diff --git a/litellm/main.py b/litellm/main.py
index 80a2f74c5713..5db85594cd9d 100644
--- a/litellm/main.py
+++ b/litellm/main.py
@@ -147,6 +147,7 @@
     token_counter,
     validate_and_fix_openai_messages,
     validate_and_fix_openai_tools,
+    validate_and_fix_thinking_param,
     validate_chat_completion_tool_choice,
     validate_openai_optional_params,
 )
@@ -1102,6 +1103,8 @@ def completion(  # type: ignore # noqa: PLR0915
     tool_choice = validate_chat_completion_tool_choice(tool_choice=tool_choice)
     # validate optional params
     stop = validate_openai_optional_params(stop=stop)
+    # normalize camelCase thinking keys (e.g. budgetTokens -> budget_tokens)
+    thinking = validate_and_fix_thinking_param(thinking=thinking)
 
     ######### unpacking kwargs #####################
     args = locals()
diff --git a/litellm/utils.py b/litellm/utils.py
index 241b9d217b7a..3c02a17d15ec 100644
--- a/litellm/utils.py
+++ b/litellm/utils.py
@@ -7666,6 +7666,23 @@ def validate_and_fix_openai_tools(tools: Optional[List]) -> Optional[List[dict]]
     return new_tools
 
 
+def validate_and_fix_thinking_param(
+    thinking: Optional["AnthropicThinkingParam"],
+) -> Optional["AnthropicThinkingParam"]:
+    """
+    Normalizes camelCase keys in the thinking param to snake_case.
+    Handles clients that send budgetTokens instead of budget_tokens.
+    """
+    if thinking is None or not isinstance(thinking, dict):
+        return thinking
+    normalized = dict(thinking)
+    if "budgetTokens" in normalized and "budget_tokens" not in normalized:
+        normalized["budget_tokens"] = normalized.pop("budgetTokens")
+    elif "budgetTokens" in normalized and "budget_tokens" in normalized:
+        normalized.pop("budgetTokens")
+    return cast("AnthropicThinkingParam", normalized)
+
+
 def cleanup_none_field_in_message(message: AllMessageValues):
     """
     Cleans up the message by removing the none field.
diff --git a/tests/test_litellm/test_utils.py b/tests/test_litellm/test_utils.py
index 6170f6b6f55d..9cb43a8dee71 100644
--- a/tests/test_litellm/test_utils.py
+++ b/tests/test_litellm/test_utils.py
@@ -3425,3 +3425,43 @@ def test_litellm_params_metadata_none(self):
         litellm_params = {"metadata": None}
         metadata = litellm_params.get("metadata") or {}
         assert metadata == {}
+
+
+class TestValidateAndFixThinkingParam:
+    """Tests for validate_and_fix_thinking_param."""
+
+    def test_none_returns_none(self):
+        from litellm.utils import validate_and_fix_thinking_param
+
+        assert validate_and_fix_thinking_param(thinking=None) is None
+
+    def test_already_snake_case(self):
+        from litellm.utils import validate_and_fix_thinking_param
+
+        thinking = {"type": "enabled", "budget_tokens": 32000}
+        result = validate_and_fix_thinking_param(thinking=thinking)
+        assert result == {"type": "enabled", "budget_tokens": 32000}
+
+    def test_camel_case_normalized(self):
+        from litellm.utils import validate_and_fix_thinking_param
+
+        thinking = {"type": "enabled", "budgetTokens": 32000}
+        result = validate_and_fix_thinking_param(thinking=thinking)
+        assert result == {"type": "enabled", "budget_tokens": 32000}
+        assert "budgetTokens" not in result
+
+    def test_both_keys_snake_case_wins(self):
+        from litellm.utils import validate_and_fix_thinking_param
+
+        thinking = {"type": "enabled", "budget_tokens": 10000, "budgetTokens": 50000}
+        result = validate_and_fix_thinking_param(thinking=thinking)
+        assert result == {"type": "enabled", "budget_tokens": 10000}
+        assert "budgetTokens" not in result
+
+    def test_original_dict_not_mutated(self):
+        from litellm.utils import validate_and_fix_thinking_param
+
+        thinking = {"type": "enabled", "budgetTokens": 32000}
+        validate_and_fix_thinking_param(thinking=thinking)
+        assert "budgetTokens" in thinking
+        assert "budget_tokens" not in thinking