Fix History replay dropping output-format reminders

dspy/adapters/base.py

@@ -542,7 +542,11 @@ def format_conversation_history(
                 else None
             )
 
-            user_content = self.format_user_message_content(signature, message)
+            # Preserve the format reminder from the original request for prompt-cache reuse.
+            # Output-only events must not acquire a synthetic user message containing just the reminder.
+            user_content = self.format_user_message_content(
+                signature, message, main_request=any(name in message for name in signature.input_fields)
+            )
             if user_content:
                 messages.append({"role": "user", "content": user_content})
 

tests/adapters/test_chat_adapter.py

@@ -370,9 +370,13 @@ class HistorySignature(dspy.Signature):
                  "[[ ## completed ## ]]\n"
                  "In adhering to this structure, your objective is: \n"
                  "        Given the fields `history`, `question`, produce the fields `answer`."},
-     {"role": "user", "content": "[[ ## question ## ]]\nWhat is 1+1?"},
+     {"role": "user", "content": "[[ ## question ## ]]\nWhat is 1+1?\n\n"
+      "Respond with the corresponding output fields, starting with the field `[[ ## answer ## ]]`, "
+      "and then ending with the marker for `[[ ## completed ## ]]`."},
      {"role": "assistant", "content": "[[ ## answer ## ]]\n2\n\n[[ ## completed ## ]]\n"},
-     {"role": "user", "content": "[[ ## question ## ]]\nWhat is 2+2?"},
+     {"role": "user", "content": "[[ ## question ## ]]\nWhat is 2+2?\n\n"
+      "Respond with the corresponding output fields, starting with the field `[[ ## answer ## ]]`, "
+      "and then ending with the marker for `[[ ## completed ## ]]`."},
      {"role": "assistant", "content": "[[ ## answer ## ]]\n4\n\n[[ ## completed ## ]]\n"},
      {"role": "user",
       "content": "[[ ## question ## ]]\n"
@@ -669,7 +673,10 @@ class RichRenderingSignature(dspy.Signature):
                  '["math", "machines"]}\n'
                  '\n'
                  '[[ ## question ## ]]\n'
-                 'Who is Ada?'},
+                 'Who is Ada?\n\n'
+                 'Respond with the corresponding output fields, starting with the field `[[ ## answer ## ]]` '
+                 '(must be formatted as a valid Python AnswerCard), and then ending with the marker '
+                 'for `[[ ## completed ## ]]`.'},
      {"role": "assistant",
       "content": '[[ ## answer ## ]]\n'
                  '{"answer": "Ada is a mathematician.", "sources": ["memory"]}\n'
@@ -1589,7 +1596,10 @@ class ToolHistorySignature(dspy.Signature):
             "        Given the fields `question`, `history`, `tools`, produce the fields `next_thought`, "
             "`tool_calls`.",
         },
-        {"role": "user", "content": "[[ ## question ## ]]\nQ1"},
+        {"role": "user", "content": "[[ ## question ## ]]\nQ1\n\n"
+         "Respond with the corresponding output fields, starting with the field `[[ ## next_thought ## ]]`, then "
+         '`[[ ## tool_calls ## ]]` (must be a JSON object like {"tool_calls": [{"name": "...", "args": {...}}]}), '
+         "and then ending with the marker for `[[ ## completed ## ]]`."},
         {
             "role": "assistant",
             "content": "[[ ## next_thought ## ]]\n"
@@ -1994,7 +2004,12 @@ class KitchenSinkSignature(dspy.Signature):
                  '[2] «older note»\n'
                  '\n'
                  '[[ ## question ## ]]\n'
-                 'Who is Ada?'},
+                 'Who is Ada?\n\n'
+                 'Respond with the corresponding output fields, starting with the field `[[ ## answer ## ]]` '
+                 '(must be formatted as a valid Python AnswerCard), then `[[ ## verdict ## ]]` '
+                 "(must be formatted as a valid Python Literal['yes', 'no']), then `[[ ## confidence ## ]]` "
+                 '(must be formatted as a valid Python float), and then ending with the marker '
+                 'for `[[ ## completed ## ]]`.'},
      {"role": "assistant",
       "content": '[[ ## answer ## ]]\n'
                  '{"answer": "Ada is a mathematician.", "sources": ["memory"]}\n'
@@ -2380,6 +2395,23 @@ class CitationGeneration(dspy.Signature):
     assert "Type description of Citations" in system_content
 
