Tools policy

Function tools and the tool union (custom, computer, server) on passthrough vs translation — when the gateway errors vs rebuilds.

How tools are handled so agents never believe a tool was registered when it was dropped.

Summary

Path	Behavior
Passthrough (OpenAI → openai / openai_compat)	Forward as-is (model rewrite only)
Translate → OpenAI	Rebuild full tool union: `function`, `custom`, `computer`, `server`
Translate → Anthropic / Google	`function` only — other kinds → HTTP 400 with a clear error

Silent skip is intentionally rejected.

Tool kinds

Kind	Wire example	OpenAI egress	Anthropic / Google
`function`	`type: "function"` + parameters schema	yes	yes
`custom`	grammar / lark / regex tools	yes	error
`computer`	computer-use styles	yes	error
`server`	`file_search`, `web_search`, …	yes	error

Empty type is treated as function. Name is required for function and custom tools.

Function tools (all families)

{
  "type": "function",
  "function": {
    "name": "lookup",
    "description": "Look up a record",
    "parameters": {
      "type": "object",
      "properties": { "id": { "type": "string" } },
      "required": ["id"]
    }
  }
}

Works on passthrough and on translate to Anthropic or Google.

Custom / grammar tools (OpenAI family)

{
  "type": "custom",
  "custom": {
    "name": "date_tool",
    "format": {
      "type": "regex",
      "definition": "[0-9]{4}-[0-9]{2}-[0-9]{2}"
    }
  }
}

Passthrough to OpenAI / openai_compat: forwarded.
Translate to Claude/Gemini: fails closed — use function tools or stay on OpenAI family.

How it works internally

OpenAI ingress parses tools into the canonical IR.
Router selects upstream kind.
Egress rebuilds vendor JSON, or returns invalid_request_error with the unsupported kind name.

Multi-turn tool loops still rely on thinking/signature fidelity (reasoning_content, Anthropic thinking blocks, Google thoughtSignature) — see Chat field parity.

Practical guidance

Vendor-specific tools → passthrough to that vendor.
OpenAI client → Claude/Gemini → function tools only.
Prefer fail-closed errors over silent drops when debugging agents.