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

Annotations already declare the tool read-only, idempotent, and non-destructive, so the safety profile is covered. The description adds behavioural context beyond annotations: latency (~10s), cost ($0.012 flat), the fact it performs more than one search pass, and the response shape. It also notes performance relative to `sonar` (three times slower) which is not in structured data.

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?

Three concise sentences, each carrying essential information: purpose, performance/relationship to sibling, and selection criteria. It is front-loaded with the outcome ('written answer with citations') and avoids repetition of schema details.

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 13-parameter tool with complete schema coverage and output schema, the description covers the key selection criteria, behavioral expectations, performance, and cost. It also routes the agent to the correct sibling tool. Minor gaps such as streaming or filtering parameters are already documented in the schema, so the description is sufficient.

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 baseline is 3. The description adds specific guidance that `model` is required and expected to be `sonar-pro`, and that `messages` is the input, which pins down the primary parameter beyond the generic enum. This adds value above the schema.

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 the tool performs a multi-pass search and returns a written answer with citations. It explicitly differentiates from siblings by naming `post_perplexity_sonar` for single lookups and `post_perplexity_sonar_reasoning_pro` for reasoning-heavy questions. This is a specific verb+resource statement with clear sibling differentiation.

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?

It explicitly says 'Use it for questions with several parts or follow-ups' and contrasts with `sonar` for single lookups and `sonar_reasoning_pro` for reasoning, giving clear when-to-use and when-not-to-use guidance. It also notes the same request/response shape as `sonar`, which helps agents migrate.

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