MCP Testing

AgentBreak can proxy and fault-inject MCP (Model Context Protocol) traffic alongside LLM calls.

Setup

1. Enable MCP in config

# .agentbreak/application.yaml
mcp:
  enabled: true
  upstream_url: http://localhost:3000/mcp   # your MCP server

2. Discover tools

agentbreak inspect

This connects to your MCP server, discovers available tools, resources, and prompts, and writes the registry to .agentbreak/registry.json.

3. Write MCP scenarios

Target MCP traffic with target: mcp_tool:

scenarios:
  - name: slow-tools
    summary: MCP tool calls are slow
    target: mcp_tool
    fault:
      kind: latency
      min_ms: 3000
      max_ms: 8000
    schedule:
      mode: random
      probability: 0.5

4. Start the proxy

agentbreak serve

MCP traffic goes through POST /mcp.

Targeting specific tools

Use match to scope faults to specific tools:

# Single tool
match:
  tool_name: search_docs

# Wildcard pattern
match:
  tool_name_pattern: "search_*"

MCP-specific faults

Fault	Behavior
`timeout`	Delays the response, then returns 504 (MCP only)
`http_error`	Returns an HTTP error for the tool call
`empty_response`	Returns an empty result
`schema_violation`	Corrupts the result shape

MCP scorecard

MCP has its own scorecard endpoint:

curl localhost:5005/_agentbreak/mcp-scorecard
curl localhost:5005/_agentbreak/mcp-requests

Presets for MCP

Preset	What it does
`mcp-slow-tools`	90% of MCP tool calls are slow
`mcp-tool-failures`	30% of MCP tool calls return 503
`mcp-mixed-transient`	Light MCP latency + errors