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

Annotations already declare this safe, deterministic and closed-world (readOnly, idempotent, openWorld=false), so the bar is lower. The description adds useful domain context — tax year 2026 and that the credit is reduced via an AGI-based phase-out — but omits the phase-out thresholds and how filing_status changes them, which is the main behavioral detail for this calculation.

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 tightly packed sentences with the purpose front-loaded and the cross-reference last. The explicit return-shape listing is slightly redundant given an output schema exists, but it costs little.

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 low-complexity, three-parameter idempotent calculator with full schema coverage, complete annotations and an output schema, the description is essentially sufficient. The only real gap is the unstated phase-out thresholds/income limits that determine the final figure, which an agent may want surfaced.

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 every parameter (agi, filing_status, children_under_17) is already documented in the schema, making 3 the baseline. The description only reinforces that agi drives the phase-out; it adds no syntax or threshold detail beyond the schema.

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 (Calculate), a specific resource (US Child Tax Credit), and pins the tax year (2026) plus the core mechanic (phase-out based on AGI). This distinguishes it cleanly from generic or other-jurisdiction siblings like calculate_us_federal_tax or calculate_belgian_income_tax.

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 tool's subject matter. The sole routing hint is 'See list_bundles for related finance-us calculators,' which points at a discovery tool rather than explaining when to pick this over adjacent US tax tools (calculate_us_federal_tax, calculate_us_paycheck). No when-not-to-use guidance.

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