WISO Steuer – Tax Declaration: API integration & ELSTER OpenFinance

Deliverables checklist

• OpenAPI 3.1 specification covering login, case sync, Steuer-Abruf and ERiC submit
• Protocol analysis report — Buhl auth chain, request signing, cloud transport
• Runnable Python and Node.js reference clients with replayable fixtures
• ERiC integration sample (Linux x86_64 + macOS arm64) with sandbox certificates
• GDPR-ready logging schema, retention table and DPIA template
• Manufacturer-ID application checklist for the ELSTER developer portal

Engagement workflow

1. Scope confirmation — which surface (cloud sync, Steuer-Abruf, ERiC submit) and which tax forms (ESt, USt, GewSt).
2. Protocol analysis on the latest WISO Steuer Android and iOS builds (3–6 business days).
3. API design and sandbox build, validated against the ELSTER acceptance servers (4–8 business days).
4. Documentation, replay tests and Postman collection (1–2 business days).
5. Hand-over with a 30-day fix window and an optional pay-per-call hosted endpoint.

1. HR & payroll reconciliation

An employer uses our wrapper around the Steuer-Abruf VaSt feed to compare its monthly Lohnsteuerbescheinigung output with what the Finanzamt actually has on file. Mismatches trigger a payroll review before the employee files. The flow consumes vast.lohnsteuer.fetch and writes a delta record into the HR system.

2. Wealth-management refund tracker

A neobank embeds the WISO Steuer refund estimator inside its app. Each time the user updates Werbungskosten in WISO Steuer, our cloud-sync API pushes the new estimate to the bank, which surfaces a refund forecast widget. The integration uses the case-list endpoint plus a webhook on case updates.

3. Tax-advisor B2B portal

A Steuerberater receives client cases from WISO Steuer cloud through our authorized hand-off endpoint, runs internal review, and uses the ERiC bridge to submit on behalf of the client. The flow keeps the Steuerberater inside their own tooling while preserving Buhl's account separation rules.

4. ERP / SME bookkeeping sync

Self-employed users in a SaaS bookkeeping product see their EÜR draft pre-populated from WISO Steuer cloud cases. Our service maps the case JSON onto the ERP's account chart, and a reverse channel writes finalized bookings back as receipts. This complements DATEV-style flows for very small firms.

5. Compliance & audit logging

For regulated lenders that request tax declarations as proof of income, we return signed, hash-anchored snapshots of the user's filed declaration. The borrower stays in WISO Steuer; the lender receives a verifiable summary plus the ELSTER Bescheid reference, with no raw credentials moving between systems.

API example: Buhl account login (pseudocode)

POST /api/v1/wiso/auth/login
Content-Type: application/json

{
  "email": "user@example.de",
  "password": "<encrypted>",
  "device_id": "android-9f4b...",
  "client_version": "2026.05"
}

200 OK
{
  "access_token": "eyJhbGci...",
  "refresh_token": "rt_...",
  "expires_in": 3600,
  "buhl_account_id": "ba_8821",
  "mfa_required": false
}

API example: Steuer-Abruf VaSt fetch

POST /api/v1/wiso/vast/fetch
Authorization: Bearer <ACCESS_TOKEN>

{
  "tax_year": 2025,
  "abruf_code": "AB-7QH-2K9X",
  "datasets": ["lohnsteuer", "rente", "kranken"]
}

200 OK
{
  "tax_year": 2025,
  "lohnsteuer": [
    {"employer": "ACME GmbH", "brutto": 58400.00,
     "lohnsteuer": 9120.55, "soli": 0.00}
  ],
  "rente": [],
  "kranken": [{"provider": "TK", "beitrag": 4180.20}],
  "fetched_at": "2026-05-08T09:14:21Z"
}

API example: ERiC submission bridge

POST /api/v1/wiso/eric/submit
Authorization: Bearer <ACCESS_TOKEN>

{
  "form": "ESt2025",
  "payload_xml": "<Anmeldungssteuern>...</Anmeldungssteuern>",
  "certificate_alias": "buhl-prod-2026",
  "test_mode": false
}

