fix(frontend): return JSON errors for invalid content types and unknown routes

[pskiran1]

Overview:

This PR fixes non-JSON error responses on the OpenAI-compatible HTTP API. OpenAI clients expect error responses to use a JSON error envelope. Previously, some invalid requests returned plain-text or empty responses.

This PR returns the standard {"message", "type", "code"} error envelope for:

• Invalid or missing JSON Content-Type headers on POST /v1/responses
• Unmatched routes, including GET /v1/responses/{response_id}

Details:

Responses request parsing

handler_responses previously used Axum's Json<T> extractor. Because extractors run before the handler, invalid or missing JSON content types were rejected by Axum with a plain-text 415 Unsupported Media Type response. The handler now reads the request body directly and uses the existing ensure_json_content_type and parse_json_request helpers, consistent with the Chat Completions and Completions handlers.

Unmatched routes

Added unmatched_route_fallback to return a JSON 404 response when no registered route matches the request path. The fallback is registered after routers are merged, since Axum permits only one custom fallback, and before service layers are applied so fallback responses include the same request-level behavior, such as x-request-id. Existing handler-level errors, such as Model not found, are unchanged.

Validation

Added tests:

• test_responses_non_json_content_type_returns_json_error
• test_responses_malformed_json_returns_json_error
• test_unknown_response_id_returns_json_404
• test_unknown_path_returns_json_404

Manual verification against a frontend serving Qwen/Qwen3.5-4B:

Request	Before	After
POST /v1/responses with Content-Type: text/plain	415 with plain-text body	415 with JSON error envelope
POST /v1/responses without Content-Type	415 with plain-text body	415 with JSON error envelope
POST /v1/responses with malformed JSON	Raw Axum error response	400 with JSON error envelope
GET /v1/responses/resp_x	404 with empty body	404 with JSON error envelope
GET /v1/not_a_route	404 with empty body	404 with JSON error envelope

Regression checks:

Request	Expected result
POST /v1/responses with application/json; charset=utf-8	Request parsing succeeds
POST /v1/chat/completions with an unknown model	Existing Model not found response
GET /v1/models and GET /health	200 responses unchanged
GET /metrics	Prometheus text response unchanged
GET /v1/responses	Existing 405 response with Allow: POST unchanged

Related Issues

⚠️ This section is required. Choose one path below and delete the other.

🔗 This PR is linked to an issue:

• Closes dis-2541

🚫 This PR is NOT linked to an issue:

• Confirmed — no related issue

Summary by CodeRabbit

• Bug Fixes
  • Improved handling of response requests with clearer validation for content types and malformed request data.
  • Added consistent, OpenAI-compatible JSON error responses for unsupported media types, invalid bodies, unknown response IDs, and unavailable paths.
  • Error responses now include the relevant request method and path when a route cannot be found.
• Tests
  • Added coverage for request validation and standardized error responses.

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[pskiran1]

/ok to test 8b3dbaf

[devin-ai-integration]

Devin Review found 2 potential issues.

[coderabbitai]

Walkthrough

The HTTP service now parses response request bodies explicitly, validates JSON-compatible content types, and uses tolerant JSON parsing. Unmatched routes return OpenAI-compatible JSON 404 responses. Integration tests cover request and routing errors.

Changes

OpenAI-compatible HTTP error handling

Layer / File(s)	Summary
Explicit response request parsing lib/llm/src/http/service/openai.rs	handler_responses accepts raw body bytes, validates the content type, and uses parse_json_request for tolerant parsing.
Fallback routing and validation lib/llm/src/http/service/openai.rs, lib/llm/src/http/service/service_v2.rs	The merged router uses unmatched_route_fallback for standardized 404 JSON errors. Integration tests cover unsupported content types, malformed bodies, unknown response IDs, and unknown paths.

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

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description covers the changes, validation, and related issue, but it omits the required reviewer-start section and leaves both issue paths selected.	Add the required "Where should the reviewer start?" section and select only the applicable Related Issues path with a valid issue reference.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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 summarizes the main change: JSON errors for invalid content types and unknown routes.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@lib/llm/src/http/service/openai.rs`:
- Around line 2953-2956: Move the JSON Content-Type validation out of
handler_responses and into route middleware or a custom extractor that examines
request parts before Axum extracts body: Bytes. Ensure invalid media types
receive an immediate 415 response, while handler_responses continues to call
parse_json_request only after validation succeeds.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

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Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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Run ID: c950200f-5e3d-40f3-b90a-63087365bdda

📥 Commits

Reviewing files that changed from the base of the PR and between f4625cd and 8b3dbaf.

📒 Files selected for processing (2)

• lib/llm/src/http/service/openai.rs
• lib/llm/src/http/service/service_v2.rs

[pskiran1]

/ok to test 8f65456

[pskiran1]

/ok to test 8f65456

[pskiran1]

/ok to test 8f65456