Pydantic Exception parsing negative integer strings (Fix)

The Fix

pip install pydantic==1.10.1

Based on closed pydantic/pydantic issue #9396 · 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

@@ -0,0 +1 @@
@@ -0,0 +1 @@
+Allow subclasses of known types to be encoded with superclass encoder
\ No newline at end of file
diff --git a/docs/examples/exporting_models_json.py b/docs/examples/exporting_models_json.py

repro.py

pydantic version: 2.7.1
        pydantic-core version: 2.18.2
          pydantic-core build: profile=release pgo=true
                 install path: /home/noam/.cache/pypoetry/virtualenvs/hl-7m4wOqun-py3.12/lib/python3.12/site-packages/pydantic
               python version: 3.12.3 (main, Apr 10 2024, 05:33:47) [GCC 13.2.0]
                     platform: Linux-5.15.133.1-microsoft-standard-WSL2-x86_64-with-glibc2.39
             related packages: typing_extensions-4.10.0 mypy-1.7.1
                       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==1.10.1\nWhen NOT to use: This fix should not be applied if the application relies on the previous validation behavior.\n\n

Why This Fix Works in Production

Trigger: In [1]: import pydantic
Mechanism: The validation logic fails to handle the '-' character in negative integer strings
Why the fix works: Addresses the issue of parsing negative integer strings in Pydantic by fixing the validation logic. (first fixed release: 1.10.1).

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 3.12 in real deployments (not just unit tests).
The validation logic fails to handle the '-' character in negative integer strings
Surfaces as: In [1]: import pydantic

Proof / Evidence

GitHub issue: #9396
Fix PR: https://github.com/pydantic/pydantic-core/pull/1291
First fixed release: 1.10.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.39

Discussion

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

Jump to Sources Open on GitHub

“I investigated this and the issue seems to be in strip_leading_zeros function where if any character occurs except (+, _, 1 ..”

@RajatRajdeep · 2024-05-13 · source

“Hey folks, Thanks for reporting this bug. Definitely something we want to fix in 2.7.2. Should be an easy fix if anyone is interested in…”

@sydney-runkle · 2024-05-14 · source

Failure Signature (Search String)

In [1]: import pydantic

Error Message

Stack trace

error.txt

Error Message
-------------
In [1]: import pydantic

In [2]: class A(pydantic.BaseModel):
   ...:     i: int
   ...: 

In [3]: A(i='4.0')
Out[3]: A(i=4)

In [4]: A(i='-4.0')
---------------------------------------------------------------------------
ValidationError                           Traceback (most recent call last)
Cell In[4], line 1
----> 1 A(i='-4.0')

File ~/.cache/pypoetry/virtualenvs/hl-7m4wOqun-py3.12/lib/python3.12/site-packages/pydantic/main.py:176, in BaseModel.__init__(self, **data)
    174 # `__tracebackhide__` tells pytest and some other tools to omit this function from tracebacks
    175 __tracebackhide__ = True
--> 176 self.__pydantic_validator__.validate_python(data, self_instance=self)

ValidationError: 1 validation error for A
i
  Input should be a valid integer, unable to parse string as an integer [type=int_parsing, input_value='-4.0', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/int_parsing

Minimal Reproduction

repro.py

pydantic version: 2.7.1
        pydantic-core version: 2.18.2
          pydantic-core build: profile=release pgo=true
                 install path: /home/noam/.cache/pypoetry/virtualenvs/hl-7m4wOqun-py3.12/lib/python3.12/site-packages/pydantic
               python version: 3.12.3 (main, Apr 10 2024, 05:33:47) [GCC 13.2.0]
                     platform: Linux-5.15.133.1-microsoft-standard-WSL2-x86_64-with-glibc2.39
             related packages: typing_extensions-4.10.0 mypy-1.7.1
                       commit: unknown

Environment

Python: 3.12
Pydantic: 2

What Broke

Parsing negative integer strings results in validation errors, causing application crashes.

Why It Broke

The validation logic fails to handle the '-' character in negative integer strings

Fix Options (Details)

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

pip install pydantic==1.10.1

When NOT to use: This fix should not be applied if the application relies on the previous validation behavior.

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

Fix reference: https://github.com/pydantic/pydantic-core/pull/1291

First fixed release: 1.10.1

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

When NOT to Use This Fix

This fix should not be applied if the application relies on the previous validation behavior.

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
1.10.1	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:configuration-error Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Data consistency OpenAPI schema Serialization

Sources

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