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

Annotations already declare readOnly, idempotent, non-destructive and open-world, and the description adds real value beyond them: it returns arithmetic plus limitations and explicitly is not a forecast, verified profitability, optimized allocation or spending approval, and it neither reads accounts nor modifies anything. The credit-cost note is a useful extra, though nothing is said about output shape in detail.

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?

Core capabilities and constraints are front-loaded in the first two sentences, and the guardrails are stated tersely. The trailing MCP/credits sentence is operational rather than semantic and slightly dilutes an otherwise tight definition.

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 nested three-variant calculation tool with no output schema, the description establishes what comes back (arithmetic plus limitations), the input discipline, and the safety posture. Full completeness would require distinguishing the three calculation types and their distinct inputs.

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 75% and the description reinforces the semantics of the key inputs: one currency with no conversion, evidence must carry sources and exact periods, and missing values must not be guessed. The three calculation subtypes are named but not explained in terms of their required fields, so it stops short of 5.

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 first sentence gives a specific verb-plus-resource ('Deterministic budget pacing, budget allocation totals, or first-order acquisition unit economics') that clearly delimits the tool to arithmetic rather than recommendation. It does not name a sibling alternative, so the agent must infer the contrast with budget_recommend.

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 usage rules: use verified inputs or clearly labelled hypotheticals, cite sources and exact periods in evidence, use one currency, never guess missing budgets/costs/time windows. It lacks an explicit 'use X instead when...' routing to siblings such as budget_recommend, keeping it at 4.

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