Exit code 134 means the Java process ended itself with SIGABRT, the abort signal, which Java raises when a fatal native error makes it unsafe to continue. In Minecraft it usually comes from a native crash in a mod or library, a broken graphics or audio driver, or a memory problem, and the crash report normally names the failing component.
The fix, in order
Read the crash report. Open the .minecraft folder, then crash-reports, and open the newest file. The Description line and the failing module name point at the cause, often a mod or a native library.
Test with a clean, unmodded game. Move the mods folder aside and remove shader packs, then launch. If the clean game works, add mods back a few at a time to find the one that aborts.
Update the graphics driver from NVIDIA, AMD, or Intel with a clean install, and update the audio driver too, since audio libraries are a common native crash source.
Check the RAM allocation. In the launcher, open Installations, edit the profile, open More Options, and set -Xmx to a reasonable value, such as 4G, leaving free memory for the rest of the system.
Reinstall the launcher's Java runtime. The official launcher bundles its own Java, so reinstalling the launcher refreshes it.
Verify the game files. In the launcher, check the installation, or reinstall the game to replace corrupt files.
Check for a broken mod loader. Update Fabric, Forge, or NeoForge to a version that matches the game version, and remove any mod that targets a different version.
If it still happens
If the crash report names a specific native library, such as a graphics or audio file, update or reinstall the software that owns it. If the crash only happens in one world, the world data may be corrupt, so test a new world to confirm before restoring a backup.
Common questions
Is exit code 134 the same as exit code 1? No. Exit code 1 is a general Java error, while 134 means the Java process aborted itself after a fatal native error. The fixes overlap, but 134 points more strongly at a native library or a mod.
Can a shader pack cause exit code 134? Yes. Shader packs and the mods that run them use native graphics code, so a broken shader or an outdated graphics driver can abort the game. Remove the shader pack first to test.