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

Annotations already declare readOnlyHint=false, destructiveHint=true, and idempotentHint=true; the description reinforces 'WRITE' and adds useful effect information ('lowers what the customer pays on the next charges'). However, it never explains the destructiveHint — e.g. whether applying a code replaces an existing discount code — nor how to undo it, which is the main behavioral unknown for an agent.

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 compact sentences with the operation type ('WRITE:') and the key scope constraint front-loaded. The trailing 'PUT /subscription-discount-code action=apply' is mildly redundant implementation detail but costs little.

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 2-parameter write tool with full schema coverage and rich annotations, the description covers purpose, scope, and customer-facing effect. The remaining gap is behavior under conflict (existing discount code) given destructiveHint=true, which would be worth one more sentence.

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 both parameters (discount_code, subscription_id, including the id-vs-internal_id distinction) are already fully documented in the schema. The description adds no syntax, format, or validation detail beyond that, so 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+resource ('Apply a Shopify discount code to an auto-charging subscription') and scopes it with an explicit exclusion ('not to recurring-invoice subscriptions'). It doesn't explicitly contrast with the sibling sealsubscriptions_remove_discount_code, though the apply/remove pairing is clear from the names.

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 a clear applicability condition (auto-charging subscriptions) and a clear non-applicability condition (recurring-invoice subscriptions). It does not name the counterpart remove_discount_code tool for the reverse operation, so the alternative-selection guidance is incomplete rather than absent.

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