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

Annotations already declare readOnlyHint=true, so the description carries a lower burden, but it adds essentially nothing beyond restating the read operation and pasting the REST path. It does not mention soft-deleted message behavior (the non-obvious effect of show_deleted) or permissions/visibility constraints.

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 short sentences, front-loaded with the action and resource, with zero filler. Every clause earns its place, though the REST path adds little for an agent.

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?

There is no output schema, but the operation is a straightforward single-resource read whose safety profile is covered by readOnlyHint. The only material omission is any note on soft-deleted messages or access restrictions.

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 both 'id' and 'show_deleted' are fully documented in the schema. The description adds no semantics beyond that, which is the correct baseline when the schema does the heavy lifting.

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 (fetch) and resource (a single message) plus the identifying key (id), which cleanly separates it from stream_search_messages, stream_query_threads, and stream_get_replies. It stops short of naming any sibling explicitly, so it is clear but not fully differentiated.

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?

Requiring an 'id' implies the tool is for when the caller already knows the message identifier, as opposed to searching or listing. However, no explicit when-to-use or when-not-to-use guidance is given, and no alternative is named.

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