invowerk

Check an e-invoice

validate_invoice

Read-only

Check an e-invoice against EN 16931 and German rules.

Input: invoice_base64 is the base64 of an XML invoice (UBL or CII) or a ZUGFeRD / Factur-X PDF, at most 4 MB decoded; or pass the file as a download link in file. explain=true adds an explanation to findings whose rule has one, in lang ("de" or "en").

valid is the result. For XRechnung the KoSIT validator decides. If it is unreachable, invowerk's own check with the same rules decides, verdict_source is "local" and official_degraded is true. disagreement is true when both ran and differ. findings lists each problem with rule_id, severity and message; pass the codes to explain_errors (free). pdfa_verdict is the PDF/A check of a PDF. hints never change valid. Unreadable input returns a finding, not an error. It fails with an error when the check is busy (engine_busy), ran longer than 20 s (engine_timeout) or the XML has more than 100,000 elements (document_too_complex).

Credits: 1 per call.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`file`	No	The file as a download link, instead of `invoice_base64`. In ChatGPT, pass the user's uploaded file here. Other clients can set `download_url` to a public https URL.
`lang`	No	Language of the explanations: "de" (default) or "en".	de
`explain`	No	true adds an explanation to each finding whose rule has one.
`invoice_base64`	No	The invoice file, base64-encoded. At most 4 MB decoded.

Output Schema

TableJSON Schema

Name	Required	Description	Default
No arguments

Schema Changelog

Changes observed during successful MCP inspections.

First observedOct 1, 2026

TDQS

A4.6/5.0

Behavior5/5

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

Goes well beyond the readOnlyHint/openWorldHint annotations: it explains the KoSIT-vs-local verdict fallback, verdict_source, official_degraded, disagreement semantics, that hints never change valid, that unreadable input yields a finding rather than an error, the three specific error codes, and the 1-credit cost.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Dense and front-loaded, with the core purpose first and output/failure semantics after. It is long, but nearly every clause carries operational information; only the output-field enumeration slightly overlaps the existing output schema.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given a mutation-free but open-world validator with 4 params, nested input objects, an output schema and a credit cost, the description covers inputs, outputs, degraded-mode behavior, and failure modes thoroughly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Although schema coverage is 100%, the description adds meaning the schema lacks: accepted input formats (UBL, CII, ZUGFeRD/Factur-X PDF) and the 4 MB decoded limit, plus the file-vs-invoice_base64 relationship. It omits detail on the nested file object, but the schema covers that.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb (check/validate) and resource (e-invoice) plus the exact rule sets applied (EN 16931, German rules), which immediately distinguishes it from parse_invoice, convert_invoice and render_invoice.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives clear context: two alternative input channels (invoice_base64 or file), when to set explain, and routes the agent to explain_errors for finding codes. It does not explicitly state when NOT to use this tool versus parse_invoice, so it stops short of a 5.

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

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