List plans

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

Annotations already declare readOnly, idempotent, open-world. The description adds real behavioral detail beyond them: results are scoped to the current API key, and the 'last-used' value resolves to whichever plan was last opened in YNAB, a non-obvious runtime quirk.

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?

Front-loads the core action in the first sentence, then adds scope and a terminology note. The budget/plan synonym explanation is slightly redundant but earns its place by preventing misidentification of the resource.

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?

For a zero-parameter read tool with an output schema and full annotation coverage, the description supplies what the structured data cannot: scope of the listing and how to consume the returned plan ids. Nothing essential is missing.

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?

The tool takes zero parameters, so there is nothing for the description to disambiguate; baseline 4 applies. The 'last-used' note concerns arguments to *other* tools rather than this one.

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 ('List all YNAB plans') plus scope ('accessible with the current API key'). It also resolves the plan/budget terminology so the agent knows this is the budget-listing tool; no sibling overlaps on that resource.

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?

Gives clear downstream context: 'Use the plan id in subsequent tool calls,' plus an explicit caution about the 'last-used' shortcut being ambiguous with multiple plans. It doesn't frame this as the entry point versus any alternatives, but none exist among siblings.

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