perf(server): cached branch PR lookups no longer start git processes

[SunkenInTime]

Background PR polling on a Windows dev server with 60 real threads now uses 59% less CPU and starts 67% fewer processes. On main it burned 151 CPU-seconds every 10 minutes, about 25% of one core around the clock, and 83 of those seconds were kernel time. With this PR it's 62 seconds.

Rerun on 2026-10-12 against current main, which now runs Git for Windows' real git.exe (#17707) and skips the extra taskkill processes (#16007). Those fixes made main cheaper, and this PR still cuts what's left. Over 8 minutes with the same 60 threads, the numbers below are averages of two runs each:

per 8 minutes	main today	this PR	change
CPU time, server plus every child process	39.2 s	14.8 s	62% less
processes started	1,035	279	73% less
git commands	518	140	73% less
server event-loop stalls over 100 ms	2.5	0

Why

PR discovery and the settlement sweep call GitManager.branchPullRequest for every thread every minute. Each call ran 4 to 6 git commands (git remote, git for-each-ref for the branch's upstream, git symbolic-ref for the remote HEAD, and one to three git config reads for the repository-identity check), even when the PR answer itself was cached and the call would return without asking GitHub.

Starting a process is expensive on Windows. A 67-minute trace from one maintainer's machine with about 40 active threads had 6,674 git launches, about 100 a minute. 794 of the 1,047 branchPullRequest calls never ran the PR cache loader and still started 3,838 git processes, 4.8 per call. Node starts each process synchronously on the main thread, so every launch also holds up the event loop, for about 2 to 3 ms on a clean Windows boot against about 1 ms on Linux. Most of the cost lands on the machine as CPU and kernel time, around the clock, for as long as T3 is open.

What we get

Measured on Windows 11 with a dev server seeded from a real install (details under Verification):

per 10 minutes	main	this PR	change
CPU time, server plus every child process	151 s	62 s	59% less
of which kernel time	83 s	35 s	58% less
processes started, counted by Windows	5,180	1,722	67% less
git commands from discovery and settlement	1,350	369	73% less
time spent waiting on git, summed	243 s	68 s	72% less
git commands per lookup with a cached PR answer	4.9	0.2	95% less

Each git command this PR removes cost the machine about 90 ms of CPU, a little over half of it kernel time. That's more than git alone (about 25 ms here) because each one also starts a console host, and Effect's spawner added two taskkill processes whenever git exited non-zero (fixed on main since by #16007).

What we trade

One thing gets slower: a git change made outside T3 can now take up to 5 minutes to reach background PR discovery. Say you run git branch --set-upstream-to, git remote set-url or git remote set-head in a terminal. On main the next one-minute pass saw it, because every lookup re-read git. Now the remembered reads are used until they expire. That's the same 5 minutes PR_LOOKUP_NO_OPEN_PR_CACHE_TTL already allows a "no PR" answer.

Everything else behaves as before:

• T3's own git actions end in invalidateStatus, which drops the checkout's remembered reads. A read already in flight when that happens is not remembered.
• refresh: true reads everything again, so turn-end refreshes see the current state.
• A merged or closed answer can settle a thread, so branchPullRequest always confirms it against fresh git reads.
• Discovery rereads git before it saves a PR link. Settlement, worktree cleanup and saved links only ever act on fresh reads.
• The PR host polling cadence is unchanged. GitHub sees the same requests.

Two smaller edge cases come from the same delay. An ambiguous-remote error built from remembered reads stays until the reads expire, a T3 git action runs, or something refreshes. A "no PR" answer built from stale reads can clear a thread's branch reference until the next pass after expiry. Neither writes a link or settles anything.

The memo holds at most PR_LOOKUP_CACHE_CAPACITY (2,048) small entries.

How

branchPullRequest keeps each git read it needs in a per-checkout map with a 5-minute expiry: remote names, the branch's upstream, the remotes tracking it, the remote's default branch, and the remote URLs for the identity check. Failed reads aren't remembered. Nothing is shared between worktrees. A remembered remote URL that disagrees with the cached answer goes through the same identity check as on main, which asks the host again and rereads git. So a remote repointed outside T3 can cost one extra host lookup, when the PR answer reloads before the remembered URL expires. Discovery's verification before it persists a link passes a new freshGitState option, which rereads git without forcing a host refresh.

I first tried invalidating on file changes, with stat fingerprints of every config and ref file git reads, shared across worktrees. Review kept finding config inputs it missed, like an include that starts empty, conditional includes that differ per worktree, and onbranch. Reproducing git's config resolution isn't a job for this cache, so the PR uses a time bound plus fresh reads on every answer that can lead to a write.

Verification

Live runs. Windows 11, Ryzen 7 5800X, git 2.55.0.windows.1, Node 24.13.1, on a fresh boot. An earlier version of this description used runs from a machine that had been up 9 days and had leaked about 3 million kernel token objects, which slowed every process launch on it. Those runs showed the same percentages with higher absolute numbers. I built the seed with migrate-dev-db from a real install. A scratch change to its family filter, not in this PR, kept every thread PR discovery polls and dropped any thread the server would resume. That left 60 threads across 18 checkout and branch pairs; the source install had 78 threads across 27. To avoid spending GitHub quota, a scratch patch made listChangeRequests answer "no PR" and the server ran with an invalid GH_TOKEN.

