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

Annotations only declare readOnly=false, idempotent=false, destructive=false, so the description carries the real behavioral load and does: flatten=true is irreversible, unknown fields error with the valid list, download links expire in 24 hours, and it costs 1 credit with 5 free calls/month. Those billing and output-lifetime details appear nowhere 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?

Front-loads the action and input shape, then layers in error behavior, widget semantics, flatten, and cost in four dense sentences with no filler. Every sentence carries information an agent would otherwise have to infer.

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 3-param nested-object tool with no schema descriptions, the description covers input encoding, per-widget value conventions, irreversible behavior, error feedback, output link lifetime, and pricing. Even though an output schema exists, briefly naming download_url adds useful context rather than redundancy.

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?

With 0% schema description coverage, the description must compensate and largely does — it defines pdf_base64, the fields name→value map, and flatten's bake-in effect, plus per-widget value conventions (checkbox true/false, radio option name, dropdown choice). Minor gap: the schema types every field value as string while the description says checkboxes accept true/false, leaving the actual wire format slightly 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 ('Fill') and resource ('fields of a fillable PDF form') plus the return value, which cleanly separates it from the siblings (merge_pdfs, images_to_pdf, pdf_tables_to_excel) — none of which touch form fields. An agent can pick this tool 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?

Explains the required input shape (base64 PDF plus a fields map) and explicitly warns that unknown field names are rejected with the real field list, which is a strong self-correcting usage hint. It never states when to prefer a sibling tool or what to do if the PDF is not fillable, so it stops short of full when/when-not guidance.

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