diff --git a/pydantic_ai/agent.py b/pydantic_ai/agent.py
index 7e9a2eac..4e3901d2 100644
--- a/pydantic_ai/agent.py
+++ b/pydantic_ai/agent.py
@@ -248,7 +248,12 @@ async def _handle_model_response(
             if self._allow_text_result:
                 return _utils.Either(left=cast(result.ResultData, model_response.content))
             else:
-                return _utils.Either(right=[_messages.PlainResponseForbidden()])
+                self._incr_result_retry()
+                assert self._result_tool is not None
+                response = _messages.UserPrompt(
+                    content='Plain text responses are not permitted, please call one of the functions instead.',
+                )
+                return _utils.Either(right=[response])
         elif model_response.role == 'llm-tool-calls':
             if self._result_tool is not None:
                 # if there's a result schema, and any of the calls match that name, return the result
diff --git a/pydantic_ai/messages.py b/pydantic_ai/messages.py
index 2401c777..5eae6b39 100644
--- a/pydantic_ai/messages.py
+++ b/pydantic_ai/messages.py
@@ -50,17 +50,6 @@ def llm_response(self) -> str:
         return f'{description}\n\nFix the errors and try again.'
 
 
-@dataclass
-class PlainResponseForbidden:
-    # TODO remove and replace with ToolRetry
-    timestamp: datetime = field(default_factory=datetime.now)
-    role: Literal['plain-response-forbidden'] = 'plain-response-forbidden'
-
-    @staticmethod
-    def llm_response() -> str:
-        return 'Plain text responses are not allowed, please call one of the functions instead.'
-
-
 @dataclass
 class LLMResponse:
     content: str
@@ -105,6 +94,6 @@ class LLMToolCalls:
 
 
 LLMMessage = Union[LLMResponse, LLMToolCalls]
-Message = Union[SystemPrompt, UserPrompt, ToolReturn, ToolRetry, PlainResponseForbidden, LLMMessage]
+Message = Union[SystemPrompt, UserPrompt, ToolReturn, ToolRetry, LLMMessage]
 
 MessagesTypeAdapter = pydantic.TypeAdapter(list[Annotated[Message, pydantic.Field(discriminator='role')]])
diff --git a/pydantic_ai/models/gemini.py b/pydantic_ai/models/gemini.py
index 3c591be8..ebb09432 100644
--- a/pydantic_ai/models/gemini.py
+++ b/pydantic_ai/models/gemini.py
@@ -176,8 +176,6 @@ def message_to_gemini(m: Message) -> _utils.Either[_GeminiTextPart, _GeminiConte
         elif m.role == 'tool-retry':
             # ToolRetry ->
             return _utils.Either(right=_GeminiContent.function_retry(m))
-        elif m.role == 'plain-response-forbidden':
-            return _utils.Either(right=_GeminiContent.user_text(m.llm_response()))
         elif m.role == 'llm-response':
             # LLMResponse ->
             return _utils.Either(right=_GeminiContent.model_text(m.content))
diff --git a/pydantic_ai/models/openai.py b/pydantic_ai/models/openai.py
index 2700984a..9f27b476 100644
--- a/pydantic_ai/models/openai.py
+++ b/pydantic_ai/models/openai.py
@@ -144,12 +144,6 @@ def map_message(message: Message) -> chat.ChatCompletionMessageParam:
                 role='assistant',
                 tool_calls=[_map_tool_call(t) for t in message.calls],
             )
-        elif message.role == 'plain-response-forbidden':
-            # PlainResponseForbidden ->
-            return chat.ChatCompletionUserMessageParam(
-                role='user',
-                content=message.llm_response(),
-            )
         else:
             assert_never(message)
 
diff --git a/tests/test_result_validation.py b/tests/test_agent.py
similarity index 74%
rename from tests/test_result_validation.py
rename to tests/test_agent.py
index d0010d30..6ce31ec9 100644
--- a/tests/test_result_validation.py
+++ b/tests/test_agent.py
@@ -2,7 +2,16 @@
 from pydantic import BaseModel
 
 from pydantic_ai import Agent, ModelRetry
-from pydantic_ai.messages import LLMMessage, LLMToolCalls, Message, ToolCall, ToolRetry, UserPrompt
+from pydantic_ai.messages import (
+    ArgsJson,
+    LLMMessage,
+    LLMResponse,
+    LLMToolCalls,
+    Message,
+    ToolCall,
+    ToolRetry,
+    UserPrompt,
+)
 from pydantic_ai.models.function import AgentInfo, FunctionModel
 from tests.conftest import IsNow
 
@@ -107,3 +116,38 @@ def validate_result(r: Foo) -> Foo:
             LLMToolCalls(calls=[ToolCall.from_json('final_result', '{"a": 42, "b": "foo"}')], timestamp=IsNow()),
         ]
     )
+
+
+def test_plain_response():
+    call_index = 0
+
+    def return_tuple(_: list[Message], info: AgentInfo) -> LLMMessage:
+        nonlocal call_index
+
+        assert info.result_tool is not None
+        call_index += 1
+        if call_index == 1:
+            return LLMResponse(content='hello')
+        else:
+            args_json = '{"response": ["foo", "bar"]}'
+            return LLMToolCalls(calls=[ToolCall.from_json(info.result_tool.name, args_json)])
+
+    agent = Agent(FunctionModel(return_tuple), deps=None, result_type=tuple[str, str])
+
+    result = agent.run_sync('Hello')
+    assert result.response == ('foo', 'bar')
+    assert call_index == 2
+    assert result.message_history == snapshot(
+        [
+            UserPrompt(content='Hello', timestamp=IsNow()),
+            LLMResponse(content='hello', timestamp=IsNow()),
+            UserPrompt(
+                content='Plain text responses are not permitted, please call one of the functions instead.',
+                timestamp=IsNow(),
+            ),
+            LLMToolCalls(
+                calls=[ToolCall(tool_name='final_result', args=ArgsJson(args_json='{"response": ["foo", "bar"]}'))],
+                timestamp=IsNow(),
+            ),
+        ]
+    )