Serving Fara with llama.cpp: use --reasoning-format none, or agent harnesses get empty content

#4
by T0mSIlver - opened

If you serve this model with llama-server and consume it from an agent harness (e.g. Magentic-UI's Fara web surfer), tool parsing can fail on every call, with the harness seeing an empty assistant message.

What happens: Fara writes its thoughts as plain text followed by a <tool_call>...</tool_call> block. It emits a <think> opening tag but never a closing </think>. With llama-server's default reasoning extraction (--reasoning-format unset/auto), the parser treats everything after <think> as reasoning β€” so the entire output (thoughts + tool call) lands in reasoning_content, and message.content comes back as "". Any client that reads only content (the OpenAI-standard field) gets an empty string. In Magentic-UI this surfaces as list index out of range. Retrying (1/3)... on every task.

Repro: identical chat completion request, same model (27B, llama.cpp b10107):

  • default reasoning format β†’ content: "", thoughts + <tool_call> block in the reasoning field, finish_reason: stop
  • --reasoning-format none β†’ full raw output in content, parses fine

Fix: launch llama-server with --reasoning-format none. If you have a router/proxy in front doing its own reasoning extraction, disable it there too. Note the raw output then starts with a stray unclosed <think> tag β€” harmless for parsers that split on <tool_call>, just cosmetic in displayed thoughts.

Thank you for bringing this to our attention. Would you mind posting this as an issue on the Fara1.5 github repo so we can mention this in the hosting instructions? Thank you!

https://github.com/microsoft/fara

Sure, here it is: https://github.com/microsoft/fara/issues/82
Thanks for the quick follow-up, and great work on Fara1.5, it's been really impressive to run locally.

Sign up or log in to comment