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diff --git a/llamachat_venv.py b/llamachat_venv.py
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+++ b/llamachat_venv.py
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+# SPDX-License-Identifier: GPL-2.0-only
+#
+# llamachat - a small native chat client for a local llama.cpp router
+# Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+"""Finding the interpreter that has PySide6.
+
+PySide6 is usually installed in a virtualenv rather than the system Python,
+so a script started with `#!/usr/bin/env python3` has to hand over to that
+interpreter. Keeping the search here means the launcher and the test script
+agree on where to look, and neither hardcodes a path.
+
+`LLAMACHAT_PYTHON` names an interpreter explicitly. `LLAMACHAT_NO_REEXEC`
+disables the handover, which is also how the re-executed process avoids
+looping.
+"""
+
+import os
+import sys
+from pathlib import Path
+
+HERE = Path(__file__).resolve().parent
+
+# Checked in order; the first interpreter that exists wins.
+CANDIDATES = (
+    HERE / ".venv" / "bin" / "python3",
+    HERE / "venv" / "bin" / "python3",
+    Path.home() / ".local" / "share" / "llamachat-venv" / "bin" / "python3",
+)
+
+
+def have_pyside() -> bool:
+    try:
+        import PySide6  # noqa: F401
+    except ImportError:
+        return False
+    return True
+
+
+def find_interpreter() -> Path | None:
+    """The first candidate interpreter that exists and is not this one.
+
+    Paths are compared unresolved on purpose. A venv created with
+    --system-site-packages has a bin/python3 that symlinks to the system
+    interpreter, so resolving both sides would make the venv look identical
+    to the interpreter we are trying to escape. What differs is sys.prefix,
+    which follows from the path used to start it, not from the real binary.
+    """
+    override = os.environ.get("LLAMACHAT_PYTHON")
+    candidates = [Path(override)] if override else list(CANDIDATES)
+    current = Path(sys.executable)
+    for candidate in candidates:
+        if candidate.is_file() and candidate != current:
+            return candidate
+    return None
+
+
+def reexec(script: str) -> None:
+    """Restart `script` under an interpreter that has PySide6.
+
+    Returns normally when the current interpreter is already usable, when
+    the handover is disabled, or when no other interpreter was found; in
+    that last case the caller fails later with a clear ImportError.
+    """
+    if os.environ.get("LLAMACHAT_NO_REEXEC") or have_pyside():
+        return
+    interpreter = find_interpreter()
+    if interpreter is None:
+        return
+    # Set before exec so the new process cannot bounce again.
+    os.environ["LLAMACHAT_NO_REEXEC"] = "1"
+    path = str(Path(script).resolve())
+    os.execv(str(interpreter), [str(interpreter), path] + sys.argv[1:])