fix(start-static-server-functions): fall back to the server function on a cache miss - #8221

theRizwan · 2026-09-02T21:48:37Z

🎯 Changes

Fixes #7876. Also fixes the failure reported in #7630.

The client half of staticFunctionMiddleware runs for every call in a production browser build, and fetched the derived cache URL with no checks at all:

result = await fetch(url, { method: 'GET' })
  .then((r) => r.json())
  .then((d) => fromJSON(d, { plugins: getDefaultSerovalPlugins() }))

A cache file is only written for calls the prerender pass actually executed, by the .server() half when TSS_CLIENT_OUTPUT_DIR is set. So with prerender disabled, or for a route the crawler never reached, nothing exists at /__tsr/staticServerFnCache/<hash>.json. The request is answered by the application's catch-all route, which serves the HTML shell, and r.json() rejects with:

SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON

That rejection propagates out of the middleware and takes the loader or navigation down with it, rather than falling through to the live server function. It is the same error reported in #7630.

The fix

Treat an unreadable cache response as a miss. A failed request, a non-ok status, a body that is not JSON, and a body that does not parse all return undefined, which the middleware already handles:

if (response) {
  return { result: response.result, ... }
}
return ctx.next()

The content type has to be checked as well as the status, because the HTML shell is served with a 200 in some setups, so the status alone cannot distinguish a hit from the fallback document.

I also wired up staticClientCache. Its lookup was immediately overwritten by the fetch result on the next line, so it never served anything and every repeat call refetched.

Not fixed: the SPA half of the report

The second half of #7876 asks that enabling spa populate the cache. I did not implement that, and I do not think it can work in general: the SPA shell prerender only runs the root route's loaders, and the build cannot know which server functions a client-only route will call at runtime. With this change that case degrades to a live server function call, which is the correct outcome, so the crash is gone even though the caching is not extended. Happy to be corrected if you had a specific mechanism in mind.

Tests

This package had no tests and its test:unit script was the exit 0; vitest placeholder, so I enabled it. Its vite.config.ts already configured vitest with jsdom, and vitest and jsdom were already devDependencies. Say the word if you would rather keep the script disabled and I will revert that line.

Five of the six new tests fail on main:

scenario	before	after
HTML shell served with 200	rejects with the `<!DOCTYPE` SyntaxError	calls the server function
404 for the cache file	rejects	calls the server function
body is not valid JSON	rejects	calls the server function
fetch itself throws	rejects	calls the server function
valid cache file, called twice	refetches on the second call	second call served from the client cache

The sixth asserts that a non-production build never requests the cache, which passes both before and after.

One test-harness note worth flagging: getDefaultSerovalPlugins reads the Start options through createIsomorphicFn, and uncompiled that chain resolves to its server implementation, which wants a Start context in AsyncLocalStorage that a browser never has. The test stubs that one function out rather than faking a server context, since the adapter list is irrelevant to the cache lookup being tested.

✅ Checklist

I have followed the steps in the Contributing guide.
I have tested code changes locally with the relevant test commands, or tests do not apply to this pull request.
I fully understand the code in this pull request, including any code generated with AI assistance.

Local verification:

pnpm nx run @tanstack/start-static-server-functions:test:unit passes, 6 tests, no type errors
pnpm nx run @tanstack/start-static-server-functions:test:types passes
pnpm nx run @tanstack/start-static-server-functions:test:eslint passes
pnpm nx run @tanstack/start-static-server-functions:test:build passes
node scripts/verify-links.ts passes for the docs change

🚀 Release Impact

This change affects published code, and I have generated a changeset.
This change is docs/CI/dev-only (no release).

Summary by CodeRabbit

Bug Fixes
- Static server function calls now fall back to server execution when prerendered cache data is unavailable, invalid, malformed, or fails to load.
- Valid cached results are reused without unnecessary refetching.
Documentation
- Clarified behavior when prerendered cache files are missing.
Tests
- Added coverage for cache hits, missing or invalid responses, malformed cached data, fetch failures, and non-production behavior.

…on a cache miss The client half of `staticFunctionMiddleware` runs for every call in a production browser build and fetched the derived cache URL with no checks: `fetch(url).then((r) => r.json())`. A cache file only exists for calls the prerender pass actually executed, so with prerendering disabled, or for a route the crawler never reached, the request is answered by the application's catch-all route with the HTML shell. `r.json()` then rejected with "Unexpected token '<', "<!DOCTYPE "... is not valid JSON" and took the loader or navigation down with it, instead of falling through to the live server function. Treat an unreadable cache response as a miss. A failed request, a non-ok status, a body that is not JSON, and a body that does not parse all return undefined, which the middleware already handles by calling `ctx.next()`. The content type has to be checked as well as the status, because the HTML shell is served with a 200 in some setups. Also wire up `staticClientCache`, whose lookup was immediately overwritten by the fetch result and so never served anything, and enable the package's unit test script now that it has tests. This does not make prerendering populate the cache for SPA mode, the second half of the report. The build cannot know which server functions a client only route will call, so a live call is the correct outcome there.

