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

Goes well beyond the annotations by disclosing per-type return behavior (image→image, PDF/docx→extracted text, text/CSV→verbatim) and, crucially, which types are named but unreadable (spreadsheets, archives, old .doc). It also adds a prompt-injection warning that returned content is data, never instructions, which annotations cannot express.

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-loads the core purpose and packs return formats, unsupported types, identifier sourcing, and a safety note into a tight block with no filler. It is dense but each clause earns its place; only the run-on length keeps it from a 5.

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?

With no output schema, the description carries return-value explanation itself, enumerating what comes back for each file type and warning about untrusted content. Given the readOnly/idempotent annotations already cover the safety profile, nothing an agent needs to call this correctly is missing.

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 and their sources are already documented in the schema. The description restates the get_thread provenance of messageId and index but adds no syntax or format detail beyond the structured fields, so the 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 (read) and resource (a file attached to a message) and defines scope as 'one file attached to a message in a thread'. It cleanly distinguishes itself from the sibling get_thread, which lists attachments rather than returning their contents.

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?

Clearly establishes the workflow: 'get_thread lists each message's attachments with the messageId and index this takes', which tells the agent the prerequisite and the identifier source. It stops short of explicitly framing when to reach for this versus other read tools, but the routing context is clear.

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