# <img src="logo.png" alt="mcp-abap-adt logo" width="36" align="absmiddle" /> mcp-abap-adt: Your Gateway to ABAP Development Tools (ADT)
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`mcp-abap-adt` is an MCP server for ABAP ADT in SAP ECC/S/4HANA (on-premise) and SAP BTP ABAP Cloud systems. It gives agents controlled access to real ABAP repositories through ADT, so analysis and changes are grounded in system data instead of assumptions. It is built for AI-assisted pair programming (AIPNV: AI Pairing, Not Vibing), not autopilot vibe coding.

**Primary workflows:**
- **Deep ABAP analysis**: where-used, object metadata, repository navigation, object structure, semantic analysis, dependency and impact exploration.
- **High-level ABAP development**: rapid CRUD and iterative updates for RAP and classic ABAP artifacts (classes, interfaces, function groups/modules, programs, DDIC, CDS/view/service artifacts), validated through ADT flows.

**Why teams use it:**
- **Full CRUD** (not read-only): create, read, update, and delete ABAP artifacts
- Works with **On-Premise (ECC/S/4HANA)**, **ABAP Cloud (BTP)**, and **Legacy** systems (BASIS < 7.50)
- **JWT/XSUAA**, **service key** (destination-based), and **RFC** authorization
- Multiple transports: **stdio**, **HTTP**, **SSE**
- Rich tool surface for ABAP objects, metadata, transports, and search

**Authorization & Destinations (Important):** A *destination* is the filename of a service key stored locally. You place service keys in the service-keys directory, and use `--mcp=<destination>` to select which one to use. This is the primary auth model for on‑prem and BTP systems. See [Authentication & Destinations](docs/user-guide/AUTHENTICATION.md).

You can configure MCP clients either manually (JSON/TOML) or via the configurator CLI (`@mcp-abap-adt/configurator`, repo: [`mcp-abap-adt-conf`](https://github.com/fr0ster/mcp-abap-adt-conf)).

## Table of Contents

1. [Getting Started](#getting-started)
2. [Architecture](#architecture)
3. [Quick Start](#quick-start)
4. [Use Cases](#use-cases)
5. [Target Users](#target-users)
6. [Capabilities (High-Level Focus)](#capabilities-high-level-focus)
7. [Terminology](#terminology)
8. [Authorization & Destinations](#authorization--destinations)
9. [Registries](#registries)
10. [Features](#features)
11. [Documentation](#documentation)
12. [Dependencies](#dependencies)
13. [Running the Server](#running-the-server)

## Getting Started

Install the server and configure your client using the configurator:

```bash
npm install -g @mcp-abap-adt/core
npm install -g @mcp-abap-adt/configurator

# stdio (destination)
mcp-conf --client cline --name abap --mcp TRIAL

# HTTP (streamable HTTP)
mcp-conf --client copilot --name abap --transport http --url http://localhost:3000/mcp/stream/http --mcp trial
```

Full configurator usage (separate repo): [CLIENT_INSTALLERS.md](https://github.com/fr0ster/mcp-abap-adt-conf/tree/main/docs/CLIENT_INSTALLERS.md).

## Terminology

**Destination**: a local service key filename. You store service keys in the standard `service-keys` directory, and pass the filename (without extension) via `--mcp=<destination>` to select which system to use.

See [docs/user-guide/TERMINOLOGY.md](docs/user-guide/TERMINOLOGY.md) for the full list.

## Authorization & Destinations

Destination-based auth is the default. Drop service keys into the standard platform folder and use the filename as your destination:

```bash
mcp-abap-adt --transport=stdio --mcp=TRIAL
```

Standard service key paths:
- Unix (Linux/macOS): `~/.config/mcp-abap-adt/service-keys/<destination>.json`
- Windows: `%USERPROFILE%\\Documents\\mcp-abap-adt\\service-keys\\<destination>.json`

For full details (paths, `.env`, direct headers), see [Authentication & Destinations](docs/user-guide/AUTHENTICATION.md).

