lookupLook up a USPS code: its row as USPS prints it in the newest edition that lists it, the codes its cells name, the editions that list it and its history, each row with its passage id.
| Name | Required | Description | Default |
|---|---|---|---|
| code | Yes | A code as USPS prints it: 055, 920, PM, 01. | |
| table | No | One table to look in: stc, extra-services, mail-classes, ptr, ptr2 or stid. Every table when absent. |
| Name | Required | Description | Default |
|---|---|---|---|
| matches | Yes |
Changes observed during successful MCP inspections.
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnly, idempotent, closed-world, and non-destructive, so safety is covered. The description adds real behavioral detail beyond that: results are drawn from the newest edition listing the code, alternate editions and history are included, and every row carries a passage id. It stops short of noting result size or ordering 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?
A single dense sentence, front-loaded with the verb and resource, then the return payload. Every clause carries information; nothing is redundant with the title or schema.
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?
With an output schema present and full parameter documentation, the description only needs to frame intent and behavior, which it does. The gap is the absence of routing guidance among the six sibling tools for what appears to be a core entry point.
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 description coverage is 100%, so both 'code' (format examples 055, 920, PM, 01) and 'table' (enumerated table names, all tables when absent) are fully documented structurally. The description adds the notion of 'newest edition' but no additional parameter-level semantics, so the 3 baseline holds.
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 ('Look up a USPS code') and enumerates the payload returned (the printed row, named codes, listing editions, history, passage ids). What it does is unambiguous, but it never distinguishes itself from sibling tools like decode or define, which plausibly operate on the same codes.
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?
There is no statement of when to use this instead of decode, define, diff, or search. The only scoping hint is inherited from the schema's 'table' parameter, not from the description, so an agent must infer that lookup is the by-code entry point and search is the free-text one.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.
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