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

Annotations already declare readOnlyHint=true. Description adds behavioral details beyond annotations: 'Read-only, no side effects', support for negative N, and dual output modes (JSON vs raw). This provides useful transparency for an agent.

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?

Four sentences, each with a distinct purpose: purpose, safety, special behavior, usage guidance. Extremely concise with no filler. Information is front-loaded.

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 key aspects: default behavior, output format, negative-N, encoding though partially via schema, and stdin support. Lacks details on return structure beyond line array, but sufficient for a simple tool. No output schema but description compensates.

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 baseline is 3. Description adds value by explaining the negative-N behavior for the 'lines' parameter and mentioning the raw output mode, which goes beyond schema descriptions.

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?

Clearly states the tool returns the first N lines of files or stdin as JSON, with specific verb and resource. Explicitly distinguishes from sibling 'tail' for viewing endings.

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?

Provides clear usage context: 'Use to preview file beginnings or extract headers.' Explicitly states when not to use it: 'Not for viewing file endings — use tail for the last N lines.' Mentions alternatives 'tail' and 'cat'.

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