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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds valuable behavioral context beyond that: it warns that 'Observation text is untrusted model output' and states the operational prerequisite 'Only works if PostHog is connected.' These are meaningful additions that help an agent anticipate failure modes.

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. The first sentence states the core purpose, the second adds the key filters, the third warns about untrusted output, and the fourth states the prerequisite. Every sentence earns its place with no redundancy.

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 read-only list tool with 100% schema coverage and no output schema, the description covers the essential context: what it lists, who it's for, when to use it, the key filters, a data-safety warning, and a prerequisite. The only minor gap is that it doesn't describe the return format, but that's not required given the tool's simplicity and the absence of an output schema.

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 documents all 5 parameters. The description adds some semantic context by explaining the purpose of scanner_id ('see one scanner's findings across sessions') and session_id ('see every scanner that ran on this recording'), which is already in the schema descriptions. The description doesn't add much beyond the schema, so baseline 3 is appropriate.

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 verb ('List'), a specific resource ('Replay Vision observations'), and clarifies what those observations are ('what scanners saw on recordings'). It also names the intended audience ('operator or analytics agent') and distinguishes itself from sibling tools like posthog_get_vision_observation and posthog_list_vision_scanners by focusing on observations rather than scanners or a single observation.

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 gives clear context for when to use the tool ('Use when reviewing scanner findings') and mentions the filter options (scanner_id and/or session_id). It doesn't explicitly name alternatives or state when not to use it, but the sibling tools are numerous and the description's context is sufficient to guide selection.

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