Tool Model

Agent Tool separates ideas that are often collapsed into one word.

Layer	Question	Examples
Declaration	What can be called?	get_weather, read_file, browser.click, generate_image.
Interface	How does this tool validate, authorize, execute, stream, render, and persist?	strict schema, read-only classifier, progress mapper, result mapper.
Surface	Which declarations are visible now?	Turn tools, task tools, skill-activated tools, tenant-limited tools, deferred tools.
Invocation	What did the agent request?	Arguments, actor, policy refs, native call id, status.
Permission decision	Was this call allowed, denied, asked, or delegated?	Rule decision, classifier decision, human approval, passthrough.
Scheduling	When and with what concurrency can it run?	Parallel reads, exclusive writes, interrupt behavior, sibling failure policy.
Execution	How was it run?	MCP server, local native command, browser executor, API adapter, model task.
Result	What came back?	Text, structured JSON, resource refs, artifact refs, errors, telemetry refs.
Persistence/rendering	How is output shown and stored?	Inline result, persisted preview, UI card, transcript search text.

A portable implementation should preserve these layers even when a provider exposes them as a single object.

Identity

Use stable tool_id for the portable declaration and preserve native ids in external_mappings. Do not rename an MCP tool, OpenAPI operation, browser action, provider function, CLI command, or A2A task/artifact in a way that loses its original identity.

Selection

Selection is context-specific. A tool may exist in a catalog but be absent from a surface because policy blocked it, setup is missing, the model lacks required capability, the current task should not see it, or the tool is deferred until discovered.

Input boundary

Do not treat tool input as a single mutable object. Keep model-provided arguments separate from observable, permission, and final call input.

Results

A tool result may be final, partial, denied, rejected, redacted, too large, synthetic, or converted into a durable artifact. The result envelope should explain which case occurred instead of hiding it behind plain text.