v2.18 defers building Pydantic models until first use, which causes errors in a multithreaded context #2991

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1. jmazella-brr

   added

   type: bugError or flaw in code with unintended results or allowing sub-optimal usage patterns.Error or flaw in code with unintended results or allowing sub-optimal usage patterns.

   priority: p2Moderately-important priority. Fix may not be included in next release.Moderately-important priority. Fix may not be included in next release.

   on Sep 21, 2026
2. Venkaiahbabuneelam

   self-assigned this

   on Sep 22, 2026
3. Venkaiahbabuneelam commented on Sep 22, 2026

   

   Venkaiahbabuneelam

   on Sep 22, 2026

   Last edited by Venkaiahbabuneelam

   More actions

   Hi @jmazella-brr, Thanks for reaching out us!

   You've accurately identified a race condition stemming from Pydantic v2's deferred model building (defer_build=True) in a multithreaded environment, where model_build() is not thread-safe. This causes validation errors when multiple threads attempt to initialize GenerateContentConfig concurrently for the first time.
   We tested the multithreaded scenario locally and we didn't encouner any errors, Please check and let us know if it resolves the issue for you.
   If issue is still presist can you please provide me full reproducible code for reference.

   Thanks
4. Venkaiahbabuneelam

   added

   status:awaiting user response

   on Sep 22, 2026
5. jmazella-brr commented on Sep 24, 2026

   

   jmazella-brr

   on Sep 24, 2026

   Author

   More actions

   @Venkaiahbabuneelam Yes I am still seeing the problem. Here is a self contained test script that shows the issue:

   """Minimal reproduction for googleapis/python-genai#2991 targeting ValidationError."""
   
   import multiprocessing as mp
   import sys
   import threading
   import pydantic
   
   THREADS = 150
   MAX_ATTEMPTS = 40
   
   
   def _worker(result_queue):
       try:
           from google.genai.types import GenerateContentConfig
           # Rebuild parent models to make the original validation error more likely
           for config in (GenerateContentConfig,):
               for base in config.__mro__[1:]:
                   if issubclass(base, pydantic.BaseModel) and base is not pydantic.BaseModel:
                       base.model_rebuild()
           # Increase chances of hitting the race condition
           sys.setswitchinterval(1e-9)
   
           validation_errors = []
           lock = threading.Lock()
           start = threading.Event()
   
           def build():
               start.wait()
               try:
                   GenerateContentConfig(max_output_tokens=1, seed=42)
               except Exception as exc:
                   with lock:
                       validation_errors.append(
                           f"{type(exc).__name__}: {str(exc)}"
                       )
           threads = [threading.Thread(target=build) for _ in range(THREADS)]
           for t in threads:
               t.start()
           start.set()
           for t in threads:
               t.join()
   
           result_queue.put(validation_errors)
       except Exception as e:
           result_queue.put([f"Worker Error: {e}"])
   
   
   def main():
       ctx = mp.get_context("spawn")
       for attempt in range(1, MAX_ATTEMPTS + 1):
           print(f"Starting attempt {attempt}...")
           queue = ctx.Queue()
           process = ctx.Process(target=_worker, args=(queue,))
           process.start()
   
           try:
               errors = queue.get(timeout=60)
           except Exception:
               errors = "Process timed out"
   
           process.join()
   
           for error in errors:
               print(f"ERROR:\n    {error}")
   
   
   if __name__ == "__main__":
       raise SystemExit(main())
   

   Here's the output i saw running it :

   Starting attempt 1...
   ERROR:
       AttributeError: type object 'GenerateContentConfig' has no attribute '__pydantic_core_schema__'
   Starting attempt 2...
   Starting attempt 3...
   ERROR:
       AttributeError: type object 'GenerateContentConfig' has no attribute '__pydantic_core_schema__'
   Starting attempt 4...
   Starting attempt 5...
   Starting attempt 6...
   ERROR:
       ValidationError: 2 validation errors for BaseModel
   max_output_tokens
     Extra inputs are not permitted [type=extra_forbidden, input_value=1, input_type=int]
       For further information visit https://errors.pydantic.dev/2.13/v/extra_forbidden
   seed
     Extra inputs are not permitted [type=extra_forbidden, input_value=42, input_type=int]
       For further information visit https://errors.pydantic.dev/2.13/v/extra_forbidden
   Starting attempt 7...
   ERROR:
       AttributeError: type object 'GenerateContentConfig' has no attribute '__pydantic_core_schema__'
   Starting attempt 8...
   Starting attempt 9...
   Starting attempt 10...
   
   
6. Venkaiahbabuneelam commented on Sep 25, 2026

   

   Venkaiahbabuneelam

   on Sep 25, 2026

   More actions

   Hi @jmazella-brr, Thank you for bringing this to our attention. We truly appreciate you flagging this issue.

   We are forwarding this issue to Our engineering team will investigate the thread-safety race condition caused by deferred Pydantic model building (defer_build=True) in multithreaded environments. We will update you here as soon as possible.
   Thanks
7. Venkaiahbabuneelam

   removed

   status:awaiting user response

   on Sep 25, 2026
8. adri22235

   added a commit that references this issue on Oct 2, 2026

   Merge pull request #1 from adri22235/claude/python-genai-issues-d5gnhj

   ...

   2cb371c
9. copybara-service

   added a commit that references this issue on Oct 28, 2026

   fix: Serialize deferred Pydantic model builds to fix a multithreaded …

   ...

   This issue will close once commit 292a2ba is merged into the 'main' branch.

   292a2ba
10. copybara-service

    added 2 commits that reference this issue on Nov 5, 2026

    fix: Serialize deferred Pydantic model builds to fix a multithreaded …

    ...

    This issue will close once commit c766056 is merged into the 'main' branch.

    c766056

    fix: Serialize deferred Pydantic model builds to fix a multithreaded …

    ...

    This issue will close once commit e393dae is merged into the 'main' branch.

    e393dae