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.
Install
pip install 3tears-mcp # server, tools, identity, PlatformHttpClient
pip install "3tears-mcp[grants]" # plus the Postgres-backed grant store
The base depends on the mcp SDK, 3tears-observe, httpx, starlette and uvicorn only. Everything
below that needs the rest of the family -- LocalGrantAuthorizer, rbac, migrations/ -- is the
grants extra. Bring your own Authorizer and the base is all you install.
What's in here
| Module | Responsibility |
|---|---|
server |
McpServer -- wraps the official mcp.server.Server. Owns tool registration, RBAC gating before handler dispatch, structured error mapping per the MCP spec. |
tool |
McpTool dataclass (name, description, input_schema, required_permission, handler) and register_tool decorator. |
http_client |
PlatformHttpClient -- typed httpx client with JWT login + refresh-on-401. Used by both MCP server tool handlers (calling /api/v1/...) and CLI scripts. One HTTP-client implementation, two transports. |
auth |
Identity dataclass + IdentityProvider Protocol + EnvVarIdentityProvider (stdio impl). Authorizer Protocol + LocalGrantAuthorizer (default impl backed by McpToolGrantCollection). |
rbac |
McpToolGrantCollection -- BaseCollection over mcp_tool_grants. Exposes the in-memory grant cache that LocalGrantAuthorizer consults. |
migrations/ |
v01_create_mcp_tool_grants -- platform-scope DDL. Consumers register via MigrationRunner.register(epoch_pkg) (same shape as threetears.epoch). |
RBAC model
Per-tool, default-deny. Each McpTool declares a required_permission string (e.g. "conversations.read", "audit.read"). On every dispatch:
- The framework calls
Authorizer.allows(identity, required_permission). LocalGrantAuthorizerchecks whether the caller's identity matches an active grant inMcpToolGrantCollectionfor the requested permission.- 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.
1 maintainer
Metadata
1 maintainer
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