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

Annotations only declare a non-read, open-world, non-destructive write. The description adds substantial context beyond that: the invoice goes offline24 and becomes uneditable after its issue day, confirmation is on by default per call, the action reaches other people, and it needs a Cotlet API key with app access. These are exactly the side-effect and auth facts an agent needs.

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?

Purpose is front-loaded in the first sentence. There is mild redundancy ('Send one invoice... Sends the invoice to KSeF...') and the follow-on sentences are dense, but each conveys a distinct prerequisite or consequence, so little is wasted.

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 no output schema, the description correctly explains the return path ('returns processing; the KSeF number appears after processing, so read it with get_ksef_status') and covers prerequisites, confirmation behavior, and irreversibility. An agent has everything needed to call it 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 description coverage is 100% for the single invoice_id parameter, so the baseline is 3. The description adds no parameter-specific detail beyond 'send one invoice', leaving the schema to carry the load.

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 (Send), resource (one invoice), and destination (Polish KSeF e-invoicing system), which cleanly distinguishes it from siblings like fakturownia_send_invoice_email. An agent can identify exactly what the tool does without opening the 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?

Provides clear context and exclusions: KSeF must be connected, send permission is required, proformas are not eligible, and it directs the agent to get_ksef_status to read the resulting KSeF number. It does not explicitly contrast itself against the sibling fakturownia_send_invoice_email, so it stops short of a full when/when-not/alternative routing.

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