Mcp Broker

mcp-broker

mcp-broker is a local Model Context Protocol process broker for MCP clients.

Think PgBouncer for MCP: one stable local endpoint in front of many upstream MCP servers. The broker owns upstream startup, reuse, cleanup, profile exposure, status, and safe tool routing.

The core idea is simple: do not make every agent session load every upstream tool definition before the user asks a task.

Why this exists

AI coding sessions with many MCP servers tend to accumulate the same problems:

• every client config repeats the same MCP server list
• every new session can start duplicate upstream processes
• OAuth, browser state, local files, and database handles spread across tools
• raw tool lists consume context before the task begins
• hosted connector caches can duplicate local MCP tools
• orphaned MCP processes survive after client sessions exit

mcp-broker puts a small broker facade in front of those upstreams. It is not a hosted workflow builder; it is local infrastructure for keeping MCP clients small, predictable, and under one config contract.

Client profile
        |
        | one local MCP entry
        v
  mcp-broker-client
        |
        | Unix socket
        v
  mcp-broker-daemon
        |
        | profile gates, namespace routing, status, cleanup
        v
  upstream MCP servers

The client sees a small set of broker tools:

broker_search_tools
broker_describe_tool
broker_call_tool
broker_status

The upstream MCPs still exist. They are discovered and called through the broker when a task needs them.

Measured context reduction

On 2026-05-24, the measured Codex setup went from many raw MCP and hosted app tool definitions to one broker facade plus a pruned codex_apps cache.

| Surface | Before | After | Reduction | |---|---:|---:|---:| | Direct Codex MCP server entries | 11 | 1 | 90.91% | | MCP tool definitions | 414 | 4 | 99.03% | | Hosted codex_apps tool definitions | 195 | 39 | 80.00% | | Combined always-loaded tool definitions | 609 | 43 | 92.94% | | Combined serialized tool payload bytes | 1,026,171 | 185,877 | 81.89% | | Combined o200k_base tool tokens | 276,989 | 45,281 | 83.65% |

The 92.94% number is a tool-definition count reduction. The 83.65% number is a token reduction for canonical serialized tool payloads measured with tiktoken o200k_base.

See [docs/context-reduction-measurement.md](docs/context-reduction-measurement.md) for evidence and caveats.

What it does

• Runs one local broker daemon over a Unix socket.
• Exposes one lightweight stdio client shim to MCP clients.
• Starts upstream MCP servers on demand.
• Reuses shared upstreams across sessions when configured.
• Isolates per-session upstreams when state must not be shared.
• Maps upstream tools into stable namespaces.
• Exposes compact search, describe, call, and status tools.
• Enforces profile-level tool budgets and exposure gates.
• Blocks mutating upstream exposure unless a profile allowlist grants it.
• Stores runtime state under $HOME/mcp/mcp-broker, outside the repo.
• Renders MCP client config entries with dry-run, backup, and rollback.
• Provides LaunchAgent install and uninstall flows for macOS.
• Provides Linux systemd user-service render, install, unload, and removal flows.
• Provides Windows PowerShell Scheduled Task render, install, and removal flows.
• Includes unit, journey, live, and e2e tests through Makefile targets.

Core differentiators:

• Profile-scoped exposure: each MCP client gets a configured view of upstreams instead of every tool by default.
• Mutating-tool gates: mutating upstreams stay hidden until a profile allowlist grants access.
• Lifecycle ownership: shared and per-session upstreams are started, watched, stopped, and reaped by the broker.
• Client parity checks: rendered profiles can be validated through the same compact broker facade before config is applied.

Who this is for

Use mcp-broker if you:

• use Codex, Claude Code, Gemini CLI, or other MCP clients
• have more MCP tools than you want in every session
• need shared local MCP servers without duplicate process startup
• want one place for OAuth state, browser state, sockets, logs, and cleanup
• need per-client profiles instead of the same tool list everywhere
• want a small broker facade instead of raw upstream tool dumps

This repo is not an enterprise MCP control plane. It is local desktop infrastructure for developer-agent workflows.

Current status

Implemented:

• YAML config loading from config/broker.private.yaml, created from config/broker.example.yaml.
• Strict YAML contract validation for runtime, clients, profiles, upstreams, and policy blocks.
• Public JSON Schema validation through config/broker.schema.json.
• Runtime path derivation from runtime.root.
• Tool namespace mapping from configured upstream prefixes.
• Local upstream subprocess lifecycle management and process-group cleanup.
• Broker daemon over Unix socket.
• MCP client shim and renderers for configured MCP client profiles, including Codex, Claude, and Gemini.
• Gemini profile rendering to .gemini/settings.json, including its MCP

allowed-server policy.

