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

Annotations only carry the generic readOnly/openWorld/destructive flags; the description adds concrete behavior the agent could not otherwise know: the email reaches external people, each call is confirmed by default (and that default can be changed), and a Cotlet API key with app access is required. This is exactly the added-value context the lower bar for annotated tools rewards.

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 and the sentences are short, but the PDF-attachment fact is stated twice ('with the PDF attached' then 'with the PDF attached when email_pdf is true'), and the confirmation point is also repeated. Trimming that redundancy would tighten it without losing information.

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 mutation that sends external email, it covers the essentials: confirmation, credential requirement, KSeF gating, and recipient limits, and no output schema exists so return values need not be described. It does not say what happens on partial/failed delivery or whether the invoice must already be finalized, which keeps it from a 5.

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 the schema already documents email_to, email_cc, email_pdf, invoice_id, and update_buyer_email. The description restates the 5-address limit and the email_pdf=true attachment rule but adds no syntax or format detail beyond the schema, so the baseline of 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 (email) and resource (an invoice) plus the delivery detail (to the client, PDF attached). It is clearly distinguishable from fakturownia_send_invoice_to_ksef and fakturownia_get_invoice_pdf, which do different things with the same invoice.

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?

Gives the KSeF precondition (emails only after a KSeF number exists) and the confirmation posture, which is real usage context. It does not, however, explicitly name alternatives or state when to prefer a different send path, so it falls short of the top band.

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