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

Goes well beyond the annotations by describing validation behavior (unknown names, unlisted options, and over-length text are refused with the reason and nothing is filled), the output artifact (a new filled PDF, editable or flattened), read-back confirmation, and reporting of still-empty required fields. It also resolves the readOnlyHint question by stating the user's original file is not changed, so the write-like operation only produces a new artifact.

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 when-to-use trigger, then packs input rules, failure behavior, and return shape into a few dense sentences with no filler. It is a single long block rather than separated lines, which makes it slightly harder to scan, but every sentence carries distinct information.

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?

Despite an output schema being present, the description still summarizes the return (filled PDF, confirmation read-back, empty required fields), and it covers all three parameters plus the failure modes that matter for a 2000-item values array. 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 coverage is 100%, so the baseline is 3; the description earns above that by tying value types to field types (text, true/false for checkboxes, one listed option for radio/dropdown, list for multi-select) and noting that case-insensitive option matches are reported. It also adds that field names must come from list_form_fields, which the schema only hints at in an example.

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 ('fill in a fillable PDF form with given answers') and immediately scopes it by naming the sibling tool list_form_fields as the source of valid field names. An agent can distinguish this from list_form_fields without opening either 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?

Opens with an explicit trigger ('Use this when the user wants to fill in a fillable PDF form') and adds negative scope: it does not sign signature fields and does not fill XFA-only forms. It stops short of naming what an agent should do instead for signature or XFA cases, so the exclusion has no forwarding path.

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