STDIO server

Most published MCP servers (the official @modelcontextprotocol/server-* packages, plenty of community ones) only speak stdio. Krypton can host them via mcp-stdio-bridge — a tiny HTTP-in-front-of-stdio shim that runs alongside your server inside the same container.

How it works

HTTP POST /  ──►  mcp-stdio-bridge  ──►  child stdin
                          ▲                    │
                          │                    ▼
                       JSON-RPC          child stdout
                       (id-routed)

• The bridge forks $MCP_COMMAND once at container startup.
• HTTP requests are serialized to the child’s stdin.
• Responses are demultiplexed by JSON-RPC id, so concurrent HTTP requests don’t cross-talk.
• Notifications (no id) are fire-and-forget.
• One-shot stdio servers (process-per-call) aren’t supported — the child must be long-lived.

Deploy

The published bridge image is a pure-Go binary on distroless/static. That’s enough for stdio binaries that are themselves statically linked. For runtime-dependent servers (Node, Python), layer the runtime onto a derived image first.

Minimal example (Node + npx)

# Dockerfile
FROM krypton/mcp-stdio-bridge:latest AS bridge
FROM node:22-slim
COPY --from=bridge /app /app
USER 65532
ENTRYPOINT ["/app"]

docker build -t my-registry/mcp-node-bridge:latest .
docker push my-registry/mcp-node-bridge:latest

apiVersion: krypton.ai/v1alpha1
kind: Agent
metadata:
  name: filesystem-mcp
  namespace: agents
spec:
  image: my-registry/mcp-node-bridge:latest
  protocol: mcp
  port: 8080
  mode: always-on
  minReplicas: 1
  env:
    - name: MCP_COMMAND
      value: "npx -y @modelcontextprotocol/server-filesystem /data"

The bridge reads MCP_COMMAND, forks it once, then accepts HTTP requests on :8080.

Configuration

Limitations

• Single child per container. If you need replicas, scale the Deployment (spec.maxReplicas) — each pod gets its own child.
• Process-per-call stdio servers aren’t supported. The bridge doesn’t fork on every request.
• No state introspection. If your stdio server reports its state via stderr or sidechannel files, the bridge passes those through unchanged but doesn’t surface them in the agent’s status.

Once it’s running

Identical to the HTTP server case — tool introspection shows up in the operator UI under /ui/mcp/<namespace>/<name>, and the typed /v1/agents/{ns}/{name}/mcp/tools[*] endpoints work the same way.