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

Annotations already declare readOnlyHint=true and openWorldHint=false. The description adds context beyond these: cost (free, pure CPU), no network, sub-millisecond performance, static nature, and the type of data returned (name, RFC anchor, category, etc.). It does not contradict annotations, and the added speed/cost details are valuable.

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 efficiently structured: a cost/read-only tag, a single-sentence purpose, a clear 'USE FIRST' directive, and a compact paired-tools list. Every sentence serves a distinct purpose without verbose padding.

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 simple, single-parameter, read-only lookup with no output schema, the description is complete. It explains what the tool returns, when to use it, and how it relates to other tools. No additional behavioral or return-value detail is necessary.

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 fully documents 'code' with type, range (100-699), and an example. The description adds marginal value by mentioning that the code maps to name/RFC category and gives a vendor-specific example (OpenSIPS 484), but these do not significantly extend the parameter's meaning beyond the schema's coverage.

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 uses a specific verb and resource: 'Instant static lookup of a SIP response code (100-699).' It clearly states the tool's scope and differentiates from siblings like troubleshoot_response_code (RAG hits) and lookup_sip_header (header lookup).

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?

Provides explicit guidance: 'USE FIRST when the user pastes or asks about any 3-digit SIP code.' It also lists paired tools with specific conditions (e.g., lint_sip_request for 4xx with offending request, validate_stir_shaken_identity for 438 with JWS), which fully covers when to use this tool vs alternatives.

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