03 — Circuit Breaker

Prerequisite: 02 — Health Checks You will need: Working setup from recipe 02 Time: 15 minutes Adds: MCP Server groups with circuit breaker for fast-fail protection

The Problem

Health checks from recipe 02 run every 30 seconds. Between checks, a flaky MCP server can accept requests, fail, get retried, fail again — wasting agent time and tokens on a MCP server that's clearly broken. Your MCP server responds to health checks (it's technically alive) but fails 80% of real tool calls. Intermittent failure. Health checks say READY. Agents suffer.

Health checks tell you the patient is dead. Circuit breakers stop you from performing surgery on a corpse.

The Config

# config.yaml — Recipe 03: Circuit Breaker

mcp_servers:
  my-mcp:
    mode: remote
    endpoint: http://localhost:8080/mcp
    description: "My remote MCP server"
    health_check_interval_s: 30
    max_consecutive_failures: 3
    http:
      connect_timeout: 10.0
      read_timeout: 30.0

  my-mcp-group:                            # NEW: added in this recipe
    mode: group                            # NEW: added in this recipe
    description: "My MCP group with circuit breaker"  # NEW: added in this recipe
    strategy: round_robin                  # NEW: added in this recipe
    min_healthy: 1                         # NEW: added in this recipe
    circuit_breaker:                       # NEW: added in this recipe
      failure_threshold: 3                 # NEW: added in this recipe
      reset_timeout_s: 30                  # NEW: added in this recipe
    members:                               # NEW: added in this recipe
      - id: my-mcp                         # NEW: added in this recipe

Save this as ~/.config/mcp-hangar/config.yaml (or update your existing file).

Try It

1. Start Hangar with the new config

   mcp-hangar --config ~/.config/mcp-hangar/config.yaml serve \
     --log-file /tmp/hangar-circuit.log &

   INFO     group_created group_id=my-mcp-group strategy=round_robin
   INFO     background_worker_started task=health_check interval_s=60
   

2. Call a tool successfully through the group

   (
     echo '{"jsonrpc":"2.0","method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}},"id":1}'
     sleep 0.5
     echo '{"jsonrpc":"2.0","method":"notifications/initialized","params":{}}'
     sleep 0.5
     echo '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"hangar_call","arguments":{"calls":[{"mcp_server":"my-mcp-group","tool":"fetch","arguments":{"url":"https://example.com"}}]}},"id":2}'
     sleep 3
   ) | mcp-hangar --config ~/.config/mcp-hangar/config.yaml serve 2>&1 | grep -E '"id":2|circuit'

   {"jsonrpc":"2.0","id":2,"result":{"content":[...]}}

   Circuit breaker is CLOSED (normal operation). Call succeeded.

3. Stop the MCP server to simulate failures

   docker stop mcp-math

   MCP Server is now dead.

4. Call the tool 3 times to trip the circuit

   for i in 1 2 3; do
     echo "Attempt $i..."
     (
       echo '{"jsonrpc":"2.0","method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}},"id":1}'
       sleep 0.5
       echo '{"jsonrpc":"2.0","method":"notifications/initialized","params":{}}'
       sleep 0.5
       echo '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"hangar_call","arguments":{"calls":[{"mcp_server":"my-mcp-group","tool":"fetch","arguments":{"url":"https://example.com"}}]}},"id":2}'
       sleep 3
     ) | mcp-hangar --config ~/.config/mcp-hangar/config.yaml serve 2>&1 | grep -E 'error|circuit' | head -2
   done

   Attempt 1...
   (error output — connection refused or timeout)
   Attempt 2...
   (error output — connection refused or timeout)
   Attempt 3...
   (error output — circuit breaker opened after 3 failures)
   

   After 3 failures, circuit opens.

5. Verify fast-fail behavior — the key demonstration

   time (
     echo '{"jsonrpc":"2.0","method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}},"id":1}'
     sleep 0.5
     echo '{"jsonrpc":"2.0","method":"notifications/initialized","params":{}}'
     sleep 0.5
     echo '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"hangar_call","arguments":{"calls":[{"mcp_server":"my-mcp-group","tool":"fetch","arguments":{"url":"https://example.com"}}]}},"id":2}'
     sleep 1
   ) | mcp-hangar --config ~/.config/mcp-hangar/config.yaml serve 2>&1 | grep -E 'circuit_open|rejected'

   (error output — request rejected immediately, circuit is open)
   
   real    0m2.1s
   

   Request rejected in ~2 seconds (no 30-second timeout). This is the protection.

6. Wait for circuit to auto-reset

   echo "Waiting 35 seconds for circuit reset..."
   sleep 35
   tail -5 /tmp/hangar-circuit.log

   After reset_timeout_s elapses, the circuit automatically transitions from OPEN back to CLOSED.

7. Restart MCP server and verify recovery

   docker start mcp-math
   sleep 2
   
   (
     echo '{"jsonrpc":"2.0","method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}},"id":1}'
     sleep 0.5
     echo '{"jsonrpc":"2.0","method":"notifications/initialized","params":{}}'
     sleep 0.5
     echo '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"hangar_call","arguments":{"calls":[{"mcp_server":"my-mcp-group","tool":"fetch","arguments":{"url":"https://example.com"}}]}},"id":2}'
     sleep 3
   ) | mcp-hangar --config ~/.config/mcp-hangar/config.yaml serve 2>&1 | grep -E '"id":2|success'

   {"jsonrpc":"2.0","id":2,"result":{"content":[...]}}

   Call succeeded. Circuit is CLOSED. Full recovery.

What Just Happened

Hangar introduced MCP server groups — a logical grouping of one or more MCP servers with shared policies. The group has a circuit breaker that tracks real tool call failures, not synthetic health probes.

Circuit breaker states:

CLOSED (normal operation): All calls pass through to group members. The circuit breaker counts consecutive failures. When failure_count reaches failure_threshold (3), the circuit opens.

OPEN (protecting): All calls are rejected immediately with a circuit-open error. No traffic reaches the MCP server — this is the protection. Instead of waiting 10+ seconds for connection timeout, Hangar fails in milliseconds. After reset_timeout_s (30 seconds), the circuit automatically closes and allows traffic again.

How this differs from health checks:

• Health checks (recipe 02): Periodic synthetic probe (tools/list every 30s). Detects "is the MCP server alive?"
• Circuit breaker (recipe 03): Tracks real tool call failures in real-time. Detects "is the MCP server working?"

They complement each other. Health checks catch dead MCP servers. Circuit breakers catch flaky MCP servers that pass health checks but fail real requests. The circuit breaker trips instantly on the Nth failure — no waiting for the next health check cycle.

Key Config Reference

What's Next

Your single MCP server is protected — but it's still a single point of failure. When the circuit opens, agents get errors instead of results. What if there was a backup MCP server ready to take over automatically?

→ 04 — Failover