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

Annotations already declare destructive and idempotent, but the description adds genuinely new behavioral context: it explains the business consequence ('Changes what the customer pays on the next charge') and pins the transport (PUT /subscription action=edit_items). It stops short of stating whether omitted fields are preserved or reset.

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?

Front-loaded with the WRITE marker, then effect and fields in a single dense sentence, with the endpoint trailing. Telegraphic ('one-time flag' vs the schema's `one_time`) but nothing wasteful.

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 mutation tool with no output schema, the description covers what changes, the immediate cost impact, and the endpoint. Partial-update semantics (merge vs replace of items[]) remain unstated, which is the main residual gap for an agent calling it safely.

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 description restates the editable fields (variant, quantity, price, SKU, one-time flag, properties) and the bounds already present in the schema (> 0, >= 0). With 50% schema description coverage it adds little beyond the schema, and it omits the required item `id`, which is the one parameter an agent most needs to source correctly.

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: 'Change existing items of a subscription', and enumerates the fields that can change. The word 'existing' implicitly separates it from add_items/remove_items, though it never names those siblings outright.

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?

The 'WRITE' marker and 'Change existing items' imply when to use it versus adding or removing items, but there is no explicit when-to-use statement, no prerequisite (e.g. fetch the item id first via get_subscription), and no mention of the sibling edit_subscription tool that also mutates a subscription.

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