PolishOpenData

.NET libraries and an MCP server for Polish public registers — company lookup in KRS, the VAT whitelist (Biała Lista) with bank-account checks, and offline validation of NIP, REGON, KRS, NRB and land-registry (KW) numbers.

🇵🇱 Polska wersja

Unofficial project. Not affiliated with the Ministry of Justice, the Ministry of Finance or any other public body. Results are informational, not legal or tax advice; the registers are the authoritative source.

Packages

Offline validation

No network, no DI:

dotnet add package PolishOpenData.Core

using PolishOpenData;

Console.WriteLine(Nip.TryParse("774-000-14-54", out var nip) ? $"Valid NIP {nip}" : "Invalid NIP"); // Valid NIP 7740001454
Console.WriteLine(KwNumber.TryParse("WA1M/00012345/1", out var kw) ? kw.Court?.Name : "Invalid KW number"); // Sąd Rejonowy dla Warszawy-Mokotowa …
Console.WriteLine(string.Join(", ", KwNumber.SuggestCorrections("WAIM/00012345/1"))); // WA1M/00012345/1

Quick start: check a counterparty

dotnet add package PolishOpenData.BialaLista
dotnet add package PolishOpenData.Krs
dotnet add package Microsoft.Extensions.Http.Resilience

using Microsoft.Extensions.DependencyInjection;
using PolishOpenData;
using PolishOpenData.BialaLista;
using PolishOpenData.Krs;
using System.Threading.Tasks;

var services = new ServiceCollection();
services.AddBialaListaClient(o => o.TrackQuota = true)
    .AddStandardResilienceHandler(o => o.Retry.ShouldHandle = _ => new ValueTask<bool>(false));
services.AddKrsClient().AddStandardResilienceHandler();
using var provider = services.BuildServiceProvider();

var vat = provider.GetRequiredService<IBialaListaClient>();
var nip = Nip.Parse("774-000-14-54");

var result = await vat.FindByNipAsync(nip);
Console.WriteLine($"{result.Value?.Name}: {result.Value?.VatStatus} (request {result.RequestId})");

var check = await vat.CheckBankAccountAsync(nip, Nrb.Parse("06 1600 1127 1843 9838 2000 0034"));
Console.WriteLine(check.Value ? "Account is on the whitelist" : "Account is NOT on the whitelist");

var krs = provider.GetRequiredService<IKrsClient>();
var extract = await krs.GetCurrentExtractAsync(result.Value!.Krs!.Value);
Console.WriteLine(extract.Extract?.ToSummary().ShareCapital);

Retries are off for Biała Lista: every retry the resilience handler would make is another upstream request that the quota guard above cannot count, and once the daily limit is reached Biała Lista blocks the whole IP address — not just this process — until midnight. Register AddBialaListaClient once per process and reuse the resulting client (or pass one shared BialaListaQuotaTracker explicitly); a client created without DI and without a shared tracker gets its own private tracker that does not see requests made by other instances.

A runnable version is in samples/CheckCounterparty.

Keep RequestId. Every BialaListaResult carries the request identifier and time returned by Biała Lista (RequestId, RequestDateTime); keep them with your payment records to document when you checked the whitelist and what it answered.

Source-generated JSON. The KRS models use public converters from PolishOpenData.Krs.Serialization (KrsDateJsonConverter, KrsTimestampJsonConverter), so on .NET 8 or later you can add KrsCurrentExtract or KrsFullExtract to your own JsonSerializerContext (for example to cache extracts) without registering any converter. If you parse raw KRS API responses yourself instead of calling IKrsClient, note two things: the extract is wrapped in an odpis object, so deserialize that inner object; and the registry's field names (such as dataRejestracjiWKRS) match the properties only case-insensitively, so use JsonSerializerDefaults.Web — [JsonSourceGenerationOptions(JsonSerializerDefaults.Web)] on your context, or new JsonSerializerOptions(JsonSerializerDefaults.Web) with the reflection-based serializer on .NET Framework. Otherwise the properties stay null without an error.

MCP server for AI assistants

Requires the .NET 10 SDK. No API keys.

Claude Code

claude mcp add --transport stdio --scope user polish-open-data -- dotnet dnx PolishOpenData.Mcp --yes

Claude Desktop / Cursor (claude_desktop_config.json or .cursor/mcp.json)

{
  "mcpServers": {
    "polish-open-data": { "command": "dotnet", "args": ["dnx", "PolishOpenData.Mcp", "--yes"] }
  }
}

VS Code (.vscode/mcp.json)

{
  "servers": {
    "polish-open-data": { "type": "stdio", "command": "dotnet", "args": ["dnx", "PolishOpenData.Mcp", "--yes"] }
  }
}

Land-registry (KW) numbers

KwNumber validates numbers such as WA1M/00012345/1 offline using the official court-code table (Dz.U. 2026 poz. 740, 342 current codes plus 7 historical ones) and suggests fixes for common typos (0S1U → OS1U). It is a .NET port of the validation logic of pyekw by mhajder (MIT). It does not query the eKW portal and deliberately offers no number generator: KW numbers are personal data (NSA, III OSK 6508/21), and enumerating them is out of scope.

Responsible use

• Data comes from official public registers; cite the source and the retrieval time. Every registry-backed MCP tool result (lookup_company, check_vat_bank_account, get_krs_extract) and BialaListaResult carry both; KrsResult does not add one itself — use ToSummary().ExtractedAt when present.
• Biała Lista allows about 100 searches and 5,000 checks per day per IP address; exceeding them blocks the IP until midnight, including the ministry's web search. Use TrackQuota and prefer checks over searches.
• KRS masks natural persons in structured fields. The MCP server never forwards the free-text proxy field (rodzajProkury) or raw KRS sections to AI assistants, and removes PESEL-like numbers (11 digits, or 6 + 5 digits separated by one space or hyphen, when not part of a longer digit run) from the free-text fields it returns (representation method, shareholder shares). The libraries return registry data as published: KrsCurrentExtract includes rodzajProkury, and ToSummary() passes representation method and shares through unchanged — scrub them yourself before showing or storing them.
• Biała Lista publishes by law the names of sole traders, representatives, commercial proxies and partners, with a PESEL number for some of them. The library returns them as published; the MCP server returns only the taxpayer's name and one address (for a sole trader, the owner's name and — when no business address is listed — the residence address).
• No scraping, no bulk harvesting, no linking of parcels to land-registry numbers.

Possible next steps

Ideas, not commitments: GUS REGON (BIR) lookups, offline checks against the VAT whitelist flat file, and property data (ULDK parcels, GUGiK geocoder, transaction prices). If one of them would help you, open an issue to show interest.

Contributing and license