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

With no annotations provided, the description carries the full burden. It discloses that bodies are decoded and messages are ordered oldest first, and implies a read-only operation. It does not mention potential side effects, but as a read tool that is acceptable. It omits details like pagination or error behavior, but these are not critical for a thread read. The provided behavioral details (decoded bodies, ordering) add real value beyond the schema.

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 concise sentences with no fluff. The first sentence states the core purpose and key details, the second gives usage guidance, the third provides a parameter sourcing hint. Information is front-loaded and every sentence earns its place.

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?

The tool is a read operation with no output schema. The description explains what it returns (every message, decoded bodies, oldest first) and how to obtain the thread ID. It does not describe the exact return structure or mention large-payload handling, but for a straightforward thread read this is adequate. It covers the essential information an agent needs to call it correctly.

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 67% (2 of 3 parameters have descriptions). The threadId parameter lacks a schema description, but the tool description compensates by stating 'Get the thread ID from search_gmail_messages.' This is essential guidance. The other parameters are already documented in the schema, and the description adds context on how to use threadId effectively. Overall, the description adds meaningful parameter semantics beyond 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?

The description clearly states the action ('Read every message in a conversation'), the resource (a Gmail thread), and specific details (oldest first, decoded bodies). It also distinguishes from the sibling get_gmail_message by explicitly saying 'Use this instead of repeated get_gmail_message calls when you need the whole exchange', making it unambiguous which tool to pick.

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?

Provides explicit when-to-use guidance: 'Use this instead of repeated get_gmail_message calls when you need the whole exchange.' It also tells the agent how to obtain a required parameter: 'Get the thread ID from search_gmail_messages.' This is clear, actionable, and leaves no ambiguity about alternatives.

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