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

Annotations already declare a non-read-only, non-destructive, closed-world write, and the description adds the owner-only privilege requirement plus the payment precondition, which is real value. It does not disclose idempotency behavior, whether credits are additive to an existing balance, or what happens if the target key is invalid.

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?

A single sentence that front-loads the most important constraint ('OWNER ONLY') before the action and its precondition. Every clause earns its place with no filler.

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 annotations describing permissions, no output schema, and 0% parameter documentation, the description covers only authorization and timing. It omits the meaning of 'plan' and 'api_key' and gives no indication of what the call returns or how a partial/failed credit addition behaves.

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 all parameter meaning, and it only implicitly maps 'credits' and 'target_key'. The 'api_key' credential and especially the 'plan' parameter (default 'paid') are never explained, leaving half the surface undocumented in both schema and description.

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 names a specific verb (add) and resource (credits) and identifies the target as a customer's key, which cleanly separates it from check_credits and the other siblings. It stops short of naming an explicit alternative, but the purpose is unambiguous.

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?

'OWNER ONLY' sets an authorization precondition and 'after payment is received' sets a timing precondition, both of which tell the agent when this tool is appropriate. No sibling alternatives or exclusions are named, so it falls short of a 5.

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