# pydantic_core._pydantic_core.ValidationError: 1 validation error for Node
children.0.children.0
  RecursionError: maximum recursion depth exceeded while calling a Python object

- **ID:** `python/pydantic-recursion-error-self-referential`
- **Domain:** python
- **Category:** runtime_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

A self-referential Pydantic model (tree/graph) was given a cyclic data structure, causing infinite recursion during validation. Pydantic has no built-in cycle detection for plain nested models.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 2.x | active | — | — |

## Workarounds

1. **** (85% success)
   ```
   def break_cycles(obj, seen=None):
    seen = seen or set()
    if id(obj) in seen:
        return None
    seen.add(id(obj))
    if isinstance(obj, dict):
        return {k: break_cycles(v, seen) for k, v in obj.items()}
    if isinstance(obj, list):
        return [break_cycles(v, seen) for v in obj]
    return obj

Node.model_validate(break_cycles(raw))
   ```
2. **** (70% success)
   ```
   from pydantic import BaseModel, field_validator

class Node(BaseModel):
    name: str
    children: list['Node'] = []

    @field_validator('children')
    @classmethod
    def limit_depth(cls, v):
        if len(v) > 100:
            raise ValueError('tree too deep')
        return v
   ```

## Dead Ends

- **** — This only defers the crash; a truly cyclic structure will still exhaust memory and eventually segfault the interpreter. (80% fail)
- **** — arbitrary_types_allowed permits non-Pydantic types, it does not detect or prevent cyclic references. (90% fail)