## Architecture

The project ships as **two packages**, because the two usage patterns want
different licences. Embedding the tools in a network service should not drag in
the obligations of a server that service never runs.

| Package | Licence | What it is |
|---|---|---|
| [`@mcp-abap-adt/lib`](https://www.npmjs.com/package/@mcp-abap-adt/lib) | Apache-2.0 | The ADT tool handlers and the embeddable MCP server. No transport: the host supplies one. |
| [`@mcp-abap-adt/core`](https://www.npmjs.com/package/@mcp-abap-adt/core) | AGPL-3.0-only | The standalone server — stdio, SSE and streamable HTTP, the launcher and the `mcp-abap-adt` CLI. Depends on the library. |

Install `@mcp-abap-adt/core` to run a server. Install `@mcp-abap-adt/lib` to
embed the tools in your own application.

### 1. Standalone MCP Server (Default)
Run as a standalone MCP server with stdio, HTTP, or SSE transport:
```bash
mcp-abap-adt                           # stdio (default)
mcp-abap-adt --transport=http          # HTTP mode
mcp-abap-adt --transport=sse           # SSE mode
```

### 2. Embeddable Server (For Integration)
Embed MCP server into existing applications (e.g., SAP CAP/CDS, Express).
This needs `@mcp-abap-adt/lib` only — not the AGPL server:
```bash
npm install @mcp-abap-adt/lib
```
```typescript
import {
  EmbeddableMcpServer,
  NoDedupStrategy, // optional: expose both Read<X> and Get<X>
} from '@mcp-abap-adt/lib/embeddable';

const server = new EmbeddableMcpServer({
  connection,              // Your AbapConnection instance
  logger,                  // Optional logger
  exposition: ['readonly', 'high'],  // Handler groups to expose
  // Default hides Read<X> when Get<X> is exposed (ReadVsGetDedupStrategy).
  // Pass NoDedupStrategy to expose both variants instead.
  // readOnlyDedupStrategy: new NoDedupStrategy(),
});
await server.connect(transport);
```

See [Handlers Management → EmbeddableMcpServer dedup strategies](docs/user-guide/HANDLERS_MANAGEMENT.md#embeddablemcpserver-dedup-strategies) for how readonly tools are deduped against high/low/compact, how to opt out with `NoDedupStrategy`, and how to plug a custom `IReadOnlyDedupStrategy` for role-based rules.

## Quick Start

1. **Install server**: See [Installation Guide](docs/installation/INSTALLATION.md)
2. **Configure client (auto)**: Use `mcp-conf` from `@mcp-abap-adt/configurator` (repo: [`mcp-abap-adt-conf`](https://github.com/fr0ster/mcp-abap-adt-conf), docs: [CLIENT_INSTALLERS.md](https://github.com/fr0ster/mcp-abap-adt-conf/tree/main/docs/CLIENT_INSTALLERS.md))
3. **Configure client (manual)**: See [Client Configuration](docs/user-guide/CLIENT_CONFIGURATION.md)
4. **Use**:
   - [Read-Only Tools](docs/user-guide/AVAILABLE_TOOLS_READONLY.md)
   - [High-Level Tools](docs/user-guide/AVAILABLE_TOOLS_HIGH.md)
   - [Low-Level Tools](docs/user-guide/AVAILABLE_TOOLS_LOW.md)
   - [Legacy System Tools](docs/user-guide/AVAILABLE_TOOLS_LEGACY.md)

## Use Cases

- **Impact analysis / where-used before changes**: map object usage and probable blast radius.
- **Dependency audit**: inspect links across classes, interfaces, DDIC, CDS/views, and RAP artifacts.
- **Migration and cleanup prep**: extract repository facts to plan refactoring or cloud-readiness work.
- **RAP and ABAP iterative development**: create/update artifacts quickly with ADT-backed operations.
- **Automated documentation and RAG ingestion**: pull structured facts from ABAP systems for downstream tooling.

## Target Users

- ABAP developers and ABAP architects
- RAP developers
- Team leads and tech leads who need fast repository visibility
- Teams building RAG/agent workflows for SAP landscapes

## Capabilities (High-Level Focus)

Key examples of high-value workflows and tools:

