diff --git a/rb/.rubocop.yml b/rb/.rubocop.yml
index 5896e31689c8a..9bcab28514e7c 100644
--- a/rb/.rubocop.yml
+++ b/rb/.rubocop.yml
@@ -64,6 +64,7 @@ Metrics/ModuleLength:
   Max: 110
   Exclude:
     - 'lib/selenium/webdriver/common/platform.rb'
+    - 'lib/selenium/webdriver/bidi/serialization/record.rb'
     - 'lib/selenium/webdriver/bidi/support/bidi_generate.rb'
     - 'spec/**/*'
 
diff --git a/rb/lib/selenium/webdriver/bidi/protocol/browsing_context.rb b/rb/lib/selenium/webdriver/bidi/protocol/browsing_context.rb
index d51de6cf9d2a3..e1973e109eb07 100644
--- a/rb/lib/selenium/webdriver/bidi/protocol/browsing_context.rb
+++ b/rb/lib/selenium/webdriver/bidi/protocol/browsing_context.rb
@@ -393,7 +393,7 @@ class ClipRectangle < Serialization::Union
           # @see https://www.selenium.dev/documentation/warnings/bidi-implementation/
           # @see https://w3c.github.io/webdriver-bidi/#cddl-type-browsingcontextsetbypasscspparameters
           SetBypassCSPParameters = Serialization::Record.define(
-            bypass: {wire_key: 'bypass', nullable: true},
+            bypass: {wire_key: 'bypass', nullable: true, const: true},
             contexts: {wire_key: 'contexts', required: false, list: true},
             user_contexts: {wire_key: 'userContexts', required: false, list: true}
           )
diff --git a/rb/lib/selenium/webdriver/bidi/protocol/emulation.rb b/rb/lib/selenium/webdriver/bidi/protocol/emulation.rb
index 4cb7a12aa7407..2066b01fbf26d 100644
--- a/rb/lib/selenium/webdriver/bidi/protocol/emulation.rb
+++ b/rb/lib/selenium/webdriver/bidi/protocol/emulation.rb
@@ -176,7 +176,7 @@ class SetGeolocationOverrideParameters < Serialization::Union
           # @see https://www.selenium.dev/documentation/warnings/bidi-implementation/
           # @see https://w3c.github.io/webdriver-bidi/#cddl-type-emulationsetscriptingenabledparameters
           SetScriptingEnabledParameters = Serialization::Record.define(
-            enabled: {wire_key: 'enabled', nullable: true},
+            enabled: {wire_key: 'enabled', nullable: true, const: false},
             contexts: {wire_key: 'contexts', required: false, list: true},
             user_contexts: {wire_key: 'userContexts', required: false, list: true}
           )
diff --git a/rb/lib/selenium/webdriver/bidi/serialization/record.rb b/rb/lib/selenium/webdriver/bidi/serialization/record.rb
index a8364bc876e53..05450bc311158 100644
--- a/rb/lib/selenium/webdriver/bidi/serialization/record.rb
+++ b/rb/lib/selenium/webdriver/bidi/serialization/record.rb
@@ -29,7 +29,8 @@ module Serialization
         # @api private
         class Record < ::Data
           # Named Field, not Member, to avoid colliding with +::Data#members+.
-          Field = ::Data.define(:name, :wire_key, :nullable, :ref, :list, :fixed, :enum, :required, :primitive, :scalar)
+          Field = ::Data.define(:name, :wire_key, :nullable, :ref, :list, :fixed, :enum, :required, :primitive,
+                                :scalar, :const)
 
           def self.define(**spec)
             extensible = spec.delete(:extensible) || false
@@ -60,7 +61,7 @@ def self.field(name, meta)
                       nullable: meta[:nullable] || false, ref: meta[:ref],
                       list: meta[:list] || false, fixed: meta.fetch(:fixed, UNSET), enum: meta[:enum],
                       required: meta.fetch(:required, true), primitive: meta[:primitive],
-                      scalar: meta[:scalar])
+                      scalar: meta[:scalar], const: meta.fetch(:const, UNSET))
           end
           private_class_method :field
 
