A model's "supports tools" flag does not guarantee structured tool calls

Fact

Model registries and serving stacks advertise tool support as a boolean capability. It means the model was trained on tool-use data and its chat template has slots for calls — not that any given response will fill them.

Smaller instruct models regularly emit the call as ordinary assistant text:

{"name": "read", "arguments": {"file_path": "src/stats.js"}}

The server sees no call in the format its parser expects, so the API returns that string as message content, an empty tool-call field, and a finish reason of "stop". Nothing errors. The agent sees a normal final answer and stops.

Why it matters

The failure is silent and reads as a model that "ignored the tools", so it gets misdiagnosed as a prompt problem or a missing capability. The real cause is a template mismatch between what the model generated and what the server parses, and it is invisible in any test that mocks the server — the mock returns whatever well-formed shape the test author wrote.

It is also the difference between an agent that works against self-hosted models and one that does not. Below roughly 7B parameters this is common enough to be the default experience.

How to apply

• Treat a missing tool call plus content that parses as a tool-call object as a recoverable case. Extract calls whose name matches a tool actually declared in the request, and ignore everything else so ordinary prose containing JSON is not misread.
• Scan for balanced brace spans rather than assuming the whole message is JSON — models wrap calls in prose, code fences, or <tool_call> tags.
• Refuse malformed JSON instead of repairing it. A half-parsed call carries arguments that write files or run commands; failing loudly is correct.
• Do not stream such text to the user before the turn ends. Withhold content that opens with { or a call wrapper, then release it as prose only if recovery does not claim it.
• Verify against a real server before believing tool use works. A mocked backend only ever returns the shape you wrote into the mock.