Add appended message request validation

types.go

@@ -123,6 +123,13 @@ type RunRequest struct {
 	// Validation optionally replaces the prompt's complete output contract. It
 	// does not merge individual fields. Nil uses the prompt contract.
 	Validation *OutputContract
+	// AppendedMessages are already-rendered messages appended after every prompt
+	// definition message. Promptkit neither templates nor resolves files in
+	// them, and preserves valid content exactly. Nil and empty slices are
+	// equivalent. Prepare, PrepareExecution, and Run validate and copy the
+	// messages before source or model work; malformed values return an error
+	// matching ErrInvalidRequest.
+	AppendedMessages []RenderedMessage
 }
 
 // PreparedRun contains prepared prompt execution state returned by
@@ -629,12 +636,13 @@ type RenderedPrompt struct {
 	Messages []RenderedMessage `json:"messages"`
 }
 
-// RenderedMessage is a rendered chat message and has a stable JSON
-// representation.
+// RenderedMessage is a prepared, provider-bound text chat message and has a
+// stable JSON representation. Its role must be one of [RoleDeveloper],
+// [RoleSystem], [RoleUser], or [RoleAssistant].
 type RenderedMessage struct {
-	// Role is the definition-supplied chat role.
+	// Role is the provider-bound chat role.
 	Role string `json:"role"`
-	// Content is the rendered message text.
+	// Content is the provider-bound message text and may be empty or whitespace.
 	Content string `json:"content"`
 	// CacheControl is optional provider cache metadata.
 	CacheControl *CacheControl `json:"cache_control,omitempty"`