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

Beyond the annotations (idempotentHint, destructiveHint=false), the description discloses three substantive traits: the idempotent merge behavior (existing senders untouched, only missing ones added), the all-or-nothing refusal when any sender is invalid, and the side effect of rescheduling a running campaign. These are exactly the behaviors an agent needs before invoking a mutating attachment.

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?

Four tight sentences, front-loaded with the core action, then escalating to idempotency, atomicity, and side effects. No filler, and each sentence carries distinct decision-relevant 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 a two-parameter mutation tool with annotations covering the safety profile and no output schema, the description covers purpose, idempotency, failure mode, and side effects. An agent has everything required to call it correctly.

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% and both parameters are documented there, so the baseline is 3. The description reinforces the constraint that every sender must be a connected workspace sender, but that validation rule is already in the schema; the added value (whole-call refusal) is more behavioral than parameter-specific.

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 opens with a specific verb and resource ('Attaches connected sender email accounts to a campaign') and immediately clarifies the effect ('this call decides which mailboxes it uses'). It is trivially distinguishable from the sibling detach_campaign_sender_email by direction of the operation. Nothing about the purpose is left ambiguous.

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?

It gives clear operational context ('a campaign only sends through the senders attached to it') that tells the agent why and when this call matters. It does not, however, explicitly name alternatives or state when-not to use it (e.g., how it relates to connect_sender_email or create_campaign), so it stops short of full when/when-not guidance.

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