> ## Documentation Index
> Fetch the complete documentation index at: https://ctpf.q-uestionable.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Live Interception

> Pause, inspect, and modify MCP messages in flight using intercept mode

Intercept mode pauses MCP messages between client and server, allowing you to inspect JSON-RPC payloads and optionally modify them before forwarding.

## Starting Intercept Mode

Enable intercept mode on startup:

```bash theme={null}
ctpf proxy start \
  --transport stdio \
  --target-command "python my_server.py" \
  --intercept
```

Or toggle intercept mode during a session by pressing `i` in the TUI.

<Tip>
  Start in passthrough mode (default) for full-session capture, then switch to intercept mode when you need to slow down and inspect specific messages.
</Tip>

## TUI Layout

The proxy interface displays three panels:

**Message List (left)**

* Scrollable list of all intercepted messages in chronological order
* Client-to-server messages prefixed with `▶`
* Server-to-client messages prefixed with `◀`
* Held messages show a pause icon (`⏸`)
* Dropped messages show a cross mark (`✕`)
* Select any message with arrow keys or mouse

**Message Detail (right)**

* Full JSON-RPC payload of the selected message
* In edit mode, becomes an editable text area
* Syntax highlighting for JSON
* Press `m` to enter edit mode

**Status Bar (bottom)**

* Current mode: `[PASSTHROUGH]` or `[INTERCEPT]`
* Total message count and held message count
* Server connection status

## Message Actions

When a message is held, you can take one of three actions:

| Action      | Command     | Effect                                                |
| ----------- | ----------- | ----------------------------------------------------- |
| **FORWARD** | `f` or `F5` | Send the message to its destination unchanged         |
| **MODIFY**  | `m` or `F6` | Edit the JSON payload, then send the modified version |
| **DROP**    | `d` or `F8` | Discard the message without forwarding                |

<Warning>
  Dropping messages may cause the other side to timeout or report protocol errors. Use carefully.
</Warning>

## Keyboard Shortcuts

| Key        | Alternate | Action                                         |
| ---------- | --------- | ---------------------------------------------- |
| `q`        |           | Quit the proxy and save session                |
| `i`        |           | Toggle intercept mode on/off                   |
| `f`        | `F5`      | Forward the held message                       |
| `d`        | `F8`      | Drop the held message                          |
| `m`        | `F6`      | Enter edit mode to modify the message          |
| `Ctrl+S`   |           | Confirm edit and send modified message         |
| `Escape`   |           | Cancel edit (forward message as-is)            |
| `s`        |           | Save session to file                           |
| `r`        | `F9`      | Replay the selected message against the server |
| `/`        |           | Focus the filter input for searching messages  |
| Arrow keys |           | Navigate message list and edit mode            |

## Edit Mode

Press `m` or `F6` on any message to enter edit mode:

1. The detail panel becomes a JSON text editor
2. Modify the JSON-RPC payload as needed
3. Press `Ctrl+S` to validate and forward the modified message
4. Press `Escape` to cancel (original message is forwarded)

**Validation:**

* The editor validates JSON structure before sending
* Invalid JSON or schema errors appear in a notification
* The editor stays open so you can fix issues and retry

**Example modification:**

```json theme={null}
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "search",
    "arguments": {
      "query": "original query"
    }
  }
}
```

Change the query and press `Ctrl+S` to send the modified request.

## Message Filtering

Press `/` to focus the filter input and search for specific messages.

**Filter syntax:**

| Prefix   | Effect                               | Example       |
| -------- | ------------------------------------ | ------------- |
| `>`      | Client-to-server only                | `>initialize` |
| `<`      | Server-to-client only                | `<result`     |
| *(none)* | Substring match on method or payload | `tools/call`  |

Press `Escape` in the filter input to clear and return to the full list.

## Mode Behavior

**When switching from INTERCEPT to PASSTHROUGH:**
All currently held messages are automatically forwarded without modification.

**When switching from PASSTHROUGH to INTERCEPT:**
All new messages are held; previously passed messages are not affected.

## Use Cases

**Inspect Before Forwarding**

* Verify tool calls match expectations before sending
* Check response payloads for unexpected data

**Modify on the Fly**

* Change tool arguments to test edge cases
* Inject test data into responses
* Alter method names to test error handling

**Block Dangerous Calls**

* Drop risky tool calls before they reach the target
* Intercept and modify sensitive parameters
* Test fallback behavior by dropping responses

<Note>
  Intercept mode slows message processing because each message waits for user action. Use it for targeted inspection of specific interactions, not for passive bulk capture. For high-volume recording, run in passthrough mode.
</Note>

## Performance Considerations

* Each held message adds latency while waiting for action
* Filter input does not block message capture
* Switching modes is instantaneous
* Saved sessions record all metadata including which messages were modified