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

Annotations already declare readOnlyHint=false, destructiveHint=false, idempotentHint=false, and openWorldHint=true, so the description doesn't need to restate those. The description adds minimal behavioral context beyond the annotations—it doesn't mention whether attaching a phone number overwrites existing numbers, whether duplicates are allowed, or what the response contains. The output schema exists, which covers return values, but the description itself adds little behavioral detail.

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, concise sentence that front-loads the core action. It is appropriately sized for a simple add-operation tool, though it could have used the space to add a note about behavior (e.g., whether multiple phone numbers are allowed).

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 mutation tool with a full output schema and 100% schema parameter coverage, the description is mostly adequate. However, it lacks guidance on edge cases like whether the phone number replaces existing numbers or is appended, and it doesn't clarify the relationship to sibling add-* tools. The annotations cover safety profile, so the main gap is behavioral nuance.

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 all three parameters (phone, extension, subjectId) with descriptions. The tool description adds no additional parameter meaning beyond what the schema provides. Baseline 3 is appropriate because the schema does the heavy lifting.

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 'Attaches a phone number to an SMI subject' clearly states the verb (attaches), resource (phone number), and target (SMI subject). It distinguishes itself from sibling tools like smi_add_subject_email or smi_add_subject_address by naming the specific data type being added, though it doesn't explicitly contrast with those siblings.

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 usage context: it is for attaching a phone number to a subject, and the sibling list shows many other add-* tools for different data types. However, it does not explicitly state when to use this tool versus alternatives, nor does it mention any prerequisites like needing a prior smi_get_subject call (though the subjectId parameter description hints at this).

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