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

Annotations show this is a write operation (readOnlyHint false, idempotentHint false), and the description's "attach" aligns with that without contradicting it. It adds the context that the target is a test-injected email, but does not disclose side effects such as whether an existing attachment is replaced or how errors are handled.

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, front-loaded sentence that says what the tool does and the payload format. There is no filler or repetition.

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?

The tool is a simple add operation with a full input schema and an output schema, so the minimal description is largely sufficient. It lacks only an explicit statement that the mail must already exist, though the required mailId parameter implies this.

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 seven parameters. The description's only parameter-related phrase, "base64 payload," duplicates the data parameter's existing description, so it adds no new meaning.

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?

"Attach a file to a test-injected email" names a specific action and target resource, and the verb clearly distinguishes it from sibling get/list/delete tools. The base64 payload parenthetical adds the technical mechanism without blurring the purpose.

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 phrase "test-injected email" gives a clear context: this tool is for adding attachments to synthetic emails, not to arbitrary real mail. It does not explicitly name alternatives or exclusions, so it stops short of a 5.

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