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

Beyond the readOnlyHint/openWorldHint annotations, the description discloses that the tool is paid, absent from MCP, returns no analysis data, and gives the concrete endpoint (POST /v1/labels/analyze/diff), pricing (0.01 USDC per request) and payment rail (x402 on Base). This is unusually rich behavioral context that annotations cannot convey, and nothing contradicts the read-only hint.

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 with the two most decision-relevant facts (paid, not available via MCP) front-loaded in parentheses. There is mild redundancy between 'each detected substance carries the cross-axis derivation' and 'This tool does not return that data', but the clarification is warranted given the counterintuitive behavior.

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 no output schema, the description carries the burden of explaining what the agent gets back, and it does so fully: not the data, but the endpoint URL, request shape and cost. Combined with 100% schema coverage on inputs, nothing an agent needs in order to call and act on this tool is missing.

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 text, axis and lang are already fully documented in the input schema, which sets the baseline at 3. The description only adds that the input matches analyze_label, which is useful for parity but not new per-parameter meaning.

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 definition states exactly what the tool is (a routing shim that returns an HTTP endpoint) and how it relates to its sibling analyze_label: same input, but the paid variant carries cross-axis derivation. An agent can tell without opening either schema that this tool does not perform the analysis itself and what it returns instead.

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 clearly signals the condition that selects this tool over analyze_label (you need cross-axis derivation) and warns that it is unavailable through MCP, which is critical for invocation decisions. It stops short of an explicit 'do not use this if...' exclusion or pointing to alternatives like get_substance_diff.

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