@@ -97,9 +98,10 @@ def from_json(json_payload)
 
             # Checks each field's value: a required field cannot be omitted (UNSET), a non-nullable
             # field cannot be nil (nil is neither a value nor the UNSET omit-sentinel, so it would be
-            # silently dropped on the wire), and an enum field must be in its allowed set. The enum
-            # constant is resolved lazily so a cross-domain enum need not be loaded first. Outbound
-            # only (from +new+); inbound presence/enum are checked separately in +wire_value+/+read+.
+            # silently dropped on the wire), a nullable-const field must carry its literal (not some
+            # other value), and an enum field must be in its allowed set. The enum constant is resolved
+            # lazily so a cross-domain enum need not be loaded first. Outbound only (from +new+);
+            # inbound presence/enum are checked separately in +wire_value+/+read+.
             def validate_values(attributes)
               fields.each do |f|
                 value = attributes[f.name]
@@ -107,11 +109,21 @@ def validate_values(attributes)
                 raise ::ArgumentError, "#{name}##{f.name} cannot be nil" if value.nil? && !f.nullable
                 next if value.nil? || UNSET.equal?(value)
 
+                validate_const(f, value)
                 check_outbound_shape(f, value)
                 Serialization.validate!("#{name}##{f.name}", value, Protocol.const_get(f.enum)) if f.enum
               end
             end
 
+            # A nullable constant (`literal / null`) is caller-settable but its only non-null value is
+            # the literal, so a value that is neither the literal nor nil (nil is handled above) is a
+            # local error rather than a wire round-trip. A non-const field carries UNSET here and passes.
+            def validate_const(field, value)
+              return if UNSET.equal?(field.const) || value == field.const
+
+              raise ::ArgumentError, "#{name}##{field.name} must be #{field.const.inspect}, got #{value.inspect}"
+            end
+
             # Outbound mirror of check_shape: a list-typed arg must be an array, a scalar-shaped one
             # (enum or ref, not a list) must not — a local ArgumentError, not a wire round-trip.
             def check_outbound_shape(field, value)
diff --git a/rb/lib/selenium/webdriver/bidi/support/bidi_generate.rb b/rb/lib/selenium/webdriver/bidi/support/bidi_generate.rb
index 0e88f5b39451f..4056e92142ee8 100644
--- a/rb/lib/selenium/webdriver/bidi/support/bidi_generate.rb
+++ b/rb/lib/selenium/webdriver/bidi/support/bidi_generate.rb
@@ -236,26 +236,35 @@ def type_entry = "'#{wire_name}' => #{payload_ref || 'nil'}"
   # ref is the Protocol-relative class path for a nested structured field (nil
   # for a scalar/opaque field); list wraps it in an array. wire_key is the exact
   # JSON payload key (the schema's `wire` name, baked verbatim).
-  FieldIR = Struct.new(:ruby_name, :wire_key, :required, :nullable, :ref, :list, :enum, :primitive, :scalar, :rbs,
-                       keyword_init: true) do
+  FieldIR = Struct.new(:ruby_name, :wire_key, :required, :nullable, :ref, :list, :enum, :primitive, :scalar, :const,
+                       :rbs, keyword_init: true) do
     # A `Serialization::Record.define` spec entry: `name: 'jsonKey'` shorthand, or
     # `name: {wire_key:, …}` when the field carries JSON facts beyond its name.
     # enum carries the allowed-values constant path, validated at construction.
     def spec_entry(indent = 0)
-      meta = []
-      meta << 'required: false' unless required
-      meta << 'nullable: true' if nullable
-      meta << "ref: '#{ref}'" if ref
-      meta << 'list: true' if list
-      meta << "scalar: #{scalar_literal}" if scalar
-      meta << "enum: '#{enum}'" if enum
-      meta << "primitive: '#{primitive}'" if primitive
+      meta = value_facts
       return "#{ruby_name}: '#{wire_key}'" if meta.empty?
 
       meta.unshift("wire_key: '#{wire_key}'")
       BiDiGenerate.wrap_call("#{ruby_name}: ", meta, indent, open: '{', close: '}')
     end
 