- **Repository and impact analysis**: `GetWhereUsed`, `DescribeByList`, `GetObjectStructure`, `GetObjectInfo`, `SearchObject`, `GetPackageTree`, `GetPackageContents`
- **Code and semantic introspection**: `GetAbapAST`, `GetAbapSemanticAnalysis`, `GetIncludesList`
- **RAP development**: `CreateBehaviorDefinition`, `UpdateBehaviorDefinition`, `CreateBehaviorImplementation`, `UpdateBehaviorImplementation`, `CreateServiceDefinition`, `UpdateServiceDefinition`, `CreateMetadataExtension`, `UpdateMetadataExtension`
- **CDS/View development**: `CreateView`, `UpdateView`, `GetView`, `DeleteView`
- **ABAP OO CRUD**: `CreateClass`, `UpdateClass`, `GetClass`, `DeleteClass`, `CreateInterface`, `UpdateInterface`, `GetInterface`, `DeleteInterface`
- **Function module/group CRUD**: `CreateFunctionGroup`, `UpdateFunctionGroup`, `GetFunctionGroup`, `DeleteFunctionGroup`, `CreateFunctionModule`, `UpdateFunctionModule`, `GetFunctionModule`, `DeleteFunctionModule`
- **Transport and activation support**: `CreateTransport`, `GetTransport`, `ActivateObject`

## Registries

Published in the official MCP Registry and listed on Glama.ai.

- MCP Registry: [docs/deployment/MCP_REGISTRY.md](docs/deployment/MCP_REGISTRY.md)
- Glama.ai:
  <a href="https://glama.ai/mcp/servers/@fr0ster/mcp-abap-adt">
    <img width="380" height="200" src="https://glama.ai/mcp/servers/@fr0ster/mcp-abap-adt/badge" />
  </a>

## Features

- **🏗️ Domain Management**: `GetDomain`, `CreateDomain`, `UpdateDomain` - Create, retrieve, and update ABAP domains
- **📊 Data Element Management**: `GetDataElement`, `CreateDataElement`, `UpdateDataElement` - Create, retrieve, and update ABAP data elements
- **📦 Table Management**: `GetTable`, `CreateTable`, `GetTableContents` - Create and retrieve ABAP database tables with data preview
- **🏛️ Structure Management**: `GetStructure`, `CreateStructure` - Create and retrieve ABAP structures
- **👁️ View Management**: `GetView`, `CreateView`, `UpdateView` - Create and manage CDS Views and Classic Views
- **🎓 Class Management**: `GetClass`, `CreateClass`, `UpdateClass` - Create, retrieve, and update ABAP classes
- **📝 Program Management**: `GetProgram`, `CreateProgram`, `UpdateProgram` - Create, retrieve, and update ABAP programs
- **🔧 Behavior Definition (BDEF) Management**: `GetBehaviorDefinition`, `CreateBehaviorDefinition`, `UpdateBehaviorDefinition` - Create and manage ABAP Behavior Definitions with support for Managed, Unmanaged, Abstract, and Projection types
- **📋 Metadata Extension (DDLX) Management**: `CreateMetadataExtension`, `UpdateMetadataExtension` - Create and manage ABAP Metadata Extensions
- **⚡ Activation**: `ActivateObject` - Universal activation for any ABAP object
- **🚚 Transport Management**: `CreateTransport`, `GetTransport` - Create and retrieve transport requests
- **🔍 Enhancement Analysis**: `GetEnhancements`, `GetEnhancementImpl`, `GetEnhancementSpot` - Enhancement discovery and analysis
- **📋 Include Management**: `GetIncludesList` - Recursive include discovery
- **🔍 System Tools**: `GetInactiveObjects` - Monitor inactive objects waiting for activation
- **🧪 Runtime Diagnostics**: `RuntimeCreateProfilerTraceParameters`, `RuntimeListProfilerTraceFiles`, `RuntimeGetProfilerTraceData`, `RuntimeGetDumpById` - Profiling and dump analysis with JSON payloads
- **📡 Runtime Feeds**: `RuntimeListFeeds`, `RuntimeListSystemMessages`, `RuntimeGetGatewayErrorLog` - Feed reader (dumps, system messages, gateway errors), SM02 system messages, Gateway error log
- **🚀 SAP BTP Support**: JWT/XSUAA authentication with browser-based token helper
- **🔑 Destination-Based Authentication**: Service key-based authentication with automatic token management (see [Client Configuration](docs/user-guide/CLIENT_CONFIGURATION.md#destination-based-authentication))
- **💾 Freestyle SQL**: `GetSqlQuery` - Execute custom SQL queries via ADT Data Preview API

> ℹ️ **ABAP Cloud limitation**: Direct ADT data preview of database tables is blocked by SAP BTP backend policies. The server returns a descriptive error when attempting such operations. On-premise systems continue to support data preview.

