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

Annotations declare a non-destructive write, and the description adds substantive behavioral context beyond that: certain rows ('source-owned and approved') and 'billed duration' remain locked and cannot be changed. That is genuinely useful constraint information. It doesn't mention return/error behavior, but the output schema covers returns.

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

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

Four short, front-loaded sentences: what it does, the selectors, the mutable fields, and the immutability caveat. Nothing redundant, nothing buried.

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

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

For a mutation tool with an output schema and annotations, the description covers the critical decision points: selector exclusivity, mutable fields, and lock constraints. It could go further on workspace-scoping behavior or failure modes, but the schema already documents those params, so nothing essential is missing.

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

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

With 60% schema coverage over 10 params, the description compensates meaningfully: it enforces the mutual-exclusivity of person_id/email (which the schema does not express), identifies the three mutable fields, and rules out numeric row keys. The remaining params (workspace_id, conversation_id, workspace_scope, idempotency_key) are documented in the schema, so this is a good balance rather than exhaustive.

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

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

Opens with a specific verb and resource: 'Change one participant of an activity.' The scope (single participant, not the whole activity) is clear, and the mutable fields are named. It doesn't explicitly distinguish itself from well_update_activity or well_create_activity/well_delete_activity, so sibling differentiation is left implicit.

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

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

Gives concrete preconditions: supply activity_id plus 'exactly one selector: person_id or email', and 'Numeric row keys are not accepted.' These are real usability rules an agent would otherwise get wrong. It stops short of naming when to prefer this over well_update_activity or the delete/create siblings.

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