app-foundation

Host-neutral foundations shared by applications in an ecosystem.

Current Status

2026.09.22.2029 is the first implementation candidate. It contains two host-neutral foundations: MCP client construction and strict native credential-value storage.

python -m pip install 'app-foundation[mcp]'
python -m pip install 'app-foundation[native-secrets]'
python -m pip install 'app-foundation[data-bus]'

The package API is under app_foundation.mcp:

• open_mcp_client(...) opens stdio, SSE, or Streamable HTTP transports;
• mcp_tool_schema(...) and normalize_mcp_tool_result(...) provide stable Python values at the protocol boundary;
• connect_remote_tools(...) and probe_remote_tools(...) provide a Streamable HTTP convenience surface for callers that already possess an endpoint and bearer.

Remote connection failures use stable, secret-safe codes. An HTTP 401 or 403, including one wrapped by the MCP transport's exception group, becomes mcp_authorization_rejected; response bodies do not cross the boundary. Timeouts and other connection failures retain their separate codes so a host can retry connectivity without treating an outage as a credential verdict.

The caller remains responsible for credential custody, authority decisions, and product-specific error language. Supplying a bearer to the transport does not grant or evaluate authority.

The native value-store API is under app_foundation.secrets:

• NativeSecretValueStore(...) selects one reviewed operating-system backend;
• replace(...), get(...), and remove(...) manage bounded text values;
• verify_ready() proves a disposable write, read, and removal;
• NativeSecretError exposes fixed, secret-safe error codes and messages.

The accepted backends are macOS Keychain (keyring.backends.macOS.Keyring), Windows Credential Manager (keyring.backends.Windows.WinVaultKeyring), and Linux Secret Service (keyring.backends.SecretService.Keyring). Null, fail, chainer, file, and wrong-platform backends are rejected. Windows values use a versioned, integrity-checked chunk manifest so replacement either selects one complete new generation or leaves the previous generation readable. The shared bound is 288 KiB of UTF-8 text, which covers the largest OAuth record accepted by the Connection Hub CLI after JSON escaping.

The shared store knows only service names, account keys, and text values. A consuming product owns serialization, logical namespaces, access policy, recovery, and the meaning of each secret.

The Data Bus client API is under app_foundation.data_bus:

• DataBusClaim.from_mapping(...) validates a short-lived, bundle-scoped claim without rendering its token;
• FederatedDataBusClient.connect() opens the authenticated Socket.IO lane;
• close() stops either a connected socket or an in-progress reconnect loop and waits until the transport task has ended;
• request(...) distinguishes ingress acceptance from the handler's correlated terminal result;
• wait_for_event(...) receives application push events that are not replies to an in-flight request.

The client constructs transport envelopes and correlates replies. The application owns subjects, operation names, domain authorization, and the meaning of pushed events. It refuses an expired claim before connection and bounds event waits by claim expiry. A Data Bus receipt broadcast by the session for another peer is ignored by this client; it cannot wake an application event loop. An ingress acknowledgement timeout leaves acceptance unknown; a terminal result received before that timeout is returned. An accepted request without a terminal result is also outcome unknown. In either unknown case, the product preserves the original message and operation identity for a retry. Client errors include the logical connection generation, Socket.IO connection ID, and connection state captured when the request began. The client logs successful connections, disconnects, reconnects, and refused connection attempts with the same generation and connection ID. A refused attempt explicitly records whether it replaced the last successful generation, so a product can correlate one request without treating reconnect activity as proof that its connection changed. A product that replaces client objects may provide non-secret lifecycle_labels; those labels appear on every lifecycle record and let the product carry its stable channel identity and replacement epoch across object-local generation resets.

Boundary

Applications serving real users repeatedly need the same host capabilities:

• principal and service-identity contracts;
• secret-reference resolution and vault adapters;
• Postgres and Redis clients, cache, compare-and-set, and distributed locks;
• HTTP, CSRF, and external-URL utilities;
• events and observability primitives;
• protocol clients and neutral result conversion.

app-foundation owns reusable application-facing mechanisms. Product authority, application-domain behavior, standalone process lifecycle, and deployment orchestration remain outside this package.

app-foundation does not import service-foundation. The two distributions can be composed by a product without creating a dependency cycle.

Extraction Contract

The production implementations being separated live in KDCube. Each extraction moves one verified contract into this package, then leaves the former KDCube path as a thin compatibility import. KDCube's MCP agent adapter and distributed server defaults remain platform-owned; they are not part of this client extraction.

License: MIT. Source: https://github.com/elenaviter/app-ecosystem