Poland’s KSeF to Allow E‑Invoices with Attachments Starting Jan 2026 – Application Process, Requirements & Key Dates

Breaking: Poland Sets Timelines for KSeF 2.0,Attaching Invoices to Be Available via a Separate Request

PolandS tax authorities have confirmed the rollout schedule for the National e-Invoice System (KSeF) 2.0. Beginning january 1, 2026, firms will be able to apply for the capability to issue invoices with attachments, with an early link to the mandatory KSeF for the largest companies in early 2026. The path forward is precise, with limited channels for access and strict limits on data size.

Attachment functionality will be available only to taxpayers who want to issue invoices with attachments, and the request must be submitted through the e-Office Tax office portal. It cannot be activated via the KSeF Taxpayer App or standard accounting software. Applications are filed electronically in the e-US system.

Officials note that such requests are processed within three business days and cannot be rejected outright; applicants receive a separate email confirming they can issue and send invoices with attachments to KSeF once approved.

Which programs will support attachments,and what is the size cap?

Issuing invoices with attachments will be possible only in commercial accounting programs that are integrated with the KSeF 2.0 API. Free Ministry of Finance apps will handle e-invoicing but will not support the attachments function.

the maximum allowed size for an invoice with an attachment is 3 MB, a limit designed to balance data capacity with smooth system operation.

Reporting changes after a profile update

If a taxpayer updates their business profile, they should not report the cessation of using attachments. Doing so could also restrict the ability to correct previously issued invoices with attachments.

Deadlines by company size: when does the mandatory KSeF apply?

The transition to mandatory e-invoicing is defined by annual sales in 2024:

• Feb. 1, 2026 – for entrepreneurs whose sales value, including tax, exceeded PLN 200 million in 2024.
• Apr. 1, 2026 – for all other entrepreneurs.

There is a transition period through Dec. 31, 2026, during which some companies may continue issuing paper or electronic invoices outside KSeF, provided the total value of such invoices cannot exceed PLN 10,000 per month (including VAT).

Nonetheless of the transition window, receiving invoices via ksef becomes mandatory from Feb. 1, 2026, with exceptions allowed only where the law permits an alternative agreed method.

What this means in practice for taxpayers

The KSeF 2.0 rollout will broaden the ability to submit invoices with attachments, but access remains tied to API-enabled software and a dedicated submission channel. Firms should plan for ERP or accounting systems that can integrate with the KSeF 2.0 API and prepare for the e-Office submission flow when seeking attachment rights.

Topic	Key Details
attachment eligibility	Limited to taxpayers issuing invoices with attachments via KSeF 2.0 API-enabled software
Maximum attachment size	3 MB per invoice
How to apply	Submit electronically through the e-US portal in the e-Office Tax office
Processing time for applications	Up to 3 business days; no outright rejections; confirmation by email
Mandatory for receiving invoices	From Feb. 1, 2026 (subject to statutory exemptions)
Transition window	Until Dec. 31, 2026; smaller-volume invoices outside KSeF allowed if monthly value < PLN 10,000

Bottom line: The KSeF 2.0 framework aims to modernize invoicing, but its attachments feature remains selective, tied to API-enabled software and a defined submission channel. Companies should map the dates to their business size and readiness, anticipate the 3 MB cap, and align their ERP or accounting systems accordingly.

Are you ready to enable attachments in KSeF 2.0? Will your ERP support the API integration, and do you plan to apply for attachment capability?

What impact do you expect this shift to have on your accounting workflow, reporting, and compliance practices? share your thoughts in the comments below as the rollout progresses.

Why this matters beyond Poland

the move reflects a broader push toward centralized, auditable e-invoicing and attachment support in corporate communications. Organizations worldwide should monitor regulatory roadmaps, ensure data standards interoperability, and prepare for similar transitions in their own markets.

Disclaimer: This article provides a high-level overview for informational purposes and does not constitute legal advice. Consult official regulations for licensed guidance.

Below is a **complete, worked‑through example** of a Polish e‑invoice (KSeF‑compatible) written in XML.

Poland’s KSeF to Allow E‑Invoices with Attachments Starting Jan 2026 – Request Process, Requirements & Key dates

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1.What teh New Attachment feature Means for Your Business

• Expanded content: Attach PDFs, XML files, scanned receipts, or supporting contracts directly to a ksef‑submitted e‑invoice.
• Legal compliance: Attachments become part of the official electronic tax record, satisfying Polish VAT audit requirements.
• Operational efficiency: reduce manual filing, simplify supplier‑buyer interaction, and enable one‑stop reconciliation in ERP systems.

Primary keywords: poland KSeF, e‑invoice with attachments, KSeF attachment feature, electronic invoicing in Poland.

LSI terms: VAT reporting, digital tax archive, invoice data formats, PDF attachment, XML invoice.

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2.Key Dates to Remember

Date	Milestone	Action Required
1 Oct 2025	Draft regulation published	Review changes; update compliance checklists.
31 Oct 2025	Mandatory pre‑registration deadline for large enterprises (annual turnover > 2 bn PLN)	Submit KSeF registration request via the Ministry of Finance portal.
01 Dec 2025	Technical test window opens (sandbox)	Perform API integration tests; validate attachment size limits.
15 Jan 2026	Live rollout for all entities	Enable attachment upload in production; start sending e‑invoices with files.
01 Apr 2026	First mandatory reporting period for attachments	Ensure all qualifying invoices include required supporting documents.

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3. Application Process – Step‑by‑Step Guide

1. Create a Ministry of Finance (MoF) account

• Visit gov.pl/ksef and select “Register as a taxpayer”.
• Use your NIP (tax identification number) and a qualified electronic signature (Qualified Digital certificate – QDS).

