fix: Steam Deck/Ubuntu crash - use system libzstd.so

The bundled libzstd.so is incompatible with glibc 2.41's stricter heap
validation, causing "free(): invalid pointer" crashes.

Solution: Automatically replace bundled libzstd.so with system version
on Linux. The launcher detects and symlinks to /usr/lib/libzstd.so.1.

- Auto-detect system libzstd at common paths (Arch, Debian, Fedora)
- Backup bundled version as libzstd.so.bundled
- Create symlink to system version
- Add HYTALE_NO_LIBZSTD_FIX=1 to disable if needed

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/managers/gameLauncher.js

@@ -285,6 +285,55 @@ exec "$REAL_JAVA" "\${ARGS[@]}"
    const gpuEnv = setupGpuEnvironment(gpuPreference);
    Object.assign(env, gpuEnv);
 
+  // Linux: Replace bundled libzstd.so with system version to fix glibc 2.41+ crash
+  // The bundled libzstd causes "free(): invalid pointer" on Steam Deck / Ubuntu LTS
+  if (process.platform === 'linux' && process.env.HYTALE_NO_LIBZSTD_FIX !== '1') {
+    const clientDir = path.dirname(clientPath);
+    const bundledLibzstd = path.join(clientDir, 'libzstd.so');
+    const backupLibzstd = path.join(clientDir, 'libzstd.so.bundled');
+
+    // Common system libzstd paths
+    const systemLibzstdPaths = [
+      '/usr/lib/libzstd.so.1',                    // Arch Linux, Steam Deck
+      '/usr/lib/x86_64-linux-gnu/libzstd.so.1',   // Debian/Ubuntu
+      '/usr/lib64/libzstd.so.1'                   // Fedora/RHEL
+    ];
+
+    let systemLibzstd = null;
+    for (const p of systemLibzstdPaths) {
+      if (fs.existsSync(p)) {
+        systemLibzstd = p;
+        break;
+      }
+    }
+
+    if (systemLibzstd && fs.existsSync(bundledLibzstd)) {
+      try {
+        const stats = fs.lstatSync(bundledLibzstd);
+
+        // Only replace if it's not already a symlink to system version
+        if (!stats.isSymbolicLink()) {
+          // Backup bundled version
+          if (!fs.existsSync(backupLibzstd)) {
+            fs.renameSync(bundledLibzstd, backupLibzstd);
+            console.log(`Linux: Backed up bundled libzstd.so`);
+          } else {
+            fs.unlinkSync(bundledLibzstd);
+          }
+
+          // Create symlink to system version
+          fs.symlinkSync(systemLibzstd, bundledLibzstd);
+          console.log(`Linux: Linked libzstd.so to system version (${systemLibzstd}) for glibc 2.41+ compatibility`);
+        } else {
+          const linkTarget = fs.readlinkSync(bundledLibzstd);
+          console.log(`Linux: libzstd.so already linked to ${linkTarget}`);
+        }
+      } catch (libzstdError) {
+        console.warn(`Linux: Could not replace libzstd.so: ${libzstdError.message}`);
+      }
+    }
+  }
+
   try {
     let spawnOptions = {
       stdio: ['ignore', 'pipe', 'pipe'],