Each run started the server from a fresh copy of the seed inside a Windows job object, waited 2 minutes, then measured 10 minutes. The job object's accounting gives CPU time and the process count for the server and every descendant, including git processes that live a few milliseconds. Git counts and durations come from runGitCommand spans in server.trace.ndjson. Runs alternated main, PR, main, PR on the same machine:

run	CPU s	kernel s	processes	git commands	git wait s	branchPullRequest calls
main 1	143.3	78.8	5,161	1,347	136	243
PR 1	63.0	34.7	1,720	368	71	270
main 2	158.8	87.8	5,198	1,353	351	243
PR 2	60.7	35.4	1,723	369	65	243

Both made about the same number of branchPullRequest calls in the same 10 minutes, and the PR runs started far fewer processes. The remaining git commands in the PR runs are the reads that expire every 5 minutes, plus calls whose PR answer had expired and ran the lookup loader, which this PR doesn't change.

I also timed git config and git status in another checkout every few seconds during each run, to see whether other git work on the machine got faster. It didn't measurably. git config took a median 33 and 38 ms on main and 35 and 35 ms with this PR, with the machine at 21 to 46% CPU from other work.

Tests. The PR adds three tests. Each one fails when the piece it covers is removed. I checked that on Windows by breaking each piece on the branch rebased onto 2026-10-12 main and rerunning the PR, settle and merge tests in GitManager.test.ts, ThreadPullRequestService.test.ts and ThreadSettlementService.test.ts (110 tests, all passing unbroken):

• a warm branch PR lookup starts no git process counts real git spawns. It fails when every lookup reads git, as on main, which starts 8 on the warm pass.
• a branch PR lookup sees an upstream change on request, after a T3 git action, or after the TTL is the only test that fails when invalidateStatus doesn't drop remembered reads.
• a git read in flight during a T3 git action does not outlive it holds one git read's output open while the upstream changes and invalidateStatus runs. Without the epoch check added after Macroscope's review, the next lookup returns the stale PR.

Skipping the fresh recheck of merged and closed answers fails main's existing branch PR lookup does not reuse a cached PR after the remote is repointed, so the PR doesn't add its own test for that.

The test spawn spy matches git by file name, because on Windows git now starts by its full path to git.exe (#17707). The old test a branch tracking origin reads origin's URL once per PR lookup is removed; the warm-lookup test asserts zero reads, which covers it.

Suites. vp test run src/git/GitManager.test.ts src/orchestration-v2/ThreadPullRequestService.test.ts src/orchestration-v2/ThreadSettlementService.test.ts: 175 passed on Windows after the 2026-10-12 rebase.

Typecheck (tsc --noEmit in apps/server) and lint on the changed files are clean.

Not checked

• macOS and Linux live runs. Process creation is much cheaper there, so expect a smaller CPU saving. CI runs the tests on both.
• Real GitHub responses. The host was stubbed for the live runs, so every lookup answered "no PR".
• Whether other git work or the UI gets faster. The probe above found no change it could separate from machine noise.

Follow-ups this PR leaves alone: caching GitHubCli.resolveRepository metadata, the negative repository-identity re-probe every minute in RepositoryIdentityResolver, and coalescing duplicate discovery refreshes in ThreadPullRequestService.

Made with Claude Opus 5.5 in Claude Code, running inside T3 Code. Reviewed over four rounds by GPT-6-Astra in Codex.

🤖 Generated with Claude Code

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR introduces substantial production cache and concurrency logic that changes how existing branch PR lookups obtain and refresh Git state, including a five-minute stale-read window. It also affects discovery persistence boundaries, so the runtime behavior and freshness tradeoffs warrant human review.

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📝 Walkthrough

GitManager caches Git state used by branch pull-request lookups for up to five minutes. Callers can request fresh state, and non-open pull-request results are checked against fresh Git state. Tests cover cache reuse, invalidation, expiry, and reads that overlap invalidation.

