fix(start-client-core): reject untagged non-JSON server function responses

[mixelburg]

🎯 Changes

Fixes #8333.

When something between the browser and the server answers /_serverFn/* itself (reverse proxy, bot challenge, captive portal), the response carries no x-tss-* header at all. If that response was 2xx and non-JSON, getResponse returned the Response object as the server function's result — the caller silently received a challenge page where it expected data, and the failure only showed up later (data.map is not a function).

An untagged response did not come from the server-functions handler, so it now rejects with its body, which is what the same function already does for the untagged non-2xx case. Deliberate raw responses keep working: the server marks them with x-tss-raw and they return before this branch.

Added packages/start-client-core/tests/server-fn-fetcher.test.ts covering the untagged 200 interstitial, the untagged error response, and the tagged raw response.

✅ 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.

🚀 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

  • Server function calls now report untagged, non-JSON responses as errors instead of returning them as successful results.
  • Error details from these responses are surfaced using the response body, improving visibility for unexpected proxy, challenge, or portal responses.
  • Responses explicitly marked for raw handling continue to be returned unchanged.

• Tests

  • Added coverage for successful and error responses, including raw response handling.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Advanced

Run ID: e1902e9b-2a0b-497e-a3a4-26ba85361c96

📥 Commits

Reviewing files that changed from the base of the PR and between a0b2ad9 and 910941b.

📒 Files selected for processing (3)

• .changeset/untagged-server-fn-response.md
• packages/start-client-core/src/client-rpc/serverFnFetcher.ts
• packages/start-client-core/tests/server-fn-fetcher.test.ts

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📝 Walkthrough

Walkthrough

The client now rejects untagged, non-JSON server function responses by throwing their body text. Tests cover 200 and 403 responses and preserve raw Response handling. A patch changeset documents the behavior.

Changes

Server function response handling

Layer / File(s)	Summary
Reject untagged non-JSON responses packages/start-client-core/src/client-rpc/serverFnFetcher.ts, packages/start-client-core/tests/server-fn-fetcher.test.ts, .changeset/untagged-server-fn-response.md	getResponse now throws the body text for every untagged, non-JSON response. Tests cover 200 and 403 responses and verify that x-tss-raw responses still return a raw Response. The changeset records a patch release.

Priority: ➖ Normal

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

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 91094

The intended handling for intermediary non-JSON responses is implemented with coverage for the primary response paths. No actionable merge risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
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 2 functions across 2 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: rejecting untagged, non-JSON server-function responses.
Description check	✅ Passed	The description explains the problem, the behavior change, preserved raw-response behavior, tests added, issue reference, and release impact. All template sections are present and the checklist is com…
Linked Issues check	✅ Passed	The change satisfies #8333. getResponse now throws the body for every untagged, non-JSON response, including successful 2xx responses. The new test covers a 200 text/html interstitial and a 403 te…
Out of Scope Changes check	✅ Passed	The changes are within #8333. The implementation changes the affected fallback, the tests cover the reported and preserved behaviors, and the changeset documents the patch. No unrelated product behavi…

Full details: Docstring Coverage

Explanation

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 2 functions across 2 files. (1 skipped: 1 unsupported.)

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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