mcp-bastion

🛡️ mcp-bastion

A reliability & security proxy for the Model Context Protocol (MCP).

Self-healing connections, runtime tool-security, and a compliance-mapped audit trail for your MCP servers.

mcp-bastion sits between your MCP client (Claude Code, Cursor, Cline, Windsurf, Zed, Claude Desktop, or any MCP-compliant agent) and your MCP servers. It is client-agnostic — it works with any compliant client through configuration alone, with zero client-specific code — and non-invasive: your servers run unchanged, and removing Bastion is a one-line config revert.

📦 Package: mcp-bastion on npm · 🗂️ Official MCP Registry: io.github.Gowthaman90/mcp-bastion

🔒 Security, measured: on the open, vendor-neutral mcp-defense-bench, Bastion covers 63% of the MCP attack surface (15.0/24 vectors; 11 enforced) at zero false positives — the broadest of the proxies measured.

📖 Launch story: Medium · dev.to

👤 Created & maintained by Gowthaman Arumugam — Independent Researcher. Companion benchmark: mcp-defense-bench.

Contents

• Why
• How it works
• Features
• Quick start
• Demo
• Control tools
• Configuration
• Transports
• Runtime security
• Audit & compliance
• Client setup
• Architecture
• Development
• Roadmap
• Contributing
• Security
• License

Why

When an MCP server disconnects mid-session, the agent only sees a generic "No such tool available" error — indistinguishable from a tool that never existed — and it cannot reconnect; only a human can. Long agent sessions silently lose capabilities and fail in confusing ways.

Bastion closes that gap. It health-checks every server, auto-reconnects with backoff, and — crucially — exposes control tools so the agent itself can inspect connection health and recover a dropped server without human intervention.

Bastion now spans three layers: reliability (v0.1), runtime security (v0.2 — tool pinning / rug-pull & poisoning detection), and audit & compliance (v0.3 — pluggable sinks mapped to NIST AI RMF / OWASP LLM Top 10). See the roadmap.

How it works

Today your client connects directly to each server. With Bastion, your client connects to Bastion, which connects to those same servers on your behalf — so it sits in the tool-call path and can add reliability (and, later, security) transparently.

Before:   Client ─▶ server A / server B / server C

After:    Client ─▶ mcp-bastion ─▶ server A
                                  ─▶ server B
                                  ─▶ server C

Bastion is a standard MCP server to your client and a standard MCP client to each upstream. Because it speaks the protocol faithfully, it works with every compliant client automatically — the only per-client difference is where you put a few lines of config.

Features

• 🔌 Client-agnostic — one binary, config-only integration; no per-client plugins.
• ♻️ Self-healing — health checks + capped exponential-backoff auto-reconnect for stdio servers.
• 🧭 Agent-recoverable — bastion__status and bastion__reconnect let the agent detect and fix drops itself, instead of hitting an opaque "no such tool" wall.
• 🧩 Transparent aggregation — merges many servers into one, with per-server tool namespacing to prevent collisions and tool-shadowing.
• 💬 Legible failures — a dropped server yields an actionable message, not a crash.
• 🛡️ Runtime security (new in v0.2) — pins each tool's definition and blocks "rug pulls" (a server changing a tool after approval); heuristically inspects descriptions for poisoning; detects cross-server shadowing. See Runtime security.
• 📝 Audit & compliance (new in v0.3, opt-in) — structured, integrity-hash-chained audit events to pluggable sinks (console / file / webhook), mapped to NIST AI RMF & OWASP LLM Top 10. See Audit & compliance.
• 🪶 Non-invasive & reversible — your servers run unchanged; uninstall is a config revert.
• 🧱 Enterprise-grade codebase — strict TypeScript, layered architecture, ESLint + Prettier, and unit + end-to-end tests.

Quick start

Bastion is published on npm as mcp-bastion — the npx command below fetches it automatically, so there's nothing to install first.

1. Add Bastion to your client, pointing it at a config file:

// your client's mcpServers config
{
  "mcpServers": {
    "bastion": {
      "command": "npx",
      "args": ["-y", "mcp-bastion", "--config", "bastion.config.json"],
    },
  },
}

2. List your real servers in bastion.config.json (moved verbatim from the client):

{
  "servers": {
    "github": { "command": "npx", "args": ["-y", "@modelcontextprotocol/server-github"] },
    "filesystem": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-filesystem", "/path/to/dir"],
    },
  },
  "reconnect": { "auto": true },
  "healthCheck": { "enabled": true },
}

3. Restart your client. Your tools now appear namespaced (e.g. github__create_issue) alongside Bastion's control tools. See bastion.config.example.json for the full set of options.

🔒 Security is on by default. Out of the box, Bastion runs the balanced enforcement profile: it blocks high-confidence attacks (rug-pulls, argument/command injection, cross-server exfiltration) and warns on heuristic ones (description/response poisoning), while redacting leaked secrets from tool results. Set security.enforcementProfile to observe (warn-only) or strict (block-all), or tune any individual control — see Runtime security.

