Pydantic mypy INTERNAL ERROR: maximum semantic analysis

The Fix

pip install pydantic==1.10.15

Based on closed pydantic/pydantic issue #8760 · PR/commit linked

Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.

Jump to Verify Open PR/Commit

@@ -126,10 +126,6 @@ def plugin(version: str) -> type[Plugin]:
 
 
-class _DeferAnalysis(Exception):
-    pass
-

repro.py

$ python -c "import pydantic.version; print(pydantic.version.version_info())"
             pydantic version: 2.6.1
        pydantic-core version: 2.16.2
          pydantic-core build: profile=release pgo=true
                 install path: .../venv/lib/python3.8/site-packages/pydantic
               python version: 3.8.18 (default, Aug 25 2023, 13:20:30)  [GCC 11.4.0]
                     platform: Linux-6.1.0-1029-oem-x86_64-with-glibc2.35
             related packages: email-validator-2.1.0.post1 typing_extensions-4.9.0 fastapi-0.109.0 pydantic-settings-2.1.0 mypy-1.8.0
                       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==1.10.15\nWhen NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\n

Why This Fix Works in Production

Trigger: mypy INTERNAL ERROR: maximum semantic analysis iteration count reached
Mechanism: The mypy plugin mishandles optionals, causing excessive semantic analysis iterations
Why the fix works: Fix handling of optionals in mypy plugin to resolve various occurrences of mypy issues with optional types. (first fixed release: 1.10.15).

Why This Breaks in Prod

Shows up under Python 3.8 in real deployments (not just unit tests).
The mypy plugin mishandles optionals, causing excessive semantic analysis iterations
Production symptom (often without a traceback): mypy INTERNAL ERROR: maximum semantic analysis iteration count reached

Proof / Evidence

GitHub issue: #8760
Fix PR: https://github.com/pydantic/pydantic/pull/9008
First fixed release: 1.10.15
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.75
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.56

Discussion

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

Jump to Sources Open on GitHub

“I believe this is a duplicate of https://github.com/pydantic/pydantic/issues/8665 which was resolved with https://github.com/pydantic/pydantic/pull/8666. That fix is now live with 2.6.1.”

@sydney-runkle · 2024-02-08 · confirmation · source

“I have this issue as well with the same mypy error”

@Cielquan · 2024-03-15 · repro detail · source

“@brianmedigate, Could you please check if you're still having this issue with 2.6.1? Thanks!”

@sydney-runkle · 2024-02-08 · source

“I have this issue as well with this sample: mypy returns error: INTERNAL ERROR: maximum semantic analysis iteration count reached I'm using python3.9 and have…”

@alexgaspard · 2024-02-27 · source

Failure Signature (Search String)

mypy INTERNAL ERROR: maximum semantic analysis iteration count reached
The code below causes mypy to crash with `INTERNAL ERROR: maximum semantic analysis iteration count reached`.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
mypy INTERNAL ERROR: maximum semantic analysis iteration count reached
The code below causes mypy to crash with `INTERNAL ERROR: maximum semantic analysis iteration count reached`.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
mypy INTERNAL ERROR: maximum semantic analysis iteration count reached
The code below causes mypy to crash with `INTERNAL ERROR: maximum semantic analysis iteration count reached`.

Minimal Reproduction

repro.py

$ python -c "import pydantic.version; print(pydantic.version.version_info())"
             pydantic version: 2.6.1
        pydantic-core version: 2.16.2
          pydantic-core build: profile=release pgo=true
                 install path: .../venv/lib/python3.8/site-packages/pydantic
               python version: 3.8.18 (default, Aug 25 2023, 13:20:30)  [GCC 11.4.0]
                     platform: Linux-6.1.0-1029-oem-x86_64-with-glibc2.35
             related packages: email-validator-2.1.0.post1 typing_extensions-4.9.0 fastapi-0.109.0 pydantic-settings-2.1.0 mypy-1.8.0
                       commit: unknown

Environment

Python: 3.8
Pydantic: 2

What Broke

Mypy crashes with an internal error during type checking, halting development.

Why It Broke

The mypy plugin mishandles optionals, causing excessive semantic analysis iterations

Fix Options (Details)

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

pip install pydantic==1.10.15

When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.

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/9008

First fixed release: 1.10.15

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

When NOT to Use This Fix

Do not use if it changes public behavior or if the failure cannot be reproduced.

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: Configuration error Data consistency OpenAPI schema

Sources

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

Pydantic mypy INTERNAL ERROR: maximum semantic analysis iteration count reached (Fix)