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

The description goes far beyond annotations. It discloses billing behavior ($0.002 USDC per call, conditions when charged), idempotent-like behavior (unchanged total is free), authentication requirements, and edge cases (empty ledger costs nothing). It also clarifies the tool never blocks. No contradiction with 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 longer than typical but every sentence carries important information about billing, limits, and authentication. It is front-loaded with the core purpose. Some phrasing like 'we will not pretend to an authority we do not have' is slightly theatrical, but the overall structure is efficient and justified given the complexity.

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?

Given the tool's complexity (billing side effects, authentication, multiple conditions) and lack of output schema, the description is remarkably complete. It covers return semantics (byte-exact, over_limit, empty ledger behavior), cost model, and authentication methods. An agent can fully understand how to invoke and interpret this tool.

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%, so baseline is 3. The description adds some context for 'limit' (pass one here) and 'agent_key' (if host cannot set headers), but does not significantly improve parameter understanding beyond the schema's own descriptions. It earns a solid 3.

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 purpose: 'What this unit of work has actually cost across every session, with a per-label breakdown and a lifetime total.' It also distinguishes itself as a reporting/checking tool (not an enforcement tool) with 'WE REPORT, WE NEVER BLOCK.' This is a specific verb+resource combination that differentiates from siblings like budget_record.

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 provides clear usage context: when to use (checking costs against limits), behavior when limits are exceeded, and billing conditions. It implicitly excludes using it for enforcement ('enforcement is the caller's decision'). However, it does not explicitly name alternative tools like budget_record, missing the 'alternatives' component for a 5.

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