User: Get_user_by_external_id

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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds the useful context that the key comes from a third-party system, but says nothing about not-found behavior or what identity fields are returned, which is a gap for an open-world lookup.

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?

One sentence, zero waste, and the disambiguating qualifier is front-loaded where the agent will see it.

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 one-parameter read tool whose annotations carry the safety profile, the definition is close to sufficient. It lacks any statement about the response shape or lookup failure, which is minor given the absence of an output schema and the tool's simplicity.

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?

With a single parameter at 100% schema coverage, the schema already documents externalId, and the baseline is 3. The description's parenthetical '(third-party system)' clarifies the id's provenance slightly beyond the schema's 'The user's external id', but adds no format or type guidance.

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 (Retrieve) and resource (user) with a disambiguating qualifier: lookup is by the external/third-party id, not the internal id used by Get_user. This implicitly separates it from Get_user and Get_user_by_credential, but no sibling is named explicitly, so it falls short of a 5.

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?

Usage is only implied: the agent infers it should call this when it holds an external-system id rather than an internal one. There is no explicit when-to-use statement, no exclusion of alternatives like Get_user or List_users, and no note on what to do if the id is unknown.

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