flask `pyright` type checking seems to be unused. `mypy`

The Fix

Upgrade to version 3.0.3 or later.

Based on closed pallets/flask issue #5537 · 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%.

Open PR/Commit

@@ -1,9 +1,7 @@
 #!/bin/bash
 set -e
-
-python3 -m venv .venv
+python3 -m venv --upgrade-deps .venv

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 3.0.3 or later.\nWhen NOT to use: Do not use this fix if mypy is required for your project's type checking.\n\n

Why This Fix Works in Production

Trigger: If this was an intentional change of type checkers, then would expect that `mypy` be completely removed, and `pyright` be the command run in `tox typing`.
Mechanism: The project was still using mypy for type checking instead of the newly added pyright
Why the fix works: Updated project files to include pyright for type checking instead of mypy. (first fixed release: 3.0.3).

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

The project was still using mypy for type checking instead of the newly added pyright
Production symptom (often without a traceback): If this was an intentional change of type checkers, then would expect that `mypy` be completely removed, and `pyright` be the command run in `tox typing`.

Proof / Evidence

GitHub issue: #5537
Fix PR: https://github.com/pallets/flask/pull/5457
First fixed release: 3.0.3
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.72

Discussion

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

Jump to Sources Open on GitHub

“We added pyright in 3.0.3 (see: https://github.com/pallets/flask/pull/5457), but mypy is still being used for type checking https://github.com/pallets/flask/blob/main/tox.ini#L29-L32, and pyright is not. If this was an intentional change of”

Issue thread · issue description · source

Failure Signature (Search String)

If this was an intentional change of type checkers, then would expect that `mypy` be completely removed, and `pyright` be the command run in `tox typing`.
It should be noted, however, that running `pyright` yields 90 errors (71 tests/19 src). Is there an upgrade path or was this an oversight?

Copy-friendly signature

signature.txt

Failure Signature
-----------------
If this was an intentional change of type checkers, then would expect that `mypy` be completely removed, and `pyright` be the command run in `tox typing`.
It should be noted, however, that running `pyright` yields 90 errors (71 tests/19 src). Is there an upgrade path or was this an oversight?

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
If this was an intentional change of type checkers, then would expect that `mypy` be completely removed, and `pyright` be the command run in `tox typing`.
It should be noted, however, that running `pyright` yields 90 errors (71 tests/19 src). Is there an upgrade path or was this an oversight?

What Broke

Type checking errors were not being reported correctly, leading to potential undetected issues.

Why It Broke

The project was still using mypy for type checking instead of the newly added pyright

Fix Options (Details)

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

Upgrade to version 3.0.3 or later.

When NOT to use: Do not use this fix if mypy is required for your project's type checking.

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

Fix reference: https://github.com/pallets/flask/pull/5457

First fixed release: 3.0.3

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

When NOT to Use This Fix

Do not use this fix if mypy is required for your project's type checking.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
Pin production dependencies and upgrade only with a reproducible test that hits the failing path.

Version Compatibility Table

Version	Status
3.0.3	Fixed

Related Issues

No related fixes found.

Cluster: flask:configuration-error All hubs All clusters

Related clusters: Data consistency Other Duplicate processing

Sources

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

flask `pyright` type checking seems to be unused. `mypy` still being used. (Fix)