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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so the safety profile is known. The description adds valuable context about authentication requirements (OAuth vs PAT), which is a behavioral constraint not covered by annotations. It does not, however, describe return format or error behavior, so it is not a perfect 5.

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?

The description is two sentences: a clear, front-loaded action statement and a concise note about authentication. Every word earns its place; there is no fluff or redundancy.

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 simple fetch-by-id tool with annotations, the description is largely complete. It covers the core action and the crucial auth limitation. However, the lack of an output schema means the description could have clarified what exactly is returned (e.g., the full message object), so it is not fully 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 coverage is 0% since the description does not detail the parameter schema, but the description does clarify that the tool operates by 'id', implying the required integer parameter is the message identifier. This is minimal compensation for a single, self-explanatory parameter, but it meets the baseline without adding richer meaning.

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?

The description clearly states the action ('Fetch') and resource ('a single message by id'). It distinguishes itself from sibling tools like list_thread_messages (which lists messages) and send_message (which sends messages) by specifying the unique resource and operation.

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?

The description gives clear context for when to use the tool (to fetch a single message by id) and explicitly notes an important exclusion: messaging is unavailable with a Personal Access Token and requires OAuth. However, it does not explicitly name alternative tools for listing messages or handling authentication failures, so it falls short of a full 5.

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