Demo

See the whole thing in action — a server crashing mid-session and healing itself:

npm run demo

It boots Bastion in front of a server that crashes on command, shows the agent getting an actionable "reconnect" message instead of a cryptic error, and then the connection auto-recovering with no human involved. To record it as a GIF: asciinema rec demo.cast -c "npm run demo" && agg demo.cast assets/demo.gif.

Control tools

Bastion injects control tools so the agent can manage connections and review security itself, using only standard MCP calls:

Tool	Purpose
bastion__status	Health of every proxied server: connected / disconnected / reconnecting / failed, tool counts, last error.
bastion__reconnect	Reconnect a named server (argument: { "server": "<name>" }) without human intervention.
bastion__security	Per-tool security report: pin status (approved vs changed), poisoning findings, and shadowing.
bastion__compliance	Audit summary of recent activity mapped to NIST AI RMF / OWASP LLM Top 10 (requires audit.enabled).

Re-approval is operator-only. Clearing a rug-pull block is a security authority, so it is not an agent-callable tool — a prompt-injected agent must not be able to re-approve the very tool it was blocked from. A changed tool stays blocked until an operator clears it out-of-band; bastion__approve is not advertised and a client call to it is refused.

Configuration

Key	Type	Default	Description
servers	map	—	Upstream servers to proxy (required, at least one).
servers.<name>.command	string	—	Executable to launch (e.g. npx, node).
servers.<name>.args	string[]	[]	Arguments to command.
servers.<name>.env	map	—	Env overrides merged over the process env.
servers.<name>.cwd	string	—	Working directory for the spawned process.
reconnect.auto	boolean	true	Auto-reconnect after an unexpected disconnect.
reconnect.maxRetries	number	10	Max attempts before giving up (-1 = unlimited).
reconnect.initialBackoffMs	number	500	Initial backoff, doubled each attempt.
reconnect.maxBackoffMs	number	30000	Backoff ceiling.
healthCheck.enabled	boolean	true	Enable periodic liveness probing.
healthCheck.intervalMs	number	30000	Interval between probes.
healthCheck.timeoutMs	number	5000	Per-probe timeout.
namespace.strategy	prefix | passthrough	prefix	How upstream tool names are exposed.
namespace.separator	string	__	Separator used by the prefix strategy.
security.pinTools	boolean	true	Pin tool definitions and detect later changes.
security.onRugPull	block | warn	block	Action when a pinned tool's definition changed.
security.inspectDescriptions	boolean	true	Run poisoning heuristics on tool descriptions.
security.onPoisoning	block | warn	warn	Action on a high-severity poisoning finding.
audit.enabled	boolean	false	Record an audit event for every tool call.
audit.includeArgs	none|redacted|full	none	How tool arguments are recorded.
audit.tamperEvident	boolean	false	Hash-chain events so tampering is detectable.
audit.sinks	array	console	Destinations: console, file, webhook, otlp.
servers.<name>.transport	stdio | http	stdio	Local subprocess or remote endpoint.
servers.<name>.url	string	—	Remote MCP URL (required for http).
servers.<name>.headers	map	—	Headers for http upstreams (e.g. Authorization).
listen.mode	stdio | http	stdio	Serve Bastion over stdio or Streamable HTTP.
listen.host / listen.port	string / number	127.0.0.1 / 3000	Bind address for http mode.

Transports

Bastion speaks two transports on both faces:

• stdio (default) — the client spawns Bastion, and Bastion spawns local servers.
• Streamable HTTP — connect to remote MCP servers (servers.<name> with transport: "http", a url, and optional auth headers), and/or serve Bastion over HTTP to multiple/remote clients (listen.mode: "http", or --http <port>).

HTTP upstreams configured without an authentication header are flagged (authenticated: false) in bastion__status and warned at connect time.

Runtime security

New in v0.2. Bastion adds a security layer in the tool-call path (an interceptor pipeline), enabled by default:

• Rug-pull detection (tool pinning). Each tool's definition is pinned on first use. If a server later changes that definition, the tool is blocked (onRugPull: "block") until an operator reviews it and re-approves it out-of-band (operator-only — not an agent-callable tool). This catches a server that looks benign at install time and turns malicious afterward.
• Poisoning inspection. Tool names and descriptions are scanned for manipulation heuristics (instruction override, secret access, data exfiltration, covert instructions, embedded directives, hidden/zero-width characters). Because heuristics can false-positive, the default is warn (logged and reported, not blocked); set onPoisoning: "block" to enforce.
• Shadowing. When two servers expose a tool with the same name, it's surfaced in the report.

Review everything with the bastion__security tool. These checks apply to local stdio servers today; authentication checks for remote servers arrive with HTTP transport support.