## Documentation

### For Users
- **[Docs Index](docs/README.md)** - Full documentation index
- **[Installation Guide](docs/installation/README.md)** - Installation overview and platform guides
- **[User Guide](docs/user-guide/README.md)** - End-user docs (auth, config, tools)
- **[Authentication & Destinations](docs/user-guide/AUTHENTICATION.md)** - Destination-based auth and service keys
- **[Handlers Management](docs/user-guide/HANDLERS_MANAGEMENT.md)** - Enable/disable handler groups
- **Configurator**: `@mcp-abap-adt/configurator` (repo: [`mcp-abap-adt-conf`](https://github.com/fr0ster/mcp-abap-adt-conf)) provides the `mcp-conf` CLI to auto-configure clients
- **Tools by level**
  - [Read-Only Tools](docs/user-guide/AVAILABLE_TOOLS_READONLY.md)
  - [High-Level Tools](docs/user-guide/AVAILABLE_TOOLS_HIGH.md)
  - [Low-Level Tools](docs/user-guide/AVAILABLE_TOOLS_LOW.md)
  - [Legacy System Tools](docs/user-guide/AVAILABLE_TOOLS_LEGACY.md)

### For Administrators
- **[Deployment Docs](docs/deployment/README.md)** - MCP Registry, Docker, release notes
- **[Server Configuration](docs/configuration/YAML_CONFIG.md)** - YAML config reference

### For Developers
- **[Architecture Documentation](docs/architecture/README.md)** - System architecture and design decisions
- **[Development Documentation](docs/development/README.md)** - Testing guides and development resources
- **[CHANGELOG.md](CHANGELOG.md)** - Version history and changes

## Dependencies

This project uses two npm packages:

- **[@mcp-abap-adt/connection](https://www.npmjs.com/package/@mcp-abap-adt/connection)** – connection/auth/session layer
- **[@mcp-abap-adt/adt-clients](https://www.npmjs.com/package/@mcp-abap-adt/adt-clients)** – Builder-first ADT clients

These packages are automatically installed via `npm install` and are published to npm.

---


## Running the Server

### Global Installation (Recommended)
After installing globally with `npm install -g`, you can run from any directory:

```bash
# Show help
mcp-abap-adt --help

# Default stdio mode (for MCP clients; requires .env file or --mcp parameter)
mcp-abap-adt

# stdio mode (explicit; default when --transport is omitted)
mcp-abap-adt --transport=stdio

# HTTP mode on custom port (HTTP requires --transport=http)
mcp-abap-adt --transport=http --port=8080

# Use stdio mode with auth-broker (--mcp parameter)
mcp-abap-adt --transport=stdio --mcp=TRIAL

# Use env destination from platform sessions store
mcp-abap-adt --env=trial

# Use explicit .env file path
mcp-abap-adt --env-path=/path/to/my.env

# SSE mode (requires .env file or --mcp parameter)
mcp-abap-adt --transport=sse --port=3001

# SSE mode with auth-broker (--mcp parameter)
mcp-abap-adt --transport=sse --mcp=TRIAL
```

### Development Mode
```bash
# Build and run locally
npm run build
npm start

# HTTP mode
npm run start:http

# SSE mode
npm run start:sse
```

### Environment Configuration

Env resolution:
1. `--env-path=<path|file>` (or `MCP_ENV_PATH`) for explicit `.env` file.
   - Absolute path: used as-is.
   - Relative path or file name only (e.g. `my.env`): resolved from current working directory.
2. `--env=<destination>` for destination file in standard sessions store:
   - Unix: `~/.config/mcp-abap-adt/sessions/<destination>.env`
   - Windows: `%USERPROFILE%\\Documents\\mcp-abap-adt\\sessions\\<destination>.env`
3. Fallback to `.env` in current working directory.

**Example .env file:**
```bash
SAP_URL=https://your-sap-system.com
SAP_CLIENT=100
SAP_AUTH_TYPE=basic
SAP_USERNAME=your-username
SAP_PASSWORD=your-password
```

For JWT authentication (SAP BTP):
```bash
SAP_URL=https://your-btp-system.com
SAP_CLIENT=100
SAP_AUTH_TYPE=jwt
SAP_JWT_TOKEN=your-jwt-token
```

For RFC connection:
```bash
SAP_URL=https://your-legacy-system.com
SAP_CLIENT=100
SAP_AUTH_TYPE=basic
SAP_USERNAME=your-username
SAP_PASSWORD=your-password
SAP_CONNECTION_TYPE=rfc
```

See [RFC Setup Guide](docs/installation/RFC_SETUP.md) for prerequisites (SAP NW RFC SDK).

