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host.tool_call

Invoke a tool this tenant has already published, by its local name -- the same real, metered call as calling it directly by its namespaced name (.), for a session that has no way to see its own namespaced tool name yet. Unlike host.tool_test, this counts toward host.usage and appears in host.tool_logs. Pass async: true for a tool that needs more than the call deadline: returns {run_id, status: "queued"} immediately instead of running inline -- see host.runs.get/wait. Pass version to pin the call to one of host.tool_history's versions instead of whichever is current.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
argsYesArguments to pass, validated against the tool's own args_schema.
nameYesLocal name of the tool to invoke.
asyncNoRun as a job instead of inline: returns {run_id, status} within ~50ms under the plan's job_max_s deadline; default false.
versionNoPin the call to this version instead of whichever is current; see host.tool_history. An unknown version is an argument error.
tenant_keyNoThe key `signup` returned. Required only when this connection carries no Authorization: Bearer header -- when both are present, the header wins.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changed
    • addedInput schema / properties / version
      Added value: +{
+  "description": "Pin the call to this version instead of whichever is current; see host.tool_history. An unknown version is an argument error.",
+  "type": "integer"
+}
  2. Changed3 schema fields changed
    • addedInput schema / properties / args / description
      Added value: +"Arguments to pass, validated against the tool's own args_schema."
    • addedInput schema / properties / async
      Added value: +{
+  "description": "Run as a job instead of inline: returns {run_id, status} within ~50ms under the plan's job_max_s deadline; default false.",
+  "type": "boolean"
+}
    • addedInput schema / properties / name / description
      Added value: +"Local name of the tool to invoke."
  3. First observed

TDQS

A4.5/5.0
Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations, the description carries the full burden. It discloses metering, logging, async queuing behavior, version pinning semantics, and auth requirements. It doesn't explicitly state that the invoked tool may have arbitrary side effects, but 'the same real, metered call' implies execution of the target tool with its inherent behavior. Overall, strong transparency for a dynamic invoker.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Four sentences, densely packed with purpose, contrasts, async behavior, and versioning. Front-loaded with the core purpose, then alternatives and nuances. No fluff, but slightly long due to necessary detail. Well structured.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a tool with 5 params, nested objects, and no output schema, the description covers all key aspects: purpose, auth, async, versioning, and differentiation from siblings. It references related tools (host.tool_history, host.runs.get/wait) for follow-up. Minor gap: doesn't explicitly describe error handling for unknown tools or validation failures, but that's not critical for invocation.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so baseline is 3. The description adds meaning beyond the schema by explaining why tenant_key is only needed without auth header, what async returns and its timing, and how version pins to history. It enriches parameter understanding without redundancy.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the verb 'invoke', the resource (a published tool by local name), and explicitly distinguishes itself from host.tool_test by noting it counts toward usage and logs. It also explains the specific use case for sessions that cannot see their own namespaced name, making it unambiguous.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description explicitly contrasts with host.tool_test, clarifies when async is needed (beyond the call deadline), when to pass version (to pin to a specific history version), and when tenant_key is required (no auth header). This gives clear when-to-use and when-not-to-use guidance with named alternatives.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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