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

Annotations only declare readOnly, idempotent, and non-destructive hints. The description adds extensive behavioral context: the opt-in transaction scan cost, the `transactions_basis` enum with exact vs estimate meanings, the authoritative nature of `totalAmount`, the interpretation of `payments[]` (payment of previous bill), and the absence of a paid/status field. 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 long but every sentence contributes. It is front-loaded with the core return fields, then layers optional behavior, then adds critical warnings. The structure flows logically from basic to advanced usage, and no sentence is redundant or filler. The depth is justified by the tool's 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 zero schema coverage, no output schema, and a complex optional transaction feature, the description covers all necessary aspects: exact return fields, the transaction basis and confidence levels, the batch response shape, the need to check `transactions_basis`, and the interpretation of `payments[]`. The only minor omission is the `transactions_detail` parameter, but overall the agent has enough to call the tool correctly without ambiguity.

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 0%, so the description carries the full burden. It explains `bill_ids` (array, discover via list), `account_id` (required when `include_transactions:true`), and `include_transactions` (opt-in, triggers extra scan). However, `transactions_detail` (enum: compact/rich/raw) is not mentioned at all, leaving a gap in parameter meaning. Otherwise, it adds substantial value beyond the schema.

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 states precisely what the tool does: returns bill-level detail for credit card bills by id, listing specific fields (`dueDate`, `billClosingDate`, `totalAmount`, etc.) and the optional transaction enrichment. It clearly distinguishes from siblings by referencing `openfinance_list_credit_card_bills` for discovery and `openfinance_list_transactions` for manual transaction fetching, making its scope unambiguous.

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 explicitly tells the agent when to use this tool: 'use openfinance_list_credit_card_bills first to discover ids', 'you can also fetch them yourself via openfinance_list_transactions', and 'prefer openfinance_list_credit_card_bills which derives payment_status'. It also warns against assuming payment status from `payments[]`, providing clear routing and exclusions.

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