Ensure first message is user message for anthropic (#1530)

Co-authored-by: Carson <cpsievert1@gmail.com>

shiny/ui/_chat.py

@@ -466,7 +466,7 @@ def messages(
 
         messages = self._messages()
         if token_limits is not None:
-            messages = self._trim_messages(messages, token_limits)
+            messages = self._trim_messages(messages, token_limits, format)
 
         res: list[ChatMessage | ProviderMessage] = []
         for i, m in enumerate(messages):
@@ -827,7 +827,8 @@ def _store_message(
     @staticmethod
     def _trim_messages(
         messages: tuple[StoredMessage, ...],
-        token_limits: tuple[int, int] = (4096, 1000),
+        token_limits: tuple[int, int],
+        format: MISSING_TYPE | ProviderMessageFormat,
     ) -> tuple[StoredMessage, ...]:
 
         n_total, n_reserve = token_limits
@@ -863,6 +864,7 @@ def _trim_messages(
             )
 
         messages2: list[StoredMessage] = []
+        n_other_messages2: int = 0
         for m in reversed(messages):
             if m["role"] == "system":
                 messages2.append(m)
@@ -871,10 +873,22 @@ def _trim_messages(
             remaining_non_system_tokens -= count
             if remaining_non_system_tokens >= 0:
                 messages2.append(m)
+                n_other_messages2 += 1
+
+        # Anthropic doesn't support `role: system` and requires a user message to come 1st
+        if format == "anthropic":
+            if n_system_messages > 0:
+                raise ValueError(
+                    "Anthropic requires a system prompt to be specified in it's `.create()` method "
+                    "(not in the chat messages with `role: system`)."
+                )
+            while n_other_messages2 > 0 and messages2[-1]["role"] != "user":
+                messages2.pop()
+                n_other_messages2 -= 1
 
         messages2.reverse()
 
-        if len(messages2) == n_system_messages and n_other_messages > 0:
+        if len(messages2) == n_system_messages and n_other_messages2 > 0:
             raise ValueError(
                 f"Only system messages fit within `.messages(token_limits={token_limits})`. "
                 "Consider increasing the 1st value of `token_limit` or setting it to "

tests/pytest/test_chat.py

@@ -9,6 +9,7 @@
 from shiny import Session
 from shiny._namespaces import ResolvedId, Root
 from shiny.session import session_context
+from shiny.types import MISSING
 from shiny.ui import Chat
 from shiny.ui._chat import as_transformed_message
 from shiny.ui._chat_normalize import normalize_message, normalize_message_chunk
@@ -66,7 +67,7 @@ def test_chat_message_trimming():
 
         # Throws since system message is too long
         with pytest.raises(ValueError):
-            chat._trim_messages(msgs, token_limits=(100, 0))
+            chat._trim_messages(msgs, token_limits=(100, 0), format=MISSING)
 
         msgs = (
             as_stored_message(
@@ -79,10 +80,10 @@ def test_chat_message_trimming():
 
         # Throws since only the system message fits
         with pytest.raises(ValueError):
-            chat._trim_messages(msgs, token_limits=(100, 0))
+            chat._trim_messages(msgs, token_limits=(100, 0), format=MISSING)
 
         # Raising the limit should allow both messages to fit
-        trimmed = chat._trim_messages(msgs, token_limits=(102, 0))
+        trimmed = chat._trim_messages(msgs, token_limits=(102, 0), format=MISSING)
         assert len(trimmed) == 2
         contents = [msg["content_server"] for msg in trimmed]
         assert contents == ["System message", "User message"]
@@ -100,7 +101,7 @@ def test_chat_message_trimming():
         )
 
         # Should discard the 1st user message
-        trimmed = chat._trim_messages(msgs, token_limits=(102, 0))
+        trimmed = chat._trim_messages(msgs, token_limits=(102, 0), format=MISSING)
         assert len(trimmed) == 2
         contents = [msg["content_server"] for msg in trimmed]
         assert contents == ["System message", "User message 2"]
@@ -121,11 +122,26 @@ def test_chat_message_trimming():
         )
 
         # Should discard the 1st user message
-        trimmed = chat._trim_messages(msgs, token_limits=(102, 0))
+        trimmed = chat._trim_messages(msgs, token_limits=(102, 0), format=MISSING)
         assert len(trimmed) == 3
         contents = [msg["content_server"] for msg in trimmed]
         assert contents == ["System message", "System message 2", "User message 2"]
 
+        msgs = (
+            as_stored_message(
+                {"content": "Assistant message", "role": "assistant"}, token_count=50
+            ),
+            as_stored_message(
+                {"content": "User message", "role": "user"}, token_count=10
+            ),
+        )
+
+        # Anthropic requires 1st message to be a user message
+        trimmed = chat._trim_messages(msgs, token_limits=(30, 0), format="anthropic")
+        assert len(trimmed) == 1
+        contents = [msg["content_server"] for msg in trimmed]
+        assert contents == ["User message"]
+
 
 # ------------------------------------------------------------------------------------
 # Unit tests for normalize_message() and normalize_message_chunk().