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

Annotations already declare readOnly, idempotent, non-destructive behavior, so the description only needs to add operational context — and it does: newest-first ordering, a 20-post page cap, a 1-credit cost, and plan-level authorization limits including the trial-key failure mode. It stops short of describing pagination behavior beyond the cursor parameter.

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, each carrying distinct information: purpose and ordering first, then page size, then cost and access constraints. No filler or repetition of the name.

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 read-only list tool with no output schema, the description covers scope, ordering, page size, cost, and access restrictions — enough to call it correctly or decide not to. The only gap is signing-posting guidance around the cursor, which the schema partially covers.

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 each parameter already carries its own description, so the schema does the heavy lifting. The description adds no parameter guidance, notably omitting the anyOf rule that either username or user_id is required, so 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 specific verb and resource ("List an X user's posts") plus result ordering ("newest first"). It is clearly distinguishable from siblings like x_search or x_comments by the resource named, though it does not explicitly contrast itself with them.

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?

It gives eligibility guidance (paid plans and pay-as-you-go only, trial keys error out) which helps an agent avoid calling it in the wrong account context. However, it never says when to prefer x_search or x_profile over this tool, so the when-to-use-vs-alternative dimension is only implied.

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