Fix ResponsesRequest marshaling when service_tier is omitted

[ell-hol]

Summary

This fixes a local JSON serialization error when using Beta.Responses.Send without explicitly setting ServiceTier.

I found the issue while testing the Responses API streaming path with this request:

res, err := s.Beta.Responses.Send(ctx, components.ResponsesRequest{
	Input: openrouter.Pointer(
		components.CreateInputsUnionStr("Hey there"),
	),
	Model:  openrouter.Pointer("openai/gpt-4o-mini"),
	Stream: openrouter.Bool(true),
}, nil)

This failed before any HTTP request was sent:

error serializing request body: json: error calling MarshalJSON for type components.ResponsesRequest:
json: error calling MarshalJSON for type json.RawMessage:
invalid character 'a' looking for beginning of value

The same failure happens with Stream: openrouter.Bool(false), so the issue is not specific to streaming. It happens during local request serialization.

Root cause

ResponsesRequest.ServiceTier has this struct tag:

ServiceTier optionalnullable.OptionalNullable[ResponsesRequestServiceTier] `default:"auto" json:"service_tier"`

When ServiceTier is omitted, the SDK applies the default value auto.

Before this patch, the default value could fall through to:

return []byte(tagValue)

That means the default was serialized as raw JSON:

auto

But auto is not valid JSON. Since service_tier is a string-like enum, the serialized JSON value must be:

"auto"

This explains why explicitly setting ServiceTier worked:

serviceTier := components.ResponsesRequestServiceTierAuto

ServiceTier: optionalnullable.From(&serviceTier)

In that case, the enum value goes through the normal marshaling path and is correctly serialized as "auto".

Behavior before

This failed:

req := components.ResponsesRequest{
	Input: openrouter.Pointer(
		components.CreateInputsUnionStr("Hey there"),
	),
	Model:  openrouter.Pointer("openai/gpt-4o-mini"),
	Stream: openrouter.Bool(false),
}

_, err := json.Marshal(req)

with:

invalid character 'a' looking for beginning of value

Behavior after

The same request now marshals successfully.

The default service_tier is emitted as valid JSON:

{
  "input": "Hey there",
  "model": "openai/gpt-4o-mini",
  "service_tier": "auto",
  "stream": false
}

Fix

The default marshaling logic now checks whether the default tag value is already valid JSON.

If it is valid JSON, it is preserved as-is. This keeps existing defaults such as true, false, null, and numeric values working the same way.

If it is not valid JSON, it is serialized as a JSON string. This fixes string-like defaults such as auto.

Test

Added a regression test covering ResponsesRequest without an explicit ServiceTier.

go test ./models/components -run TestResponsesRequestMarshalWithoutServiceTier -v

Full test suite passes:

go test ./...

[perry-the-pr-reviewer]

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Perry's Review

Fixes invalid JSON marshaling for ResponsesRequest.ServiceTier: when the field is unset, the default:"auto" tag now correctly produces "auto" (quoted) rather than bare auto (invalid JSON).

Verdict: ✅ LGTM

Details

Risk: 🟢 Low

CI: no checks recorded (0 checks)

Findings: none

Security: no concerns

Test coverage: new test TestResponsesRequestMarshalWithoutServiceTier directly exercises the fixed path — constructs ResponsesRequest without ServiceTier, marshals, verifies both json.Valid and presence of "service_tier":"auto". Only one other OptionalNullable[EnumType] with a string default: tag exists in the codebase (ResponsesRequest.ServiceTier) and it is covered.

Unresolved threads: 1 prior Perry suggestion (companion test for ChatRequest.ServiceTier) — field is *ChatRequestServiceTier (plain pointer), already handled by the Kind==String branch; suggestion stands as optional documentation.

Scope: first review (full)

Review: tier=small · model=claude-sonnet-latest · score=?