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

The description goes beyond annotations by providing exact formulas for series and parallel calculations, and it explicitly states 'ANTI-FABRICATION: exact closed-form. Quote verbatim.' This communicates that the tool performs deterministic, exact math and should not be approximated. The readOnlyHint and idempotentHint are consistent, so no contradiction exists.

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?

The description is compact and efficient. Each sentence serves a purpose: defining inputs/output, giving formulas, citing use cases, explaining composition, and setting anti-fabrication expectations. No fluff or redundant restatement of schema fields.

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 two-parameter calculator with rich schema and annotations, the description covers all necessary operational context: what it computes, how it computes it, when to use it, how to handle mixed structures, and that it returns an exact value. The lack of an output schema is mitigated by the explicit formula, which implies the return type is a numeric reliability.

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?

Even though the schema already fully describes parameters (100% coverage), the description adds mathematical meaning: series = product, parallel = 1 − product(1−Rᵢ). This clarifies how the 'components' and 'structure' are used in the calculation, and it reinforces that component order is irrelevant. This is meaningful semantic value beyond the schema's property descriptions.

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 opens with a clear, specific statement: 'Given per-component reliabilities and a structure (series or parallel), return the system reliability.' It ties directly to the tool's name and provides an unambiguous resource (reliability calculation). It further distinguishes itself from broader RBD tooling by positioning itself as a 'back-of-envelope' calculator, which sets expectations.

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?

Explicit guidance is given: 'Useful for back-of-envelope RBD calcs before reaching for full RBD tooling' states when to use it, while 'For mixed-structure systems (series with parallel sub-blocks), call this tool repeatedly on the sub-blocks' prescribes how to handle larger structures. This is a clear directive with a practical decomposition strategy.

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