diff --git a/index.bs b/index.bs
index c4b946e0..b33553aa 100644
--- a/index.bs
+++ b/index.bs
@@ -2682,7 +2682,8 @@ The <dfn export for=commands>browser.createUserContext</dfn> command creates a
 
       browser.CreateUserContextParameters = {
         ? acceptInsecureCerts: bool,
-        ? proxy: session.ProxyConfiguration
+        ? proxy: session.ProxyConfiguration,
+        ? unhandledPromptBehavior: session.UserPromptHandler
       }
       </pre>
    </dd>
@@ -2716,6 +2717,10 @@ The [=remote end steps=] with |session| and |command parameters| are:
       [=user context to accept insecure certificates override map=][|user context|]
       to |acceptInsecureCerts|.
 
+1. If |command parameters| [=map/contains=] "<code>unhandledPromptBehavior</code>",
+   [=map/set=] [=user context to unhandled prompt behavior override map=][|user context|] to
+   |command parameters|["<code>unhandledPromptBehavior</code>"].
+
 1. If |command parameters| [=map/contains=] "<code>proxy</code>":
 
    1. Let |proxy configuration| be |command parameters|["<code>proxy</code>"].
@@ -3081,6 +3086,10 @@ or null and an [=struct/item=] named <dfn attribute for="viewport-configuration"
 
 A [=remote end=] has a <dfn>viewport overrides map</dfn> which is a weak map between [=user contexts=] and [=viewport configuration=].
 
+A [=remote end=] has an <dfn>user context to unhandled prompt behavior override map</dfn>
+which is a weak map between [=user contexts=] and [=/map=] matching the
+<code>session.UserPromptHandlerType</code> production.
+
 ### Types ### {#module-browsingcontext-types}
 
 #### The browsingContext.BrowsingContext Type #### {#type-browsingContext-Browsingcontext}
@@ -5597,6 +5606,26 @@ closed</dfn> steps given |window|, |type|, |accepted| and optional |user text|
    </dd>
 </dl>
 
+<div algorithm>
+To <dfn>get navigable's user prompt handler</dfn> given |type| and |navigable|:
+
+1. Let |user context| be |navigable|'s [=associated user context=].
+
+1. If [=user context to unhandled prompt behavior override map=] contains |user context|:
+
+  1. Let |behavior map| be [=user context to unhandled prompt behavior override map=][|user context|].
+
+  1. If |behavior map| contains |type|, return |behavior map|[|type|].
+
+  1. If |behavior map| contains <code>"default"</code>, return
+     |behavior map|[<code>"default"</code>].
+
+1. Let |handler configuration| be [=get the prompt handler=] with |type|.
+
+1. Return |handler configuration|'s [=prompt handler configuration/handler=].
+
+</div>
+
 <div algorithm>
 The [=remote end event trigger=] is the <dfn export>WebDriver BiDi user prompt
 opened</dfn> steps given |window|, |type|, |message|, and optional |default value|
@@ -5606,10 +5635,8 @@ opened</dfn> steps given |window|, |type|, |message|, and optional |default valu
 
 1. Let |navigable id| be the [=navigable id=] for |navigable|.
 
-1. Let |handler configuration| be [=get the prompt handler=] with |type|.
-
-1. Let |handler| be |handler configuration|'s [=prompt handler
-   configuration/handler=].
+1. Let |handler| be [=get navigable's user prompt handler=] with |type| and
+   |navigable|.
 
 1. Let |params| be a [=/map=] matching the
    <code>browsingContext.UserPromptOpenedParameters</code> production with the