git"], { cwd: workingRepo });
+
+      const restore = setTestEnv(workingRepo);
+      try {
+        // origin/shallow-fetch-fail still points to the base commit even though fetch will fail
+        const result = await generateGitPatch("shallow-fetch-fail", "main", { mode: "incremental" });
+
+        // Should succeed using the existing remote tracking ref as the base
+        expect(result.success).toBe(true);
+        expect(result.patchPath).toBeDefined();
+
+        const patchContent = fs.readFileSync(result.patchPath, "utf8");
+
+        // Should contain the agent's new commit
+        expect(patchContent).toContain("Agent commit - should appear in patch");
+
+        // Should NOT include the already-pushed base commit
+        expect(patchContent).not.toContain("Base commit (already on remote)");
+      } finally {
+        // Restore remote URL before cleanup so afterEach can delete the directory
+        execGit(["remote", "set-url", "origin", bareRepoDir], { cwd: workingRepo });
+        restore();
+      }
+    });
+
     it("should include all commits in full mode even when origin/branch exists", async () => {
       // Create a feature branch with first commit
       execGit(["checkout", "-b", "full-mode-branch"], { cwd: workingRepo });

go.mod

@@ -7,6 +7,7 @@ require (
 	charm.land/bubbletea/v2 v2.0.2
 	charm.land/lipgloss/v2 v2.0.2
 	github.com/charmbracelet/huh v0.8.0
+	github.com/charmbracelet/lipgloss v1.1.1-0.20250319133953-166f707985bc
 	github.com/charmbracelet/x/exp/golden v0.0.0-20251215102626-e0db08df7383
 	github.com/cli/go-gh/v2 v2.13.0
 	github.com/creack/pty v1.1.24
@@ -42,7 +43,6 @@ require (
 	github.com/charmbracelet/bubbletea v1.3.10 // indirect
 	github.com/charmbracelet/colorprofile v0.4.2 // indirect
 	github.com/charmbracelet/harmonica v0.2.0 // indirect
-	github.com/charmbracelet/lipgloss v1.1.1-0.20250319133953-166f707985bc // indirect
 	github.com/charmbracelet/ultraviolet v0.0.0-20260205113103-524a6607adb8 // indirect
 	github.com/charmbracelet/x/ansi v0.11.6 // indirect
 	github.com/charmbracelet/x/cellbuf v0.0.15 // indirect