From f29b4981a0fca42eb7c3918da1649a9036d74e49 Mon Sep 17 00:00:00 2001
From: Sameer Kankute <sameer@berri.ai>
Date: Wed, 18 Mar 2026 17:13:42 +0530
Subject: [PATCH] fix(prompting): preserve separator for
 assistant(tc)->assistant edge case

When scanning backward over counted messages, preserve old behavior for
adjacent assistant turns by inserting user_continue if the immediate
previous raw message is assistant. This handles malformed
assistant(tool_calls)->assistant(no-tool-calls) inputs without splitting
valid assistant(tool_calls)->tool chains.

Made-with: Cursor
---
 .../prompt_templates/common_utils.py          | 27 +++++++----
 tests/llm_translation/test_prompt_factory.py  | 48 +++++++++++++++++++
 2 files changed, 65 insertions(+), 10 deletions(-)

diff --git a/litellm/litellm_core_utils/prompt_templates/common_utils.py b/litellm/litellm_core_utils/prompt_templates/common_utils.py
index eb3755b71fe..2efd90e0c2f 100644
--- a/litellm/litellm_core_utils/prompt_templates/common_utils.py
+++ b/litellm/litellm_core_utils/prompt_templates/common_utils.py
@@ -298,16 +298,23 @@ def _insert_user_continue_message(
         curr_message = result_messages[i]
         inserted_continue_message = False
         if _counts_for_alternation(curr_message) and curr_message["role"] == "assistant":
-            j = i - 1
-            while j >= 0:
-                previous_message = result_messages[j]
-                if _counts_for_alternation(previous_message):
-                    if previous_message["role"] == "assistant":
-                        result_messages.insert(i, continue_message)
-                        i += 2
-                        inserted_continue_message = True
-                    break
-                j -= 1
+            # Preserve old behavior for malformed adjacent assistant sequences like
+            # assistant(tool_calls) -> assistant(no-tool-calls) with no tool message.
+            if i > 0 and result_messages[i - 1].get("role") == "assistant":
+                result_messages.insert(i, continue_message)
+                i += 2
+                inserted_continue_message = True
+            else:
+                j = i - 1
+                while j >= 0:
+                    previous_message = result_messages[j]
+                    if _counts_for_alternation(previous_message):
+                        if previous_message["role"] == "assistant":
+                            result_messages.insert(i, continue_message)
+                            i += 2
+                            inserted_continue_message = True
+                        break
+                    j -= 1
         if not inserted_continue_message:
             i += 1
 
diff --git a/tests/llm_translation/test_prompt_factory.py b/tests/llm_translation/test_prompt_factory.py
index fe46c24a298..64556c3f26d 100644
--- a/tests/llm_translation/test_prompt_factory.py
+++ b/tests/llm_translation/test_prompt_factory.py
@@ -903,6 +903,54 @@ def test_ensure_alternating_roles_does_not_split_tool_call_chain():
     ]
 
 
+def test_ensure_alternating_roles_assistant_tool_call_then_assistant():
+    """
+    Preserve old behavior for malformed adjacent assistant turns:
+    [assistant(tool_calls), assistant(no-tool-calls), user] should insert
+    user_continue between assistant messages.
+    """
+    messages = [
+        {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [
+                {
+                    "id": "c1",
+                    "type": "function",
+                    "function": {"name": "search", "arguments": "{}"},
+                }
+            ],
+        },
+        {"role": "assistant", "content": "Here's what I found."},
+        {"role": "user", "content": "Thanks"},
+    ]
+
+    transformed_messages = get_completion_messages(
+        messages=messages,
+        assistant_continue_message=None,
+        user_continue_message=None,
+        ensure_alternating_roles=True,
+    )
+
+    assert transformed_messages == [
+        {"role": "user", "content": "Please continue."},
+        {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [
+                {
+                    "id": "c1",
+                    "type": "function",
+                    "function": {"name": "search", "arguments": "{}"},
+                }
+            ],
+        },
+        {"role": "user", "content": "Please continue."},
+        {"role": "assistant", "content": "Here's what I found."},
+        {"role": "user", "content": "Thanks"},
+    ]
+
+
 def test_ensure_alternating_roles_trailing_tool_call_assistant():
     messages = [
         {"role": "user", "content": "What's the weather?"},