diff --git a/pydantic_ai_slim/pydantic_ai/_parts_manager.py b/pydantic_ai_slim/pydantic_ai/_parts_manager.py
index 3988f1ea8c..76b3f2b482 100644
--- a/pydantic_ai_slim/pydantic_ai/_parts_manager.py
+++ b/pydantic_ai_slim/pydantic_ai/_parts_manager.py
@@ -198,16 +198,16 @@ def handle_thinking_delta(
                 existing_thinking_part_and_index = existing_part, part_index
 
         if existing_thinking_part_and_index is None:
-            if content is not None:
+            if content is not None or signature is not None:
                 # There is no existing thinking part that should be updated, so create a new one
                 new_part_index = len(self._parts)
-                part = ThinkingPart(content=content, id=id, signature=signature, provider_name=provider_name)
+                part = ThinkingPart(content=content or '', id=id, signature=signature, provider_name=provider_name)
                 if vendor_part_id is not None:  # pragma: no branch
                     self._vendor_id_to_part_index[vendor_part_id] = new_part_index
                 self._parts.append(part)
                 return PartStartEvent(index=new_part_index, part=part)
             else:
-                raise UnexpectedModelBehavior('Cannot create a ThinkingPart with no content')
+                raise UnexpectedModelBehavior('Cannot create a ThinkingPart with no content or signature')
         else:
             if content is not None or signature is not None:
                 # Update the existing ThinkingPart with the new content and/or signature delta
diff --git a/pydantic_ai_slim/pydantic_ai/models/anthropic.py b/pydantic_ai_slim/pydantic_ai/models/anthropic.py
index 509b7fdf59..49b69f9567 100644
--- a/pydantic_ai_slim/pydantic_ai/models/anthropic.py
+++ b/pydantic_ai_slim/pydantic_ai/models/anthropic.py
@@ -641,7 +641,6 @@ async def _get_event_iterator(self) -> AsyncIterator[ModelResponseStreamEvent]:
                     yield self._parts_manager.handle_thinking_delta(
                         vendor_part_id=event.index,
                         id='redacted_thinking',
-                        content='',
                         signature=current_block.data,
                         provider_name=self.provider_name,
                     )
diff --git a/pydantic_ai_slim/pydantic_ai/models/bedrock.py b/pydantic_ai_slim/pydantic_ai/models/bedrock.py
index a7a2182146..2b53ac0a30 100644
--- a/pydantic_ai_slim/pydantic_ai/models/bedrock.py
+++ b/pydantic_ai_slim/pydantic_ai/models/bedrock.py
@@ -681,7 +681,6 @@ async def _get_event_iterator(self) -> AsyncIterator[ModelResponseStreamEvent]:
                             yield self._parts_manager.handle_thinking_delta(
                                 vendor_part_id=index,
                                 id='redacted_content',
-                                content='',
                                 signature=redacted_content.decode('utf-8'),
                                 provider_name=self.provider_name,
                             )
diff --git a/pydantic_ai_slim/pydantic_ai/models/google.py b/pydantic_ai_slim/pydantic_ai/models/google.py
index 5b724fca3c..75d2adbe17 100644
--- a/pydantic_ai_slim/pydantic_ai/models/google.py
+++ b/pydantic_ai_slim/pydantic_ai/models/google.py
@@ -596,7 +596,6 @@ async def _get_event_iterator(self) -> AsyncIterator[ModelResponseStreamEvent]:
                     signature = base64.b64encode(part.thought_signature).decode('utf-8')
                     yield self._parts_manager.handle_thinking_delta(
                         vendor_part_id='thinking',
-                        content='',  # A thought signature may occur without a preceding thinking part, so we add an empty delta so that a new part can be created
                         signature=signature,
                         provider_name=self.provider_name,
                     )
diff --git a/tests/models/test_google.py b/tests/models/test_google.py
index bfdeebda5a..4b143646bc 100644
--- a/tests/models/test_google.py
+++ b/tests/models/test_google.py
@@ -1226,9 +1226,7 @@ def dummy() -> None: ...  # pragma: no cover
             PartDeltaEvent(index=0, delta=ThinkingPartDelta(content_delta=IsStr())),
             PartDeltaEvent(index=0, delta=ThinkingPartDelta(content_delta=IsStr())),
             PartDeltaEvent(index=0, delta=ThinkingPartDelta(content_delta=IsStr())),
-            PartDeltaEvent(
-                index=0, delta=ThinkingPartDelta(content_delta='', signature_delta=IsStr(), provider_name='google-gla')
-            ),
+            PartDeltaEvent(index=0, delta=ThinkingPartDelta(signature_delta=IsStr(), provider_name='google-gla')),
             PartStartEvent(index=1, part=TextPart(content=IsStr())),
             FinalResultEvent(tool_name=None, tool_call_id=None),
             PartDeltaEvent(index=1, delta=TextPartDelta(content_delta=IsStr())),