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

Annotations already declare idempotentHint=true and destructiveHint=true, so the safety profile is partly covered. The description still adds genuinely new context: the credit rule and 90 s cap, the already:true return, and the auth requirement to act as the session player. The auth/ownership constraint is the most valuable addition since annotations cannot express it.

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 side effect, then idempotency, then auth. No filler; every sentence carries distinct information.

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 single-parameter mutating tool with no output schema, the description covers the mutation effect, idempotent response field, and auth requirement. It omits any error conditions (e.g., invalid session_id) or confirmation of what the credited time looks like, which keeps it just short of complete.

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?

Only one parameter, and schema coverage is 100% (the schema already documents session_id as 'from start_play'). The description adds no syntax, format, or sourcing detail beyond what the schema provides, so the baseline 3 applies.

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 and resource ('End a play session') plus the side effect (credits time since last beat). It is clearly distinguishable from family siblings like start_play, play_heartbeat, and now_playing without opening any schema.

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?

The idempotency note ('ending a session that is already over returns it unchanged with already:true') tells the agent it is safe to call even when uncertain, which is real usage context. It stops short of explicitly naming alternatives or a when-not-to-use condition, so it doesn't reach the top of the range.

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