3tears-mcp

3tears-mcp

Shared MCP (Model Context Protocol) framework. Per-product MCP servers compose this framework instead of reimplementing stdio transport, JWT auth, error mapping, and per-tool RBAC.

What's in here

RBAC model

Per-tool, default-deny. Each McpTool declares a required_permission string (e.g. "conversations.read", "audit.read"). On every dispatch:

1. The framework calls Authorizer.allows(identity, required_permission).
2. LocalGrantAuthorizer checks whether the caller's identity matches an active grant in McpToolGrantCollection for the requested permission.
3. If denied, the framework returns a structured MCP error to the client (not a Python exception in the response body).

The configured admin identity (env-var creds in the stdio impl) is auto-granted in memory at server startup. The grant is logged but NOT written to mcp_tool_grants. This keeps the table truthful (only operator-added grants live there).

Grant changes propagate cross-pod via the mcp.rbac epoch broadcast. LocalGrantAuthorizer subscribes to Subjects.mcp_rbac_epoch() via an EpochListener; on bump it reloads the grant cache from L3. Cold-start primes from EpochClient.current(...). Missed broadcasts recover via the standard pull-on-stale path.

Identity in v1

EnvVarIdentityProvider returns one fixed Identity for the lifetime of the server, derived from env-var credentials. v2 (HTTP transport + per-call bearer-token identity) plugs in by adding a BearerTokenIdentityProvider; the rest of the framework is unchanged. The Authorizer.allows(identity, permission) interface is unchanged between v1 and v2.

Stdio transport discipline

Every byte on stdout/stderr from a stdio MCP server confuses the client. The framework configures logging to a file or to NATS; no module under threetears.mcp should write to sys.stdout / sys.stderr. An AST enforcement test guards this.

Postgres backing

CREATE TABLE IF NOT EXISTS mcp_tool_grants (
    grant_id UUID PRIMARY KEY,
    principal_type TEXT NOT NULL,   -- 'user' | 'group' | 'role'
    principal_id UUID NOT NULL,
    tool_name TEXT NOT NULL,
    permission TEXT NOT NULL,
    date_created TIMESTAMPTZ NOT NULL DEFAULT now()
);

Mutation paths (admin POST /admin/mcp/grants, DELETE /admin/mcp/grants/{id}) bump Subjects.mcp_rbac_epoch() after the row commit; sibling pods reload via the epoch listener.