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

Annotations only cover the safety profile (readOnlyHint=false, idempotentHint=true, destructiveHint=false). The description adds behavior beyond that: a conditional fallback where the caller receives a link instead of a join when the person's 'groups' setting isn't 'send', plus the side effect that hitting a threshold promotes the circle to a group chat and notifies everyone. That is meaningful side-effect and branching disclosure; auth/permission requirements remain unstated, keeping it off a 5.

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?

Three short sentences, front-loaded with the action and its gating condition, then the fallback and side effect. No filler. Slightly terse phrasing in the opening clause ('for the person') costs it the top mark.

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 one-parameter mutation tool with an output schema and annotation coverage, the description supplies what structured fields cannot: the conditional return-link path and the group-chat promotion side effect. Return-value shape is delegated to the output schema, which is acceptable, leaving only minor gaps around permissions.

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?

Schema description coverage is 100% and the single parameter is already documented as 'circleId or slug from find_circles'. The description adds no syntax, format, or lookup guidance beyond that, so the baseline 3 for a fully documented schema is appropriate.

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?

States a specific verb (join) and resource (circle), and the qualifying clause 'for the person — only when they said yes' frames the action's scope. It does not explicitly contrast with close siblings like propose_circle or find_circles, so it stops short of a 5, but the action itself is unambiguous.

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 an explicit precondition for invocation ('only when they said yes'), which tells the agent this is a consent-gated action rather than a free-form join. No alternatives are named, so the agent must consult sibling names/parent descriptions to know when to prefer propose_circle, but the 'when' condition is real guidance.

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