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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is covered structurally. The description adds nothing beyond that: no indication of result shape, ordering, pagination, or behavior for a user with zero passkeys or an unknown user id.

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?

It is very short with no wasted words, but it is a sentence fragment lacking a verb and front-loads nothing useful. Brevity here comes from under-specification rather than efficiency.

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 simple one-parameter read tool with annotations and full schema coverage, the bar is low, but there is no output schema and the description says nothing about what is returned (passkey metadata, credentials, empty list) — an agent cannot tell what a successful call yields.

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% and the single parameter's meaning ('Your own id for the user') is fully documented in the schema. The description only echoes the parameter's existence ('one user id'), adding no format or scope detail. Baseline 3 is appropriate.

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 is a noun phrase that essentially restates the title 'List a user's passkeys' — it adds only the scope 'for one user id'. No verb, and no differentiation from siblings such as list_devices or get_authenticator, which an agent could easily confuse with passkey/authenticator lookups.

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?

There is no guidance on when to use this versus list_devices, get_authenticator, or remove_authenticator. The only implied usage is 'given a user id, get their passkeys', which the agent could infer from the name alone.

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