Fiddler MCP Setup

Fiddler MCP Setup

Configure the Fiddler Everywhere MCP server so that agent tools can call Fiddler's traffic inspection, status, and session APIs.

Operating rules

• Shell-first. MCP is not yet configured, so you cannot use MCP tools.
• Sequential execution only. Follow steps strictly in order — do not run steps or scripts in parallel. Each step may depend on values produced by the previous one.
• On Windows — detect opened terminal. Only if it is not PowerShell, wrap scripts with: powershell.exe -Command 'script'. Use single quotes to wrap the script.

Step 1 — Verify Fiddler is installed

macOS:

if [ -d "/Applications/Fiddler Everywhere.app" ]; then echo "INSTALLED"; else echo "NOT_INSTALLED"; fi

Linux:

if command -v fiddler-everywhere &>/dev/null; then echo "INSTALLED"; else echo "NOT_INSTALLED"; fi

Windows (PowerShell):

$installed = Get-ItemProperty `
  "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\*",
  "HKCU:\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\*",
  "HKLM:\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Uninstall\*" `
  -ErrorAction SilentlyContinue |
  Where-Object { $_.DisplayName -like "*Fiddler Everywhere*" }
if ($installed) { "INSTALLED" } else { "NOT_INSTALLED" }

If NOT_INSTALLED: stop and tell the user:

"Fiddler Everywhere is not installed. Please install it first."

Step 2 — Detect the provider

Identify which agent is being configured and set PROVIDER.

Parent process name

Inspect the parent process:

macOS / Linux:

ps -p $PPID -o comm= 2>/dev/null

Windows (PowerShell):

(Get-Process -Id (Get-CimInstance Win32_Process -Filter "ProcessId=$PID").ParentProcessId).Name

Match the output against known process names:

If matched unambiguously, set PROVIDER to Provider value from the table and proceed to Step 3.

If still unrecognised, ask the user:

"Which agent are you setting Fiddler MCP up for? (VS Code / GitHub Copilot, Cursor, Claude Code CLI, GitHub Copilot CLI, or OpenAI Codex CLI)"

Use the user's answer to set PROVIDER from the table above.

Step 3 — Discover the port

Fiddler Everywhere listens on port 8868 by default. Confirm it is reachable and set PORT for all subsequent steps.

macOS / Linux:

curl -s -o /dev/null -w "%{http_code}" "http://localhost:8868/api/McpManagement/Detect?provider=PROVIDER"

Windows (PowerShell):

curl.exe -s -o NUL -w "%{http_code}" "http://localhost:8868/api/McpManagement/Detect?provider=PROVIDER"

Port discovery script:

macOS / Linux:

python3 -c "
import json, glob, os
ports = set()
for f in glob.glob(os.path.expanduser('~/.fiddler/*/Settings/appsettings.json')):
    try:
        p = json.load(open(f)).get('MCPServerSettings', {}).get('Port')
        if p and p != 8868:
            ports.add(p)
    except: pass
print(' '.join(str(p) for p in ports) if ports else 'none')
"

Windows (PowerShell):

$ports = Get-ChildItem "$env:USERPROFILE\.fiddler\*\Settings\appsettings.json" -ErrorAction SilentlyContinue |
  ForEach-Object { (Get-Content $_ | ConvertFrom-Json).MCPServerSettings.Port } |
  Where-Object { $_ -and $_ -ne 8868 } | Select-Object -Unique
Write-Output $(if ($ports) { $ports -join ' ' } else { 'none' })

For each port returned, probe it:

macOS / Linux:

curl -s -o /dev/null -w "%{http_code}" "http://localhost:PORT/api/McpManagement/Detect?provider=PROVIDER"

Windows (PowerShell):

curl.exe -s -o NUL -w "%{http_code}" "http://localhost:PORT/api/McpManagement/Detect?provider=PROVIDER"

Set PORT to the first port that returns any HTTP response. If no port responds, Fiddler is not running — launch it and retry from the top of Step 3.

Launch Fiddler:

macOS:

open -a "Fiddler Everywhere" && sleep 15

Linux:

(nohup fiddler-everywhere &>/dev/null &); sleep 15

Windows: Important On windows if the current terminal used is bash, use the specific GitBash script.

PowerShell

$candidates = @(
  "$env:LOCALAPPDATA\Programs\Fiddler Everywhere\Fiddler Everywhere.exe",
  "C:\Program Files\Fiddler Everywhere\Fiddler Everywhere.exe",
  "C:\Program Files (x86)\Fiddler Everywhere\Fiddler Everywhere.exe"
)
$fiddlerExe = $candidates | Where-Object { Test-Path $_ } | Select-Object -First 1
if ($fiddlerExe) {
  $cmdLine = '"' + $fiddlerExe + '"'
  Invoke-CimMethod -ClassName Win32_Process -MethodName Create -Arguments @{ CommandLine = $cmdLine } | Out-Null
  Start-Sleep 15
}

Git Bash

FIDDLER_EXE=""
for dir in "$LOCALAPPDATA/Programs/Fiddler Everywhere" \
           "/c/Program Files/Fiddler Everywhere" \
           "/c/Program Files (x86)/Fiddler Everywhere"; do
  if [ -f "$dir/Fiddler Everywhere.exe" ]; then
    FIDDLER_EXE="$dir/Fiddler Everywhere.exe"
    break
  fi
done
if [ -n "$FIDDLER_EXE" ]; then
  "$FIDDLER_EXE" &
  sleep 15
fi

Important: Wait 15 seconds for Fiddler to launch, before continuing with the next steps!

