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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so safety is covered. The description adds the computational semantics (annualized, date-driven), but omits material behavior: XIRR requires at least one negative and one positive flow to converge, and can fail to converge — neither the schema nor the description warns of that, so an agent could pass two same-signed flows and get an error it cannot anticipate.

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 plus a link, front-loaded with the definition and immediately followed by the disambiguation. The interactive-calculator URL is marginally promotional but plausibly useful for a human, and no sentence is wasted.

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 one-parameter, read-only, no-output-schema tool, the description says what comes back (an annualized return) and how it differs from the sibling. The only real gap is the undisclosed convergence/sign requirement, which matters for a numerically solved function.

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 coverage is 100% and the schema already documents sign convention, date format, ordering and the 2–500 item bounds. The description adds only the notion of irregular timing, which is already implied by the per-item dates, so it sits at the baseline rather than adding meaning.

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 computation (annualized return / XIRR) over a specific resource (dated cash flows) and anchors it to a familiar reference ('like Excel XIRR'). It also explicitly distinguishes itself from the sibling tool irr, so an agent can pick correctly without opening a schema.

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 explicit when-to-use ('real investments with irregular timing — capital calls, distributions, follow-ons') and an explicit when-not with the named alternative ('use irr only for evenly spaced yearly flows'). This is the routing information an agent needs.

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