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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the bar is lower; the description still adds real behavioral context by stating that the signature is appended server-side on send/reply/forward/draft/scheduled messages and that source can be null when unset. It does not cover pagination or error behavior, but for a single-object read that is minor.

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?

Front-loaded with the core action in the first clause, and the second sentence about server-side appending is genuinely useful. The middle enumeration of return fields is somewhat redundant given an output schema exists, but it is compact and not padded.

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 annotations covering safety and an output schema covering return shape, the description's job is largely done; it goes further by enumerating key return fields and explaining where the signature is applied. The only real gap is explicit routing guidance versus siblings like signature_set.

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 both inbox and inbox_id are already documented as alternatives and inbox_id as optional with a single inbox. The description adds no additional parameter semantics, which is the expected baseline when 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?

States a specific verb+resource ('Read the email signature configured for an inbox') and immediately distinguishes itself from the sibling signature_set, which is the write counterpart. An agent can tell what it does and what it does not do without opening the schema.

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?

Usage is implied by the verb 'read' and the sibling naming (signature_set), but there is no explicit 'use this when...' statement, no mention of prerequisites, and no guidance on the two identifier alternatives beyond what the schema already says. Minimum viable.

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