For client certificate (mTLS) authentication — on-prem HTTP only:
```bash
SAP_URL=https://your-sap-system.com
SAP_AUTH_TYPE=certificate

# PEM format (provide both files):
SAP_CERT_PATH=/path/to/client.crt
SAP_CERT_KEY_PATH=/path/to/client.key

# Or PKCS#12 format (alternative to PEM):
# SAP_CERT_PFX_PATH=/path/to/client.pfx
# SAP_CERT_PASSPHRASE=your-passphrase
```

For Kerberos (SPNEGO) authentication — on-prem HTTP only:
```bash
SAP_URL=https://your-sap-system.com
SAP_AUTH_TYPE=kerberos

# Optional: explicit SPN (default: HTTP@<host>)
# SAP_KERBEROS_SPN=HTTP@mysaphost.corp.example
# Optional: service class used to derive the SPN when SAP_KERBEROS_SPN is unset (default: HTTP)
# SAP_KERBEROS_SERVICE=HTTP
```

**Certificate auth notes:**
- Identifies the client via mTLS — no `SAP_USERNAME` / `SAP_PASSWORD` required.
- Provide either PEM files (`SAP_CERT_PATH` + `SAP_CERT_KEY_PATH`) or a PKCS#12 file (`SAP_CERT_PFX_PATH`), not both.
- On-prem HTTP connections only (`SAP_CONNECTION_TYPE=rfc` is not supported).

**Kerberos auth notes:**
- Requires a valid Kerberos ticket on the host before starting the server. Obtain one with `kinit` or a keytab.
- The optional [`kerberos`](https://www.npmjs.com/package/kerberos) npm package must be installed (needs GSSAPI dev libs on Linux / build tools on Windows): `npm i kerberos`.
- No `SAP_USERNAME` / `SAP_PASSWORD` required — identity comes from the TGT.
- Both auth types bypass the auth-broker; use `.env` directly.
- **NTLM is hard-rejected:** if the SAP system offers NTLM instead of Kerberos/SPNEGO, the connection fails with a clear error rather than silently downgrading. Ensure the system accepts Kerberos (SPNEGO) for your user.

> **⚠️ Help wanted — not yet validated on a live system.** Certificate and Kerberos auth pass full unit coverage but have not been tested against a real SAP system. If you have on-prem **client-certificate** or **Kerberos/SPNEGO** SSO, please try it and [open an issue](https://github.com/fr0ster/mcp-abap-adt/issues) with results — especially whether Kerberos succeeds with a single-leg Negotiate token or your system needs mutual-auth continuation.

**Generate .env from Service Key (JWT):**
```bash
# Install the connection package globally (one-time setup)
npm install -g @mcp-abap-adt/connection

# Generate .env file from service key JSON
mcp-auth auth -k path/to/service-key.json
```

This will automatically create/update `.env` file with JWT tokens and connection details.

**.env comments rule:** only full-line comments are supported (lines that start with `#`).  
Inline comments are not parsed, so keep comments on separate lines.

**Claude recommendation:** place the service key in the service-keys directory and use `--mcp=<destination>` (avoid manual JWT tokens).

### Command-Line Options

**Authentication:**
- `--auth-broker` - Force use of auth-broker (service keys), ignore .env file
- `--auth-broker-path=<path>` - Custom path for auth-broker service keys and sessions
- `--browser-auth-port=<port>` - Override OAuth browser callback port (default: 5000 for HTTP, 4000 for SSE, 4001 for stdio)
- `--connection-type=<http|rfc>` - SAP connection transport: `http` (default) or `rfc`
- `--unsafe` - Enable file-based session storage (persists tokens to disk). By default, sessions are stored in-memory (secure, lost on restart)

When `--mcp=<destination>` is specified, automatic fallback loading of `./.env` is skipped.

