fix: use json.RawMessage for Instructions field in OpenAIResponsesResponse

[linbin524]

Problem

When proxying /v1/responses requests to upstream providers, both non-streaming and streaming calls return HTTP 500 if the upstream response contains "instructions": null (or any non-string value).

This happens because OpenAIResponsesResponse.Instructions is defined as string, but the OpenAI Responses API spec allows this field to be null, a string, or potentially other JSON types. Go's json.Unmarshal fails when trying to decode null into a string field, causing the entire response deserialization to fail.

Root Cause

In dto/openai_response.go, the OpenAIResponsesResponse struct defines:

Instructions       string             `json:"instructions"`

Other fields in the same struct (Status, ToolChoice, Truncation, PreviousResponseID, User, Metadata) already use json.RawMessage to handle polymorphic JSON values. The Instructions field was the only one left as string.

Notably, the request-side struct in dto/openai_request.go (line 828) already defines Instructions as json.RawMessage — this fix makes the response-side consistent with the request-side.

Fix

Change Instructions from string to json.RawMessage in OpenAIResponsesResponse, matching the pattern used by all other polymorphic fields in the same struct.

Impact

• One-line change, no behavioral difference for valid string values
• Fixes 500 errors when upstream returns "instructions": null
• Aligns response DTO with request DTO

Summary by CodeRabbit

• No user-facing changes
  • Internal technical adjustment to data handling.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 6907087d-9b46-4c82-ae06-7c3fc555f34b

📥 Commits

Reviewing files that changed from the base of the PR and between 8c8661d and 4184059.

📒 Files selected for processing (1)

• dto/openai_response.go

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Walkthrough

The Instructions field in the OpenAIResponsesResponse struct is changed from a string type to json.RawMessage type, altering how the field is decoded from JSON and represented in memory while maintaining the same struct tag.

Changes

Cohort / File(s)	Summary
Type Change dto/openai_response.go	OpenAIResponsesResponse.Instructions field type changed from string to json.RawMessage to preserve raw JSON representation during decoding.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐰 A string once held instructions tight,
Now raw JSON keeps formats right!
No parsing delays, just JSON pure—
The rabbit approves, that's for sure! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically describes the main change: converting the Instructions field from string to json.RawMessage in OpenAIResponsesResponse.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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