---
title: Precursor
description: Client-side, session-based verification that continuously evaluates visitor behavior to identify automation.
image: https://developers.cloudflare.com/core-services-preview.png
---

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# Precursor

Precursor is a client-side, session-based verification system that continuously evaluates a visitor's behavior over time. Instead of relying on a single challenge event, Precursor runs ongoing verification in the browser to detect automation that appears legitimate in individual requests but exhibits non-human patterns across a session.

## How it works

Precursor operates as a continuous client-side verification loop:

* A client-side script is injected into the page
* The script continuously collects signals and performs verification
* Each execution produces signals that are evaluated by Cloudflare
* Results are used to update session state stored in the `cf_clearance` cookie
* The process repeats throughout the session

This enables Cloudflare to continuously evaluate session behavior over time.

## Get started

Enable Precursor for your zone:

1. In the Cloudflare dashboard, select your zone.
2. Go to **Security** \> **Settings**.
3. Locate **Precursor**.
4. Turn on Precursor.
![Security Settings page in the Cloudflare dashboard, showing the Precursor card with the on/off toggle](https://developers.cloudflare.com/images/precursor/precursor-settings.png) 
1. **Choose a mode:** To fully verify a user session, visitors may need to complete a lightweight Challenge to establish a valid session. Precursor provides two modes depending on whether you want to prioritize user experience or strict verification:

  * **Minimize Friction (default)**Does not show an interstitial Challenge to the visitor. Instead, Precursor attempts to establish session state in the background. This provides a smoother user experience, but cannot guarantee that every session is fully verified.
  * **Maximize Security (recommended)**Shows a lightweight interstitial Challenge to establish a valid session if one does not already exist. This ensures every session is verified before a user can proceed, but may introduce additional friction.
![Precursor mode selector showing Minimize Friction and Maximize Security options](https://developers.cloudflare.com/images/precursor/precursor-rules.png) 

For most customers, selecting a mode is the only configuration required.

### Precursor rules (optional)

Precursor runs across your zone by default. Rules do not enable or disable Precursor — they determine which mode applies to each request.

For example:

* Run **Minimize Friction** across your site, but run **Maximize Security** to enforce a valid session on `/checkout`.
* Run **Maximize Security** on all pages, except your homepage.

## Relationship to JavaScript Detections

Precursor supersedes JavaScript Detections (JSD):

* moves from one-time execution to continuous verification
* introduces session-based state
* enables dynamic runtime control

If you enable Precursor, you should disable JavaScript Detections (JSD).

## Relationship to Challenge Pages

Precursor and Challenges serve different roles:

* Challenges provide point-in-time verification
* Precursor provides continuous, session-level verification

Precursor does not replace Challenges. Instead, it strengthens them by:

* determining when additional Challenges should be required
* re-evaluating visitors after they have already passed a Challenge
* identifying automation that emerges over time

## Relationship to `cf_clearance` cookie

Precursor is tightly integrated with [cf\_clearance](https://developers.cloudflare.com/cloudflare-challenges/concepts/clearance/#cf%5Fclearance-cookies). When running Precursor:

* effective clearance may be reduced or invalidated
* additional Challenges may be triggered
* the visitor may be re-verified during the same session

## Visibility in Security Analytics

Once Precursor runs on a zone, its detections appear in the zone's Analytics view. To open it, select your zone in the Cloudflare dashboard, then go to **Security** \> **Analytics** \> **Traffic** \> **Bot analysis**. The bot score distribution and WAF rule-match counts now include Precursor's behavioral and biometric detections.

For more information, refer to [Security Analytics](https://developers.cloudflare.com/waf/analytics/security-analytics/).

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