Intercept mode pauses MCP messages in transit between client and server, letting you inspect and modify JSON-RPC payloads before they reach their destination.
Enabling intercept mode
Start the proxy with the --intercept flag:
qai proxy start \
--transport stdio \
--target-command "python my_server.py" \
--intercept
i key.
TUI layout
The proxy TUI has three panels:
- Message list (left) — Scrollable list of all proxied messages. Client-to-server messages show
>, server-to-client show <. Held messages are prefixed with a pause icon, dropped messages with a cross mark.
- Message detail (right) — Full JSON-RPC payload of the selected message. In edit mode, this becomes an editable text area.
- Status bar (bottom) — Shows current mode (
[PASSTHROUGH] or [INTERCEPT]), total message count, held message count, and connection status.
Intercepted message actions
When a message is held, you can take one of three actions:
| Action | Effect |
|---|
| FORWARD | Send the message to its destination unchanged |
| MODIFY | Edit the JSON-RPC payload, then send the modified version |
| DROP | Discard the message without forwarding |
Keyboard shortcuts
| Key | Alternate | Action |
|---|
q | | Quit the proxy |
i | | Toggle intercept mode on/off |
f | F5 | Forward the held message |
d | F8 | Drop the held message |
m | F6 | Enter edit mode (modify) |
Ctrl+S | | Confirm edit and forward modified message |
Escape | | Cancel edit (forwards message as-is) |
s | | Save session to file |
r | F9 | Replay selected message against the server |
/ | | Focus the message filter input |
Edit mode
Press m or F6 on a held message to enter edit mode. The detail panel becomes a JSON text editor. Edit the JSON-RPC payload, then press Ctrl+S to validate and forward the modified message. Press Escape to cancel and forward the original.
Invalid JSON or schema errors are reported in a notification — the editor stays open so you can fix the issue.
Filtering messages
Press / to focus the filter input. Filter syntax:
| Prefix | Effect | Example |
|---|
> | Show client-to-server only | >initialize |
< | Show server-to-client only | <result |
| (none) | Substring match on method or payload | tools/call |
Escape in the filter input to return to the message list.
Mode switching
When you switch from INTERCEPT to PASSTHROUGH (press i), all currently held messages are automatically forwarded.
Intercept mode slows traffic because every message waits for user action. Use it for targeted inspection of specific interactions, not bulk capture. For passive recording, run without --intercept.