docs/STEAMDECK_CRASH_INVESTIGATION.md

@@ -0,0 +1,123 @@
+# Steam Deck / Ubuntu LTS Crash Investigation
+
+## Status: SOLVED
+
+**Last updated:** 2026-01-27
+
+**Solution:** Replace bundled `libzstd.so` with system version.
+
+---
+
+## Problem Summary
+
+The Hytale F2P launcher's client patcher causes crashes on Steam Deck and Ubuntu LTS with the error:
+```
+free(): invalid pointer
+```
+or
+```
+SIGSEGV (Segmentation fault)
+```
+
+The crash occurs after successful authentication, specifically right after "Finished handling RequiredAssets".
+
+**Affected Systems:**
+- Steam Deck (glibc 2.41)
+- Ubuntu LTS
+
+**Working Systems:**
+- macOS
+- Windows
+- Older Arch Linux (glibc < 2.41)
+
+---
+
+## Root Cause
+
+The **bundled `libzstd.so`** in the game client is incompatible with glibc 2.41's stricter heap validation. When the game decompresses assets using this library, it triggers heap corruption detected by glibc 2.41.
+
+The crash occurs in `libzstd.so` during `free()` after "Finished handling RequiredAssets" (asset decompression).
+
+---
+
+## Solution
+
+Replace the bundled `libzstd.so` with the system's `libzstd.so.1`.
+
+### Automatic (Launcher)
+
+The launcher automatically detects and replaces `libzstd.so` on Linux systems. No manual action needed.
+
+### Manual
+
+```bash
+cd ~/.hytalef2p/release/package/game/latest/Client
+
+# Backup bundled version
+mv libzstd.so libzstd.so.bundled
+
+# Link to system version
+# Steam Deck / Arch Linux:
+ln -s /usr/lib/libzstd.so.1 libzstd.so
+
+# Debian / Ubuntu:
+ln -s /usr/lib/x86_64-linux-gnu/libzstd.so.1 libzstd.so
+
+# Fedora / RHEL:
+ln -s /usr/lib64/libzstd.so.1 libzstd.so
+```
+
+### Restore Original
+
+```bash
+cd ~/.hytalef2p/release/package/game/latest/Client
+rm libzstd.so
+mv libzstd.so.bundled libzstd.so
+```
+
+---
+
+## Why This Works
+
+1. The bundled `libzstd.so` was likely compiled with different allocator settings or an older toolchain
+2. glibc 2.41 has stricter heap validation that catches invalid memory operations
+3. The system `libzstd.so.1` is compiled with the system's glibc and uses compatible memory allocation patterns
+4. By using the system library, we avoid the incompatibility entirely
+
+---
+
+## Previous Investigation (for reference)
+
+### What Was Tried Before Finding Solution
+
+| Approach | Result |
+|----------|--------|
+| jemalloc allocator | Worked ~30% of time, not stable |
+| GLIBC_TUNABLES | No effect |
+| taskset (CPU pinning) | Single core too slow |
+| nice/chrt (scheduling) | No effect |
+| Various patching approaches | All crashed |
+
+### Key Insight
+
+The crash was in `libzstd.so`, not in our patched code. The patching just changed timing enough to expose the libzstd incompatibility more frequently.
+
+---
+
+## GDB Stack Trace (Historical)
+
+```
+#0  0x00007ffff7d3f5a4 in ?? () from /usr/lib/libc.so.6
+#1  raise () from /usr/lib/libc.so.6
+#2  abort () from /usr/lib/libc.so.6
+#3-#4  ?? () from /usr/lib/libc.so.6
+#5  free () from /usr/lib/libc.so.6
+#6  ?? () from libzstd.so    <-- CRASH POINT (bundled library)
+#7-#24  HytaleClient code (asset decompression)
+```
+
+---
+
+## Branch
+
+`fix/steamdeck-libzstd`

docs/STEAMDECK_DEBUG_COMMANDS.md

@@ -0,0 +1,65 @@
+# Steam Deck / Linux Crash Fix
+
+## SOLUTION: Use system libzstd
+
+The crash is caused by the bundled `libzstd.so` being incompatible with glibc 2.41's stricter heap validation.
+
+### Automatic Fix
+
+The launcher automatically replaces `libzstd.so` with the system version. No manual action needed.
+
+### Manual Fix
+
+```bash
+cd ~/.hytalef2p/release/package/game/latest/Client
+
+# Backup and replace
+mv libzstd.so libzstd.so.bundled
+ln -s /usr/lib/libzstd.so.1 libzstd.so
+```
+
+### Restore Original
+
+```bash
+cd ~/.hytalef2p/release/package/game/latest/Client
+rm libzstd.so
+mv libzstd.so.bundled libzstd.so
+```
+
+---
+
+## Debug Commands (for troubleshooting)
+
+### Check libzstd Status
+
+```bash
+# Check if symlinked
+ls -la ~/.hytalef2p/release/package/game/latest/Client/libzstd.so
+
+# Find system libzstd
+find /usr/lib -name "libzstd.so*"
+```
+
+### Binary Validation
+
+```bash
+file ~/.hytalef2p/release/package/game/latest/Client/HytaleClient
+ldd ~/.hytalef2p/release/package/game/latest/Client/HytaleClient
+```
+
+### Restore Client Binary
+
+```bash
+cd ~/.hytalef2p/release/package/game/latest/Client
+cp HytaleClient.original HytaleClient
+rm -f HytaleClient.patched_custom
+```
+
+---
+
+## Environment Variables
+
+| Variable | Description | Example |
+|----------|-------------|---------|
+| `HYTALE_AUTH_DOMAIN` | Custom auth domain | `auth.sanasol.ws` |
+| `HYTALE_NO_LIBZSTD_FIX` | Disable libzstd replacement | `1` |