diff --git a/src/phoenix/server/api/subscriptions.py b/src/phoenix/server/api/subscriptions.py
index fb984500d2..49768f95cd 100644
--- a/src/phoenix/server/api/subscriptions.py
+++ b/src/phoenix/server/api/subscriptions.py
@@ -65,7 +65,7 @@
 
 PLAYGROUND_PROJECT_NAME = "playground"
 
-ToolCallIndex: TypeAlias = int
+ToolCallID: TypeAlias = str
 
 
 @strawberry.enum
@@ -315,8 +315,10 @@ def _build_anthropic_messages(
                 anthropic_messages.append({"role": "assistant", "content": content})
             elif role == ChatCompletionMessageRole.SYSTEM:
                 system_prompt += content + "\n"
+            elif role == ChatCompletionMessageRole.TOOL:
+                raise NotImplementedError
             else:
-                raise ValueError(f"Unsupported role: {role}")
+                assert_never(role)
 
         return anthropic_messages, system_prompt
 
@@ -369,7 +371,7 @@ async def chat_completion(
         ) as span:
             response_chunks = []
             text_chunks: List[TextChunk] = []
-            tool_call_chunks: DefaultDict[ToolCallIndex, List[ToolCallChunk]] = defaultdict(list)
+            tool_call_chunks: DefaultDict[ToolCallID, List[ToolCallChunk]] = defaultdict(list)
 
             async for chunk in llm_client.chat_completion_create(
                 messages=messages,
@@ -382,8 +384,7 @@ async def chat_completion(
                     text_chunks.append(chunk)
                 elif isinstance(chunk, ToolCallChunk):
                     yield chunk
-                    tool_call_index = int(chunk.id) if chunk.id.isdigit() else 0
-                    tool_call_chunks[tool_call_index].append(chunk)
+                    tool_call_chunks[chunk.id].append(chunk)
 
             span.set_status(StatusCode.OK)
             llm_client_attributes = llm_client.attributes
@@ -499,7 +500,7 @@ def _llm_input_messages(
 
 def _llm_output_messages(
     text_chunks: List[TextChunk],
-    tool_call_chunks: DefaultDict[ToolCallIndex, List[ToolCallChunk]],
+    tool_call_chunks: DefaultDict[ToolCallID, List[ToolCallChunk]],
 ) -> Iterator[Tuple[str, Any]]:
     yield f"{LLM_OUTPUT_MESSAGES}.0.{MESSAGE_ROLE}", "assistant"
     if content := "".join(chunk.content for chunk in text_chunks):