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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint, so the safety and purity profile is largely covered. The description usefully clarifies that filling targets named form fields and that flattening is optional, but says nothing about what happens to unfilled fields, existing values, or how failures (e.g., a non-interactive PDF, a refused private URL) surface.

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?

Two tight sentences with no filler. The core action and supported field types lead, and the optional behaviors (flatten, inspectOnly) follow in the order an agent would need them.

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?

An output schema exists, so return values need no explanation, and all four parameters are named in prose. What remains thin is failure behavior and the semantics of partial fills, which matters for a form-filling tool but is not essential to selecting or invoking it.

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 the schema already documents file, values, flatten and inspectOnly in detail, including the 10 MB cap and checkbox/radio value rules. The description only mirrors those points at a high level and adds no syntax or format detail the schema lacks.

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 (fill) on a specific resource (an interactive PDF form) and enumerates the field types it handles: text fields, checkboxes, dropdowns, radios. This cleanly separates it from the other PDF siblings (extract_pdf_text, sign_pdf, flatten-style tools) without the agent needing to open a 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?

Recommendations the workflow order explicitly: 'Pass inspectOnly to list the fields first,' which tells the agent how to discover field names before filling. It does not name any when-not conditions or alternatives (e.g., for non-interactive PDFs the agent should use another tool), so it stops short of the top band.

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