From 98448deb9c69b114dc98f37924f00215b3b26a20 Mon Sep 17 00:00:00 2001
From: Yash Singh <saiansh2525@gmail.com>
Date: Tue, 22 Sep 2026 19:40:40 -0500
Subject: [PATCH] fix(server): generate PR diffs from branch changes (#13170)

(cherry picked from commit f22331240ed1a97fae560e314bffb75fe39db8ca)
---
 apps/server/src/git/GitManager.test.ts  | 17 ++++++++++++++++-
 apps/server/src/vcs/GitVcsDriverCore.ts | 10 ++++++----
 2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/apps/server/src/git/GitManager.test.ts b/apps/server/src/git/GitManager.test.ts
index d4d22be1d2..69ebed8214 100644
--- a/apps/server/src/git/GitManager.test.ts
+++ b/apps/server/src/git/GitManager.test.ts
@@ -4191,7 +4191,7 @@ it.layer(GitManagerTestLayer)("GitManager", (it) => {
     }),
   );
 
-  it.effect("generates PR content against the remote base when the local base is stale", () =>
+  it.effect("generates PR content from branch changes when the remote base advances", () =>
     Effect.gen(function* () {
       const repoDir = yield* makeTempDir("t3code-git-manager-");
       yield* initRepo(repoDir);
@@ -4223,7 +4223,15 @@ it.layer(GitManagerTestLayer)("GitManager", (it) => {
       yield* runGit(repoDir, ["push", "-u", "origin", "feature/remote-base"]);
       yield* runGit(repoDir, ["config", "branch.feature/remote-base.gh-merge-base", "main"]);
 
+      NodeFS.writeFileSync(NodePath.join(peerDir, "later-main.txt"), "unrelated\n");
+      yield* runGit(peerDir, ["add", "later-main.txt"]);
+      yield* runGit(peerDir, ["commit", "-m", "Later main commit"]);
+      yield* runGit(peerDir, ["push", "origin", "main"]);
+      yield* runGit(repoDir, ["fetch", "origin"]);
+
       let generatedCommitSummary = "";
+      let generatedDiffSummary = "";
+      let generatedDiffPatch = "";
       const { manager } = yield* makeManager({
         ghScenario: {
           prListSequence: ["[]", "[]"],
@@ -4231,6 +4239,8 @@ it.layer(GitManagerTestLayer)("GitManager", (it) => {
         textGeneration: {
           generatePrContent: (input) => {
             generatedCommitSummary = input.commitSummary;
+            generatedDiffSummary = input.diffSummary;
+            generatedDiffPatch = input.diffPatch;
             return Effect.succeed({ title: "Feature PR", body: "Feature body" });
           },
         },
@@ -4244,6 +4254,11 @@ it.layer(GitManagerTestLayer)("GitManager", (it) => {
       expect(result.pr.status).toBe("created");
       expect(generatedCommitSummary).toContain("Feature commit");
       expect(generatedCommitSummary).not.toContain("Remote base commit");
+      expect(generatedCommitSummary).not.toContain("Later main commit");
+      expect(generatedDiffSummary).toContain("feature.txt");
+      expect(generatedDiffSummary).not.toContain("later-main.txt");
+      expect(generatedDiffPatch).toContain("feature.txt");
+      expect(generatedDiffPatch).not.toContain("later-main.txt");
     }),
   );
 
diff --git a/apps/server/src/vcs/GitVcsDriverCore.ts b/apps/server/src/vcs/GitVcsDriverCore.ts
index 58191a9129..2e06c57584 100644
--- a/apps/server/src/vcs/GitVcsDriverCore.ts
+++ b/apps/server/src/vcs/GitVcsDriverCore.ts
@@ -2282,13 +2282,15 @@ export const makeGitVcsDriverCore = Effect.fn("makeGitVcsDriverCore")(function*
   const readRangeContext: GitVcsDriver.GitVcsDriver["Service"]["readRangeContext"] = Effect.fn(
     "readRangeContext",
   )(function* (cwd, baseRef) {
-    const range = `${baseRef}..HEAD`;
+    const commitRange = `${baseRef}..HEAD`;
+    // PR diffs start at the common ancestor when the base branch has advanced.
+    const diffRange = `${baseRef}...HEAD`;
     const [commitSummary, diffSummary, diffPatch] = yield* Effect.all(
       [
         runGitStdoutWithOptions(
           "GitVcsDriver.readRangeContext.log",
           cwd,
-          ["log", "--oneline", range],
+          ["log", "--oneline", commitRange],
           {
             maxOutputBytes: RANGE_COMMIT_SUMMARY_MAX_OUTPUT_BYTES,
             appendTruncationMarker: true,
@@ -2297,7 +2299,7 @@ export const makeGitVcsDriverCore = Effect.fn("makeGitVcsDriverCore")(function*
         runGitStdoutWithOptions(
           "GitVcsDriver.readRangeContext.diffStat",
           cwd,
-          ["diff", "--stat", range],
+          ["diff", "--stat", diffRange],
           {
             maxOutputBytes: RANGE_DIFF_SUMMARY_MAX_OUTPUT_BYTES,
             appendTruncationMarker: true,
@@ -2306,7 +2308,7 @@ export const makeGitVcsDriverCore = Effect.fn("makeGitVcsDriverCore")(function*
         runGitStdoutWithOptions(
           "GitVcsDriver.readRangeContext.diffPatch",
           cwd,
-          ["diff", "--no-ext-diff", "--patch", "--minimal", range],
+          ["diff", "--no-ext-diff", "--patch", "--minimal", diffRange],
           {
             maxOutputBytes: RANGE_DIFF_PATCH_MAX_OUTPUT_BYTES,
             appendTruncationMarker: true,