• Dry-run client config rendering, apply-time backups, and rollback.
• LaunchAgent render and install scripts with dry-run defaults.
• Compact broker facade for search, describe, call, and status.
• Profile validation from YAML smoke probes.
• Discovery parity checks between compact client profiles.
• Public and maintainer quality gates through Makefile targets.

Wiring status:

• Codex is wired through the broker.
• Claude is wired through the broker after profile validation and manual /mcp acceptance.
• Gemini is wired through the broker by rendering .gemini/settings.json.

Public release status:

• The repo is designed to stay public-safe.
• Private upstream inventory, account paths, OAuth state, secrets, sockets, logs, and generated client configs stay outside git.
• Stable release metadata is validated by make release-version-check; publication proof is tracked in docs/distribution.md.
• Docker image support is available for container-friendly configs. Docker MCP Catalog submission still requires Docker review.
• MCPB metadata is present at mcpb/manifest.json for local directory review.

See [ROADMAP.md](ROADMAP.md) for public-facing release work.

Architecture

mcp-broker has three runtime layers:

| Layer | Responsibility | |---|---| | Client shim | Presents one stdio MCP server entry to each MCP client and forwards JSON-RPC over the broker socket. | | Broker daemon | Owns profile gates, namespace routing, upstream lifecycle, status, logging, and cleanup. | | Upstream MCP servers | Run as configured stdio, HTTP, streamable HTTP, or SSE connectors with shared or per-session process policy. |

The config file is the contract. Profiles decide exposure, upstreams define transport and lifecycle behavior, and smoke probes define safe read calls for validation.

Comparison

| Approach | Best fit | Tradeoff | |---|---|---| | Raw MCP client config | Small setups with a few tools. | Every session loads the full tool list and each client repeats config. | | Simple MCP proxy | Forwarding one server to one client. | Does not own upstream lifecycle, profile budgets, or cross-client cleanup. | | Hosted app connectors | SaaS tools managed by the client provider. | Local MCP state and cross-client parity remain outside user control. | | mcp-broker | Local developers with many upstream MCPs across MCP clients. | Adds a local daemon and config contract that must be installed and monitored. |

Screenshots Or GIF

The quickstart flow should look like this:

make config-init
make config-validate
make broker-status
make codex-facade-smoke

In an MCP client, /mcp should show one mcp-broker entry. Use broker_status to inspect profile-visible upstream state.

Quickstart

Prerequisites:

• macOS with launchctl for LaunchAgent use.
• Python 3.10 or newer available as python3.
• make.
• Node.js and npx for npm-based upstream MCP servers.
• A clone of this repo.

Package installs:

pipx install mcp-broker
uv tool install mcp-broker
brew tap ${HOMEBREW_TAP_REF}
brew install mcp-broker

Homebrew installs the same console scripts as the Python package. Package installs do not write MCP client config; client wiring stays an explicit Makefile action.

Docker is for container-friendly configs:

docker build -t mcp-broker:local .
docker run --rm -i mcp-broker:local

Local stdio clients and MCPB-style installs use the package-owned lifecycle:

mcp-broker stdio --init-if-missing

Create the local venv, install dependencies, and verify runtime layout:

make setup

Create private config from the public template:

make config-init

config-init creates the destination directory when needed and copies the public template as the starting point. It does not import local MCP inventory, user paths, or secrets.

Edit config/broker.private.yaml for local upstreams. Keep secret values out of config. Use environment variable names or files under:

$HOME/mcp/mcp-broker/secrets/

Run the quality gate:

make quality-gate

Validate the configured YAML contract:

make config-validate

Start the broker:

make broker-start

Check status:

make broker-status

For the full install flow, see [docs/install.md](docs/install.md).

Runtime layout

Default runtime root:

$HOME/mcp/mcp-broker/
|- backups/
|- logs/
|- renders/
|- run/
|- secrets/
|- sockets/
`- state/
   `- upstreams/

Runtime files are not repo files. Upstream OAuth state, browser state, secret files, sockets, logs, rendered client configs, backups, and daemon state belong under the runtime root.

See [docs/runtime-layout.md](docs/runtime-layout.md).

Client wiring

Back up a client config:

make config-backup CLIENT=codex

Dry-run render:

make config-render CLIENT=codex CONFIG_RENDER_APPLY=0

Apply after reviewing the rendered file under $HOME/mcp/mcp-broker/renders/:

make config-render CLIENT=codex CONFIG_RENDER_APPLY=1

Rollback:

make config-rollback CLIENT=codex

Use CLIENT=claude or CLIENT=gemini after that profile smoke passes and that client is intended to use the broker. For new JSON-based MCP clients, generate a starter block:

make profile-snippet NEW_PROFILE=local-client NEW_CLIENT_FORMAT=mcp-settings-json

See [docs/add-profile.md](docs/add-profile.md) for the full new-profile flow.

