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

The annotations already declare this as a non-read-only, non-destructive, non-idempotent update. The description adds the critical behavioral detail that the line list is fully replaced and that all existing lines must be passed, which goes beyond the annotations and prevents accidental data loss.

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 three tight sentences with the core action and replacement warning front-loaded. Every sentence contributes useful guidance, and there is no redundant restatement of the title.

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 five-parameter mutation tool with no output schema, the description covers the main invocation requirements: draft-only scope, required ID provenance, and full line replacement. It does not explain what the tool returns or mention the optional notes/dueDate parameters, but the schema covers those fields adequately.

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 description coverage is 80%, so the schema already documents most parameters. The description adds operational meaning by stressing that the lines parameter must contain the complete new line list, including unchanged lines, which is more actionable than the schema's 'Complete new line list' phrasing alone.

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 a specific verb and resource: update an existing draft invoice by adding, removing, or changing lines. It distinguishes this from create_invoice_draft by tying it to a draft that was just created and by sourcing the IDs from the create response.

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?

It clearly says to use this when the user wants to modify a draft that was just created, and it explains where to get invoiceId and contactId. It does not explicitly state when not to use it or name an alternative for non-draft invoices, but the context is clear.

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