Pydantic Missing `eval-type-backport` as required

The Fix

pip install pydantic==2.10.2

Based on closed pydantic/pydantic issue #10958 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -109,7 +109,7 @@ def collect_model_fields(  # noqa: C901
             parent_fields_lookup.update(model_fields)
 
-    type_hints = _typing_extra.get_cls_type_hints(cls, ns_resolver=ns_resolver, lenient=True)
+    type_hints = _typing_extra.get_model_type_hints(cls, ns_resolver=ns_resolver)

repro.py

pydantic version: 2.10.1
        pydantic-core version: 2.27.1
          pydantic-core build: profile=release pgo=false
                 install path: /usr/local/lib/python3.9/site-packages/pydantic
               python version: 3.9.20 (main, Nov 12 2024, 23:14:42)  [GCC 12.2.0]
                     platform: Linux-6.10.11-linuxkit-aarch64-with-glibc2.36
             related packages: typing_extensions-4.12.2 fastapi-0.115.5 mypy-1.9.0
                       commit: unknown

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install pydantic==2.10.2\nWhen NOT to use: This fix should not be used if the application does not utilize forward annotations.\n\n

Why This Fix Works in Production

Trigger: Missing `eval-type-backport` as required dependency in 2.10.* Pydantic line for Python 3.8/3.9
Mechanism: Prevents evaluation of annotations for private fields, addressing the issue where Pydantic fails to import classes with forward annotations in Python 3.8 and 3.9.
Why the fix works: Prevents evaluation of annotations for private fields, addressing the issue where Pydantic fails to import classes with forward annotations in Python 3.8 and 3.9. (first fixed release: 2.10.2).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

Shows up under Python 3.8 in real deployments (not just unit tests).
Production symptom (often without a traceback): Missing `eval-type-backport` as required dependency in 2.10.* Pydantic line for Python 3.8/3.9

Proof / Evidence

GitHub issue: #10958
Fix PR: https://github.com/pydantic/pydantic/pull/10962
First fixed release: 2.10.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.55

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“Happy to test #10962 once it's merged to main - we know know that it should work for the right gitHub syntax in our CI”

@potiuk · 2024-11-24 · confirmation · source

“> Happy to test #10962 once it's merged to main It's not clear to me if the suggested syntax https://github.com/apache/airflow/pull/44260#discussion_r1852542551 worked in your CI, but…”

@Viicos · 2024-11-25 · confirmation · source

“> If you are curious, you can take a look at the implementation of the eval-type-backport. It is interesting stuff, but a bit scary at…”

@potiuk · 2024-11-25 · source

“Runing the test here https://github.com/apache/airflow/pull/44339”

@potiuk · 2024-11-25 · source

Failure Signature (Search String)

Missing `eval-type-backport` as required dependency in 2.10.* Pydantic line for Python 3.8/3.9
- this only broke private fields (i.e. prefixed by `_`), less common that "normal" fields. For "normal" fields, the exception would have been raised anyway in Pydantic < 2.10.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Missing `eval-type-backport` as required dependency in 2.10.* Pydantic line for Python 3.8/3.9
- this only broke private fields (i.e. prefixed by `_`), less common that "normal" fields. For "normal" fields, the exception would have been raised anyway in Pydantic < 2.10.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Missing `eval-type-backport` as required dependency in 2.10.* Pydantic line for Python 3.8/3.9
- this only broke private fields (i.e. prefixed by `_`), less common that "normal" fields. For "normal" fields, the exception would have been raised anyway in Pydantic < 2.10.

Minimal Reproduction

repro.py

pydantic version: 2.10.1
        pydantic-core version: 2.27.1
          pydantic-core build: profile=release pgo=false
                 install path: /usr/local/lib/python3.9/site-packages/pydantic
               python version: 3.9.20 (main, Nov 12 2024, 23:14:42)  [GCC 12.2.0]
                     platform: Linux-6.10.11-linuxkit-aarch64-with-glibc2.36
             related packages: typing_extensions-4.12.2 fastapi-0.115.5 mypy-1.9.0
                       commit: unknown

Environment

Python: 3.8
Pydantic: 2

What Broke

Users experience import failures when using Pydantic with base classes containing forward annotations.

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install pydantic==2.10.2

When NOT to use: This fix should not be used if the application does not utilize forward annotations.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Option D — Guard side-effects with OnceOnly Guardrail for side-effects

Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.

Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.

Show example snippet (optional)

onceonly.py

from onceonly import OnceOnly
import os

once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)

# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..."  # replace
key = f"stripe:webhook:{event_id}"

res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
    return {"status": "already_processed"}

# Safe to execute the side-effect exactly once.
handle_event(event_id)

See OnceOnly SDK

When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Fix reference: https://github.com/pydantic/pydantic/pull/10962

First fixed release: 2.10.2

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix should not be used if the application does not utilize forward annotations.
Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.

Version Compatibility Table

Version	Status
2.10.2	Broken
2.10.2	Fixed

Related Issues

No related fixes found.

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Related clusters: Data consistency OpenAPI schema Serialization

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Pydantic Missing `eval-type-backport` as required dependency in 2.10.* Pydantic line for Python (Fix)