Compact broker facade

The compact facade keeps chat-facing profiles small:

| Tool | Purpose | |---|---| | broker_search_tools | Search configured upstream tools by query. Results carry name, description, upstream, purpose, tags, and mutating flag; the heavy inputSchema is omitted and fetched on demand from broker_describe_tool. | | broker_describe_tool | Return schema and metadata for one upstream tool. | | broker_call_tool | Call one upstream tool through broker routing. Accepts an optional projection ({"paths": [...], "max_array_items": N}) that trims the response server-side before it reaches the client. | | broker_status | Show profile-visible upstream state, passive auth probes, and last errors without starting tools. |

Codex /mcp shows the single mcp-broker entry by design. Per-upstream visibility, status, and socket path come from broker_status.

Profiles and safety

Profiles decide which upstreams a client can see and call.

Supported concepts:

• max_tools protects clients from huge tool lists.
• compact_tools_enabled exposes broker facade tools instead of raw upstream tools.
• broker_tool_name_style adapts broker facade names for clients that cannot surface dotted tool names.
• mcp_allowed_servers renders client settings for MCP clients that require an explicit server allowlist.
• allow_mutating_upstreams is required before a mutating upstream can be exposed.
• shared mode reuses one upstream process where shared account state is acceptable.
• per_session mode isolates upstream state per client session.
• disabled mode keeps compatibility records without exposing the upstream.

Protected surfaces such as OAuth, browser state, filesystem roots, and databases require explicit config and validation. Public examples stay disabled or placeholder-based.

See [docs/security-review.md](docs/security-review.md) and [docs/upstream-compatibility-matrix.md](docs/upstream-compatibility-matrix.md). For a deeper safety checklist, see [docs/safety.md](docs/safety.md).

Config contract

The public template is [config/broker.example.yaml](config/broker.example.yaml). The matching JSON Schema is [config/broker.schema.json](config/broker.schema.json).

Supported top-level sections:

schema_version: 1
runtime: {}
broker: {}
profiles: {}
clients: {}
upstreams: {}

The loader rejects unknown keys. Runtime placeholders such as {runtime.root}, {runtime.state_dir}, and {runtime.secrets_dir} can be used in upstream command, args, working directory, and env file paths.

make config-validate checks the selected CONFIG_PATH against the public JSON Schema first, then runs the runtime loader so semantic rules are enforced from the same code path the broker uses.

Each enabled upstream exposed to a profile should define a safe smoke probe:

smoke:
  query: read example graph
  tool: example-store.read_graph
  arguments: {}
  call: true

make profile-validation PROFILE= validates every enabled upstream visible to that profile through broker_status, broker_search_tools, broker_describe_tool, and the configured safe broker_call_tool.

Codex operator acceptance

Repo-owned tests validate broker behavior through the local client shim. The last Codex-specific check has to run inside an active Codex session because that is where the deferred MCP wrapper tools exist.

Generate the current acceptance steps from YAML:

make codex-deferred-acceptance

The target reads the configured smoke probes and prints the exact mcp__mcp_broker__ wrapper calls for search, describe, and safe call. It does not invoke Codex, does not call an external LLM session, and is not part of make quality-gate.

See [docs/codex-deferred-tool-acceptance.md](docs/codex-deferred-tool-acceptance.md).

LaunchAgent

Render without writing:

make launchagent-install

Apply and load:

make launchagent-install LAUNCHAGENT_APPLY=1
make launchagent-load
make broker-status

Unload or remove:

make launchagent-unload
make launchagent-uninstall LAUNCHAGENT_APPLY=1

systemd

Linux user-service install uses the same runtime root and config path contract:

make systemd-install
make systemd-install SYSTEMD_APPLY=1
make systemd-load

For package installs, set MCP_BROKER_DAEMON_COMMAND to the installed daemon path before applying the service.

Windows

Windows startup uses PowerShell Scheduled Task commands with the same runtime root and config path contract:

make windows-install
make windows-install WINDOWS_APPLY=1
make windows-load

Remove it with:

make windows-unload
make windows-uninstall WINDOWS_APPLY=1

Test and release gates

Run all test tiers:

make test

Run the public quality gate:

make quality-gate

The coverage gate uses line and branch coverage for Python source.

Run the release gate when preparing a tag:

make release-gate

release-gate runs package, smoke, and mutation checks. Mutation receives a release-scoped child count derived from LOCAL_CPU_BUDGET and RELEASE_GATE_JOBS, so it does not take the full CPU budget while other release children run. Mutation runs public unit and journey tests last and writes var/quality/mutation_stats.json with total counts, score, and ranked

…

Source & license

This open-source MCP server is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: doricstack
• Source: doricstack/mcp-broker
• License: MIT
• Homepage: https://github.com/NavinAgrawal/mcp-broker#readme

Install and usage instructions live in the source repository linked above.