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

Annotations already mark the tool as readOnly and idempotent, but the description adds meaningful behavioral details: the default resting heart rate of 80 bpm, the ability to override it, and the specific output fields (heartbeats, breaths, minutes alive, days alive). This explains underlying assumptions without contradicting annotations.

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 three well-structured sentences: purpose, default assumption, and return values. Every sentence adds necessary information with no redundancy, and the main purpose is front-loaded.

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 simple stateless calculator, the description combined with the input schema and output schema provides complete context. It covers inputs, defaults, and returns, and edge cases like birthMs <= nowMs are handled in the schema. No critical information 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 input schema provides 100% parameter description coverage, including defaults and constraints for birthMs, nowMs, restingBpm, and breathsPerMinute. The description only reiterates the 80 bpm default and override capability, adding no extra parameter-level semantics.

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 clearly states the tool's function: 'Estimate the total number of heartbeats a person has had since birth' with specific verb (estimate) and resource (heartbeats since birth). It also specifies inputs (DOB, now timestamp) and outputs, distinguishing it from sibling date calculators like days-between.

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 description gives clear context for when to use the tool (given birth date and reference timestamp), but does not explicitly mention exclusions or alternatives. Since no sibling tool offers heartbeat estimation, this level of guidance is sufficient.

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