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

Annotations already mark readOnlyHint=true and destructiveHint=false, so the read-only nature is known. The description adds value by disclosing the exact output fields, the meaning of the 'kind' values (applied vs. scheduled), and the auth/scope requirement. No hidden behaviors are left uncovered.

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?

Two sentences, densely informative with zero filler. The first sentence front-loads the action and output, the second covers auth. Every clause adds value, and the parentheticals clarify the 'kind' enum without bloating the text.

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?

Given the simple single-param schema and no output schema, the description fully compensates by listing all return fields and their semantics. It also includes prerequisite auth details, making it complete for an agent to invoke and interpret results correctly.

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%: the only parameter 'plan' has an enum and a description ('Target plan to preview a switch to.'). The tool description merely refers to 'this plan' without adding extra format or semantics beyond the schema. Baseline 3 is appropriate.

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 begins with a specific verb-resource pair ('Preview what would happen if change_plan were called'), clearly distinguishing this from the actual sibling tool change_plan. It also enumerates the exact output fields (immediate-charge amount, next-invoice amount, effective date, kind), fully specifying its purpose.

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 phrase 'if change_plan were called' establishes the tool as a preview companion to change_plan, implying usage before making a change. It also states prerequisites (Owner-only, billing scope token). It does not explicitly name alternatives or exclusion cases, but the context and sibling list make the intended use clear.

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