Reply in fmsg thread

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

The description goes well beyond the annotations. It discloses immutability ('fmsg messages are immutable: once sent they cannot be edited or recalled'), which aligns with and deepens destructiveHint=true; it details the reply-all default recipient set and how recipients overrides it; and it surfaces failure cases (terminal parent, no-reply refusal, secrets redaction). This is rich behavioral disclosure for a write tool. No contradiction with the annotations.

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 four sentences, longer than minimal but tightly packed: purpose comes first, then immutability, usage authorization, recipient behavior, then failure modes. Each sentence earns its place and the most decision-relevant facts are front-loaded. It could be trimmed slightly, but it reads efficiently and is well-ordered.

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 complex, destructive tool with 8 parameters and a real failure surface, the description covers the essential operational behavior: what it does, the immutability consequence, the default recipient scope, when it fails, and secret handling. With an output schema present, return values need not be explained. The remaining gap is individual param semantics (body, type, important, attachments), which the schema does not fully cover, so completeness is strong but not total.

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 low (38%), so the description carries part of the burden. It does add meaning for two params — recipients ('pass recipients to narrow or widen that') and allow_no_reply ('a parent marked no-reply is refused unless allow_no_reply is true'). But body, type, important, and attachments have no schema description and no description compensation, and body is a required field. The description partially compensates but leaves meaningful gaps.

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 states a specific verb and resource: 'Send an immediate reply to a message (linking it into that thread).' This clearly identifies the action and its scope, and the parenthetical distinctively marks it as a thread-linked reply. However, it does not explicitly differentiate itself from the sibling send_message tool, which could plausibly overlap in an agent's decision, so it falls just short of 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 description provides real usage context — 'Send within the user's requested task or authorized automation' — and documents failure conditions (terminal parent, no-reply refusal). But it never names an alternative tool or a 'use X instead' routing, despite having a closely-related sibling (send_message) that an agent would need to distinguish against. The guidance is present but not contrastive.

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