 
+@pytest.mark.parametrize("adapter", [dspy.ChatAdapter(), dspy.JSONAdapter(), dspy.XMLAdapter()])
+def test_history_replay_preserves_previous_request_prefix(adapter):
+    class Conversation(dspy.Signature):
+        question: str = dspy.InputField()
+        history: dspy.History = dspy.InputField()
+        answer: str = dspy.OutputField()
+
+    history = dspy.History(messages=[])
+    previous = None
+    for question, answer in [("What is 3+4?", "7"), ("Double that?", "14"), ("Subtract five?", "9")]:
+        messages = adapter.format(Conversation, [], {"question": question, "history": history})
+        if previous is not None:
+            assert messages[:len(previous)] == previous
+        history.messages.append({"question": question, "answer": answer})
+        previous = messages
+
+
 def test_chat_adapter_formats_conversation_history():
     class MySignature(dspy.Signature):
         question: str = dspy.InputField()
@@ -2397,9 +2429,13 @@ class MySignature(dspy.Signature):
     messages = adapter.format(MySignature, [], {"question": "What is the capital of France?", "history": history})
 
     assert len(messages) == 6
-    assert messages[1]["content"] == "[[ ## question ## ]]\nWhat is the capital of France?"
+    reminder = (
+        "\n\nRespond with the corresponding output fields, starting with the field `[[ ## answer ## ]]`, "
+        "and then ending with the marker for `[[ ## completed ## ]]`."
+    )
+    assert messages[1]["content"] == "[[ ## question ## ]]\nWhat is the capital of France?" + reminder
     assert messages[2]["content"] == "[[ ## answer ## ]]\nParis\n\n[[ ## completed ## ]]\n"
-    assert messages[3]["content"] == "[[ ## question ## ]]\nWhat is the capital of Germany?"
+    assert messages[3]["content"] == "[[ ## question ## ]]\nWhat is the capital of Germany?" + reminder
     assert messages[4]["content"] == "[[ ## answer ## ]]\nBerlin\n\n[[ ## completed ## ]]\n"
 
 

tests/adapters/test_json_adapter.py

@@ -305,7 +305,9 @@ class RichRenderingSignature(dspy.Signature):
                  '["math", "machines"]}\n'
                  '\n'
                  '[[ ## question ## ]]\n'
-                 'Who is Ada?'},
+                 'Who is Ada?\n\n'
+                 'Respond with a JSON object in the following order of fields: `answer` '
+                 '(must be formatted as a valid Python AnswerCard).'},
      {"role": "assistant",
       "content": '{\n'
                  '  "answer": {\n'
@@ -1226,9 +1228,10 @@ class MySignature(dspy.Signature):
     messages = adapter.format(MySignature, [], {"question": "What is the capital of France?", "history": history})
 
     assert len(messages) == 6
-    assert messages[1]["content"] == "[[ ## question ## ]]\nWhat is the capital of France?"
+    reminder = "\n\nRespond with a JSON object in the following order of fields: `answer`."
+    assert messages[1]["content"] == "[[ ## question ## ]]\nWhat is the capital of France?" + reminder
     assert messages[2]["content"] == '{\n  "answer": "Paris"\n}'
-    assert messages[3]["content"] == "[[ ## question ## ]]\nWhat is the capital of Germany?"
+    assert messages[3]["content"] == "[[ ## question ## ]]\nWhat is the capital of Germany?" + reminder
     assert messages[4]["content"] == '{\n  "answer": "Berlin"\n}'
 
 

tests/adapters/test_xml_adapter.py

@@ -829,7 +829,10 @@ class RichRenderingSignature(dspy.Signature):
                  '\n'
                  '<question>\n'
                  'Who is Ada?\n'
-                 '</question>'},
+                 '</question>\n\n'
+                 'Respond with the corresponding output fields wrapped in XML tags `<answer>`. '
+                 'Use this nested XML structure: '
+                 '<answer><answer>...</answer><sources><item>...</item></sources></answer>'},
      {"role": "assistant",
       "content": "<answer><answer>Ada is a mathematician.</answer><sources><item>memory</item></sources></answer>"},
      {"role": "user",