Reply to a thread

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

With no annotations, the description carries the full behavioral burden and does well: it discloses the 1-credit cost, that it sends a real email, that it only reaches the thread's participant from the conversation's mailbox, cannot add recipients, and that idempotencyKey guards retries. It stops short of stating auth requirements or what a successful call returns, so it falls just short of the top bar.

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 tightly packed sentences with zero waste. The core action and its constraints are front-loaded, and each subsequent sentence adds a distinct constraint (routing, cost, idempotency).

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 3-parameter mutation with no annotations and no output schema, the description covers the essential behavioral facts an agent needs (scope, cost, retry safety). It could say more about the response or irreversibility, but it is largely 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 coverage is 100%, so the schema already documents all three parameters including idempotencyKey's retry semantics and threadId's source. The description's mention of idempotencyKey largely restates the schema field rather than adding new meaning, so the baseline 3 is appropriate.

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 and resource (send a real email answering an existing thread) and immediately scopes it against siblings by noting it cannot start new conversations. An agent can distinguish it from send_email and create_sequence without opening any 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?

Explicitly names the when-not condition and the alternatives: 'use send_email or a sequence for new outreach.' The routing decision between reply versus new outreach is fully spelled out, leaving nothing to inference.

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