Extract the verification code from an email

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

With no annotations, the description notes two behavioral facts: 'Requires a paid API key' and the no-guessing policy: 'If no code is found with confidence, that is reported rather than guessed, because a wrong code is worse than none.' This gives the agent expectations about authentication prerequisites and failure behavior, but doesn't cover whether the operation is strictly read-only or how the no-code result is returned.

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 sentences, no filler: action, usage context, key requirement, and failure behavior. The most important information is front-loaded. Every sentence earns its place.

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 two-parameter read-like tool with high schema coverage, the description includes the trigger, required auth, and failure mode. It does not specify the output structure or edge cases like multiple codes, but these are minor given 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?

The schema already covers both parameters with descriptions ('The inbox address'; 'Message id from wait_for_email or list_messages'). Description reiterates the wait_for_email relationship but adds no new format, constraints, or semantics beyond the schema, so baseline 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?

The description opens with the specific action 'Pull the verification or one-time code out of a message,' naming the verb, resource, and result. This clearly distinguishes it from siblings like list_messages or read_message by focusing on extraction of a code rather than retrieving whole messages or managing inboxes.

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?

It gives an explicit invocation condition: 'Use this after wait_for_email when the user needs a code to complete a signup or login.' This tells the agent when to call it relative to wait_for_email. However, it does not name two or more alternatives or state when not to use other tools like read_message, so it falls short of a full when/when-not guide.

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