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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), so the description's real contribution is disclosing the pricing rule max(declared, parsed duration, 16 kbps size floor) and that a URL triggers an SSRF-protected HEAD for Content-Length. That is substantive behavior an agent could not derive from annotations or schema, though the return shape of the estimate is not described.

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 sentences, front-loaded with the free/price-estimate purpose and followed by the one billing rule that matters. No filler and nothing repeated from the schema.

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?

Covers purpose, inputs, and the pricing algorithm for a tool with no output schema and no required parameters. The remaining gap is that with zero required parameters the description doesn't say what happens if all four are omitted or which input is preferred when several are supplied.

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, but the description adds the cross-parameter precedence rule — that declared minutes/seconds are hints and pricing takes the maximum of declared, parsed, and the 16 kbps floor. That interaction between otherwise-optional parameters is genuine added meaning beyond the per-field 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 and resource: 'estimate the exact x402 price', with the accepted inputs (minutes, seconds, bytes, audio URL) named. This is unmistakably a pricing-preview tool, clearly distinct from speech_transcribe or speech_report, which an agent can tell apart 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?

The 'Free:' prefix plus the billing formula implies this is the no-cost way to preview cost before committing to a paid transcription, but the description never says so explicitly and names no alternative tool or when-not condition. Usage is inferable rather than stated.

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