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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint, so the safety profile is covered. The description adds genuinely non-structured information: the 10 free calls/day, the $0.001/call price, and the accepted payment paths (x402 via _meta or HTTP, or X-API-Key credits) – real operational context an agent cannot obtain from annotations.

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?

The metric list is front-loaded and the payment caveat is compressed into one short bracket sentence, with no filler. It is appropriately sized for a utility tool, though the payment detail is dense enough that it competes with the purpose sentence for attention.

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-shape explanation is unnecessary, and the safety and billing picture is covered. What is missing is the input contract: no indication of size limits, whether very large texts cost extra, or how the free-tier cap is enforced, which matters for a tool whose main risk is a paid call.

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?

Only one required parameter exists and schema coverage is 100%, so the baseline is 3. The description earns above baseline by flagging CJK-aware tokenization and giving a mixed-script example (Chinese plus English plus digits), which tells the agent the input may be non-ASCII and multiline where sentence and paragraph counts matter.

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?

The description enumerates the exact metrics produced (characters, CJK-aware word count, sentences, paragraphs, reading time, token estimate, top words), which is a specific and inspectable statement of what the tool does. It stops short of differentiating itself from close siblings like token_count or mcp_token_measure, which also touch token estimation.

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?

There is no statement of when to use this over token_count, mcp_token_measure, or text_chunk; the only guidance given concerns billing. The CJK-aware note implies a use case but the agent must infer the selection criteria itself.

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