Intercept (Breakpoints & Map Local)

Overview

The Intercept feature lets you modify MCP traffic in real-time. There are two mechanisms:

• Breakpoints — pause matching messages so you can inspect, edit, then forward or drop them
• Map Local — automatically replace matching responses with canned payloads from disk

Access intercept rules from the Traffic tab by switching to the Rules view mode in the header.

Breakpoints

Creating a Breakpoint

1. Switch to the Rules view in the Traffic tab
2. Click Add Breakpoint
3. Configure the matching criteria:
   • Method — which JSON-RPC method to match (e.g. tools/call, resources/read)
   • Direction — client-to-server, server-to-client, or both
   • Server — optionally limit to a specific server
4. Enable the breakpoint

When a Breakpoint Hits

When a matching message arrives:

1. A banner appears at the top of the timeline showing the paused frame
2. The Breakpoint Editor opens with the full JSON payload in a Monaco editor
3. You have three options:
   • Forward — send the message through unchanged
   • Forward Modified — edit the payload and send the modified version
   • Drop — discard the message entirely

Caution

Breakpoints have a 30-second timeout. If you don’t resolve a hit within 30 seconds, the message is automatically forwarded to prevent the MCP connection from hanging.

Map Local

Map Local lets you define canned responses that automatically replace matching server responses.

Creating a Map Local Rule

1. Switch to the Rules view in the Traffic tab
2. Click Add Map Local
3. Configure:
   • Method — which method’s response to replace
   • Server — optionally limit to a specific server
   • Response file — path to a JSON file containing the canned response payload
4. Enable the rule

When a matching request comes in, Conduit intercepts the response from the server and replaces it with the contents of your file.

Use Cases

• Test error handling — return error responses without modifying the server
• Mock unavailable servers — return cached responses when a server is down
• Reproducible testing — ensure consistent responses for specific tool calls
• Development — iterate on client behavior without touching server code

Graceful Degradation

If Conduit is closed while breakpoints or map-local rules are active, the shim automatically falls back to transparent passthrough. No messages are lost or blocked.