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

The annotations already declare readOnlyHint=true, so the read-only nature is covered. The description adds meaningful context about live sessions, idle age, auto-eviction, and the 8-minute idle limit, which helps the agent understand the lifecycle without contradicting annotations.

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 purposeful sentences. The first states the core function and output fields, and the second gives usage guidance and expiration policy. No filler or 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 zero-parameter, read-only list tool with no output schema, the description is complete. It tells the agent what the tool returns (idle age, eviction time), when to use it, and a key behavioral constraint (8-minute idle expiry). Nothing essential 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?

The tool has zero parameters, so the schema is trivially complete at 100% coverage. Baseline for 0 params is 4; the description adds no parameter details because none are needed, and that 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 uses a specific verb ('List') and resource ('live browser sessions') and clearly states what information is returned ('idle age and the time left before auto-eviction'). It distinguishes itself from session_create by framing the tool as a discovery mechanism for reuse.

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?

Explicitly states when to use the tool: 'discover a session to reuse instead of creating a new one'. It also provides a critical operational condition ('sessions expire after 8 min idle'), giving the agent a clear decision rule.

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