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

Annotations declare a non-read-only write with openWorldHint=true and destructiveHint=false; the description adds a real constraint beyond them — no new threads, older accepted threads only — and clarifies the key travels in the JSON body rather than the path. It does not describe failure modes, auth requirements, or rate limits, so it falls short of a 5.

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 critical routing information is front-loaded and no sentence is wasted, but 'No new threads' partially repeats 'older accepted threads only', and the body-vs-path remark duplicates the schema.

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 2-parameter write tool with no output schema, the description covers usage, the key distinction from the sibling, and the negative constraint. It omits what a successful write returns and any error/permission behavior, leaving a small but real gap.

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 both parameters are already documented. The description's note that the thread key goes in the JSON body restates what the schema property description already says, adding no new meaning. Baseline 3 applies.

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 ('Write the next message on older accepted threads') and immediately scopes it against the sibling write_first_reply. An agent can distinguish this tool 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 gives the when ('you already have the thread key for an older accepted thread and need to send the next line'), the when-not ('Do not use this for a first reply'), and the named alternative 'write_first_reply'. 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.