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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so safety is covered. The description adds real value beyond that by disclosing the required credential and the workspace-claim requirement, including the 403 refusal for self-registered sandbox keys. It stops short of describing return shape or 404 behavior.

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?

Three short sentences, front-loaded with the core action and parameter, followed by prerequisites. No sentence is wasted, and the 403 detail is stated compactly.

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 read-only lookup tool, the description covers the parameter, authentication requirement, workspace eligibility, and the specific failure mode. No output schema exists, but the description supplies everything needed to decide whether and how to call it.

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 coverage is 100% for the single parameter, and its schema description already explains that external_id is the id from your own system. The description restates 'pass external_id' without adding format or validation details, so the baseline of 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 and resource ('Read one customer') and scopes it by external id ('the id your own system gives it'). That phrasing inherently distinguishes it from the sibling get_customer, which uses the internal customer id.

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?

Makes clear when to use it: when you have an external_id from your own system. It also states prerequisites (credential and claimed workspace) and the 403 failure mode. It does not explicitly name alternatives such as get_customer, but the context is otherwise unambiguous.

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