tool_choice: 'required' incompatible with thinking-enabled models when using structured output

[bralca]

Description

When using format: { type: "json_schema" } with the prompt_async API, OpenCode unconditionally sets toolChoice: "required" for the internal StructuredOutput tool. For reasoning models like Kimi K2.5
that have thinking automatically enabled, the downstream provider rejects this with:

tool_choice 'required' is incompatible with thinking enabled

This affects OpenCode Go (opencode-go). The same model + structured output works via OpenCode Zen (opencode), suggesting Zen reconciles the conflict server-side.

OpenCode Version

1.2.14

Plugins

No response

OpenCode version

No response

Steps to reproduce

1. Start an OpenCode server with an opencode-go API key
2. Create a session
3. Send prompt_async with json_schema format targeting a reasoning model:

curl -X POST http://127.0.0.1:/session/<session_id>/prompt_async
-H "Authorization: Bearer "
-H "Content-Type: application/json"
-d '{
"parts": [{"type":"text","text":"Describe the directory. Return JSON."}],
"format": {
"type": "json_schema",
"schema": {
"type": "object",
"properties": {
"description": {"type":"string"},
"files": {"type":"array","items":{"type":"string"}}
},
"required": ["description","files"]
},
"retryCount": 2
},
"model": {"providerID":"opencode-go","modelID":"kimi-k2.5"}
}'

4. SSE /event endpoint fires session.error immediately:

{
"error": {
"message": "Provider returned error",
"code": 400,
"metadata": {
"raw": "{"error":{"message":"tool_choice 'required' is incompatible with thinking enabled"}}",
"provider_name": "Moonshot AI"
}
}
}

Expected Behavior

Structured output should work with all models including reasoning/thinking models.

Root Cause

Tracing through the v1.2.14 source:

1. SessionPrompt.loop() creates a StructuredOutput tool when format.type === "json_schema"
2. Unconditionally sets toolChoice: "required":
   toolChoice: format4.type === "json_schema" ? "required" : undefined
3. Separately, ProviderTransform.options() enables thinking for reasoning models:
   result["thinking"] = { type: "enabled", budgetTokens: ... }
4. Both tool_choice: "required" and thinking params are sent together to the downstream provider, which rejects the combination.
   Affected Models

Any model with reasoning: true accessed via providers that enforce thinking + tool_choice incompatibility. Confirmed: kimi-k2.5 (Moonshot AI) via opencode-go.

Screenshot and/or share link

No response

Operating System

No response

Terminal

No response

[github-actions]

This issue might be a duplicate of or related to existing issues. Please check:

• 400 Error using ZenMux with Kimi K2.5: "reasoning_content is missing" #15197: Similar issue with Kimi K2.5 and thinking-enabled models causing errors (different error message but same underlying conflict area: thinking mode incompatibility with tool usage)

[lucad87]

I had the same issue using the version 1.2.27 of opencode with Qwen3.5-9B as model.

It seems that I found a workaround specifying the following prompt for an agent in opencode.json:

"prompt": "If a tool or a command returns null, none, or empty values, do not assume it is a failure. Instead, analyze the situation and make a decision on how to proceed based on the context. Consider alternative approaches, gather more information, or adjust your strategy as needed to achieve the best outcome."

in opencode.json I also have the following configuration:

"reasoning": true,
"tool_call": true,
"interleaved": true

[alex-wesolowski-cb]

We're running into this issue as well when setting format as well as an anthropic variant with thinking: true for the model config. Is there a solution that allows structured output and thinking for anthropic models?