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

Even though readOnlyHint already communicates read-only behavior, the description adds useful behavioral nuance: probes occur on a fixed clock, every reading is tied to a retained capture, and blocked/rate-limited endpoints are distinguished from unreachable ones at capture time, so defended endpoints are never reported as dead. This is property information an agent cannot infer from the annotations alone.

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 first sentence front-loads the purpose and the later sentences add meaningful contextual guidance, but the language is deliberately embellished ('Machine Commerce Observatory', 'alive right now, quoting what, and how fast?'). Every sentence earns its place topically, yet the style is more promotional than a plain description would require for a one-parameter read-only tool.

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 single-parameter, read-only history lookup, the description covers what the data includes (liveness, quote, speed), how it is collected (fixed clock, retained captures), and what the classification means (blocked vs unreachable). The output shape is not specified, but with no output schema and a simple fetch-by-id operation, the provided context is sufficient for an agent to call it correctly.

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 lone parameter, endpoint_id, is already documented as 'probe402 endpoint id, from the free /coverage list'. The description only refers to the endpoint in prose and does not add new format, source, or fallback semantics, so it meets the baseline without exceeding it.

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 states a specific capability: it returns independent historical readings of x402 and MPP endpoints, with liveness, quote, and response speed, all traceable to a retained capture. This goes beyond a generic 'get history' and distinguishes the tool from its sibling probe402_endpoint_grade, which is a grade rather than a temporal record. An agent can tell what this tool is for without opening the 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?

The description gives concrete usage triggers: call before paying an unfamiliar endpoint and to check whether a known endpoint has repriced or rotated its payTo. It does not explicitly name the alternative probe402_endpoint_grade or state when to prefer that sibling, so it misses a crisp exclusions, but the when-to-use guidance is clear.

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