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

Annotations already cover the safety profile (readOnlyHint=false, destructiveHint=false, idempotentHint=false), so the bar is lower. The description adds genuinely non-obvious context the annotations cannot express: the call executes under the connection owner's identity and is unavailable outside a personal connection. It stops short of saying whether the thread notifies anyone or what the response contains.

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?

Two sentences, purpose front-loaded, with no filler. The second sentence is slightly indirect ('so the answer lands where the question was asked'), though that rationale does justify the alternative-tool rule rather than being decorative.

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 7-parameter mutation with strong schema coverage and no output schema, the description covers the critical unknowns: identity of the actor and the availability constraint. Minor gaps remain — whether creating a thread notifies mentioned people, and what the call returns — but nothing blocks correct invocation.

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 86%, so the schema already documents nearly every parameter, including the 'me' convention for assignee and mentions. The description only reinforces the identity framing ('as the person whose connection you are using'), which subtly explains what 'me' resolves to but adds little else beyond the schema. 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 concrete verb and resource — 'Open a new thread on a page' — and immediately pins down the acting identity ('as the person whose connection you are using'). It explicitly separates itself from the sibling post_update, so an agent can route between them 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?

Gives an explicit availability constraint ('Only available on a person's own connection') and names the alternative tool with the condition that selects it ('an agent reporting what it found or changed uses post_update on the thread it was working'). That is a complete when-this/when-that routing rule.

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