perf(router-core): avoid async scaffolding for synchronous hooks

[Sheraff]

Summary

• keep synchronous lifecycle hook values out of waitFor's Promise, AbortController, and abort-listener scaffolding
• retain the existing cancellation path for Promise-returning hooks, with an explicit post-await currentness/abort check for synchronous beforeLoad results
• document the architectural boundary and add focused adversarial tests plus repeatable client-navigation benchmarks

Why

Most client navigation hooks are typed as Awaitable<T>, but the internal bridge treated every result as Promise-like. Even entirely synchronous navigation therefore allocated a wrapper Promise/controller, installed and removed an abort listener, and crossed an extra microtask boundary.

This change makes the fast path part of the existing awaitable architecture: raw values remain raw, while actual Promises use the existing cancellation machinery. It intentionally does not add a generalized thenable adapter or a separate after-the-fact optimization layer.

Impact

Local A/B runs:

• 80 synchronous waits: 0.0222 ms → 0.0035 ms (-84.2%, ~6.3× faster)
• 10 navigations through 8 synchronous beforeLoad hooks: 0.6411 ms → 0.6223 ms (-2.9%)
• existing nested-params client-nav benchmark: mean 5.7912 ms → 5.7436 ms (-0.82%), p75 -0.80%, p99 -1.80%
• minimal React Router bundle: +3 B gzip; affected bundle scenarios ranged from +1 B to +8 B gzip

Full measurements and methodology are in RESULT-optimization-promise-controller-context.md.

Validation

• router-core focused cancellation/adversarial tests: 26 passed
• router-core unit suite: 1,539 passed, 3 expected failures
• router-core type tests: TypeScript 5.6–7 passed
• router-core ESLint: 0 errors (26 existing warnings)
• React basic file-based redirect E2E: 33 Chromium cases passed
• Prettier, git diff --check, and the affected bundle suite passed

Summary by CodeRabbit

• Performance

  • Improved client-side navigation performance for synchronous route hooks.
  • Reduced unnecessary cancellation overhead when processing immediate results.

• Bug Fixes

  • Prevented aborted navigations from continuing into loaders or accepting stale results.
  • Preserved cancellation handling for asynchronous operations.

• Tests

  • Added coverage for synchronous and asynchronous cancellation behavior.
  • Added benchmarks for navigation scenarios across nested routes.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: e43acd53-7f08-4f41-ac23-2b7b844bb92e

📥 Commits

Reviewing files that changed from the base of the PR and between f97188f and 1ca28da.

📒 Files selected for processing (6)

• RESULT-optimization-promise-controller-context.md
• packages/router-core/INTERNALS.md
• packages/router-core/src/load-client.ts
• packages/router-core/tests/client-lane-adversarial.test.ts
• packages/router-core/tests/client-load-sync.bench.ts
• packages/router-core/tests/load-client-wait-for.test.ts

---

📝 Walkthrough

Walkthrough

The client loading path now handles synchronous values without cancellation listeners, rechecks cancellation after beforeLoad, adds adversarial coverage, and introduces benchmarks with correctness, bundle-size, type, lint, and E2E validation results.

Changes

Client-lane optimization

Layer / File(s)	Summary
Synchronous waitFor handling packages/router-core/src/load-client.ts, packages/router-core/tests/load-client-wait-for.test.ts	waitFor returns synchronous values directly. Promise inputs retain abort-aware handling and listener cleanup.
Contextualized cancellation checks packages/router-core/src/load-client.ts, packages/router-core/INTERNALS.md, packages/router-core/tests/client-lane-adversarial.test.ts	beforeLoad rechecks the abort signal and lane currentness after completion. An adversarial test confirms that an aborted navigation does not run its loader.
Benchmark and validation coverage packages/router-core/tests/client-load-sync.bench.ts, RESULT-optimization-promise-controller-context.md	The benchmark measures synchronous and asynchronous route work across parameterized navigations. The report records runtime, bundle-size, correctness, type, lint, and E2E results.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 1ca28

The change keeps synchronous navigation hooks on a faster path while preserving cancellation for asynchronous hooks; no actionable merge-blocking risk remains beyond normal checks and review.

Sequence Diagram(s)

sequenceDiagram
  participant RouterNavigation
  participant beforeLoad
  participant AbortSignal
  participant RouteLoader
  RouterNavigation->>beforeLoad: Execute synchronously
  beforeLoad->>AbortSignal: Queue abort
  RouterNavigation->>AbortSignal: Check after beforeLoad
  RouterNavigation-->>RouteLoader: Skip loader when aborted

Loading

Possibly related PRs

• TanStack/router#7805: Introduced the lane-based client loading implementation refined by this change.
• TanStack/router#8067: Also changes contextualize in load-client.ts, but addresses retained ancestor context during pending navigation.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main optimization: avoiding async scaffolding for synchronous router hooks.
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.

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch agent/optimize-sync-client-navigation

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View your CI Pipeline Execution ↗ for commit 1ca28da

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---

☁️ Nx Cloud last updated this comment at 2026-08-15 12:03:26 UTC

[github-actions]

🚀 Changeset Version Preview

1 package(s) bumped directly, 4 bumped as dependents.

