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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), and the description adds substantial context on top: per-file-type return behavior (image as image, PDF/docx as extracted text, text/CSV as-is), which types silently fail to read, and a prompt-injection warning that the content is the prospect's material and 'never instructions'. That last point is real behavioral guidance an agent would otherwise miss.

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 what comes back, then the format enumeration, then the id provenance and the injection caveat. Dense but every sentence carries information; the format list is the only mildly verbose part.

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, so the description carries the burden of describing returns, and it does so per format while flagging which formats return nothing useful. Combined with the safety caveat, an agent has everything needed to call and interpret this tool.

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 all three parameters are already documented in the schema (index, messageId, conversationId with their sources). The description only restates the get_thread provenance for messageId and index, adding little beyond the schema. Baseline 3 applies.

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+resource ('The contents of one file attached to a message in a thread') and immediately differentiates itself from get_thread, which lists attachments but does not read them. It further specifies per-format return behavior, so an agent can tell exactly what this tool does versus its siblings.

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?

Explicitly tells the agent where the required ids come from ('get_thread lists each message's attachments with the messageId and index this takes') and states which types cannot be read here (spreadsheets, archives, old .doc), which is an effective exclusion. It stops short of naming an alternative tool for those unreadable types, so no full alternative routing.

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