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

Annotations only declare readOnlyHint=true, so the description carries the behavioral burden and does so well: default scope is the user's own messages, `senders` expands to teammates, non-sharing teammates or out-of-workspace emails are silently dropped, and a shared agent is scoped to that agent's campaign. It stops short of describing pagination edge cases, but the filtering semantics are transparent.

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-loads the one-line scope summary before use cases, and the return shape sits in a separate <returns> block rather than bloating the prose. It is a bit long, but each sentence (scope, senders behavior, tagging use cases, shared-agent caveat) carries distinct information.

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 an 8-parameter, no-required-arg list tool with no output schema, the description is complete: it defines the result set, the default filtering behavior, and the full return shape (totals, counts, per-message fields, nested tags/groups/outcome) with clarifying notes on `sender`, `groups`, and `outcome`. Nothing an agent needs to call it or interpret the response is missing.

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 baseline is 3, but the description adds value beyond the schema: it motivates `tag_state='untagged'` for finding untaggable items, explains why `senders` may yield an empty list, and ties `tag_id` and `q` to the tagging workflow. The added context is meaningful rather than redundant.

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?

Names a specific verb and resource with explicit scope: 'Every message the user sent to a prospect, tagged or not, each with its tags from every tag group.' It anchors this to the Analytics → Messages tab, so an agent knows exactly what set this returns and can distinguish it from tagging/mutation 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?

Gives concrete when-to-use cases (find untagged messages via tag_state='untagged', read tags across groups, locate a message to tag) and routes to the sibling correct_message_classification. It does not, however, differentiate from the closely related query_tagged_messages sibling, so the routing guidance is not exhaustive.

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