DeepSeek Custom Harness: "all 10 agents failed to start" – buzz-acp crashes on DeepSeek response

When setting up DeepSeek as a custom harness via buzz-acp with an OpenAI-compatible base URL, the agent connects successfully to the relay but crashes as soon as a message needs to be processed.

Configuration:

Command: /Applications/Buzz.app/Contents/MacOS/buzz-acp
Env var: OPENAI_BASE_URL=https://api.deepseek.com/v1
Env var: OPENAI_API_KEY=sk-...
Model: deepseek-chat

Steps to reproduce:

Add a custom harness in Buzz with the configuration above
Create an agent using the DeepSeek harness with model deepseek-chat
Add the agent to a channel and mention it

Actual behavior:

Agent starts and connects to relay ✓
presence set to online ✓
As soon as a message arrives: Error: all 10 agents failed to start — cannot continue
Broken pipe (os error 32)

Expected behavior: Agent responds using the DeepSeek model.

Related: Likely connected to #5399 (reasoning_content missing in DeepSeek responses causes buzz-acp to crash). PR #5487 appears to fix this but has not been released yet.

Buzz version: Desktop v0.5.11
macOS: MacBook Air (Apple Silicon)