diff --git a/docs/runtimes.md b/docs/runtimes.md
index b2262ad..991cae5 100644
--- a/docs/runtimes.md
+++ b/docs/runtimes.md
@@ -16,9 +16,14 @@ The following runtimes are available:
 
 ## null
 
-The default runtime. Assumes you have any necessary toolchain installed on your
-system. If you request a build that requires a (cross) toolchain you don't have
-installed locally, the build will just fail.
+The default runtime. Uses toolchains installed on your system. You need to
+install any required (cross) compilers yourself. For example, to cross-compile
+for arm64 on Debian you would need to install `gcc-aarch64-linux-gnu`.
+
+If a required compiler is not installed, the build will fail.
+
+The support matrix (`tuxmake -p`) checks which compilers are available in your
+`$PATH` and only reports combinations as supported when the compiler is found.
 
 ## docker
 
diff --git a/test/conftest.py b/test/conftest.py
index a9cc2c4..d8c4342 100644
--- a/test/conftest.py
+++ b/test/conftest.py
@@ -63,6 +63,8 @@ def fake_cross_compilers(tmpdir_factory):
             binary = arch.makevars["CROSS_COMPILE"] + tool
             if not shutil.which(binary):
                 missing[binary] = tool
+    if not shutil.which("clang"):
+        missing["clang"] = "true"
     if missing:
         testbin = tmpdir_factory.mktemp("bin")
         for p, real in missing.items():
diff --git a/test/integration/fakelinux b/test/integration/fakelinux
index b0680ae..1ed16b4 100755
--- a/test/integration/fakelinux
+++ b/test/integration/fakelinux
@@ -132,6 +132,7 @@ test_dtbs() {
 }
 
 test_dtbs_armv5() {
+  skip_if not program_installed arm-linux-gnueabi-gcc
   run tuxmake --target-arch=armv5 dtbs
   assertTrue "build dtbs on armv5" "grep 'dtbs: PASS' stderr"
 }
diff --git a/test/test_build.py b/test/test_build.py
index 6ac4e05..3fa2be4 100644
--- a/test/test_build.py
+++ b/test/test_build.py
@@ -889,7 +889,7 @@ def test_errors(self, build):
 
 class TestUnsupportedToolchainArchitectureCombination:
     def test_exception(self, linux, mocker):
-        mocker.patch("tuxmake.runtime.Runtime.is_supported", return_value=False)
+        mocker.patch("tuxmake.runtime.NullRuntime.is_supported", return_value=False)
         with pytest.raises(
             tuxmake.exceptions.UnsupportedArchitectureToolchainCombination
         ):
diff --git a/test/test_runtime.py b/test/test_runtime.py
index 73491a9..3b73929 100644
--- a/test/test_runtime.py
+++ b/test/test_runtime.py
@@ -2,6 +2,7 @@
 import subprocess
 import pytest
 
+from tuxmake.arch import Architecture
 from tuxmake.build import Build
 from tuxmake.exceptions import InvalidRuntimeError
 from tuxmake.exceptions import RuntimePreparationFailed
@@ -13,6 +14,7 @@
 from tuxmake.runtime import PodmanRuntime
 from tuxmake.runtime import PodmanLocalRuntime
 from tuxmake.runtime import Terminated
+from tuxmake.toolchain import Toolchain
 
 
 @pytest.fixture
@@ -81,6 +83,14 @@ def test_create_output_directory_on_prepare_already_exists(self, tmp_path):
         assert runtime.output_dir.exists()
 
 
+class TestRuntimeIsSupported:
+    def test_base_class_returns_true(self):
+        runtime = Runtime.get("docker")
+        arch = Architecture("x86_64")
+        toolchain = Toolchain("gcc")
+        assert Runtime.is_supported(runtime, arch, toolchain) is True
+
+
 class TestNullRuntime:
     def test_get_command_line(self, build):
         assert NullRuntime().get_command_line(
@@ -91,6 +101,20 @@ def test_toolchains(self):
         runtime = NullRuntime()
         assert "gcc" in runtime.toolchains
 
+    def test_is_supported_compiler_found(self, mocker):
+        mocker.patch("shutil.which", return_value="/usr/bin/gcc")
+        runtime = NullRuntime()
+        arch = Architecture("x86_64")
+        toolchain = Toolchain("gcc")
+        assert runtime.is_supported(arch, toolchain) is True
+
+    def test_is_supported_compiler_not_found(self, mocker):
+        mocker.patch("shutil.which", return_value=None)
+        runtime = NullRuntime()
+        arch = Architecture("x86_64")
+        toolchain = Toolchain("gcc")
+        assert runtime.is_supported(arch, toolchain) is False
+
 
 @pytest.fixture
 def container_id():
diff --git a/tuxmake/runtime.py b/tuxmake/runtime.py
index 3520e13..bbea49c 100644
--- a/tuxmake/runtime.py
+++ b/tuxmake/runtime.py
@@ -2,6 +2,7 @@
 import re
 import json
 import shlex
+import shutil
 import subprocess
 import sys
 import time
@@ -361,6 +362,10 @@ def run_cmd(
 class NullRuntime(Runtime):
     name = "null"
 
+    def is_supported(self, arch, toolchain):
+        compiler = toolchain.compiler(arch)
+        return shutil.which(compiler) is not None
+
 
 class Image:
     def __init__(