diff --git a/pydantic_ai_slim/pydantic_ai/_thinking_part.py b/pydantic_ai_slim/pydantic_ai/_thinking_part.py
index fa10b4e0e..a7ba38fc0 100644
--- a/pydantic_ai_slim/pydantic_ai/_thinking_part.py
+++ b/pydantic_ai_slim/pydantic_ai/_thinking_part.py
@@ -16,21 +16,22 @@ def split_content_into_text_and_thinking(content: str) -> list[ThinkingPart | Te
     something else, we just match the tag to make it easier for other models that don't support the `ThinkingPart`.
     """
     parts: list[ThinkingPart | TextPart] = []
-
-    start_index = content.find(START_THINK_TAG)
-    while start_index >= 0:
-        before_think, content = content[:start_index], content[start_index + len(START_THINK_TAG) :]
-        if before_think:
-            parts.append(TextPart(content=before_think))
-        end_index = content.find(END_THINK_TAG)
-        if end_index >= 0:
-            think_content, content = content[:end_index], content[end_index + len(END_THINK_TAG) :]
-            parts.append(ThinkingPart(content=think_content))
-        else:
-            # We lose the `<think>` tag, but it shouldn't matter.
-            parts.append(TextPart(content=content))
-            content = ''
-        start_index = content.find(START_THINK_TAG)
-    if content:
-        parts.append(TextPart(content=content))
+    
+    start_index = 0
+    while True:
+        start = content.find('<think>', start_index)
+        if start < 0:
+            if start_index < len(content):
+                parts.append(TextPart(content=content[start_index:]))
+            break
+        if start > start_index:
+            parts.append(TextPart(content=content[start_index:start]))
+        think_start = start + 7  # len('<think>')
+        end = content.find('</think>', think_start)
+        if end < 0:
+            parts.append(TextPart(content=content[think_start:]))
+            break
+        if end > think_start:
+            parts.append(ThinkingPart(content=content[think_start:end]))
+        start_index = end + 8  # len('</think>')
     return parts