Tool calling · Fermion Docs

Send tools

json

{
  "model": "neutrino",
  "messages": [{"role": "user", "content": "What is the weather in Paris?"}],
  "tools": [{
    "type": "function",
    "function": {
      "name": "get_weather",
      "description": "Get current weather for a city",
      "parameters": {
        "type": "object",
        "properties": {"city": {"type": "string"}},
        "required": ["city"]
      }
    }
  }],
  "tool_choice": "auto"
}

Neutrino emits Hermes-format tool markers internally. The server parses them into OpenAI message.tool_calls entries and returns finish_reason: "tool_calls". Only Neutrino-8B reliably produces tool calls. The 0.6B models generally answer in prose.

Tool choice behavior

Value	Behavior
auto or omitted	Render every declared tool and parse calls when the model emits them.
none	Render no tools and return a normal assistant response.
required	Best effort. No grammar constraint forces a call.
named function	Render only that function. The model may still answer in prose.

Parser and name matching

The parser supports one or several calls, JSON arrays and wrapper objects, missing closing tags, trailing commas, single-quoted arguments, and balanced JSON inside surrounding reasoning. Arguments always leave the server as a strict JSON string.

When a model emits a short function name for a uniquely matching namespaced tool, the server rewrites the call to the registered name and logs the mapping. Ambiguous names are never guessed.

Loop protection and tool profiles

The serve-only stuck detector watches the conversation history supplied by the client. It can detect repeated identical calls, strict A-B alternation, and repeated identical tool errors. Polling tools are automatically exempt unless you disable that behavior.

Mode	Behavior
enforce	Default. On a detected loop, produce a diagnosis and a final answer without exposing tools.
observe	Log and count signals without changing output.
off	Disable analysis.

Affected responses include a top-level fermion_stuck object. Counters and configuration are available on /health.

Profile	Temperature	Top p	Repetition penalty
graded	0.01	1.0	1.05 / 256
vendor	0.7	0.8	1.10 / 256
vendor-thinking	0.6	0.95	1.10 / 256
antiloop	0.7	0.8	1.15 / 512