sGTM Live Debug
Updated August 2026 · 6 min read
Route selected live requests from your server-side container into Google Tag Manager Preview, so you can debug real traffic instead of only your own session.
What Live Debug does
Google Tag Manager's Preview mode normally shows you only your own session. That is fine for checking a tag you can trigger yourself, and useless for the problems that matter most — the checkout that fails for one payment method, the event that only appears on mobile Safari, the request that arrives malformed from a third-party integration.
Live Debug routes selected live requests arriving at your server-side container into your Google Tag Manager Preview session. You see real traffic, from real visitors, in the debug pane.
It is part of sGTM Test Lab and holds its own one-hour lease.
When Live Debug is useful
- A tag misbehaves for some visitors but never for you.
- You need to see the actual shape of an incoming request rather than an assumed one.
- You are validating a change against live traffic before trusting it.
- You want to watch a specific customer's path through the container while they are on the phone to support.
If you only need to know whether events are arriving at all, the lighter option is viewing incoming container requests.
Obtain the X-Gtm-Server-Preview value
In Google Tag Manager, open your server container and start Preview. Google issues a preview header value for that session. Copy it and paste it into the Preview header token field.
TagVolt is explicit about how it treats the value:
Shown while you review it. After activation it is encrypted at rest, never returned by TagVolt, and removed when this one-hour session ends.
Understand the exact incoming path selector
The Exact path (optional) field narrows which requests get routed. The in-app help says it precisely:
This is the incoming sGTM request path, such as /g/collect—not the webpage URL where the event started.
This is the single most common misunderstanding. It is not https://example.com/checkout. It
is the path of the request your container receives — the endpoint your tags post to.
# correct — the incoming request path at your container
/g/collect
# wrong — the page the visitor was on
https://example.com/checkout
Understand the source IP selector
Source IP (optional) narrows by the origin address of the incoming request. Use it when you want one specific person's traffic — your own, or a colleague's — rather than one specific endpoint.
Understand matching when both selectors are present
Setting both fields does not narrow further. The panel states the rule:
Matching either field is enough.
The two selectors are combined as or, not and. A request matching the path or the IP is routed. If you expected an intersection, you will route considerably more traffic than intended.
Understand what happens when both selectors are blank
Leaving both empty routes everything. The confirmation line reads:
This session will route all incoming requests.
Read that line before you press start — it always tells you exactly what the session will route. On a busy container, routing all incoming requests fills the Preview pane faster than you can read it.
Start Live Debug
With the token in place and your selectors set, press Start Live Debug.
Live Debug cannot start if a session is already in progress or still shutting down. You will see one of:
sGTM Live Debug is already active; stop it before starting a new session
sGTM Live Debug is already changing; wait for the current operation
Expired sGTM Live Debug cleanup is still pending; end it before restarting
sGTM Live Debug needs attention; stop it before restarting
Confirm runtime activation
Starting is not instant. The panel confirms only once the serving runtime has actually applied the change:
Live Debug is runtime-confirmed.
Wait for that before concluding a request should have appeared. Until it shows, requests are still being handled normally.
Verify requests in Google Tag Manager Preview
Switch to the Google Tag Manager Preview window for your server container. Matching requests appear as they arrive. If nothing appears, work through the troubleshooting below before changing your selectors.
Stop Live Debug
Press Stop Live Debug to release this lease. Ad-Safe Test Mode, if running, is unaffected — it holds a separate lease.
End the entire Test Lab session
End Test Session stops both tools at once:
Stop sGTM Live Debug and Ad-Safe Mode together.
It also clears the session's stored state and credential. Use it when you are finished, and whenever a session has expired.
One-hour expiry
The lease lasts exactly one hour from activation. When it elapses while Live Debug is still on, the status becomes needs attention rather than turning itself off, and a new session cannot start until the expired one is ended. Press End Test Session to clear it.
Credential protection and fail-open behaviour
The preview credential is encrypted at rest, never returned by the API, and removed when the session ends. Its purpose is to bind genuine Preview traffic to your session so that ordinary visitors are never affected.
Routing fails open: a request that cannot be positively identified is handled as ordinary traffic. In practice that means an occasional request you expected to see may not appear in Preview, which is the safer failure — the alternative would risk interfering with real traffic.
Limitations
- One hour per session, with no extension.
- Owners and administrators only.
- Selectors combine as or, so they widen rather than narrow when used together.
- An expired session must be ended manually before a new one can start.
Troubleshooting
Nothing appears in Preview. Check three things in order: that the panel shows Live Debug is runtime-confirmed; that your Google Tag Manager Preview session is still open and using the same token you pasted; and that your Exact path is the incoming request path, not a page URL.
Far more traffic than expected. Either both selectors are set — remember they combine as or — or both are blank, which routes all incoming requests. The confirmation line above the start button states which.
Cannot start a session. The message names the cause. "Needs attention" and "cleanup is still pending" both mean the previous session must be ended first with End Test Session.
The token was rejected. Copy the X-Gtm-Server-Preview value again from a freshly started
Preview session in your server container. Preview values do not survive indefinitely.
FAQ
Does Live Debug affect ordinary visitors?
No. Only requests matching your selectors are routed into Preview, and only while the session is active. Delivery of ordinary traffic is unchanged.
Is the exact path the page URL?
No. It is the path of the request arriving at your server-side container, such as /g/collect.
The page the visitor was on is not what this field matches.
What if I set both the path and the IP?
You will route requests matching either one. The fields widen the selection, they do not intersect it.
Does Live Debug stop conversions being sent?
No. It only routes requests into Preview so you can watch them. To stop reviewed advertising destinations receiving deliveries, use Ad-Safe Test Mode.
Can I run Live Debug and Ad-Safe Test Mode together?
Yes, and it is a common combination. Start Live Debug first and Ad-Safe Test Mode will reuse the credential you already supplied.
What happens when the hour is up?
The session stops being valid and its status becomes needs attention. Press End Test Session to clear it before starting another.