Pydantic Type `IPv4Address` not parsed (Fix)

The Fix

pip install pydantic==1.10.15

Based on closed pydantic/pydantic issue #8942 · 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

@@ -54,7 +54,7 @@
 from .annotated_handlers import GetJsonSchemaHandler
 from .config import JsonDict, JsonSchemaExtraCallable, JsonValue
-from .errors import PydanticInvalidForJsonSchema, PydanticUserError
+from .errors import PydanticInvalidForJsonSchema, PydanticSchemaGenerationError, PydanticUserError

repro.py

@pytest.mark.parametrize('ip_address', (IPv4Address('127.0.0.1'), (IPv6Address('::1'))))
def test_ipv4_address_parsing(ip_address):
    class IpModel(BaseModel):
        ip: IPvAnyAddress = ip_address

    print(IpModel.model_json_schema())

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 the serialization of other non-implemented types is required.\n\n

Why This Fix Works in Production

Trigger: PydanticJsonSchemaWarning: Default value 127.0.0.1 is not JSON serializable; excluding default from JSON schema [non-serializable-default]
Mechanism: The type `IPv4Address` was not being serialized correctly in Pydantic V2
Why the fix works: Fixes a bug where default values of model fields won't be serialized into the JSON schema. (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 3.11 in real deployments (not just unit tests).
The type `IPv4Address` was not being serialized correctly in Pydantic V2
Production symptom (often without a traceback): PydanticJsonSchemaWarning: Default value 127.0.0.1 is not JSON serializable; excluding default from JSON schema [non-serializable-default]

Proof / Evidence

GitHub issue: #8942
Fix PR: https://github.com/pydantic/pydantic/pull/9066
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: 0.62

Discussion

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

Jump to Sources Open on GitHub

“@sydney-runkle I'd like to work on this if it's still available 😄”

@NeevCohen · 2024-03-04 · source

“I think as IPv4Address is so far handled just in pydantic, I think it'd be better to keep the change out of core for now”

@davidhewitt · 2024-03-19 · source

“@Kyomotoi, Thanks for the report. This should be relatively easy to fix. I'll note - this can't be solved with validate_default=True.”

@sydney-runkle · 2024-03-04 · source

“@NeevCohen, Great job looking into this. Let's see what @davidhewitt thinks first thing tomorrow morning :).”

@sydney-runkle · 2024-03-19 · source

Failure Signature (Search String)

PydanticJsonSchemaWarning: Default value 127.0.0.1 is not JSON serializable; excluding default from JSON schema [non-serializable-default]
warnings.warn(message, PydanticJsonSchemaWarning)

Copy-friendly signature

signature.txt

Failure Signature
-----------------
PydanticJsonSchemaWarning: Default value 127.0.0.1 is not JSON serializable; excluding default from JSON schema [non-serializable-default]
warnings.warn(message, PydanticJsonSchemaWarning)

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
PydanticJsonSchemaWarning: Default value 127.0.0.1 is not JSON serializable; excluding default from JSON schema [non-serializable-default]
warnings.warn(message, PydanticJsonSchemaWarning)

Minimal Reproduction

repro.py

@pytest.mark.parametrize('ip_address', (IPv4Address('127.0.0.1'), (IPv6Address('::1'))))
def test_ipv4_address_parsing(ip_address):
    class IpModel(BaseModel):
        ip: IPvAnyAddress = ip_address

    print(IpModel.model_json_schema())

Environment

Python: 3.11
Pydantic: 2

What Broke

Users received warnings about non-serializable default values in JSON schema.

Why It Broke

The type `IPv4Address` was not being serialized correctly in Pydantic V2

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 the serialization of other non-implemented types is required.

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

Fix reference: https://github.com/pydantic/pydantic/pull/9066

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 the serialization of other non-implemented types 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.

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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.