A developer closed a bug saying it is not reproducible, but you see it 100% of the time. How to act?

In such a situation, it is important to act constructively and systematically:

1. Confirm reproduction: Offer the developer to reproduce the bug together in their environment, showing specific steps, data, and environment where the bug manifests.

2. Gather additional data: Take screenshots, videos, logs, state dumps to demonstrate the problem as clearly as possible.

3. Automate testing: Write an automated test that reproduces the bug to eliminate human factors.

4. Check the environment: Ensure that the developer and tester have matching versions of software, configurations, data, and testing conditions.

5. Discuss with the team: If the problem persists, raise the issue in a team meeting or with the team lead to find a common solution.

6. Document: Record all steps and results to avoid misunderstandings.

This approach helps to avoid conflict and jointly find the cause and fix the bug.