sdk:responses ignores reasoning.effort and uses the model's default reasoning variant

[Juddd]

Summary

The local sdk:responses bridge accepts OpenAI Responses requests containing reasoning.effort, but silently drops that value before calling the Cursor SDK.

As a result, clients such as Codex can select a reasoning level through model_reasoning_effort or /model, yet Cursor still uses the model's default reasoning variant.

Reproduction

Use Codex with the local Responses proxy:

model = "gpt-5.6-sol"
model_provider = "cursor"
model_reasoning_effort = "high"

[model_providers.cursor]
name = "Cursor Composer"
base_url = "http://127.0.0.1:8791/sdk/v1"
wire_api = "responses"

Start the proxy:

CURSOR_SDK_PROXY_CWD="$PWD" npm run sdk:responses

A request captured at the proxy boundary contains:

{
  "model": "gpt-5.6-sol",
  "reasoning": {
    "effort": "high",
    "context": "all_turns"
  },
  "stream": true
}

However, the Cursor Usage Dashboard records the run as:

gpt-5.6-sol-medium

Changing the reasoning level through Codex /model produces the same result.

Root cause

In scripts/cursor-sdk-responses-proxy.mjs:

• handleCreateResponse() reads and normalizes body.model, but does not read body.reasoning.
• agent.send(), Agent.resume(), and Agent.create() receive only:

model: { id: model }

The Cursor SDK supports model parameters through ModelSelection.params. For gpt-5.6-sol, Cursor.models.list() currently reports:

reasoning: none, low, medium, high, xhigh, max
context:   272k, 1m
fast:      false, true
default:   context=1m, reasoning=medium, fast=false

This explains why the resolved variant is always medium when the proxy omits the parameter.

Expected behavior

When the request supplies a supported reasoning.effort, map it to the Cursor SDK model selection:

{
  id: model,
  params: [{ id: "reasoning", value: effort }]
}

The selection should be applied per request to Agent.create(), Agent.resume(), and agent.send(), so changing the level through Codex /model works without restarting the proxy.

If the model does not advertise a reasoning parameter, the proxy should omit it or return a clear validation error rather than sending an unsupported value. If reasoning.effort is absent, the current Cursor default should remain unchanged.

Environment

• Linux
• @cursor/sdk@1.0.13
• Codex CLI 0.144.4
• Local npm run sdk:responses bridge

The issue appears to be in the cross-platform Node bridge rather than Linux-specific behavior.

Related precedent

The project already maps public model variants to Cursor SDK params in sdkModelSelection():

• fix: preserve composer model selection in SDK bridge #18 fixed Composer model selection using ModelSelection.params
• Add Grok 4.5 support (macOS app + SDK bridge) #26 added parameter mapping for Grok fast variants