**Examples:**
```bash
# Use auth-broker with file-based session storage (persists tokens)
mcp-abap-adt --auth-broker --unsafe

# Use auth-broker with in-memory session storage (default, secure)
mcp-abap-adt --auth-broker

# Custom path for service keys and sessions
mcp-abap-adt --auth-broker --auth-broker-path=~/prj/tmp/ --unsafe
```

See [Client Configuration](docs/user-guide/CLIENT_CONFIGURATION.md) for complete configuration options.

### Handler logging switches
- `AUTH_LOG_LEVEL=error|warn|info|debug` — sets base log level for handler logger; `DEBUG_AUTH_LOG=true` also enables `debug`.
- `HANDLER_LOG_SILENT=true` — fully disables handler logging.
- `DEBUG_CONNECTORS=true` — verbose connection logging in high-level handlers.
- `DEBUG_HANDLERS=true` — enables verbose logs for selected read-only/system handlers.

## Development

### Testing
```bash
npm test
```

#### Test logging switches
- `TEST_LOG_LEVEL=error|warn|info|debug` — controls test logger verbosity (DEBUG_TESTS/DEBUG_ADT_TESTS/DEBUG_CONNECTORS force `debug`).
- `TEST_LOG_FILE=/tmp/adt-tests.log` — writes test logs to a file (best-effort).
- `TEST_LOG_SILENT=true` — disables test logging pipeline (console output muted).
- `TEST_LOG_COLOR=true` — adds colored/prefixed tags to test log lines.
- All `console.*` in tests are routed through the test logger with a `[test]` prefix.

### Building
```bash
npm run build
```

### Developer Tools
```bash
# Generate tool documentation
npm run docs:tools

# See tools/README.md for more developer utilities
```

## Contributors

Thank you to all contributors! See [CONTRIBUTORS.md](CONTRIBUTORS.md) for the complete list.

---

**Acknowledgment**: This project was originally inspired by [mario-andreschak/mcp-abap-adt](https://github.com/mario-andreschak/mcp-abap-adt). We started with the core concept and then evolved it into an independent project with our own architecture and features.

## License

**Two packages, two licences.** Which one applies depends on which you install.

| Package | Licence | |
|---|---|---|
| `@mcp-abap-adt/lib` | Apache-2.0 | [`LICENSE`](LICENSE), [`NOTICE`](NOTICE) |
| `@mcp-abap-adt/core` | AGPL-3.0-only | [`server/LICENSE`](server/LICENSE) |

Both are published from this repository with one command, in the order the
dependency requires:

```bash
npm run release:dry        # rehearses both, touches nothing
npm run release:publish    # @mcp-abap-adt/lib, then @mcp-abap-adt/core
```

`release:publish` skips a version already on the registry, so re-running after
a failure resumes rather than starting over. It aborts on the first failure
instead of publishing the server on top of a library that is not there.

Note that `npm publish` and `npm run` are different commands. `npm publish
release` asks npm to publish a package *named* `release`, which is somebody
else's package on the registry.

Copyright © 2025–2026 Oleksii Kyslytsia

Both are distributed in the hope that they will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.

**What this means.** Running either, on your own data, carries no conditions.

Embedding the library in your own application carries no obligation to open your
application: Apache-2.0 asks for the notice and the licence text to travel with
it, and nothing more.

Distributing the standalone server, or running a modified version of it as a
network service, means passing on the same freedoms under AGPL section 13 —
including the source. That is why the two are separate packages: installing the
library never puts the server in your dependency tree.

**The libraries underneath are LGPL-3.0-only** — `@mcp-abap-adt/adt-clients`,
`connection`, `interfaces` and `logger` — and the library links them at runtime.
LGPL does not reach your own code, but its terms do travel with those four
packages whatever this project is licensed as. Plan for that, not for the
notice on this repository.

**Other terms are possible.** Apache-2.0 is what the library is offered under
publicly, not the only way it can be offered. The copyright holder may license
the same code separately to a party who needs different terms; that takes
nothing away from anyone who received it under Apache-2.0, which is permanent.
[`CONTRIBUTORS.md`](CONTRIBUTORS.md#licensing) records what keeps that option
open, including the rule that no LGPL code from the packages underneath is ever
copied into this tree.

**History.** Releases through 8.13.0 were MIT and stay MIT; 9.x was
`GPL-3.0-only`. A licence change is not retroactive — anyone may still take an
earlier release under the licence it carried. See [`CONTRIBUTORS.md`](CONTRIBUTORS.md#licensing)
for the full account of how the relicensing was lawful.