Pydantic `multiple_of` failing on some negative floats (Fix)

The Fix

pip install pydantic==1.10.1

Based on closed pydantic/pydantic issue #9871 · 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 +1,15 @@
@@ -0,0 +1,15 @@
+from typing import List, Dict
+from pydantic import BaseModel, ValidationError
+

repro.py

pydantic version: 2.7.2
        pydantic-core version: 2.18.3
          pydantic-core build: profile=release pgo=true
                 install path: /Users/lucas/{redacted}/venv/lib/python3.12/site-packages/pydantic
               python version: 3.12.3 (main, Apr  9 2024, 08:09:14) [Clang 15.0.0 (clang-1500.3.9.4)]
                     platform: macOS-14.5-arm64-arm-64bit
             related packages: typing_extensions-4.12.0 fastapi-0.111.0 pydantic-settings-2.2.1
                       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 is not applicable if using float types is required for other functionalities.\n\n

Why This Fix Works in Production

Trigger: Why does it work on `x` but fails on `-x`? Is this intended behavior?
Mechanism: Validation of negative floats with the 'multiple_of' constraint fails due to rounding issues
Why the fix works: Fixed a bug related to the validation of negative floats with the 'multiple_of' constraint in Pydantic. (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).
Validation of negative floats with the 'multiple_of' constraint fails due to rounding issues
Production symptom (often without a traceback): Why does it work on `x` but fails on `-x`? Is this intended behavior?

Proof / Evidence

GitHub issue: #9871
Fix PR: https://github.com/pydantic/pydantic-core/pull/1373
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: 0.62

Discussion

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

Jump to Sources Open on GitHub

“Hi, I've been investigating this today, seems that using decimal.Decimal instead of float makes the trick. Results in: This should be fixed at pydantic-core though.”

@aldorvv · 2024-07-21 · confirmation · source

“Hi @AlessandroMiola Thank you for your suggestion:). I agree with your point, so I have added the same test cases to test_decimal_multiple_of as well.”

@K-dash · 2024-07-22 · source

“Hi @K-dash, I was looking at the same issue, but you made it quicker:)”

@AlessandroMiola · 2024-07-22 · source

“@plutaniano, Huh, this is definitely a bug. Thanks for reporting this! Likely an issue due to undesired rounding. A PR with a fix is more…”

@sydney-runkle · 2024-07-17 · source

Failure Signature (Search String)

Why does it work on `x` but fails on `-x`? Is this intended behavior?
print(f"FAIL: {i}")

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Why does it work on `x` but fails on `-x`? Is this intended behavior?
print(f"FAIL: {i}")

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Why does it work on `x` but fails on `-x`? Is this intended behavior?
print(f"FAIL: {i}")

Minimal Reproduction

repro.py

pydantic version: 2.7.2
        pydantic-core version: 2.18.3
          pydantic-core build: profile=release pgo=true
                 install path: /Users/lucas/{redacted}/venv/lib/python3.12/site-packages/pydantic
               python version: 3.12.3 (main, Apr  9 2024, 08:09:14) [Clang 15.0.0 (clang-1500.3.9.4)]
                     platform: macOS-14.5-arm64-arm-64bit
             related packages: typing_extensions-4.12.0 fastapi-0.111.0 pydantic-settings-2.2.1
                       commit: unknown

Environment

Python: 3.12
Pydantic: 2

What Broke

Negative float values fail validation unexpectedly, causing incorrect data handling.

Why It Broke

Validation of negative floats with the 'multiple_of' constraint fails due to rounding issues

Fix Options (Details)

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

pip install pydantic==1.10.1

When NOT to use: This fix is not applicable if using float types is required for other functionalities.

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/1373

First fixed release: 1.10.1

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

When NOT to Use This Fix

This fix is not applicable if using float types is required for other functionalities.

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.