Tool use
How tool calls work through Syntax — across providers, across engines, across model families.
Tool use — the model deciding to call a function and the harness executing it — is a core part of any modern coding workflow. Syntax normalizes tool use across model families so your harness sees a consistent interface.
What works through the Bridge
When your harness sends a request with tools defined, you get:
- Auto tool choice. The model picks when to call tools and when to answer directly.
- Streamed tool calls. Tool-call deltas stream as they're generated; your harness can render or execute them as they arrive.
- Tool-call preservation across turns. Tool calls and their results are preserved in conversation history so the model can reason about them later.
- Mixed content. A response can mix natural-language text and tool calls in one turn.
Across model families
Different model families expose tool calling differently. Some use OpenAI-style function calls, some use Anthropic-style tool blocks, some use a model-specific chat template. Syntax handles the translation:
- Provider-hosted models (OpenAI, Anthropic, Google, etc.) get their native tool-call format.
- Open-weight models with a native tool-call chat template get their template's expected format. Where applicable, the Bridge also enables the engine's auto-tool-choice path so the model reliably emits structured tool calls instead of free-form text.
- Open-weight models without a native tool-call template fall back to a structured-output approach that produces parseable tool calls regardless.
The harness sees the same OpenAI- or Anthropic-shaped tool calls either way.
Specialists as tools
When you deploy a multi-model party, every specialist is registered as a tool the main agent can call.
Approvals on tool calls
Tool-call execution is gated by your active Runtime Mode. In Default, every meaningful tool call asks for confirmation before running; in AutoEdit, common edits and routine commands run unattended; in Bypass, everything runs without asking. See Concepts → Runtime Modes.
Where to go next
- Models → Reasoning models — reasoning composes with tool use.
- Concepts → Specialist Models — how specialists become tools.
- Differentiators → First-class inter-compatibility — how the Bridge surfaces tool definitions through Chat Completions and Messages.