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

Annotations already declare readOnlyHint=false and destructiveHint=false, so the write-safety profile is covered. The description adds one genuinely useful behavioral detail the annotations cannot: when owner is omitted the caller key hash is used. It omits duplicate-commit behavior, overwrite semantics, and failure modes, which matter for a write tool.

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 core action front-loaded and optional-parameter notes after it. No padding, though 'scope (max 64)' restates schema maxLength rather than earning its place.

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?

A four-parameter mutation tool with no output schema and zero schema description coverage needs more than this: the meaning of result_ref and the identity/format of key are never explained, and the outcome of a successful commit (what gets stored, when it is reused by idem.begin) is left implicit.

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 0%, so the description carries the burden. It covers scope (including the max-64 constraint, which merely repeats the schema) and the owner default, but says nothing about the two required parameters, key and result_ref, leaving their format and meaning undocumented anywhere.

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?

States a specific verb and resource: 'Store the outcome reference for an idempotency key.' An agent can distinguish it from idem.begin by the commit-vs-begin naming, but the description never explicitly contrasts the two phases, so the differentiation is by inference rather than statement.

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 by the name and the paired sibling idem.begin. There is no explicit 'call this after idem.begin' sequencing, no note on whether committing an already-committed key is an error or an overwrite, and no mention of alternatives.

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