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

Annotations already declare readOnlyHint=false, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds genuinely non-redundant behavior: partial-update semantics, an ownership constraint, and the source of required ids. It does not mention error behavior for an invalid event_id or whether type must match event-vs-period kind.

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?

Two compact sentences, front-loaded with the mutation verb and a field list that earns its length by mapping to the schema. The long enumeration is dense but purposeful; only the jargon ("side of the line") costs a little clarity.

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 12-param mutating tool with a sparse schema and no output schema, the definition supplies the missing essentials: ownership scope, id provenance, and patch semantics, while annotations handle reversibility and world-boundary. Nothing critical to invoking it correctly is absent.

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 only 42%, so the description must carry weight — and it largely does, giving plain-language mappings for otherwise undocumented params (link=url, size=scale, side of the line=over_line, icon type=type) and covering title/date/color/description. It does not address date_to or the timeline_id/event_id pairing beyond "Ids come from get_timeline".

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 ("Change") and a precisely scoped resource ("one event or period of a timeline the user owns"), then enumerates the mutable fields. It is clearly distinguishable from add_events/delete_events, though it never names those siblings explicitly.

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?

"Ids come from get_timeline" gives a concrete prerequisite and names the tool that supplies the required identifiers. "Only the fields you send are changed" clarifies the patch semantics so an agent knows it need not resend the whole object. No explicit when-not-to-use guidance versus add_events, but the context is otherwise clear.

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