From ae734e2254fda58e044defd46986a48c4227c95a Mon Sep 17 00:00:00 2001
From: David Montague <35119617+dmontagu@users.noreply.github.com>
Date: Fri, 17 Oct 2025 12:52:48 -0600
Subject: [PATCH] Improve validation error retry message

---
 pydantic_ai_slim/pydantic_ai/messages.py                 | 5 ++++-
 .../test_gemini/test_gemini_drop_exclusive_maximum.yaml  | 2 +-
 tests/test_agent.py                                      | 9 ++++++---
 3 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/pydantic_ai_slim/pydantic_ai/messages.py b/pydantic_ai_slim/pydantic_ai/messages.py
index 6e91b78bda..0e9ba35871 100644
--- a/pydantic_ai_slim/pydantic_ai/messages.py
+++ b/pydantic_ai_slim/pydantic_ai/messages.py
@@ -882,7 +882,10 @@ def model_response(self) -> str:
                 description = self.content
         else:
             json_errors = error_details_ta.dump_json(self.content, exclude={'__all__': {'ctx'}}, indent=2)
-            description = f'{len(self.content)} validation errors: {json_errors.decode()}'
+            plural = isinstance(self.content, list) and len(self.content) != 1
+            description = (
+                f'{len(self.content)} validation error{"s" if plural else ""}:\n```json\n{json_errors.decode()}\n```'
+            )
         return f'{description}\n\nFix the errors and try again.'
 
     def otel_event(self, settings: InstrumentationSettings) -> Event:
diff --git a/tests/models/cassettes/test_gemini/test_gemini_drop_exclusive_maximum.yaml b/tests/models/cassettes/test_gemini/test_gemini_drop_exclusive_maximum.yaml
index fe2ce34d2f..7bd4c0a19c 100644
--- a/tests/models/cassettes/test_gemini/test_gemini_drop_exclusive_maximum.yaml
+++ b/tests/models/cassettes/test_gemini/test_gemini_drop_exclusive_maximum.yaml
@@ -259,7 +259,7 @@ interactions:
             name: get_chinese_zodiac
             response:
               call_error: |-
-                1 validation errors: [
+                1 validation error: [
                   {
                     "type": "greater_than",
                     "loc": [
diff --git a/tests/test_agent.py b/tests/test_agent.py
index c8beb08312..01daabc0cf 100644
--- a/tests/test_agent.py
+++ b/tests/test_agent.py
@@ -185,7 +185,7 @@ def return_model(messages: list[ModelMessage], info: AgentInfo) -> ModelResponse
             ),
             ModelResponse(
                 parts=[ToolCallPart(tool_name='final_result', args='{"a": 42, "b": "foo"}', tool_call_id=IsStr())],
-                usage=RequestUsage(input_tokens=87, output_tokens=14),
+                usage=RequestUsage(input_tokens=89, output_tokens=14),
                 model_name='function:return_model:',
                 timestamp=IsNow(tz=timezone.utc),
             ),
@@ -244,7 +244,9 @@ def check_b(cls, v: str) -> str:
     retry_prompt = user_retry.parts[0]
     assert isinstance(retry_prompt, RetryPromptPart)
     assert retry_prompt.model_response() == snapshot("""\
-1 validation errors: [
+1 validation error:
+```json
+[
   {
     "type": "value_error",
     "loc": [
@@ -254,6 +256,7 @@ def check_b(cls, v: str) -> str:
     "input": "foo"
   }
 ]
+```
 
 Fix the errors and try again.""")
 
@@ -1762,7 +1765,7 @@ class CityLocation(BaseModel):
             ),
             ModelResponse(
                 parts=[TextPart(content='{"city": "Mexico City", "country": "Mexico"}')],
-                usage=RequestUsage(input_tokens=85, output_tokens=12),
+                usage=RequestUsage(input_tokens=87, output_tokens=12),
                 model_name='function:return_city_location:',
                 timestamp=IsDatetime(),
             ),