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

Annotations only mark readOnlyHint false, so the description adds meaningful behavioral context: edits affect the real calendar immediately, other attendees can see them, and only the provided fields change rather than a full replace. This discloses side effects and partial-update behavior beyond what annotations convey.

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 sentences, each earning its place: the core action, partial-update semantics, prerequisite lookup, usage gate and side effects, then the timestamp format. The description is front-loaded with the most important information and contains no filler.

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 side-effecting update tool with no output schema, the description covers prerequisites, side effects, partial-update behavior, and the timestamp format. It does not explain the response value, but that is less critical for a mutation tool and the parameter schema is fully documented.

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 baseline is 3. The description adds value by explaining that start/end are RFC3339 timestamps with an example, and by clarifying that event_id and calendar_id come from get_calendar_events. It also reinforces that unpassed fields remain unchanged, which helps interpret the optional parameters.

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 states a specific verb (Update), a specific resource (one Google Calendar event), and the exact fields that can be changed (title, description, location, start/end times). It also clearly implies the distinction from creating a new event by focusing on editing an existing event identified by event_id.

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 description gives explicit context: only use when the user asked for the change, and first fetch event_id (and calendar_id when needed) via get_calendar_events. The 'only when the user asked for the change' instruction clarifies the appropriate trigger, but it does not explicitly name create_calendar_event as the alternative for creating a new event.

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