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

Annotations already declare idempotent, non-destructive, non-read-only, open-world. The description adds genuinely new behavioral context: the return is a base64 resource and the call consumes one render from the monthly quota. That quota-cost disclosure is exactly the kind of operational fact annotations cannot express. It stops short of detailing failure modes for protected or oversized PDFs.

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 short sentences, front-loaded with the core operation, then output format, then cost. Nothing is redundant and every clause carries information an agent needs.

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 tool with no output schema, the description usefully states the return shape (base64 resource) and the quota cost, and the input contract is mostly covered. The only shortfall is the undescribed "filename" parameter and no note on how password-protected or failing URLs are handled.

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?

The nested schema already documents url, pages, base64, and password thoroughly; the description reinforces this by flagging the per-file page selection and ordering semantics. The one gap is the top-level "filename" parameter, which is undocumented in both schema and description, leaving its effect on the returned resource ambiguous.

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 and resource ("Join 2-50 PDFs into one") plus the key scoping rule ("in order, optionally picking pages from each"), which cleanly separates it from split_pdf, compress_pdf, and watermark_pdf. An agent can identify the operation and its inverse sibling at a glance.

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 a hard input constraint (2-50 PDFs) and a quota note, which implicitly bounds usage, but it never explicitly says when to choose this over split_pdf or compress_pdf, nor states prerequisites. Usage is inferable from the verb rather than spelled out.

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