coderabbitai · 2026-09-02T21:49:30Z

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📒 Files selected for processing (2)

packages/start-static-server-functions/src/staticFunctionMiddleware.ts
packages/start-static-server-functions/tests/staticFunctionMiddleware.test.ts

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packages/start-static-server-functions/src/staticFunctionMiddleware.ts
packages/start-static-server-functions/tests/staticFunctionMiddleware.test.ts

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📝 Walkthrough

Walkthrough

The static server-function middleware now treats missing or invalid prerendered responses as cache misses, invokes the server function, and reuses valid client-cached results. Tests, documentation, a changeset, and the unit-test script were updated.

Changes

Static cache fallback

Layer / File(s)	Summary
Cache lookup and fallback handling `packages/start-static-server-functions/src/staticFunctionMiddleware.ts`	`fetchItem` returns client-cached results and treats failed, non-JSON, non-OK, or invalid responses as cache misses.
Behavior tests and runtime contract `packages/start-static-server-functions/tests/staticFunctionMiddleware.test.ts`, `docs/start/framework/react/guide/static-server-functions.md`, `.changeset/static-server-fn-cache-miss-fallback.md`, `packages/start-static-server-functions/package.json`	Tests cover cache hits, cache misses, fetch failures, and non-production behavior. Documentation and release notes describe the fallback. The unit-test script runs Vitest directly.

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

Merge Risk: ⚪ Minimal · up to 8bcd0

Static cache failures now fall back to live server-function calls, while valid client-cached results are reused. No unresolved merge-readiness risk is identified.

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant staticFunctionMiddleware
  participant StaticCache
  participant ServerFunction

  Client->>staticFunctionMiddleware: Request static server function
  staticFunctionMiddleware->>StaticCache: Check client cache
  alt Cached result exists
    StaticCache-->>staticFunctionMiddleware: Return cached result
  else Cache miss
    staticFunctionMiddleware->>StaticCache: Fetch prerendered JSON
    alt Valid cache response
      StaticCache-->>staticFunctionMiddleware: Return serialized result
    else Missing or invalid response
      staticFunctionMiddleware->>ServerFunction: Invoke server function
      ServerFunction-->>Client: Return live result
    end
  end

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The PR addresses the missing-cache failure by falling back to the live server function and adds payload validation [`#7876`]. However, it does not implement the issue's SPA caching behavior or populate …	Implement the required SPA behavior for `#7876`, or update and split the issue if SPA cache population is intentionally out of scope. Document the revised acceptance criteria before merging.
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the primary change: cache misses now fall back to the server function.
Description check	✅ Passed	The description follows the repository template and provides detailed motivation, implementation scope, testing results, checklist status, and release impact.
Out of Scope Changes check	✅ Passed	The documentation, changeset, test-script update, client-cache wiring, validation logic, and unit tests directly support the cache-miss fallback fix and are within scope.

Full details: Linked Issues check

Explanation

The PR addresses the missing-cache failure by falling back to the live server function and adds payload validation [#7876]. However, it does not implement the issue's SPA caching behavior or populate cache files when SPA mode is enabled.

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Inline comments:
In `@packages/start-static-server-functions/src/staticFunctionMiddleware.ts`:
- Around line 155-157: Validate the value returned by fromJSON in the static
function middleware before treating it as a cache hit: require an object with
own result and context fields, otherwise continue through the cache-miss path
and invoke ctx.next(). Add a regression test covering a valid Seroval payload
that decodes to a non-object value.

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📒 Files selected for processing (5)

.changeset/static-server-fn-cache-miss-fallback.md
docs/start/framework/react/guide/static-server-functions.md
packages/start-static-server-functions/package.json
packages/start-static-server-functions/src/staticFunctionMiddleware.ts
packages/start-static-server-functions/tests/staticFunctionMiddleware.test.ts

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… a miss Decoding can succeed for a payload that is not a cached result, for instance a seroval encoded plain string left at the cache path by another build step. Such a value is truthy, so the caller took it for a hit, skipped the server function and handed back an undefined result. Require the decoded value to be an object carrying both `result` and `context` before accepting it. `addItemToCache` always writes both keys and seroval keeps them even when the context is undefined, so a genuine hit is unaffected. Plain JSON that is not seroval encoded already threw inside `fromJSON` and was caught, so only the valid-seroval-wrong-shape case needed the check.

coderabbitai Bot reviewed Sep 2, 2026

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Comment thread packages/start-static-server-functions/src/staticFunctionMiddleware.ts

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fix(start-static-server-functions): fall back to the server function on a cache miss - #8221