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

Annotations already declare readOnlyHint=false, idempotentHint=true and destructiveHint=true; the description adds real value on top by stating partial-update semantics ('Only provided fields change'), the permission model, and the external GA4 side effect with a link to the underlying Admin API resource. It does not, however, explain the destructiveHint=true flag or what data a bad edit could clobber, which is the one behavioral gap.

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?

Front-loaded with the core action and the when-to-use condition, and each sentence carries information. It is slightly bloated: 'Optionally re-sync to Google Analytics (GA4) after editing' duplicates the sync_to_ga4 explanation already given earlier and repeated in the schema, and the raw documentation URL adds length for marginal benefit.

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 10-parameter mutation tool with no output schema, the description covers permissions, partial-update behavior, the GA4 external dependency, and ID sourcing, which is close to sufficient. It stops short of describing what the tool returns or how a failed GA4 sync surfaces, which would matter given the external API call.

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 goes beyond the schema by explaining the sync_to_ga4 special case ('pass sync_to_ga4 on its own to push an existing annotation to Google Analytics without editing it') and by pointing at the tools that supply annotation_id. That is genuine semantic amplification rather than restatement.

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?

Opens with a precise verb+resource+scope: 'Update an existing custom annotation on an AIdata property.' It is immediately distinguishable from the sibling create-annotation, and the follow-on sentences make clear it is an edit-in-place operation rather than a creation.

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 trigger ('when an annotation turned out to be inaccurate, unclear, or too vague'), an explicit restriction ('Only manually created annotations can be edited, by their creator or a property admin'), and routes the agent to the right siblings for IDs (get-custom-annotations, get-recent-annotations). It also carves out the sync-only use case, which no alternative tool covers.

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