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

Annotations already declare destructiveHint=false and idempotentHint=true, so the safety profile is covered. The description adds non-obvious behavior: billing semantics ('No additional close fee; outstanding browser time is settled') and the idle auto-close timeout, which an agent cannot infer from structured fields.

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 short sentences, front-loaded with the action, with no padding. The HTTP-equivalent line is mildly extraneous for a chat agent but is compact and occasionally useful.

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 one-parameter lifecycle tool with no output schema and annotations already covering safety, the description supplies billing, idle-timeout, and endpoint context. It does not state the return shape, but with no output schema that is a minor gap.

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 single sessionId parameter is already documented in the schema as coming 'from web_session_open.' The description adds no syntax or format detail beyond that, so the baseline 3 applies.

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?

States a specific verb and resource ('Closes the browser session') that pairs cleanly with the sibling web_session_open, and the schema reference back to 'web_session_open' reinforces the lifecycle pairing. An agent can tell exactly what this does without opening the schema.

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 gives implicit when-to-use guidance by noting the session 'closes itself after a minute without use,' which tells the agent closing is optional and only needed for immediate teardown. It does not name an explicit alternative or exclusion, but the auto-close caveat is real routing context.

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