+    # The JSON facts beyond the field's name, in the order Record.define reads them. A
+    # nullable const (`literal / null`) carries `const:` so the runtime rejects a value that
+    # is neither the literal nor null; `const.nil?` means the field has no const at all.
+    def value_facts
+      facts = []
+      facts << 'required: false' unless required
+      facts << 'nullable: true' if nullable
+      facts << "const: #{BiDiGenerate.ruby_literal(const)}" unless const.nil?
+      facts << "ref: '#{ref}'" if ref
+      facts << 'list: true' if list
+      facts << "scalar: #{scalar_literal}" if scalar
+      facts << "enum: '#{enum}'" if enum
+      facts << "primitive: '#{primitive}'" if primitive
+      facts
+    end
+
     # The `scalar` primitive(s) a bare non-object wire value must match at a scalar-tolerant
     # union position: a single primitive string, or an array when the union's scalar arms differ.
     def scalar_literal
@@ -655,7 +664,23 @@ def field_ir(field)
                   required: field['required'], nullable: resolved[:nullable],
                   ref: resolved[:ref], list: resolved[:list], enum: enum_const(field['type']),
                   primitive: leaf_primitive(field['type']), scalar: resolved[:scalar],
-                  rbs: resolved[:rbs])
+                  const: leaf_const(field['type']), rbs: resolved[:rbs])
+    end
+
+    # The literal value of a const field, following alias chains, so the runtime can reject a
+    # value that is neither the literal nor null (a `literal / null` param such as
+    # emulation.setScriptingEnabled's `enabled`). Nil for any non-const node — const literals are
+    # never nil, so nil unambiguously means "no const" (a null value is carried by `nullable`).
+    def leaf_const(node, seen = {})
+      return node['const'] if node.key?('const')
+      return nil unless node.key?('ref')
+
+      name = node['ref']
+      type = @types[name]
+      return nil if seen[name] || type.nil? || type['kind'] != 'alias'
+
+      seen[name] = true
+      leaf_const(type['type'], seen)
     end
 
     # The runtime-checkable scalar primitive of a field, following alias chains so a
diff --git a/rb/sig/lib/selenium/webdriver/bidi/serialization.rbs b/rb/sig/lib/selenium/webdriver/bidi/serialization.rbs
index ba4fbf6504b69..214a9958dd4a6 100644
--- a/rb/sig/lib/selenium/webdriver/bidi/serialization.rbs
+++ b/rb/sig/lib/selenium/webdriver/bidi/serialization.rbs
@@ -60,6 +60,8 @@ module Selenium
 
             def validate_values: (Hash[Symbol, untyped] attributes) -> void
 
+            def validate_const: (untyped field, untyped value) -> void
+
             def check_outbound_shape: (untyped field, untyped value) -> void
 
             def fixed?: (untyped field) -> bool
diff --git a/rb/spec/unit/selenium/webdriver/bidi/serialization_spec.rb b/rb/spec/unit/selenium/webdriver/bidi/serialization_spec.rb
index b0adce8300080..3cbb110fe79ae 100644
--- a/rb/spec/unit/selenium/webdriver/bidi/serialization_spec.rb
+++ b/rb/spec/unit/selenium/webdriver/bidi/serialization_spec.rb
@@ -199,6 +199,13 @@ def valid_cookie_attrs
               expect(BrowsingContext::SetBypassCSPParameters.new(bypass: true).as_json).to eq('bypass' => true)
               expect(BrowsingContext::SetBypassCSPParameters.new(bypass: nil).as_json).to eq('bypass' => nil)
             end
+
+            it 'rejects a value that is neither the literal nor null, before it reaches the wire' do
+              expect { BrowsingContext::SetBypassCSPParameters.new(bypass: false) }
+                .to raise_error(ArgumentError, /bypass must be true/)
+              expect { Emulation::SetScriptingEnabledParameters.new(enabled: true) }
+                .to raise_error(ArgumentError, /enabled must be false/)
+            end
           end
 
           describe 'extensible records' do