Bug Description

When using DeepSeek v4 series models (deepseek-v4-pro) with thinking mode enabled, multi-turn conversations with tool calls intermittently fail with HTTP 400: The reasoning_content in the thinking mode must be passed back to the API.

Root Cause

In multi-turn tool-calling scenarios, Hermes sometimes fails to include reasoning_content in assistant messages when constructing subsequent requests.

Evidence from request dumps:

1. Direct hit (Session 20260429_175349_88d049, Apr 29 18:14):

   • Message [40]: assistant + tool_calls + reasoning_content ✓
   • Message [42]: assistant + tool_calls + reasoning_content ✓
   • Message [44]: assistant + tool_calls WITHOUT reasoning_content ✗ ← causes 400

2. Null reasoning_content (Session 20260428_160911_013162, Apr 28 16:09):

   • Message [133]: assistant with reasoning_content: null

The pattern: all other models (qwen3.6-plus, kimi-k2.6, etc.) work fine because they don't enforce reasoning_content echo-back. Only DeepSeek enforces this requirement in thinking mode.

Steps to Reproduce

1. Configure Hermes with deepseek-v4-pro via an OpenAI-compatible endpoint (e.g., OpenCode Go API at opencode.ai/zen/go/v1)
2. Enable thinking mode (the model defaults to xhigh)
3. Have a multi-turn conversation with multiple tool calls (5+ rounds of tool usage)
4. Eventually the 400 error appears when Hermes sends a request missing reasoning_content in an assistant message

Expected Behavior

All assistant messages (especially those with tool_calls) that the API returned with reasoning_content should have that field preserved when echoed back in subsequent requests.

Actual Behavior

Some assistant messages lose their reasoning_content field. The API then rejects the request with:

Error code: 400 - {'error': {'message': 'Error from provider (DeepSeek): The reasoning_content in the thinking mode must be passed back to the API.'}}

Configuration

model:
  default: deepseek-v4-pro
  provider: go-custom
  base_url: https://opencode.ai/zen/go/v1

compression:
  enabled: true
  threshold: 0.5
  target_ratio: 0.2
  protect_last_n: 20

Related

• DeepSeek API docs: https://api-docs.deepseek.com/guides/thinking_mode (requires reasoning_content echo-back in tool-calling turns)
• Similar issue in CLIProxyAPI: DeepSeek v4 系列模型在调用工具轮次中未正确回传reasoning_content 导致报错 router-for-me/CLIProxyAPI#2999 (closed, same symptom, different root cause)

Logs

2026-04-29 18:03:42,992 ERROR root: Non-retryable client error: Error code: 400 - {'error': {'message': 'Error from provider (DeepSeek): The reasoning_content in the thinking mode must be passed back to the API.', 'type': 'invalid_request_error', 'param': None, 'code': 'invalid_request_error'}}
2026-04-29 18:14:10,896 ERROR root: Non-retryable client error: Error code: 400 - {'error': {'message': 'Error from provider (DeepSeek): The reasoning_content in the thinking mode must be passed back to the API.', 'type': 'invalid_request_error', 'param': None, 'code': 'invalid_request_error'}}

Full request dumps available at ~/.hermes/sessions/request_dump_*_deepseek*.json