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

Annotations only cover safety (readOnly, idempotent, non-destructive, openWorld); the description adds the operationally critical facts the agent cannot get elsewhere: the text is sent to an external language model, the default path uses a small shared daily allowance via Google Gemini, and an X-OpenRouter-Key header switches to the caller's own key. That is exactly the kind of auth/quota/external-service disclosure that matters for a tool whose input leaves the process.

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, no filler. The purpose is front-loaded and the operational caveat about model routing follows immediately, so an agent reading only the first clause still knows what the tool does.

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?

Auth, quota and external-model routing are covered, and the safety profile comes from annotations, so the main gaps are minor: the return shape (corrected text vs. change list) and behavior when the shared allowance is exhausted are unstated, and there is no output schema to fill that in.

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?

There is a single parameter with 100% schema description coverage ('Text to proofread, at most 2,000 words'), so the schema already carries the semantics. The description adds nothing about the parameter itself, which is the expected baseline of 3 when the schema does the heavy lifting.

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 set (fix) and resource (spelling, grammar, punctuation, word choice) plus the constraint that meaning and voice are preserved. None of the sibling tools (word_count, character_count, extract_strings, font_generator) overlap with proofreading, so no sibling differentiation is needed for an agent to pick this tool.

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?

Usage is implied by the tool's obvious purpose but never stated as a when-to-use rule, and there is no when-not guidance (e.g. length limits beyond the schema, or what to do for non-English text). No alternative tool is named, which is acceptable here since no sibling competes, but the routing/quota sentence reads as behavioral disclosure rather than selection guidance.

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