src/ai_reviewer/diff.py

@@ -53,10 +53,27 @@ def run_git_diff(repo: str, base: str, head: str) -> str:
         "--no-color",
     ]
     result = subprocess.run(cmd, check=False, capture_output=True, text=True)
-    if result.returncode not in (0, 1):
-        raise RuntimeError(result.stderr.strip() or "git diff failed")
+    if result.returncode in (0, 1):
         return result.stdout
 
+    # If the diff failed because revisions are missing (common in CI when the
+    # PR head/base aren't fetched), try fetching from origin and retry once.
+    stderr = (result.stderr or "").strip()
+    try:
+        fetch_cmd = ["git", "-C", repo, "fetch", "origin", f"{base}", f"{head}"]
+        subprocess.run(fetch_cmd, check=False, capture_output=True, text=True)
+    except Exception:
+        # Ignore fetch errors; we'll raise the original error below.
+        pass
+
+    # Retry diff once
+    result2 = subprocess.run(cmd, check=False, capture_output=True, text=True)
+    if result2.returncode in (0, 1):
+        return result2.stdout
+
+    # If still failing, raise a helpful error including stderr from git
+    raise RuntimeError(result2.stderr.strip() or stderr or "git diff failed")
+
 
 def parse_diff(diff_text: str) -> list[FileDiff]:
     files: list[FileDiff] = []