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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint, so the safety profile is covered. The description adds genuinely new behavioral context: a ±15% accuracy bound, CJK awareness, context-window coverage of 8k-1M, and the exact rate limit and payment/auth path (x402 via params._meta or X-API-Key credits) — an agent knows before calling that it may be throttled or charged.

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 capability statement is front-loaded in the first clause, followed by accuracy and then commercial terms. It is dense but every clause carries information; the payment/server parenthetical is slightly cluttered but not wasted.

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?

With an output schema present, return values need not be described, and annotations cover the safety profile, so what remains — accuracy, input scope, rate limits, payment — is largely present. It omits any input size limit for the text field, which is the one thing an agent could still trip over.

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?

There is a single required parameter with 100% schema coverage, so the schema carries the load and the baseline is 3. The description adds only that the input can be 'any text' with CJK handling, which is marginal meaning beyond the schema.

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 ('Estimate LLM tokens, words and characters for any text') plus a concrete accuracy band and CJK support, so the operation is unambiguous. It does not explicitly distinguish itself from close siblings like llm_cost_estimate or mcp_token_measure, leaving the agent to infer the boundary from names.

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?

Usage is only implied by 'for any text' and by the context-window fit mention; there is no explicit when-to-use versus when to prefer llm_cost_estimate. The pricing tiers tell the agent about cost exposure but not about tool selection.

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