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

The annotations already declare this is a non-read-only, non-idempotent mutation. The description adds meaningful context beyond that: it is a paid operation, uses stablecoins, follows the create_session quote/payment flow, and is capped by max_expires_at.

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 a single front-loaded sentence with no filler. It communicates the paid extension action and the relevant constraints efficiently.

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 paid mutation with three required parameters and no output schema, important details are missing: secret semantics, session_id format/ownership expectations, and what a successful payment/extension returns. The description is adequate but not complete enough for an agent to call this confidently without inspecting the 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 0%, so the description must carry parameter meaning. It mentions the duration values, but this only mirrors the enum and leaves secret and session_id undocumented, including their authentication and session-ownership roles.

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 and resource: "Pay for another 10m, 30m or 60m on a session you own." The phrase "another" clearly distinguishes it from create_session, while "up to max_expires_at" scopes the operation.

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 states the prerequisite that the session must be owned and gives the upper bound via max_expires_at. It also points to the create_session quote/payment flow, though it does not explicitly say when to choose extension versus creating a new session.

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