diff --git a/mfc.sh b/mfc.sh
index 3a4ec38195..94c2faf8d1 100755
--- a/mfc.sh
+++ b/mfc.sh
@@ -103,7 +103,7 @@ if [ $code -ne 0 ]; then
 fi
 
 # Deactivate the Python virtualenv in case the user "source"'d this script.
-# In prebuilt/spack mode the venv was never created, so there's nothing to deactivate.
+# Guard against the case where no venv was activated (e.g. system Python).
 if type deactivate > /dev/null 2>&1; then
     log "(venv) Exiting the$MAGENTA Python$COLOR_RESET virtual environment."
     deactivate
diff --git a/toolchain/bootstrap/python.sh b/toolchain/bootstrap/python.sh
index 4193903f15..2809918b76 100644
--- a/toolchain/bootstrap/python.sh
+++ b/toolchain/bootstrap/python.sh
@@ -4,12 +4,6 @@ MFC_PYTHON_MIN_MAJOR=3
 MFC_PYTHON_MIN_MINOR=9
 MFC_PYTHON_MIN_STR="$MFC_PYTHON_MIN_MAJOR.$MFC_PYTHON_MIN_MINOR"
 
-# Prebuilt/spack mode: skip the venv entirely. The caller must already have
-# python3 + the MFC toolchain deps + the `mfc` package importable on PYTHONPATH.
-if [ -n "${MFC_PREBUILT_PREFIX:-}" ]; then
-    return 0
-fi
-
 is_python_compatible() {
     if ! ${1:-python3} -c "import sys; exit(int(not (sys.version_info[0]==$MFC_PYTHON_MIN_MAJOR and sys.version_info[1] >= $MFC_PYTHON_MIN_MINOR)))"; then
         return 1
diff --git a/toolchain/mfc/build.py b/toolchain/mfc/build.py
index d5a13fe182..01efb1a9b1 100644
--- a/toolchain/mfc/build.py
+++ b/toolchain/mfc/build.py
@@ -14,7 +14,7 @@
 from rich.text import Text
 
 from .case import Case
-from .common import MFCException, create_directory, debug, delete_directory, format_list_to_string, get_prebuilt_prefix, system
+from .common import MFCException, create_directory, debug, delete_directory, format_list_to_string, system
 from .printer import cons
 from .run import input
 from .state import ARG, CFG, gpuConfigOptions
@@ -336,9 +336,6 @@ def get_home_dirpath(self) -> str:
         return os.sep.join([os.getcwd()])
 
     def get_install_binpath(self, case: Case) -> str:
-        prebuilt = get_prebuilt_prefix()
-        if prebuilt:
-            return os.sep.join([prebuilt, "bin", self.name])
         # <root>/install/<slug>/bin/<target>
         return os.sep.join([self.get_install_dirpath(case), "bin", self.name])
 
@@ -362,9 +359,6 @@ def is_buildable(self) -> bool:
         if ARG("no_build"):
             return False
 
-        if get_prebuilt_prefix():
-            return False
-
         if self.isDependency and ARG(f"sys_{self.name}", False):
             return False
 
diff --git a/toolchain/mfc/cli/commands.py b/toolchain/mfc/cli/commands.py
index d396c95d36..1e1898b117 100644
--- a/toolchain/mfc/cli/commands.py
+++ b/toolchain/mfc/cli/commands.py
@@ -274,14 +274,6 @@
             default=False,
             dest="no_build",
         ),
-        Argument(
-            name="prebuilt-prefix",
-            help="Use prebuilt MFC binaries from <PREFIX>/bin (e.g. a Spack install prefix). Implies --no-build. Also settable via MFC_PREBUILT_PREFIX.",
-            type=str,
-            default=None,
-            metavar="PREFIX",
-            dest="prebuilt_prefix",
-        ),
         Argument(
             name="wait",
             help="(Batch) Wait for the job to finish.",
@@ -445,14 +437,6 @@
             default=False,
             dest="no_build",
         ),
-        Argument(
-            name="prebuilt-prefix",
-            help="Use prebuilt MFC binaries from <PREFIX>/bin (e.g. a Spack install prefix). Implies --no-build. Also settable via MFC_PREBUILT_PREFIX.",
-            type=str,
-            default=None,
-            metavar="PREFIX",
-            dest="prebuilt_prefix",
-        ),
         Argument(
             name="no-examples",
             help="Do not test example cases.",
diff --git a/toolchain/mfc/common.py b/toolchain/mfc/common.py
index 6ca1441409..ff066c8672 100644
--- a/toolchain/mfc/common.py
+++ b/toolchain/mfc/common.py
@@ -42,17 +42,6 @@ def debug(msg: str):
 MFC_BENCH_FILEPATH = abspath(join(MFC_TOOLCHAIN_DIR, "bench.yaml"))
 MFC_MECHANISMS_DIR = abspath(join(MFC_TOOLCHAIN_DIR, "mechanisms"))
 
-
-def get_prebuilt_prefix() -> typing.Optional[str]:
-    # Returns the prebuilt install prefix when MFC is run against pre-built binaries
-    # (e.g. a Spack install). The CLI flag --prebuilt-prefix takes precedence over the
-    # MFC_PREBUILT_PREFIX environment variable. Returns None when running normally.
-    from .state import ARG
-
-    cli = ARG("prebuilt_prefix", "") or None
-    return cli or os.environ.get("MFC_PREBUILT_PREFIX") or None
-
-
 MFC_LOGO = """\
      .=++*:          -+*+=.
     :+   -*-        ==   =* .