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

readOnlyHint=true already establishes the safety profile, and the description adds the scoping behavior for user_id plus the underlying POST endpoint. It says nothing about pagination, ordering defaults, or result volume, so the added behavioral context is real but thin.

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 tight sentences, front-loaded with the resource definition and followed by the one actionable constraint. The trailing endpoint path is marginally useful and slightly offsets the otherwise lean structure.

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 read-only, non-destructive query tool with full schema parameter coverage and no output schema, the description covers the essentials and adds the non-obvious scoping rule. Remaining gaps (pagination, default sort) are minor given the annotations and schema.

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 100%, so sort, limit, user_id, and reply_limit are all documented in the schema and the baseline is 3. The description does add meaning for user_id by explaining why server-side callers must pass it (read-state scoping), but contributes nothing for the other three parameters.

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 ('List threads') and usefully clarifies that a thread is a message with replies, which distinguishes it from stream_get_message. It does not explicitly contrast with the closest sibling, stream_get_replies, so it stops short of 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?

Provides one concrete usage rule: server-side callers should pass user_id to scope read-state. That is implied-usage guidance rather than an explicit when-to-use/when-not framing, and it never tells the agent when to prefer stream_get_replies or stream_search_messages over this tool.

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