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

Beyond annotations (readOnlyHint false), the description discloses key behaviors: creates on first use (mutation), returns api_key and sender_address, explains idempotency (existing_account flag), and warns that failing to pass the api_key later will cause failures. All critical for safe invocation.

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?

The description is fairly long but every sentence adds value. It is front-loaded with purpose, then usage, then result details, then important warnings. Slight redundancy in explaining the domain format could be tightened, but overall well-organized and concise.

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?

Given the tool's complexity (1 parameter, nested, output schema present), the description fully covers the input context (terms_accepted must be true), the output structure (address, sender_address, api_key, existing_account, upgrade_link), idempotency, and critical api_key handling. Complete enough for an agent to use correctly.

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 input schema already describes both parameters (device_name, terms_accepted) with adequate detail. The tool description adds no further information about these parameters. Baseline 3 is appropriate since schema coverage is good and the description focuses on output and usage instead.

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 clearly states the tool's purpose: 'Get this user's Primitive managed inbox domain, creating the account on first use.' It specifies the resource (managed inbox domain) and action (get/create), and distinguishes from siblings by calling it first when no API key exists. No other sibling tool has this role.

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?

Explicitly tells when to use: 'CALL THIS FIRST whenever you do not already have a Primitive API key and the user wants to send or receive email, instead of telling them to sign up or visit a website.' Also warns against re-calling: 'calling again will not produce a different one.' Provides clear alternatives (signup/website) and context.

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