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

Annotations cover the safety profile (readOnlyHint=false, destructiveHint=true, idempotentHint=false), so the description's real contribution is the merge semantics: each supplied part replaces that part and omitted parts persist, with live messages required to remain complete. That partial-replace behavior is genuinely non-obvious for an 11-parameter mutation and is exactly the kind of context annotations cannot convey. It stops short of saying what is lost or whether the update is reversible.

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?

Three short sentences, front-loaded with the action, then the replace rule, then the constraint. No filler. The opening 'Changes a message' is a touch thin for a mutation tool, but nothing is wasted.

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 11-parameter mutation with no output schema, the description covers the essential write semantics but leaves gaps: it does not say whether the target must be a draft, what happens when an update would invalidate a live message, or how the messageId relates back to list_proactive_messages. Adequate, not complete.

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 raises it by explaining that each listed field is swapped wholesale rather than merged field-by-field, which the schema does not state. Minor gap: the enumerated list omits name, priority, and productId, which are also present in the schema.

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 ('Changes') on a specific resource and enumerates the mutable parts, plus the non-obvious patch semantics. The resource is only called 'a message' rather than 'proactive message', so the agent relies on the title and sibling names (create_/delete_/get_proactive_message) to disambiguate, but the intent is clear.

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?

No when-to-use guidance and no named alternatives. Nothing tells the agent this is the edit counterpart to create_proactive_message or that list_proactive_messages supplies the required messageId (that is only in the schema). The lone constraint, 'A live message must stay complete', is a hint at validation rather than usage routing.

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