Reply in fmsg thread

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

The description adds rich behavioral detail beyond the annotations: immutability (cannot be edited or recalled), default recipient expansion, failure in terminal threads, refusal on no-reply unless allow_no_reply is true, secret redaction, and redaction count reporting. This substantially exceeds what readOnlyHint/destructiveHint alone convey.

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 information-dense but organized: core action first, then immutability warning, then recipient default, then failure modes, then redaction behavior. Every sentence adds non-redundant value and no filler is present.

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 destructive, non-idempotent, world-open tool with 8 parameters and an output schema, the description covers the essential operational nuances: when to call it, recipient behavior, failure conditions, and post-send redaction reporting. No critical behavior an agent would need to invoke it safely 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 only 38%, so the description must compensate. It explains recipients ('override the reply-all recipient set' is implied by 'narrow or widen') and allow_no_reply explicitly. Some parameters like no_reply and important are not elaborated, but they are relatively self-explanatory, and the essential behavioral parameters receive meaningful clarification.

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: 'Send an immediate reply to a message (linking it into that thread).' It clearly distinguishes this from sending a new message by emphasizing thread linking and immutable fmsg behavior, and the sibling list includes send_message, so the differentiation is effective.

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 description gives strong usage context: only send when the user has clearly asked, default recipients behavior, and failure conditions for terminal or no-reply parents. It does not explicitly name sibling alternatives like send_message, but the conditions for using reply are concrete enough for an agent to decide correctly.

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