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

The annotations already declare idempotentHint=true and destructiveHint=false. On top of that, the description adds real behavioral detail: the update is partial, so unspecified fields are left alone; the datetimes must be naked local event-timezone values; and the ID arrays replace rather than append. Those are exactly the traits an agent could get wrong. It does not cover permissions or failure behavior, so a 4, not 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?

Four short sentences put the purpose first, then the partial-update rule, then the two gotchas an agent is most likely to trip over: timezone handling and array replacement. Every sentence earns its place, and none repeats the title.

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?

Because an output schema exists, return values need not be explained. The annotations already cover the safety profile. In this nested-payload mutation, the description covers partial updates, time formatting, and array replacement well. It only leaves out authorization requirements and what happens when session_id does not belong to event_id.

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%, so the schema already documents every field, including the array-replacement semantics and the no-timezone-designer rule. The description restates those rules rather than adding new detail about the nested session payload. With the schema doing the heavy lifting, 3 is the right baseline.

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 first sentence names a specific verb and resource: 'Update an existing agenda session.' That distinguishes it from create_sessions and delete_session by implication. It never names a sibling explicitly, though, so the boundary with create_sessions or update_session_category has to be inferred.

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?

'Provide only the fields to change inside the session object' is a concrete rule about how to call the tool. The description never says when to choose this tool over create_sessions or delete_session, and it gives no prerequisites, such as needing the session to already exist under that event. Usage is implied rather than stated.

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