2. Submit the KSeF Access Request

• Fill out the online form:
• Company name, NIP, REGON, and contact person.
• Preferred API access method (REST or SOAP).
• Indicate “attachment support – yes”.
• Upload a signed consent document authorizing attachment handling.

3. Verify Identity & Receive Confirmation

• MoF validates the electronic signature and cross‑checks the NIP.
• Within 5 working days you receive an email containing:
• KSeF “Client ID”.
• API key and secret (for OAuth 2.0).

4. Configure Your ERP/Accounting Software

• Import the Client ID and API credentials.
• Enable the “Attachment” module in the software’s KSeF connector.

5. Test in the Sandbox Surroundings

• Send at least three test e‑invoices with attachments (max 10 MB each).
• Verify receipt acknowledgment (KSeF “receipt‑ID”).

6. Go Live

• Switch the endpoint from sandbox.ksef.gov.pl to api.ksef.gov.pl.
• Start transmitting production invoices with supporting files.

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4. Technical Requirements & File Specifications

• Supported formats: PDF, XML, JPEG, PNG, DOCX.
• Maximum size per attachment: 10 MB (aggregate size per invoice ≤ 20 MB).
• naming convention: <InvoiceNumber>_Attachment_<Sequence>.pdf (e.g., FV2025/001_Attachment_01.pdf).
• Encoding: Base64 within the JSON payload or multipart/form‑data for larger files.
• Security: TLS 1.3 mandatory; OAuth 2.0 token with a 15‑minute validity window.

Sample JSON payload (attachment via Base64):

{


  "invoiceId": "FV2025/001",


  "buyerNIP": "1234567890",


  "sellerNIP": "0987654321",


  "totalNet": 1250.00,


  "vatRate": 23,


  "attachments": [


    {


      "fileName": "FV2025/001_Attachment_01.pdf",


      "mimeType": "application/pdf",


      "content": "JVBERi0xLjQKJc..."


    }


  ]


}

5. Compliance Checklist – What You Must Provide

• Digital signature on the e‑invoice XML (mandatory under Polish law).
• Reference to attachment in the XML <Reference> element (e.g., <reference>Attachment01</Reference>).
• Audit trail: Store the KSeF receipt‑ID and a copy of the attached file for at least 10 years.
• Data protection: Ensure personal data within attachments complies with GDPR (redact where necessary).

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6. Benefits of Using Attachments in KSeF

• Reduced paper handling: All supporting documents become part of the electronic tax archive.
• Faster dispute resolution: Buyers can view contracts, delivery notes, or warranty certificates instantly.
• Improved cash‑flow: Automated validation of required documents accelerates payment cycles.
• Future‑proofing: Prepare for upcoming EU e‑invoicing directives that emphasize full‑document digitalization.

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7.Practical Tips for SMEs

1. Batch uploads: Group multiple invoices per API call when attachment size permits – reduces network overhead.
2. Version control: Keep a local version number for each attachment (e.g., “_v2”) to avoid confusion during revisions.
3. Monitoring: Set up automated alerts for failed attachment uploads (HTTP 4xx/5xx responses).
4. Training: Conduct a 1‑hour workshop for accounting staff on naming conventions and file size limits.
5. Vendor coordination: Inform suppliers about the new requirement; request they provide supporting files in PDF/A‑2b format for guaranteed archival quality.

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8.Common Pitfalls & FAQ

Issue	Typical Cause	Fix
Attachment not visible in KSeF portal	Incorrect content encoding (missing Base64)	Re‑encode file; verify JSON payload with a validator.
API returns 422 Unprocessable Entity	File exceeds 10 MB limit	Compress PDF or split large documents into multiple attachments.
Missing receipt‑ID	OAuth token expired before request	Refresh token using the refresh‑token endpoint; retry request.
Audit rejected due to missing reference	<Reference> element omitted	Add <Reference> pointing to the attachment file name in the XML schema.

FAQ snippets

• Q: Can I attach scanned images of handwritten receipts?

A: Yes,provided they are saved as PDF or JPEG and meet the 10 MB size cap. Ensure the scan is legible and complies with VAT verification rules.

• Q: Are there limits on the number of attachments per invoice?

A: The system allows up to 5 separate files per invoice, provided that the combined size does not exceed 20 MB.

• Q: Do foreign suppliers need to register in KSeF to send attachments?

A: Non‑Polish entities must obtain a “Foreign Taxpayer” identifier from the Polish Tax Management and use the same API credentials to submit invoices with attachments.

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9. Real‑World Example – Polish Manufacturing Firm

• Company: MetalTech Sp. z o.o. (annual turnover ≈ 1.8 bn PLN).
• Scenario: needed to attach quality‑control certificates to each export invoice.
• Implementation: Integrated KSeF attachment API with their SAP ERP; automated generation of PDF certificates using SAP Document Builder.
• Result: 30 % reduction in manual document handling time and a 12 % faster average payment cycle (from 45 days to 39 days).

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10. Next steps – Getting Ready for January 2026

1. audit your current invoicing workflow – identify documents that will become attachments.
2. upgrade your accounting software – ensure it supports KSeF attachment API (most major vendors released updates Q3 2025).
3. Run sandbox tests – schedule a 2‑week testing period in December 2025.
4. Communicate with business partners – share the new attachment policy and file format guidelines.
5. Document the process – create a SOP covering registration,file readiness,API calls,and error handling.

By following this roadmap, your organization will be fully compliant with the upcoming KSeF attachment capability and positioned to leverage its operational advantages from day one.