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

Annotations already mark the tool as read-only, and the description adds cost and network behavior ('pure CPU, no network') and performance ('sub-millisecond'). It does not cover error handling or result formatting, but the added context is useful beyond annotations.

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 well-structured with a cost banner, core functionality, usage guidance, and pairing suggestions. Every sentence serves a purpose, and the length is appropriate given the lack of an output 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 only one parameter and no output schema, the description fully enumerates the return fields, gives use-case examples, and points to complementary tools. It is comprehensive for the tool's complexity and leaves no critical gap for an agent deciding whether to invoke it.

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 already describes the name parameter thoroughly, including case-insensitivity and examples. The description reinforces this with additional examples of canonical and compact forms, adding practical context without redundancy.

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 opens with 'Instant lookup of a SIP header' and provides concrete examples, making the action and resource unmistakable. It distinguishes itself from sibling tools like lint_sip_request or search_sip_docs by focusing on the definitional lookup use case.

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?

It explicitly states 'USE FIRST when the user asks about a specific header they saw in a trace' and names specific alternative tools ('lint_sip_request', 'search_sip_docs') for checking or vendor-specific headers. This gives clear when-to-use and when-to-defer guidance.

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