fix: accept empty delta strings in content events (#1419)

Models like GPT-5 legitimately send empty deltas during streaming.
Removed min_length=1 (Python) and .refine(s.length > 0) (TypeScript)
from TextMessageContentEvent, ThinkingTextMessageContentEvent, and
ReasoningMessageContentEvent. Empty deltas are harmless — concatenating
an empty string is a no-op.

Fixes #337

Author: AlemTuzlak

sdks/python/ag_ui/core/events.py

@@ -77,7 +77,7 @@ class TextMessageContentEvent(BaseEvent):
     """
     type: Literal[EventType.TEXT_MESSAGE_CONTENT] = EventType.TEXT_MESSAGE_CONTENT  # pyright: ignore[reportIncompatibleVariableOverride]
     message_id: str
-    delta: str = Field(min_length=1)
+    delta: str
 
 
 class TextMessageEndEvent(BaseEvent):
@@ -108,7 +108,7 @@ class ThinkingTextMessageContentEvent(BaseEvent):
     Event indicating a piece of a thinking text message.
     """
     type: Literal[EventType.THINKING_TEXT_MESSAGE_CONTENT] = EventType.THINKING_TEXT_MESSAGE_CONTENT  # pyright: ignore[reportIncompatibleVariableOverride]
-    delta: str = Field(min_length=1)
+    delta: str
 
 class ThinkingTextMessageEndEvent(BaseEvent):
     """
@@ -312,7 +312,7 @@ class ReasoningMessageContentEvent(BaseEvent):
     """
     type: Literal[EventType.REASONING_MESSAGE_CONTENT] = EventType.REASONING_MESSAGE_CONTENT  # pyright: ignore[reportIncompatibleVariableOverride]
     message_id: str
-    delta: str = Field(min_length=1)
+    delta: str
 
 
 class ReasoningMessageEndEvent(BaseEvent):

sdks/python/tests/test_events.py

@@ -424,14 +424,13 @@ def test_event_union_deserialization(self):
             self.assertEqual(event.type.value, data["type"])
             self.assertEqual(event.timestamp, data["timestamp"])
 
-    def test_validation_constraints(self):
-        """Test validation constraints for different event types"""
-        # TextMessageContentEvent delta cannot be empty
-        with self.assertRaises(ValueError):
-            TextMessageContentEvent(
-                message_id="msg_123",
-                delta=""  # Empty delta, should fail
-            )
+    def test_empty_delta_accepted(self):
+        """Models like GPT-5 legitimately send empty deltas during streaming"""
+        event = TextMessageContentEvent(
+            message_id="msg_123",
+            delta=""
+        )
+        self.assertEqual(event.delta, "")
 
     def test_serialization_round_trip(self):
         """Test serialization and deserialization for different event types"""

sdks/typescript/packages/core/src/__tests__/event-factories.test.ts

@@ -43,10 +43,10 @@ describe("event factories", () => {
     expect(event.role).toBe("user");
   });
 
-  it("rejects empty deltas in TEXT_MESSAGE_CONTENT", () => {
-    expect(() => createTextMessageContentEvent({ messageId: "msg-3", delta: "" })).toThrow(
-      /Delta must not be an empty string/,
-    );
+  it("accepts empty deltas in TEXT_MESSAGE_CONTENT", () => {
+    const event = createTextMessageContentEvent({ messageId: "msg-3", delta: "" });
+    expect(event.type).toBe(EventType.TEXT_MESSAGE_CONTENT);
+    expect(event.delta).toBe("");
   });
 
   it("creates TEXT_MESSAGE_CONTENT when delta provided", () => {

sdks/typescript/packages/core/src/events.ts

@@ -78,7 +78,7 @@ export const TextMessageStartEventSchema = BaseEventSchema.extend({
 export const TextMessageContentEventSchema = BaseEventSchema.extend({
   type: z.literal(EventType.TEXT_MESSAGE_CONTENT),
   messageId: z.string(),
-  delta: z.string().refine((s) => s.length > 0, "Delta must not be an empty string"),
+  delta: z.string(),
 });
 
 export const TextMessageEndEventSchema = BaseEventSchema.extend({
@@ -260,7 +260,7 @@ export const ReasoningMessageStartEventSchema = BaseEventSchema.extend({
 export const ReasoningMessageContentEventSchema = BaseEventSchema.extend({
   type: z.literal(EventType.REASONING_MESSAGE_CONTENT),
   messageId: z.string(),
-  delta: z.string().refine((s) => s.length > 0, "Delta must not be an empty string"),
+  delta: z.string(),
 });
 
 export const ReasoningMessageEndEventSchema = BaseEventSchema.extend({