Audit & compliance

New in v0.3, opt-in. Enable audit to record a structured, versioned event for every tool call — including calls blocked by the security layer:

"audit": {
  "enabled": true,
  "includeArgs": "redacted",     // none | redacted | full
  "tamperEvident": true,          // integrity hash-chain (detects naive edits; unkeyed, not signed)
  "sinks": [
    { "type": "file", "path": "./bastion-audit.jsonl" },
    { "type": "webhook", "url": "https://collector.example/v1/audit" }
  ]
}

• Pluggable sinks. console (stderr JSONL), file (JSONL append), webhook (batched POST), and otlp (native OpenTelemetry logs export — point it at an OTel Collector to fan out to any SIEM/cloud backend). The sink interface makes new destinations additive.
• Compliance mapping. Each event is mapped to NIST AI RMF functions and OWASP LLM Top 10 categories; bastion__compliance returns an aggregate report of recent activity.
• Integrity hash chain. With tamperEvident, events are linked by a SHA-256 hash chain, and the exported verifyChain helper detects naive or partial edits within an intact log. The chain is unkeyed: a party who can rewrite the log file can recompute a consistent chain, and truncating the most-recent events links cleanly — so treat this as corruption-detection, not cryptographic tamper-proofing. For stronger guarantees, ship events to an append-only external sink (webhook / OTLP → a WORM store or SIEM). Keyed/signed attestation is on the roadmap.
• Redaction (best-effort). Arguments are omitted by default; includeArgs: "redacted" keeps structure while masking values under known credential key-names and common secret patterns. It is a heuristic, not a guarantee — a secret under an unrecognized key can still be recorded, so prefer none for high-sensitivity deployments.

Standards alignment

Every security check maps to recognized frameworks — the NIST AI Risk Management Framework (a U.S. federal standard), the OWASP Top 10 for LLM (2025) and Agentic (2026) Applications, and STRIDE. The full per-check mapping (with reference links to the MCP-security literature the checks are drawn from) is in docs/CHECKS-MAPPING.md.

Coverage is measured independently — bastion is scored against a 22-vector attack surface by the vendor-neutral mcp-defense-bench benchmark, with a public leaderboard and framework mapping.

Client setup

The steps are identical for every client — only the config file location differs:

Client	Where to add the bastion entry
Claude Code	project .mcp.json (or claude mcp add)
Cursor	~/.cursor/mcp.json or project .cursor/mcp.json
Claude Desktop	claude_desktop_config.json
Cline	cline_mcp_settings.json
Windsurf	~/.codeium/windsurf/mcp_config.json

Gradual adoption: you don't have to route every server through Bastion — put only your flaky or untrusted servers behind it and leave the rest connected directly.

Architecture

Bastion is organized into clear layers with a one-directional dependency flow, so each concern is independently testable and easy to evolve:

src/
├── cli.ts              # thin CLI entrypoint (parse → wire → serve)
├── index.ts            # public library API
├── errors.ts           # error hierarchy (BastionError, …)
├── config/             # schema (Zod) + loader
├── core/               # domain: upstream connection lifecycle, aggregation & routing
├── proxy/              # client-facing MCP server + control tools
├── observability/      # logging (audit sinks in v0.3)
└── internal/           # small cross-cutting utilities

Design details — including the client-agnostic rationale, the interceptor pipeline, and the audit-sink strategy — live in the project's design docs.

Development

npm install
npm run check      # format:check + lint + typecheck + test (the full gate)
npm test           # unit + end-to-end (in-memory transport) tests
npm run build      # bundle to dist/ (CLI + library)
npm run dev -- --config bastion.config.json

Script	Does
build	Bundle CLI + library with tsup.
dev	Run the CLI from source with tsx.
typecheck	tsc --noEmit (strict).
lint / lint:fix	ESLint (flat config).
format / format:check	Prettier.
test / test:watch	Vitest.
check	Everything above, as one gate.

Roadmap

Version	Theme	Highlights
v0.1 ✅	Reliability	Aggregating proxy, auto-reconnect, bastion__status / __reconnect.
v0.2 ✅	Runtime security	Tool-definition pinning (rug-pull detection), poisoning inspection, shadowing detection.
v0.3 ✅	Audit & compliance	Pluggable audit sinks (console / file / webhook), NIST AI RMF / OWASP LLM Top 10 mapping.

Both stdio and Streamable HTTP transports are supported (see Transports).

Contributing

Contributions are very welcome — this project is built to be community-owned. Please read CONTRIBUTING.md for the dev setup, project layout, and PR workflow, and our Code of Conduct.

In short: open an issue for non-trivial changes, keep PRs focused with tests, and make sure npm run check passes (CI runs it on Node 18/20/22). Good first areas: additional client setup recipes, more upstream test fixtures, and Streamable HTTP transport support.

Security

mcp-bastion is security-adjacent software, so we hold it to a high bar. Please report vulnerabilities privately — do not open a public issue. See SECURITY.md for the disclosure process.

License

Apache-2.0 © Gowthaman Arumugam and mcp-bastion contributors