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

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:

  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.

Project links

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Key dates

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  • Released:
Newer release available (0.35.1)

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Credits

Author: pace

GitHub Statistics

Data verified by PyPI on Sep 3, 2026
The GitHub source repository was provided by the project maintainers and verified by PyPI at the time of upload. Stars, forks, and open issues/PRs are derived from that repository and have not been independently verified.

License expression

MIT
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Requires

Python >=3.14

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