The Fix
pip install pydantic==1.10.15
Based on closed pydantic/pydantic issue #9019 · 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%.
@@ -1067,9 +1067,26 @@ class StaticFoobarModel(BaseModel):
Here `StaticFoobarModel` and `DynamicFoobarModel` are identical.
-Fields are defined by a tuple of the form `(<type>, <default value>)`. The special keyword
-arguments `__config__` and `__base__` can be used to customise the new model. This includes
-extending a base model with extra fields.
repro.py
from pydantic.v1 import BaseModel as BaseModelV1
from pydantic import BaseModel as BaseModelV2
class MessageV1(BaseModelV1):
test: str
class MessageV2(BaseModelV2):
test: str
message_v1 = MessageV1(test="¿Cómo estás?")
message_v2 = MessageV2(test="¿Cómo estás?")
print("V1", message_v1.json())
print("V2", message_v2.model_dump_json())
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==1.10.15\nWhen NOT to use: This fix should not be used if backward compatibility with V1's JSON output is required.\n\nOption C — Workaround\nin the meantime: https://github.com/pydantic/pydantic/issues/8825.\nWhen NOT to use: This fix should not be used if backward compatibility with V1's JSON output is required.\n\n
Why This Fix Works in Production
- Trigger: When I read V2 migration guide, I did not expect major changes to how `model_dump_json` will work compared to V1's `json`. However, it seems like there's a…
- Mechanism: The change in JSON serialization behavior between Pydantic V1 and V2 affects non-ASCII character handling
- Why the fix works: Updates the documentation to address the differences in JSON serialization between Pydantic V1 and V2, specifically regarding the handling of non-ASCII characters. (first fixed release: 1.10.15).
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).
- The change in JSON serialization behavior between Pydantic V1 and V2 affects non-ASCII character handling
- Production symptom (often without a traceback): When I read V2 migration guide, I did not expect major changes to how `model_dump_json` will work compared to V1's `json`. However, it seems like there's a difference in how the new method handles non-ASCII characters.
Proof / Evidence
- GitHub issue: #9019
- Fix PR: https://github.com/pydantic/pydantic/pull/9110
- First fixed release: 1.10.15
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.60
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“I've also marked this with a documentation tag, and will open a PR later this week to update the migration guide with a note on…”
“@pkontis, You can use model_dump(mode='json') to make this work!”
“@pkotnis, Thanks for reporting this! Indeed, this is a V1 -> V2 change that hasn't yet been well documented”
“@sydney-runkle , thanks for the update”
Failure Signature (Search String)
- When I read V2 migration guide, I did not expect major changes to how `model_dump_json` will work compared to V1's `json`. However, it seems like there's a difference in how the
Copy-friendly signature
signature.txt
Failure Signature
-----------------
When I read V2 migration guide, I did not expect major changes to how `model_dump_json` will work compared to V1's `json`. However, it seems like there's a difference in how the new method handles non-ASCII characters.
python version: 3.8.17 (default, Nov 6 2023, 20:47:47) [Clang 15.0.0 (clang-1500.0.40.1)]
Error Message
Signature-only (no traceback captured)
error.txt
Error Message
-------------
When I read V2 migration guide, I did not expect major changes to how `model_dump_json` will work compared to V1's `json`. However, it seems like there's a difference in how the new method handles non-ASCII characters.
python version: 3.8.17 (default, Nov 6 2023, 20:47:47) [Clang 15.0.0 (clang-1500.0.40.1)]
Minimal Reproduction
repro.py
from pydantic.v1 import BaseModel as BaseModelV1
from pydantic import BaseModel as BaseModelV2
class MessageV1(BaseModelV1):
test: str
class MessageV2(BaseModelV2):
test: str
message_v1 = MessageV1(test="¿Cómo estás?")
message_v2 = MessageV2(test="¿Cómo estás?")
print("V1", message_v1.json())
print("V2", message_v2.model_dump_json())
Environment
- Python: 3.8
- Pydantic: 2
What Broke
Users experience unexpected JSON output differences when migrating from V1 to V2.
Why It Broke
The change in JSON serialization behavior between Pydantic V1 and V2 affects non-ASCII character handling
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==1.10.15
When NOT to use: This fix should not be used if backward compatibility with V1's JSON output is required.
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Option C — Workaround Temporary workaround
in the meantime: https://github.com/pydantic/pydantic/issues/8825.
When NOT to use: This fix should not be used if backward compatibility with V1's JSON output is required.
Use only if you cannot change versions today. Treat this as a stopgap and remove once upgraded.
Fix reference: https://github.com/pydantic/pydantic/pull/9110
First fixed release: 1.10.15
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be used if backward compatibility with V1's JSON output is required.
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 |
|---|---|
| 1.10.15 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.