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

Annotations only declare readOnlyHint=true, so the description carries the real informational load — and it does, spelling out the exact definitional facets returned and the two conditional facets (1P `basis`, S&S `readingNote` including period reading and finality of a day). It does not cover behavior on an unknown or ambiguous key, which is the remaining gap.

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 sentence, front-loaded with the verb and resource, followed by a parenthetical that lists return facets — every clause earns its place and nothing is repeated from the schema or annotations.

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 convey the return shape, and it does so precisely for a two-parameter read tool whose inputs are fully specified in schema. Only the failure/ambiguity path for `key` is left unaddressed.

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 both parameters are thoroughly documented in the schema, including that `key` comes from list_metrics and that `reason` must be English and is user-visible. The description adds no parameter meaning beyond that, 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 gives a specific verb+resource ("Get the full definition of a single metric") and enumerates what that definition contains: meaning, unit, valid grains, 1P `basis`, and Subscribe & Save `readingNote`. It is clearly distinguishable from list_metrics by the word "single," though it never names the sibling explicitly.

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 only implied — an agent infers it should call this when it needs to understand what a metric means or how it reads. There is no explicit when-to-use, no exclusion, and no reference to list_metrics as the source of keys (that routing only lives in the schema's `key` description).

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