diff --git a/backends.go b/backends.go
index 33774fd..4343a98 100644
--- a/backends.go
+++ b/backends.go
@@ -19,10 +19,13 @@ const BackendLocal = "local"
 // explicit zero or blank values. A nil ExtraParams map is omitted; a non-nil
 // empty map supplies an empty mapping. A nil SmokeTests pointer is omitted; a
 // non-nil pointer supplies a smoke_tests mapping. Use raw ConfigKit Mapping
-// inputs for explicit null. BackendConfig has no stable JSON representation.
+// inputs for explicit null. Extends names a sibling backend in the same list.
+// BackendConfig has no stable JSON representation.
 type BackendConfig struct {
 	// ID is the local backend identity within its resource list.
 	ID string
+	// Extends names a sibling backend to inherit through ConfigKit.
+	Extends *string
 	// Endpoint is the required absolute HTTP(S) base URL after all layers merge.
 	Endpoint *string
 	// APIKeyEnv optionally names an environment variable, not a credential.
@@ -84,6 +87,9 @@ func BackendSource(label string, backends []BackendConfig) configkit.Source {
 	definitions := make([]any, 0, len(backends))
 	for _, config := range backends {
 		fields := map[string]any{"id": config.ID}
+		if config.Extends != nil {
+			fields["extends"] = map[string]any{"id": *config.Extends}
+		}
 		if config.Endpoint != nil {
 			fields["endpoint"] = *config.Endpoint
 		}
diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index dd5f2c5..074864d 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -470,6 +470,10 @@ Do not introduce benchmark thresholds or additional release gates.
 
 ## Stage 9: Add Typed Backend Inheritance And Prompt Sources
 
+**Completed:** Backend inheritance and the typed prompt mapping constructor
+follow the existing ConfigKit source pattern. Existing focused programmatic
+source and inspection tests pass; composition coverage follows next.
+
 **Scope:** Public mapping-constructor implementation and GoDoc.
 
 1. Add `Extends *string` to `BackendConfig` and encode it in `BackendSource`
diff --git a/prompts.go b/prompts.go
new file mode 100644
index 0000000..982b7e5
--- /dev/null
+++ b/prompts.go
@@ -0,0 +1,129 @@
+package promptkit
+
+import "go.maximumdirect.net/configkit"
+
+// PromptConfig is a presence-aware source input for one prompt. Extends names
+// a sibling prompt. Nil optional pointers and slices are omitted; non-nil
+// pointers and slices preserve explicit zero, blank, and empty values. These
+// inputs are mapping constructors, not serialized inspection values. Raw
+// ConfigKit mappings can express explicit null and other composition controls.
+type PromptConfig struct {
+	ID                                              string
+	Extends, DefaultProfile, Description, SessionID *string
+	Inputs                                          []PromptInputConfig
+	Messages                                        []PromptMessageConfig
+	Output                                          *PromptOutputConfig
+}
+
+// PromptInputConfig supplies an input declaration. Name is always supplied;
+// nil optional pointers omit their fields.
+type PromptInputConfig struct {
+	Name                     string
+	Required                 *bool
+	ContentType, Description *string
+}
+
+// PromptMessageConfig supplies message fields, including inline content or a
+// content-file reference. Final content selection is validated by NewEngine.
+type PromptMessageConfig struct {
+	Role, Content, ContentFile *string
+	CacheControl               *PromptCacheControlConfig
+}
+
+// PromptCacheControlConfig supplies optional cache-control source fields.
+type PromptCacheControlConfig struct{ Type, TTL *string }
+
+// PromptOutputConfig supplies optional output contract source fields.
+type PromptOutputConfig struct {
+	Format, ValidationMode, SchemaPath *string
+	RepairAttempts                     *int
+}
+
+// PromptSource creates a labeled ConfigKit mapping in caller order. Nil
+// top-level input supplies an empty prompts sequence. Records may be partial:
+// sibling inheritance and layered sources can complete them before NewEngine
+// validates the final configuration. Mapping snapshots supported values when
+// constructed; the helper neither resolves inheritance nor reads files.
+// Relative content files use the ConfigKit Input filesystem context of the
+// contributing field. Raw mappings remain available for explicit null and
+// generic ConfigKit operations.
+func PromptSource(label string, prompts []PromptConfig) configkit.Source {
+	definitions := make([]any, 0, len(prompts))
+	for _, config := range prompts {
+		fields := map[string]any{"id": config.ID}
+		if config.Extends != nil {
+			fields["extends"] = map[string]any{"id": *config.Extends}
+		}
+		if config.DefaultProfile != nil {
+			fields["default_profile"] = *config.DefaultProfile
+		}
+		if config.Description != nil {
+			fields["description"] = *config.Description
+		}
+		if config.SessionID != nil {
+			fields["session_id"] = *config.SessionID
+		}
+		if config.Inputs != nil {
+			inputs := make([]any, 0, len(config.Inputs))
+			for _, input := range config.Inputs {
+				item := map[string]any{"name": input.Name}
+				if input.Required != nil {
+					item["required"] = *input.Required
+				}
+				if input.ContentType != nil {
+					item["content_type"] = *input.ContentType
+				}
+				if input.Description != nil {
+					item["description"] = *input.Description
+				}
+				inputs = append(inputs, item)
+			}
+			fields["inputs"] = inputs
+		}
+		if config.Messages != nil {
+			messages := make([]any, 0, len(config.Messages))
+			for _, message := range config.Messages {
+				item := make(map[string]any)
+				if message.Role != nil {
+					item["role"] = *message.Role
+				}
+				if message.Content != nil {
+					item["content"] = *message.Content
+				}
+				if message.ContentFile != nil {
+					item["content_file"] = *message.ContentFile
+				}
+				if message.CacheControl != nil {
+					cache := make(map[string]any)
+					if message.CacheControl.Type != nil {
+						cache["type"] = *message.CacheControl.Type
+					}
+					if message.CacheControl.TTL != nil {
+						cache["ttl"] = *message.CacheControl.TTL
+					}
+					item["cache_control"] = cache
+				}
+				messages = append(messages, item)
+			}
+			fields["messages"] = messages
+		}
+		if config.Output != nil {
+			output := make(map[string]any)
+			if config.Output.Format != nil {
+				output["format"] = *config.Output.Format
+			}
+			if config.Output.ValidationMode != nil {
+				output["validation_mode"] = *config.Output.ValidationMode
+			}
+			if config.Output.SchemaPath != nil {
+				output["schema_path"] = *config.Output.SchemaPath
+			}
+			if config.Output.RepairAttempts != nil {
+				output["repair_attempts"] = *config.Output.RepairAttempts
+			}
+			fields["output"] = output
+		}
+		definitions = append(definitions, fields)
+	}
+	return configkit.Mapping(label, map[string]any{"prompts": definitions})
+}