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

Beyond the readOnly/openWorld annotations, it discloses the critical behavior that this tool does NOT return the diff data but instead returns the endpoint (GET /v1/substances/{id}/diff), along with cost (0.01 USDC per request) and the payment rail (x402 on Base). This is exactly the kind of non-obvious operational context an agent cannot get from the schema or annotations.

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 paid/HTTP-only warning is correctly front-loaded in parentheses. The first sentence is dense and describes data the tool does not itself return, which costs some clarity, but it still earns its place by telling the agent what the paid endpoint would yield before routing them there.

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 must explain the return value, and it does state that the response is the HTTP endpoint rather than the diff. It could be more explicit about the exact response shape (bare URL vs. structured field), but for a 1-parameter routing tool the essential context is present.

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 'id' parameter with 100% schema description coverage, including format examples, so the schema carries the semantics. The description adds nothing about the id beyond the {id} path placeholder, matching the baseline-3 case where the schema does the heavy lifting.

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 name/title and description clearly state that this retrieves the cross-axis derivation for one substance (all axes other than the substance's own), and the second sentence uniquely clarifies that the tool returns an HTTP endpoint rather than the data itself. That non-obvious distinction separates it from siblings like get_substance and analyze_label_diff, though the purpose sentence is wordy and indirect.

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 implies when to use it (you want the per-axis derivation with source separation and refusals) and flags it as paid/HTTP-only, but it never explicitly contrasts it with get_substance or analyze_label_diff, nor states conditions under which an agent should prefer those siblings instead.

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