If you are using passlib with bcrypt and your hashing flow suddenly fails, version mismatch is often the reason.

In setups like this, pinning bcrypt to 4.3.0 can be the stable fix.

Common Symptoms

You may see issues such as:

• password hash generation failing at runtime
• verification errors even with correct passwords
• backend-loading/import errors tied to bcrypt

These usually show up after dependency upgrades, especially when bcrypt gets updated without validating Passlib compatibility.

Practical Fix

Pin bcrypt explicitly in your environment:

pip install "bcrypt==4.3.0"

If you use a requirements file:

passlib
bcrypt==4.3.0

Then reinstall dependencies and re-run your auth tests.

Why Pinning Helps

Passlib relies on bcrypt backend behavior and interfaces. When versions drift, subtle compatibility gaps can break hashing code that previously worked.

Pinning to a known-good version avoids random breakage across local, staging, and production environments.

Recommended Team Practice

• lock dependency versions for auth/security libraries
• run login/register/reset-password test flows in CI
• update bcrypt only after validating Passlib behavior end-to-end

For authentication code, stability beats surprise upgrades.