fix(review): use merge-base SHA for PR file contents

gh pr diff computes diffs against the merge-base (common ancestor), but
file contents were fetched at baseSha (base branch tip). When the base
branch has moved since the branch point, line counts don't match the
diff hunks, causing iterateOverDiff trailing context mismatch crashes.

Fetch the merge-base SHA via GitHub's compare API and use it for old
file contents. Added try/catch around processFile as a safety net.
Fixed in both Bun and Pi servers.

For provenance purposes, this commit was AI assisted.

Author: backnotprop

apps/pi-extension/server/serverReview.ts

@@ -531,8 +531,9 @@ export async function startReviewServer(options: {
 
 			if (isPRMode && prRef && prMeta) {
 				try {
+					const oldSha = prMeta.mergeBaseSha ?? prMeta.baseSha;
 					const [oldContent, newContent] = await Promise.all([
-						fetchPRFileContent(prRef, prMeta.baseSha, oldPath || filePath),
+						fetchPRFileContent(prRef, oldSha, oldPath || filePath),
 						fetchPRFileContent(prRef, prMeta.headSha, filePath),
 					]);
 					json(res, { oldContent, newContent });

packages/review-editor/components/DiffViewer.tsx

@@ -279,11 +279,16 @@ export const DiffViewer: React.FC<DiffViewerProps> = ({
   // against the complete file (isPartial: false), enabling expansion.
   const augmentedDiff = useMemo(() => {
     if (!fileContents || fileContents.forPath !== filePath || (fileContents.old == null && fileContents.new == null)) return fileDiff;
-    const result = processFile(patch, {
-      oldFile: fileContents.old != null ? { name: oldPath || filePath, contents: fileContents.old } : undefined,
-      newFile: fileContents.new != null ? { name: filePath, contents: fileContents.new } : undefined,
-    });
-    return result || fileDiff;
+    try {
+      const result = processFile(patch, {
+        oldFile: fileContents.old != null ? { name: oldPath || filePath, contents: fileContents.old } : undefined,
+        newFile: fileContents.new != null ? { name: filePath, contents: fileContents.new } : undefined,
+      });
+      return result || fileDiff;
+    } catch {
+      // Fall back to partial diff if file contents don't match hunks
+      return fileDiff;
+    }
   }, [patch, filePath, oldPath, fileContents, fileDiff]);
 
   const previousScrollFilePathRef = useRef(filePath);

packages/server/review.ts

@@ -462,10 +462,13 @@ export async function startReviewServer(
               return Response.json(result);
             }
 
-            // PR mode: fetch from platform API using base/head SHAs
+            // PR mode: fetch from platform API using merge-base/head SHAs.
+            // The diff is computed against the merge-base (common ancestor), not the
+            // base branch tip. File contents must match the diff for hunk expansion.
             if (isPRMode) {
+              const oldSha = prMetadata.mergeBaseSha ?? prMetadata.baseSha;
               const [oldContent, newContent] = await Promise.all([
-                fetchPRFileContent(prRef!, prMetadata.baseSha, oldPath || filePath),
+                fetchPRFileContent(prRef!, oldSha, oldPath || filePath),
                 fetchPRFileContent(prRef!, prMetadata.headSha, filePath),
               ]);
               return Response.json({ oldContent, newContent });

packages/shared/pr-github.ts

@@ -101,6 +101,21 @@ export async function fetchGhPR(
     url: string;
   };
 
+  // Fetch the merge-base SHA — the common ancestor commit GitHub uses to compute the PR diff.
+  // baseSha (baseRefOid) is the tip of the base branch, which may have moved since the branch point.
+  // File contents must be fetched at the merge-base to match the diff hunks.
+  let mergeBaseSha: string | undefined;
+  try {
+    const compareResult = await runtime.runCommand("gh", hostnameArgs(ref.host, [
+      "api",
+      `repos/${ref.owner}/${ref.repo}/compare/${raw.baseRefOid}...${raw.headRefOid}`,
+      "--jq", ".merge_base_commit.sha",
+    ]));
+    if (compareResult.exitCode === 0 && compareResult.stdout.trim()) {
+      mergeBaseSha = compareResult.stdout.trim();
+    }
+  } catch { /* fallback to baseSha if compare API fails */ }
+
   const metadata: PRMetadata = {
     platform: "github",
     host: ref.host,
@@ -114,6 +129,7 @@ export async function fetchGhPR(
     headBranch: raw.headRefName,
     baseSha: raw.baseRefOid,
     headSha: raw.headRefOid,
+    mergeBaseSha,
     url: raw.url,
   };
 

packages/shared/pr-provider.ts

@@ -70,6 +70,8 @@ export interface GithubPRMetadata {
   headBranch: string;
   baseSha: string;
   headSha: string;
+  /** Merge-base SHA — the common ancestor commit used to compute the PR diff. Differs from baseSha when the base branch has moved. */
+  mergeBaseSha?: string;
   url: string;
 }