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

Beyond the readOnly/idempotent annotations it discloses semantic caveats that materially change interpretation: 'Observations, not settlement tests,' 'Stale rows are last-known-good,' and 'Absent fees are not zero.' It also covers failure and cost behavior (503 without charge, ~1 s latency, USD 0.001, no free tier) and a prompt-injection warning about returned page text and on-chain strings.

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?

Front-loaded with purpose, then packed with short, load-bearing clauses; nothing is padding, and the caveats (stale rows, absent fees) each earn their place. It is telegraphic rather than elegantly structured, mixing transport, latency, price, and security into one run-on block, but it stays compact.

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?

An output schema exists, so return shape need not be described. Given a one-parameter read tool, the description still covers the things an agent could get wrong: transport, empty-result handling with billing implications, freshness semantics, latency, price, and untrusted-data handling.

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 the single `since` parameter already carries a full description (inclusive UTC YYYY-MM-DD, default all recorded changes). The description only restates that it is optional and date-based, adding no format or boundary detail the schema lacks, so the 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?

The description opens with a specific verb and resource: 'Compare x402 facilitator support and changes since a date,' and the title enumerates the dimension set (schemes, networks, signers, fee changes). That is enough to separate it from siblings like get_x402_bazaar_pulse or x402_seller_report, though it never explicitly names or excludes an alternative.

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 implied by 'changes since a date' and the optional `since` input, but there is no statement of when to prefer this tool over sibling history tools such as get_endpoint_history or agent_index_delta. The transport note ('GET query or POST JSON') is invocation mechanics, not selection guidance.

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