MCP Bridge Patterns

MCP Bridge Patterns

Recurring patterns for building MCP servers that bridge Claude Desktop to Hermes Agent. v2.0: HTTP Bridge replaces subprocess spawning.

When to Use

• Building a new MCP bridge (Node.js, stdio transport)
• Debugging "MCP error -32001" or socket hang up from Hermes Gateway API
• Adding tools to an existing MCP server
• Routing tasks to Kanban boards based on project path

Pattern 1: HTTP Bridge (v2.0)

Problem: spawn("hermes chat -q") is slow, fragile, and fails in Claude Desktop because MCP stdio servers inherit a limited subset of PATH. hermes binary is not available.

Solution: HTTP POST to Hermes Gateway API. Zero subprocess. Retry logic handles transient failures.

function hermesApi(messages, profile, timeoutMs, retries = 2) {
  return new Promise((resolve, reject) => {
    const attempt = (n) => {
      const req = request(`${API_URL}/v1/chat/completions`, {
        method: "POST",
        headers: { "Authorization": `Bearer ${API_KEY}`, "Content-Type": "application/json" },
        timeout: timeoutMs,
      }, (res) => { /* accumulate data, parse, resolve */ });
      req.on("error", (e) => {
        if (n > 0 && isTransient(e)) setTimeout(() => attempt(n-1), backoff(n));
        else reject(e);
      });
      req.write(body); req.end();
    };
    attempt(retries);
  });
}

Retry: Only socket hang up, ECONNRESET, timeout. Backoff 2s→4s. Max 3 attempts.

API key source: Read from the Hermes environment file at server startup. Set the Authorization header on every request.

Pattern 2: Non-blocking Dispatch with File Persistence

All tools (quick_task, lead_delegate, kanban views) should be non-blocking. Return task_id immediately. Background HTTP requests persist state to disk. Client polls via lead_task_status.

Key features: file persistence (survives restarts), content-hash dedup (duplicate EPICs get same ID), max 2 concurrent, auto-cleanup after 1h.

Pattern 3: Kanban-Aware lead_task_status

lead_task_status accepts two ID formats:

Never use exec() to query Kanban. MCP servers don't inherit the user's PATH. Always route through the Gateway API with the Lead Architect profile.

Pattern 4: Board Routing via project_path

Map project_path to board slug. Inject into Lead Architect prompt:

const boardMap = { "Azdal": "azdal", "CarSah": "carsah" }; // key = project folder basename
const board = boardMap[projectPath] || "default";

Boards created with hermes kanban boards create <slug>. Isolated per-project history.

Pattern 5: project_path Enforcement

lead_delegate MUST require project_path. Validate with fs.existsSync(). No path → hard refusal. Prevents the #1 contamination class: EPICs routed to wrong project via stale memory.

Pattern 6: MCP PATH Limitations

Per MCP spec: stdio servers inherit a limited subset of environment variables. Never use exec(), spawn(), or any subprocess that depends on the user's PATH. Route everything through the Gateway API — it runs in the full user environment.

Zero-Dependency MCP Server Boilerplate (Node.js)

#!/usr/bin/env node
import { spawn, exec } from "child_process";

// Stdio JSON-RPC transport
process.stdin.on("data", data => {
  for (const line of data.toString().trim().split("\n")) {
    if (!line) continue;
    try { handleMessage(JSON.parse(line)); } catch(e) { /* log to stderr */ }
  }
});

function send(msg) { process.stdout.write(JSON.stringify(msg) + "\n"); }

function handleMessage(msg) {
  if (msg.method === "initialize") {
    send({ jsonrpc: "2.0", id: msg.id, result: {
      protocolVersion: "2024-11-05",
      capabilities: { tools: {} },
      serverInfo: { name: "...", version: "1.0.0" }
    }});
  } else if (msg.method === "tools/list") {
    send({ jsonrpc: "2.0", id: msg.id, result: { tools: [...] }});
  } else if (msg.method === "tools/call") {
    runTool(msg.params.name, msg.params.arguments)
      .then(r => send({ jsonrpc: "2.0", id: msg.id, result: r }))
      .catch(e => send({ jsonrpc: "2.0", id: msg.id, result: { content: [{ type: "text", text: e.message }], isError: true }}));
  }
}

// Keep process alive (MCP stdio server must never exit)
setTimeout(() => {}, 86400000);

package.json requirements:

{ "type": "module", "main": "server/index.js" }