fix(server): generate PR diffs from branch changes

[Yash-Singh1]

Problem

When the PR base advances after a feature branch splits, T3's two-dot diff includes the reverse of unrelated base-branch changes. Its generated PR title and description can then describe those changes instead of the feature branch.

Fix

Generate the PR diff summary and patch from the common ancestor with the base branch. Keep the commit summary limited to commits on the feature branch. The regression test advances the remote base after the branch splits and checks that unrelated files stay out of the generated diff.

Verification

• Focused GitManager regression test passed.
• Server typecheck, targeted lint, formatting, and git diff --check passed.

Model: GPT-6; harness: Codex.

[coderabbitai]

Navigate logical layers of code changes, visualize relationships, and explore their blast radius.

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository: pingdotgg/t3code/.coderabbit.yaml

Review profile: CHILL

Plan: Advanced

Run ID: 28716655-4f03-470c-8fbf-d32316cce989

📥 Commits

Reviewing files that changed from the base of the PR and between 5d493d6 and 123d821.

📒 Files selected for processing (2)

• apps/server/src/git/GitManager.test.ts
• apps/server/src/vcs/GitVcsDriverCore.ts

Included review availability: Your plan provides up to 10 included reviews per hour; 7 remain after this review.

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📝 Walkthrough

Walkthrough

readRangeContext now uses separate Git ranges for commit summaries and branch diffs. Tests advance the remote base and verify that generated commit and diff content includes feature changes while excluding later base-branch changes.

Changes

Branch diff context

Layer / File(s)	Summary
Separate commit and diff ranges apps/server/src/vcs/GitVcsDriverCore.ts	readRangeContext keeps the two-dot range for git log and uses the three-dot range for diff statistics and patches.
Validate an advanced remote base apps/server/src/git/GitManager.test.ts	The test advances the remote base and verifies that generated commit summaries, diff summaries, and diff patches exclude the later base commit.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: t3dotgg

Merge Risk: ⚪ Minimal · up to 123d8

Generated PR content remains limited to feature-branch changes when the base branch advances. No actionable merge risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main change: generating server-side PR diffs from branch-specific changes.
Description check	✅ Passed	The description explains the problem, the fix, and verification results. It is mostly complete, although it uses Problem and Fix headings instead of the template's What Changed and Why headings, and i…

✨ Finishing Touches

📝 Generate docstrings

• Commit to this branch
• Create a new PR

🧪 Generate unit tests (beta)

• Commit to this branch
• Create a new PR

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_{Comment @coderabbitai help to get the list of available commands.}

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.5 KiB	13.5 KiB	+10 B (+0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.0 KiB	−4 B (−0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.5 KiB	+14 B (+0.2%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.3 KiB	56.3 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	10	10	0 (0.0%)	21	✅
Claude	Total thread wire	13.5 KiB	13.5 KiB	−2 B (−0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	+1 B (+0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.4 KiB	6.4 KiB	−3 B (−0.0%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.0 KiB	57.0 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: 5d493d6 · PR result: 123d821 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

[macroscopeapp]

Approvability

Verdict: Approved at 123d821

Macroscope's review found this PR approvable — This is a small, self-contained server bug fix that keeps generated PR diffs anchored at the common ancestor while preserving branch-only commit summaries. A focused regression test verifies that unrelated commits added later to the base branch are excluded.

Notes:

• Diff unchanged. Approvability was decided on eligibility alone.

^{You can add or adjust custom eligibility rules. Learn more.}