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

Beyond the annotations (idempotentHint=true), the description reveals significant behavioral context: the Durable Object design ensures N simultaneous signals produce exactly one notification; already-fired joins still record the signal but cannot double-fire; expired joins refuse with signalled:false and reason 'expired'; and the tool is free. No contradictions with annotations.

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?

The description is long but every section adds value: core action, idempotency, concurrency, edge cases (already-fired, expired), and auth/route. It's front-loaded with the primary verb and uses headers-like caps (FREE, IDEMPOTENT) to highlight key properties. No redundant sentences.

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 complex tool with no output schema, the description covers behavior thoroughly (edge cases, failure modes, and next steps). However, it does not explicitly state the response shape for a successful signal (only references signalled:false in the expiry case), leaving a minor gap in expectations. Overall it's sufficiently complete for most use cases.

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?

Although the input schema covers all 4 parameters (100% coverage), the description adds semantic meaning: it explains that payload is 'what the joiner will receive' and that on expiry the payload will not reach the joiner ('your payload did NOT reach the joiner'), and it ties participant_id to the idempotency behavior. It does not add syntax details but clarifies the lifecycle implications, so it goes beyond the schema baseline of 3.

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?

The description opens with a specific verb+resource: 'Report that one participant finished, optionally attaching a payload the joiner will receive.' This clearly distinguishes it from sibling tools like barrier_create and barrier_status, which handle creation and status checks respectively. The scope (one participant at a time) is explicit.

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?

Provides explicit when-not and alternative guidance: signals to a timed-out or TTL-passed join are refused with reason 'expired', and in that case the caller should 'call barrier_create with it to open a new generation.' Also notes the idempotency property ('the same participant_id signalling twice counts once'), which informs safe retry behavior. It doesn't explicitly compare with barrier_status, but the guidance is concrete.

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