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

Adds substantial context beyond annotations: debit happens exactly once with idempotent repeat behavior (reinforcing idempotentHint), sandbox keys pay nothing, and policy refusal semantics (max_cost_cents refuses before any debit). The destructive/mutation profile is conveyed with concrete consequences rather than just a hint.

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 core action and idempotency are front-loaded, and the policy/sandbox details follow in a compact sequence. Dense but nearly every clause earns its place; only the restated max_cost_cents phrasing is slightly redundant.

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?

With an output schema present, return values needn't be explained. The description covers the mutation semantics, idempotency, sandbox behavior, and error-gating policies that an agent needs to call this correctly.

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 description largely restates the schema's own max_cost_cents semantics ('refuses before any debit') and batch_id provenance ('from create_batch_mail or list_batch_mail'). It adds little meaning beyond the already-complete schema, so baseline 3 applies.

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 verb+resource ('Pay for a draft... and queue it for printing') and immediately clarifies the billing mechanism ('prepaid credits', 'debits exactly batch.pricing.total_cents'). An agent can distinguish this from get_batch_mail/create_batch_mail without opening either 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?

Explicit precondition ('Only after the person has seen the proof'), explicit policy gates (approval-required keys cannot pay, max_daily_pieces, max_cost_cents), and a named alternative path for sandbox ('poll get_batch_mail until submitted and use advance_test_batch_mail'). When-to-use, when-not, and alternatives are all present.

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