If still unreachable after relaunch:

"Fiddler Everywhere is not reachable. Please verify it is running and try again."

PORT is used in all subsequent steps.

Step 4 — Check for existing configuration

Call the Detect endpoint to see if Fiddler MCP is already configured for this provider:

macOS / Linux:

curl -s "http://localhost:$PORT/api/McpManagement/Detect?provider=PROVIDER"

Windows (PowerShell):

curl.exe -s "http://localhost:$PORT/api/McpManagement/Detect?provider=PROVIDER"

Replace PROVIDER with the value from Step 2.

Login (if Fiddler reports the user is not logged in)

Use the MCP protocol to trigger the login flow. Follow these sub-steps in order.

1. Open an MCP session and get the session ID:

macOS / Linux:

SESSION_ID=$(curl -si -X POST "http://localhost:$PORT/mcp" \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"mcp-setup","version":"1.0"}}}' \
  | grep -i "^mcp-session-id:" | awk '{print $2}' | tr -d '\r')
echo "Session ID: $SESSION_ID"

Windows (PowerShell):

$initResp = curl.exe -si -X POST "http://localhost:$PORT/mcp" `
  -H "Content-Type: application/json" `
  -H "Accept: application/json, text/event-stream" `
  -d '{\"jsonrpc\":\"2.0\",\"id\":1,\"method\":\"initialize\",\"params\":{\"protocolVersion\":\"2024-11-05\",\"capabilities\":{},\"clientInfo\":{\"name\":\"mcp-setup\",\"version\":\"1.0\"}}}'
$SESSION_ID = ($initResp | Select-String "(?i)mcp-session-id:\s*(\S+)").Matches.Groups[1].Value.Trim()
Write-Host "Session ID: $SESSION_ID"

If SESSION_ID is empty, Fiddler may not be fully started — wait a few seconds and retry.

2. Fetch the tools list to confirm initiate_login is available:

macOS / Linux:

curl -s -X POST "http://localhost:$PORT/mcp" \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -H "Mcp-Session-Id: $SESSION_ID" \
  -d '{"jsonrpc":"2.0","id":2,"method":"tools/list","params":{}}'

Windows (PowerShell):

curl.exe -s -X POST "http://localhost:$PORT/mcp" `
  -H "Content-Type: application/json" `
  -H "Accept: application/json, text/event-stream" `
  -H "Mcp-Session-Id: $SESSION_ID" `
  -d '{\"jsonrpc\":\"2.0\",\"id\":2,\"method\":\"tools/list\",\"params\":{}}'

Confirm initiate_login appears in the response before continuing.

3. Call initiate_login:

macOS / Linux:

curl -s -X POST "http://localhost:$PORT/mcp" \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -H "Mcp-Session-Id: $SESSION_ID" \
  -d '{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"initiate_login","arguments":{}}}'

Windows (PowerShell):

curl.exe -s -X POST "http://localhost:$PORT/mcp" `
  -H "Content-Type: application/json" `
  -H "Accept: application/json, text/event-stream" `
  -H "Mcp-Session-Id: $SESSION_ID" `
  -d '{\"jsonrpc\":\"2.0\",\"id\":3,\"method\":\"tools/call\",\"params\":{\"name\":\"initiate_login\",\"arguments\":{}}}'

Tell the user:

"A login window has been opened in your browser. Please complete sign-in, then let me know when done."

Wait for the user to confirm sign-in is complete, then retry the step that triggered this login flow.

Step 5 — Autoconfigure

Call the Configure endpoint. Fiddler will automatically generate or reuse the API key and write the correct config file for the provider:

macOS / Linux:

curl -s -X POST "http://localhost:$PORT/api/McpManagement/Configure" \
  -H "Content-Type: application/json" \
  -d '{"provider":"PROVIDER"}'

Windows (PowerShell):

curl.exe -s -X POST "http://localhost:$PORT/api/McpManagement/Configure" `
  -H "Content-Type: application/json" `
  -d '{\"provider\":\"PROVIDER\"}'

Replace PROVIDER with the value from Step 2.

Step 6 — Confirm success

Tell the user:

"Fiddler MCP has been configured for PROVIDER. Restart your agent (reload the VS Code window, restart Cursor, etc.) to pick up the new configuration, then try using a Fiddler tool."