200 OK
{
  "transferTicket": "T123456789",
  "telnummer": null,
  "validation": {"errors": [], "warnings": []},
  "submitted_at": "2026-05-08T09:15:02Z"
}

// Error contract follows ERiC return codes
// 610001801 = pin/zertifikat invalid
// 610001823 = data plausibility failed

Webhook: case update (cloud sync)

POST https://yourapp.example/webhooks/wiso
X-OFL-Signature: sha256=...

{
  "event": "case.updated",
  "buhl_account_id": "ba_8821",
  "case_id": "case_2025_a1",
  "tax_year": 2025,
  "estimated_refund_eur": 1674.20,
  "updated_at": "2026-05-08T09:16:44Z"
}

Regulatory anchors

Tax data is special-category material under EU GDPR and the German Abgabenordnung §30 (Steuergeheimnis). Every endpoint we deliver tracks the legal basis for processing, supports purpose limitation, and emits an immutable consent log. Submission paths align with the official ELSTER developer programme and the ERiC client certified by the Finanzverwaltung.

Identity & KYC

Buhl already partners with finAPI GiroIdent and Verimi for paperless identity. Our integrations call those flows behind a clean facade, so you can plug them into onboarding without wiring each provider yourself. Both partners are BaFin-licensed and meet GwG (anti-money-laundering) requirements for full identity verification.

Data minimisation

The default field mask returns only what the integration scenario needs — refund forecasts surface aggregate amounts, audit hand-offs surface signed hashes rather than raw PDFs. Retention timers default to the seven-year German bookkeeping window for finalised cases and 30 days for working drafts.

• Taxfix — Chat-driven employee tax app, popular with English-speaking expats; integration here is mostly about pulling final filed declarations and refund estimates back into a portfolio view.
• Smartsteuer — Browser-based, supports up to five returns per license; commonly paired with WISO Steuer when families split tooling between spouses.
• SteuerGo — English-language interface from Forium, strong for cross-border cases; teams often need a unified field map between SteuerGo and WISO Steuer.
• Lohnsteuer kompakt — Long-running online tool from Forium; integrations focus on Bescheid reconciliation and historical data import.
• Wundertax — Targets students and immigration-related cases; the data model overlaps with WISO Steuer on basic salary and Werbungskosten fields.
• Tax (also by Buhl) — Buhl's classic desktop sibling to WISO Steuer; shares the Buhl cloud account, which makes the integration story very similar.
• ELSTER Online (official) — The free Finanzamt portal; downstream of all third-party tools because every submission ultimately lands here via ERiC.
• Quicksteuer — Lemförde's lightweight desktop product; integrates with similar ELSTER ERiC entry points.
• Check24 Steuer — Marketplace-driven flow; useful for refund-comparison widgets that also surface WISO Steuer estimates.
• Accountable — Self-employed-focused app; pairs well with WISO Steuer when a freelancer needs both an EÜR feed and a personal tax declaration.

About OpenFinance Lab

We are an independent studio focused on OpenData, OpenBanking and tax-tech integration. Our engineers come from German banks, regulated payment providers and protocol-analysis backgrounds, and we have shipped end-to-end integrations across DACH, the EU and a handful of cross-border financial corridors.

• German tax tooling, ELSTER ERiC, DATEV adjacent flows
• OpenBanking PSD2 wiring, KYC/GwG identity providers
• Custom Python, Node.js and Go SDKs with replay test harnesses
• Full pipeline: protocol analysis → build → validation → compliance review
• Source code delivery from $300 — runnable API source plus full documentation; pay after delivery upon satisfaction
• Pay-per-call API billing — access our hosted endpoints, pay only per call, no upfront fee; ideal for teams that prefer usage-based pricing

Contact

For quotes or to submit your target app and requirements, open our contact page:

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