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

Annotations only supply destructiveHint=true, so the description carries most of the burden and does so well — it discloses the mutation of live store data, the forward-looking billing impact, the underlying PATCH endpoint and the partial-write contract. It does not address reversibility (e.g., rescheduling skipping a charge) or any permission requirements, so it is not fully complete.

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 tight sentences with the mutating scope and target fields front-loaded, followed by the risk note and the endpoint reference. Nothing is padded, though the endpoint string is of marginal value and slightly dilutes an otherwise dense paragraph.

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?

The description covers what changes and what the change affects, but with no output schema present an agent still does not know what the call returns (updated subscription object, empty body, error shape) or what happens to a pending charge when next_transaction_date is rescheduled. Adequate for calling the tool, incomplete for interpreting the result.

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 each of the four parameters is already documented in the schema, and this sets the baseline at 3. The description adds grouping ('reschedule its next run', 'change its frequency', 'activate/pause') that reinforces intent but introduces no syntax, format, or constraint details beyond what the schema states.

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 ('UPDATES a subscription') and goes on to enumerate exactly what can be changed: next_transaction_date, frequency, is_active. An agent can immediately distinguish this from the sibling read tools (foxy_get_subscription, foxy_list_subscriptions) without opening any 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?

Gives real operational guidance: 'Provide only the fields you want to change,' which establishes partial-update semantics, plus a caution that this modifies live store data and affects future billing. It stops short of explicit when-not-to-use conditions or pointing at alternate tools for bulk/other updates.

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