Improve validation error retry message (#3193)

Author: dmontagu

pydantic_ai_slim/pydantic_ai/messages.py

@@ -888,7 +888,10 @@ 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:

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": [

tests/test_agent.py

@@ -201,7 +201,7 @@ 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),
             ),
@@ -260,7 +260,9 @@ 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": [
@@ -270,6 +272,7 @@ def check_b(cls, v: str) -> str:
     "input": "foo"
   }
 ]
+```
 
 Fix the errors and try again.""")
 
@@ -1796,7 +1799,7 @@ 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(),
             ),