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

The annotations already declare that this is a non-destructive write, idempotent, and closed-world. The description only adds a precondition hint: the message must be visible in your firm's mailboxes and the client must be one you're allowed to change. It says nothing about what happens if the email is already linked to another client, or what a failed link returns. This adds some value over the annotations but not much.

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?

One sentence, and the action is stated up front. The relative clause 'that you may change' is awkward and adds little clarity, but the description is otherwise tight and earns its space.

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?

Because an output schema exists, the description doesn't need to explain return values, and the annotations cover the safety profile. What is missing is the meaning of the identifiers, how duplicates or conflicts are handled, and the purpose of the optional firm_id. That makes it adequate for a simple write tool but not complete.

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 0%, so the description has to carry the full burden. It only indirectly maps 'email message' and 'client' to message_id and client_id, and it never mentions firm_id at all. Format expectations and constraints for the three identifiers are undocumented in both the schema and the description.

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 gives a specific verb and resource pair: link an email message to a client. That is enough to separate it from siblings like get_email_messages or get_email_detail, which only read. The trailing phrase 'that you may change' is ambiguous and reads like a fragment, which keeps this from being a 5.

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 use case is implied: you have an email visible in your firm's mailboxes and you want to associate it with a client. The description never states when to use this instead of another path, or any prerequisites like the email or client needing to exist and be accessible. The intent is inferable but there is no explicit guidance.

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