Changes: Branch pull-request lookup

Layer / File(s)	Summary
Cache Git state reads apps/server/src/git/GitManager.ts, apps/server/src/git/GitManager.test.ts	GitManager caches per-checkout Git-state reads for five minutes. Fresh reads bypass the cache, and status invalidation clears cached state. Tests cover cache reuse, invalidation, expiry, and reads that overlap invalidation.
Verify pull-request answers apps/server/src/git/GitManager.ts, apps/server/src/git/GitManager.test.ts, apps/server/src/orchestration-v2/ThreadPullRequestService.ts	Branch pull-request lookups use cached Git-state reads for remote and branch information. Non-open results are rechecked with fresh Git state, and the orchestration service requests fresh state for its final check. Tests cover changes to upstream refs.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Refactor

Sequence Diagram(s)

sequenceDiagram
  participant ThreadPullRequestService
  participant GitManager
  participant GitStateCache
  participant Git
  participant PRProvider
  ThreadPullRequestService->>GitManager: Request final branch pull-request check
  GitManager->>GitStateCache: Read remote and branch state
  GitStateCache->>Git: Run Git commands when needed
  Git-->>GitStateCache: Return Git state
  GitManager->>PRProvider: Fetch pull-request results
  PRProvider-->>GitManager: Return pull-request results
  GitManager->>GitStateCache: Request fresh state for a non-open result
  GitManager-->>ThreadPullRequestService: Return pull-request result

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Suggested reviewers: juliusmarminge

Merge Risk: 🔵 Low · up to 21253

Branch pull-request lookups reuse cached Git reads for up to five minutes, which reduces Git process load. In a narrow same-millisecond overlap, a slightly older read could briefly stand in for newer state. This fits the documented freshness window, so the change is mergeable with owner awareness.

Security architecture risk: 🔵 Low · up to 21253

The cache is scoped to each checkout, and important persistence and cleanup decisions still request fresh reads. No newly introduced security issue was established. Concurrent repository changes remain a consistency limitation, and the available security coverage is incomplete.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• observed — The reviewed flow consumes checkout paths, saved branch names and mutable remote configuration. Its consequential outcomes include persisted PR associations, thread settlement and policy-controlled worktree removal. The new cache is separated by checkout rather than globally sharing repository identity.

Trust Boundaries and Controls

• observed — Freshness controls remain at consequential consumers: discovery requests freshGitState before applying a detected association, and settlement requests refresh before checking whether an open PR on a reused branch should prevent settlement.
• observed — Worktree cleanup requests refresh and does not accept merged metadata alone for a non-integrated checkout. It requires the PR head branch, base branch and head SHA to match the local worktree; alternatively, local integration must already be established.

Resilience and Maintainability Implications

• inferred — Cache insertion is protected against invalidation and newer reads, but returned lookup decisions are not atomic snapshots: a superseded in-flight read can still return its value. The baseline also performed separate direct reads without a decision-level epoch barrier. This is an existing consistency limitation, not an established newly worsened attack path.

Pre-merge checks · 4

Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely describes the main change: caching branch pull-request lookups to avoid starting Git processes.
Description check		The description provides detailed problem context, implementation changes, performance results, verification steps, limitations, and agent attribution. It does not use the template headings and does n…

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Review comments at @apps/server/src/git/GitManager.ts:
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[SunkenInTime]

A correction to "the server waits on git asynchronously, so this never blocked the event loop": on Windows it does. Node's spawn() runs CreateProcess synchronously on the main thread, and only the wait for exit is async.

Measured on the current nightly (0.0.46-nightly.20261008) on the same Windows machine:

• A 90 s CPU profile of the live desktop server had the main thread blocked inside spawn() for 38.8 s, 43% of wall time. That's 376 launches, most of them git. Most take 15 ms, but 1 in 10 took over 430 ms and the worst 1.5 s.
• Probing /.well-known/t3/environment four times a second for 4 minutes: median 3 ms, 31 replies over 1 s, p99 5.8 s, worst 8.5 s.
• The RPC client pings every 5 s and reconnects when the previous ping is still unanswered. Stalls that long are what drop clients to "Connecting". After a restart, the server trace showed the WebSocket reconnect 5 times in 5 minutes.
• In the hour of trace before that, 68% of the 13,400 git launches came from ThreadSettlementServiceV2.sweep and ThreadPullRequestServiceV2.synchronize through branchPullRequest.

So beyond CPU, this PR removes most of the main-thread stalls behind the "Connecting" drops on Windows. #16850 removes most of the rest: the two taskkill launches Effect's spawner makes after every non-zero exit (numbers in #16850 (comment)).

[SunkenInTime]

Follow-up to my comment above: those numbers came from a machine that had been up 9 days and had leaked about 3 million kernel token objects. A kernel CPU trace put most of each process launch in the token index that leak bloats (SepAddLuidToIndexEntry), so every launch was slow and some took 350 ms or more. After a reboot, the same machine measured:

• Starting a process blocks the main thread for 2 to 3 ms, worst 36 ms in 600 launches. Before it was 9 ms typical with 350 ms to 1.1 s spikes.
• The same 4-minute probe of /.well-known/t3/environment saw 3 replies over 1 s instead of 31, p99 102 ms instead of 5.8 s, and a worst case of 1.7 s instead of 8.5 s.

So the "Connecting" drops needed both the leak and this volume of git launches. On a healthy machine each launch still holds up the event loop briefly, and this PR still removes 73% of the git commands these sweeps start. I reran the PR's comparison on the fresh boot and updated the description. The percentages held, and the absolute CPU numbers came down by about a third.

I haven't found what leaks the tokens. Launching git, cmd or taskkill from Node or .NET doesn't, even 1,500 times in a row.