🟩 Patch bumps

Package	Version	Reason
@tanstack/react-router	1.170.29 → 1.170.30	Changeset
@tanstack/react-start	1.168.46 → 1.168.47	Dependent
@tanstack/react-start-client	1.168.27 → 1.168.28	Dependent
@tanstack/react-start-rsc	0.1.45 → 0.1.46	Dependent
@tanstack/react-start-server	1.167.34 → 1.167.35	Dependent

[pkg-pr-new]

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commit: 1ca28da

[github-actions]

Bundle Size Benchmarks

• Commit: 26aeebc3615f
• Measured at: 2026-08-15T11:49:45.679Z
• Baseline source: history:f97188fdb4c3
• Dashboard: bundle-size history

The following scenarios have bundle-size changes compared with the baseline:

Scenario	Current (gzip)	Delta vs baseline	Initial gzip	Raw	Brotli	Trend
react-router.minimal	83.83 KiB	+4 B (+0.00%)	83.69 KiB	262.46 KiB	72.95 KiB	▂▂▂▃██▆▂▂▁▁▁
react-router.full	87.32 KiB	-2 B (-0.00%)	87.18 KiB	274.17 KiB	76.04 KiB	▄▄▄▄██▅▂▂▁▁▁
solid-router.minimal	33.13 KiB	+10 B (+0.03%)	33.00 KiB	96.16 KiB	29.93 KiB	▆▇████▅▁▁▁▁▂
solid-router.full	37.94 KiB	+3 B (+0.01%)	37.82 KiB	110.80 KiB	34.15 KiB	▆▇████▅▁▁▁▁▁
vue-router.minimal	49.51 KiB	+4 B (+0.01%)	49.38 KiB	138.34 KiB	44.68 KiB	▁█▇███▅▃▃▃▃▃
vue-router.full	55.12 KiB	+4 B (+0.01%)	54.99 KiB	156.54 KiB	49.55 KiB	▁█████▆▄▄▄▄▅
react-start.minimal	96.63 KiB	+8 B (+0.01%)	96.49 KiB	304.69 KiB	83.79 KiB	▄▄▄▄██▅▂▂▁▁▂
react-start.full	99.80 KiB	+2 B (+0.00%)	99.67 KiB	314.41 KiB	86.43 KiB	▄▄▄▄██▄▁▁▁▁▁
react-start.rsbuild.minimal	99.97 KiB	+6 B (+0.01%)	99.80 KiB	315.07 KiB	86.15 KiB	▁▁▁▂███▃▃▁▁▂
react-start.rsbuild.minimal-iife	100.38 KiB	+8 B (+0.01%)	100.21 KiB	316.01 KiB	86.57 KiB	▁▁▁▂███▂▂▁▁▂
react-start.rsbuild.full	103.29 KiB	+2 B (+0.00%)	103.12 KiB	325.16 KiB	88.86 KiB	▁▁▁▂███▂▂▁▁▁
solid-start.minimal	45.87 KiB	+7 B (+0.01%)	45.75 KiB	137.22 KiB	40.84 KiB	▇█████▄▁▁▁▁▂
solid-start.deferred-hydration	48.96 KiB	+5 B (+0.01%)	45.82 KiB	144.68 KiB	43.66 KiB	▇▇████▄▁▁▁▁▁
solid-start.full	50.97 KiB	+4 B (+0.01%)	50.84 KiB	152.61 KiB	45.16 KiB	▆█████▄▁▁▁▁▁
vue-start.minimal	65.57 KiB	+5 B (+0.01%)	65.44 KiB	189.16 KiB	58.35 KiB	▁█████▆▅▅▅▅▅
vue-start.full	69.38 KiB	+2 B (+0.00%)	69.26 KiB	201.46 KiB	61.66 KiB	▁█████▆▅▅▅▅▅

Current gzip tracks all emitted client JS chunks. Initial gzip tracks only the entry/import graph. Trend sparkline is historical current gzip ending with this PR measurement; lower is better.

[codspeed-hq]

Merging this PR will degrade performance by 0.49%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 6 improved benchmarks
❌ 9 (👁 2) regressed benchmarks
✅ 165 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Memory	mem server serialization-payload (react)	4.1 MB	5.6 MB	-27.74%
❌	Memory	mem server request-churn (react)	657.9 KB	804.5 KB	-18.22%
❌	Memory	mem client navigation-churn (vue)	1.5 MB	1.7 MB	-10.81%
❌	Memory	mem server peak-large-page (vue)	1 MB	1.1 MB	-6.6%
❌	Memory	mem client unique-location-churn (vue)	539.8 KB	577.5 KB	-6.54%
❌	Memory	mem server error-paths not-found (react)	416.3 KB	436.9 KB	-4.7%
❌	Simulation	ssr control-flow route headers (react)	302.4 ms	312 ms	-3.06%
⚡	Memory	mem server serialization-payload (solid)	7.4 MB	4.5 MB	+66.47%
⚡	Memory	mem client navigation-churn (solid)	960.6 KB	683.9 KB	+40.47%
⚡	Memory	mem server error-paths error (solid)	1,044.4 KB	974 KB	+7.23%
⚡	Memory	mem server peak-large-page (solid)	1.1 MB	1 MB	+3.56%
⚡	Memory	mem client interrupted-navigations (vue)	478 KB	461.8 KB	+3.52%
👁	Memory	mem server error-paths redirect (react)	309.3 KB	325 KB	-4.86%
👁	Memory	mem server error-paths redirect (vue)	403.5 KB	504.5 KB	-20.03%
👁	Memory	mem server server-fn-churn (vue)	379.5 KB	354.9 KB	+6.92%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing agent/optimize-sync-client-navigation (1ca28da) with main (f97188f)}