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

Annotations already declare it is a non-read-only, non-destructive, open-world write. The description adds the sequencing context ('next message'/'next line' implies an existing thread) and notes the key travels in the body, but does not disclose error behavior, the empty/over-500-char drop behavior, or any rate limits. With annotations carrying the safety profile, this is an adequate-but-shallow addition.

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?

Three short sentences, front-loaded with the core action, then usage conditions, then a placement caveat. Every sentence carries information; no padding.

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 simple two-parameter write tool with no output schema and full annotation coverage, the description supplies purpose, routing to the sibling, and key placement. It does not explain what a successful write returns or the thread-state prerequisites, but those are minor for this tool's complexity.

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% and both parameters are documented, including the 500-character drop rule in the schema itself. The description's note about the thread key being in the JSON body is largely redundant with the schema's own description. Baseline 3 applies when the schema does the heavy lifting.

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+resource ('Write the next message on a thread') and clearly distinguishes itself from the first-reply case. An agent can tell it apart from write_first_reply without opening either 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 condition for use ('already have the thread key after accept') and the exclusion ('Do not use this for a first reply - use write_first_reply'), pointing to the correct alternative by name. Nothing is left to inference.

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