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

The annotations (readOnlyHint=false, destructiveHint=false) convey little, so the description carries the burden. It adds meaningful behavioral detail: returns revised text, changes only what the instruction requests, and costs about 97 credits per call. No side effects are disclosed, but the wording implies a contained transformation.

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 description is compact and front-loaded: it states the primary action first, then adds pairing guidance, scope constraints, and cost. Every sentence earns its place with no filler or repetition.

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 two-parameter tool with no output schema and minimal annotations, the description is complete enough. It explains what the tool does, what it returns, how it pairs with a sibling tool, what its behavioral scope is, and the cost. No critical operational detail 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 description coverage is 100%, so the schema already fully documents both parameters. The description adds only the workflow-level notion of applying 'one instruction,' which is also present in the instruction parameter description. This is the baseline score for schema-covered parameters.

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 states a specific action ('Apply one instruction to an existing letter') and a specific deliverable ('return the revised text'). The additional scope constraint ('Changes only what the instruction asks for') makes it clear what this tool does and distinguishes it from broader editing tools.

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 description explicitly positions this tool in a workflow: 'Pair it with letterlens: review to find the problems, revise to fix them.' This gives clear usage context. It does not enumerate exclusions relative to all siblings, but the pairing guidance is strong enough.

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