How do UTM parameters affect Plausible's referral classification?

Definition: UTM parameters are campaign labels added to a URL's query string. In Plausible, a source parameter takes precedence over the browser referrer when determining a new session's source, while channel classification also considers campaign and medium values.

Key facts

• As of October 2026, Plausible's traffic sources documentation lists AI Assistants as a channel for recognized sources including ChatGPT, Claude and Perplexity.
• Plausible's public source resolver prioritizes utm_source, then source, then ref, ahead of the browser referrer.
• The public channel classifier checks paid and organic social rules before its AI Assistants rule.
• Plausible's troubleshooting documentation attributes UTMs only on the first pageview of a new session. A session remains active for 30 minutes after the last pageview.

Which value determines the source?

A referrer identifies the page or site from which a visitor arrives. Plausible's source resolver documentation says that a source tag overrides the browser's Referer information. Without a source tag, the resolver uses a valid external referrer. Its source-tag lookup is:

request.query_params["utm_source"] ||
  request.query_params["source"] ||
  request.query_params["ref"]

Thus, utm_campaign alone does not replace the source through this lookup. When both utm_source and ref contain source labels, utm_source wins. The resolver also normalizes recognized names and aliases.

Plausible's custom source mapping maps chatgpt.com and chat.openai.com to ChatGPT. Applying those rules, a new session tagged utm_source=chatgpt.com receives that recognized source even when the browser supplies a different referrer.

Can an AI assistant source appear in another channel?

A channel groups sources according to classification rules. Plausible's traffic sources documentation separates Channels, Sources and Campaigns. Sources consolidates recognized aliases, while Campaigns provides UTM Source and UTM Medium breakdowns; UTM Source also includes ref and source tags.

The public channel classifier evaluates rules in order. Organic Social matches a recognized social source or a medium such as social, and this rule precedes AI Assistants. Applying that code to a recognized ChatGPT source with utm_medium=social therefore yields Organic Social. A medium of ppc matches Paid Other before the AI Assistants check.

With utm_medium=referral, the AI Assistants check precedes the Referral check. The source label and the channel can therefore describe different aspects of the same tagged session.

How can the attribution be checked?

Plausible's traffic sources documentation allows dashboard filtering by multiple sources, channels or UTM tags. The Campaigns tab exposes the tag values; the Sources tab consolidates recognized aliases. For example, the documentation combines facebook, fb and facebook-ads under Facebook in Sources.

The troubleshooting guide recommends testing with a device and network combination that has not visited the site in the past 30 minutes, then checking Campaigns. Repeated tagged clicks within an active session retain its initial attribution. A redirect must preserve the query string: if it removes the parameters before the page running Plausible loads, attribution falls back to the referrer or Direct / None.

Related terms

• UTM parameters
• HTTP Referer header
• Where does Plausible show AI assistant traffic?
• What does Direct or None mean in Plausible's source report?

FAQ

Does a UTM tag always create a separate source row?

Plausible's traffic sources documentation says Sources consolidates recognized aliases and capitalization differences. You can inspect tagged traffic separately in Campaigns.

Why does my second test keep the earlier source?

The troubleshooting guide says only the first tagged arrival in a session receives attribution. You can test after 30 minutes without a pageview or use a fresh device and network combination.

Sources

1. Plausible Analytics, "Traffic sources, referrers and UTM campaigns", updated 15 October 2026
2. Plausible Analytics, "Plausible.Ingestion.Source", undated public source code
3. Plausible Analytics, "Plausible.Ingestion.Acquisition", undated public source code
4. Plausible Analytics, "Troubleshooting Plausible Analytics", updated 1 October 2026
5. Plausible Analytics, "custom_sources.json", undated public source code

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