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

Annotations supply only readOnlyHint=false, so the description carries nearly the full behavioral burden, and it delivers the critical mutation context: partial-update semantics ('Only the fields you pass change'), immediate effect on the user's real calendar, and downstream visibility to other attendees. It stops short of covering recurring-event behavior, notification semantics, or error cases when the event_id is stale.

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 tightly-packed sentences, front-loaded with the verb/resource and side effects; the prerequisite and format notes follow. No filler, no restatement of 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?

For an 8-parameter mutation with no output schema, the description covers purpose, prerequisites, side effects, and timestamp format adequately. It does not say what the call returns (the updated event) or how failures surface, which is a minor gap given there is no output schema to compensate.

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 coverage is 100% so the baseline is 3, but the description adds meaning beyond it: patch semantics for unspecified fields, a concrete RFC3339 example ('2026-08-25T19:00:00-04:00') that the terse schema strings lack, and the linkage of account/calendar_id back to their source tools.

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 (update) and resource (one calendar event) plus the exact mutable fields (summary, description, location, start/end), and it names Google/Outlook as the scope. An agent can distinguish it from create_calendar_event and get_calendar_events purely from the text.

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 ('Get the event's id, its account... from get_calendar_events first') and an explicit restriction ('Use only when the user asked for the change'), which is a real when-not rule for an agent that might otherwise edit proactively. The only omission is that it never names create_calendar_event as the alternative for new events.

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