lookup_merchant

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

No annotations are provided, so the description carries the full burden. It discloses that this is a 'probe=false' operation (likely a read-only check) and indicates it returns security-relevant fields (pass, score, payToMismatch, tier), which suggests it may affect payment routing. However, it doesn't state whether it makes any external calls, has side effects, or requires authentication. For a tool that gates payments, this is a notable gap, but it does provide some behavioral hints beyond the minimal.

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 two sentences long, front-loads the critical 'DEFAULT before pay' warning, and includes a clear instruction. Every sentence contributes: the first defines the action and output, the second specifies when to use. No wasted words.

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 single-parameter lookup tool with a well-documented schema, the description is nearly complete. It explains the purpose, key return fields, and when to call it. It doesn't explain error handling or the meaning of 'profile URL', but given it's a verification check before payment, an agent likely needs this. An explicit note about what 'pass' means would be helpful but is a minor gap.

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 description for the only parameter (origin) is already detailed, covering URL, domain, or slug with examples. The description adds context by saying it's for 'merchant origin/slug' but doesn't add meaningful new semantics beyond what the schema covers. Given 100% schema coverage, the baseline of 3 is appropriate; the tool adds minimal extra value.

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 action: 'look up a merchant origin/slug on Cleared Index via Check'. It specifies the resource (Cleared Index) and the method (Check), and lists key return fields (pass, score, payToMismatch, tier, profile URL). This is distinct from siblings like 'cleared_check' and 'list_cleared_services', as it focuses on merchant lookup before payment.

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?

The description explicitly states 'Call this before any x402 payment to an unknown merchant', which gives strong usage context. However, it doesn't explicitly mention alternatives or when not to use this tool. The context is clear enough for an agent to know when to use it, but it lacks exclusions (e.g., 'use cleared_check for ...') to fully distinguish from similar siblings.

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