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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), and the description adds genuinely new operational context: the credit cost formula (ceil(total pages / 20), minimum 1), the aggregate 10 MB input ceiling, the 1 MiB inline output cap, and the returned filename/meta shape. These are the kinds of constraints an agent must know before invoking and are not available anywhere in the structured fields.

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?

Extremely dense and front-loaded: the core action leads, then limits, then cost, then I/O shape. The telegraphic fragments ('Output: ... merged.pdf; meta: {...}') are slightly run-on and the filename placement is awkward, but virtually no sentence is wasted.

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 correctly carries the return-value burden and does so: inline application/pdf, 1 MiB cap, filename, and meta fields. Combined with input limits and cost, an agent has everything needed to call this correctly on the first attempt.

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 100%, so the baseline is 3; the schema already documents both parameters including the accepted file input forms. The description adds marginal value by restating the accepted encodings and by emphasizing order-sensitivity ('in the order given'), which reinforces the semantic meaning of the files array beyond its schema text.

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 (merge) plus resource (PDF files) plus the exact scope (2 to 20 files, in the given order, bookmarks optional). It is trivially distinguishable from siblings like pdf_split, pdf_info, and pdf_text without opening any schema.

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?

Usage is implied by the cardinality and ordering constraints ('2 to 20 PDF files into one document in the order given'), which tells the agent when this tool is applicable. However, there is no explicit routing guidance — e.g. it never says to use pdf_split to go the other direction, or when to prefer this over other document assembly paths. Implied usage only.

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