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

With no annotations provided, the description carries the full behavioral burden and largely succeeds. It discloses the full return composition and, notably, a subtle behavioral trait: 'Those action fields are empty when the value follows an underlying cause rather than lifestyle, which is itself the answer' — teaching the agent that empty fields are meaningful, not an error. It stops short of stating error behavior for unknown slugs or explicitly confirming read-only semantics, though 'Return' implies it.

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?

A single front-loaded sentence that states the core operation first, then enumerates the return sections, then delivers the essential empty-fields caveat. Every clause earns its place; there is no repetition of schema content or filler.

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?

Despite having no output schema and no annotations, the description compensates well by enumerating what the explainer contains, effectively substituting for return-value documentation. The behavioral caveat about empty action fields adds completeness. The only gap is error behavior for an invalid or unknown slug, which is minor for a simple 2-parameter lookup tool.

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 the schema already documents slug ('Mediora canonical slug, e.g. hba1c, ferritin') and lang ('Language code. Defaults to en.') fully. The description's 'by slug' adds no parameter meaning beyond what the schema provides, 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+resource+method: 'Return the full explainer for a single blood-test marker by slug.' The 'single blood-test marker' scoping cleanly distinguishes it from siblings explain_condition, explain_panel, and explain_symptom by entity type, and the detailed contents list ('what it measures, what high/low values mean...') leaves no ambiguity about what the tool produces.

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?

Clear context is provided: the tool is for a single marker, identified by canonical slug, and the schema's slug parameter explicitly points to mediora__list_markers / mediora__list_conditions as the discovery route. However, the description itself does not name exclusions or alternatives (e.g., 'for panels use explain_panel'), so routing is implied rather than explicit.

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