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

Annotations already declare readOnlyHint, idempotentHint, and openWorldHint=false, so safety is covered structurally. The description usefully adds that this is a public library requiring no account, and it enumerates the response content (meaning, severity, drive-safety, probable causes, recommended tests, canonical URL), which is non-obvious behavioral information.

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?

Two sentences, well front-loaded with the action and resource before the return-value list. Slightly dense in the return enumeration, but every clause conveys information an agent would otherwise lack.

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 no output schema, the description compensates by listing exactly what is returned, and it notes the no-account requirement. For a single-required-param read tool with full annotation coverage, an agent has everything needed to call it correctly; only sibling routing is missing, which is scored elsewhere.

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 the schema already documents 'code', 'lang' (with enum), and 'engine_family'. The description restates the code format examples from the schema but says nothing about 'lang' or 'engine_family' semantics, so it adds no meaning beyond the schema. 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 states a specific verb and resource ('look up a diesel truck fault code') and enumerates the accepted code formats (J1939 SPN/FMI, OBD-II). It does not, however, differentiate itself from overlapping siblings such as explain_fault_code, search_fault_code_library, buscar_codigo, or es_seguro_manejar, which appear to cover the same ground.

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?

'Look up a fault code ... in DERATE's public fault-code library' implies the usage context, and 'Works without any account' hints at when it's viable (unauthenticated lookup). But it never states when to prefer this tool over explain_fault_code, buscar_codigo, or search_fault_code_library, which is a real selection risk given the crowded sibling set.

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