What a diagnostic trouble code (P0420, U0100, …) means where the OBD-II standard defines it, and — the part no other source publishes — which vehicles' federal record actually names it: how many manufacturer service bulletins and owner complaints mention it, and on which model years.
| Name | Required | Description | Default |
|---|---|---|---|
| code | Yes | A trouble code, e.g. P0420. |
Changes observed during successful MCP inspections.
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already mark the tool as read-only, idempotent, and non-destructive, so the description only needs to add behavioral context. It does so by explaining exactly what data is returned: standard meaning, counts of bulletins and complaints, and affected model years. This goes beyond the annotations without contradicting them.
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 a single, information-dense sentence that front-loads the core purpose and then adds the unique value. The promotional aside 'the part no other source publishes' is slightly unnecessary, but the overall length is reasonable and the structure is clear.
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 one-parameter lookup with strong read-only and idempotent annotations, the description covers the main purpose and the shape of the result. It lacks an explicit output schema but effectively summarizes return dimensions. It could be more explicit about code formatting or validation, but the examples and schema description cover the essentials.
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% and the single parameter 'code' is already described as 'A trouble code, e.g. P0420.' The description reinforces this with additional examples and context, but it does not add significant new constraints or format guidance, so the schema can carry most of the semantic weight.
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 job: explain an OBD-II diagnostic trouble code and surface which vehicles' federal records mention it, with bulletin/complaint counts and model years. It is specific about the verb and resource, though it does not explicitly distinguish itself from sibling tools like lookup_bulletin.
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 implies that the tool is for looking up a trouble code's meaning and federal record mentions, but it gives no explicit guidance about when to prefer it over alternatives such as lookup_bulletin, decode_vin, or search_vehicles. There are no exclusions, prerequisites, or decision rules.
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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