diff --git a/scripts/ada_d_eh_matrix.py b/scripts/ada_d_eh_matrix.py
index 4f795ae..0d9877d 100644
--- a/scripts/ada_d_eh_matrix.py
+++ b/scripts/ada_d_eh_matrix.py
@@ -249,14 +249,16 @@ def compiler_flags(
 ) -> tuple[str, ...]:
     mapped = f"-ffile-prefix-map={checkout_prefix}=/testbins"
     if cell.toolchain == "gnat":
+        # -cargs is a one-way door: gnatmake hands every later argv token to
+        # gcc, including the source file and -o if they follow it.
         return (
             "-q",
             "-gnat2022",
+            f"-O{variant.optimization[-1]}",
             "-cargs",
             "-fexceptions",
             "-g0",
             mapped,
-            f"-O{variant.optimization[-1]}",
         )
     if cell.toolchain == "gdc":
         return (
diff --git a/scripts/build_language_eh_corpus.py b/scripts/build_language_eh_corpus.py
index 4010392..5297d8d 100644
--- a/scripts/build_language_eh_corpus.py
+++ b/scripts/build_language_eh_corpus.py
@@ -100,6 +100,16 @@ def runner_image() -> str:
     return f"local-{platform.system().lower()}-{platform.machine().lower()}"
 
 
+def gnatmake_command(
+    compiler: str, flags: list[str], source: str, output: str
+) -> list[str]:
+    """Keep the source and -o on gnatmake's side of -cargs."""
+    if "-cargs" in flags:
+        index = flags.index("-cargs")
+        return [compiler, *flags[:index], source, "-o", output, *flags[index:]]
+    return [compiler, *flags, source, "-o", output]
+
+
 def build_artifact(
     cell: matrix.MatrixCell,
     variant: matrix.Variant,
@@ -114,7 +124,7 @@ def build_artifact(
 
     if cell.toolchain == "gnat":
         with tempfile.TemporaryDirectory(prefix="ada-eh-") as temporary:
-            command = [compiler, *flags, str(source), "-o", str(output)]
+            command = gnatmake_command(compiler, flags, str(source), str(output))
             run(command, cwd=Path(temporary), environment=environment)
     elif cell.toolchain == "gdc":
         command = [compiler, *flags, cell.source_path, "-o", str(output)]
diff --git a/scripts/tests/test_ada_d_eh_matrix.py b/scripts/tests/test_ada_d_eh_matrix.py
index 3b76859..66dbf67 100644
--- a/scripts/tests/test_ada_d_eh_matrix.py
+++ b/scripts/tests/test_ada_d_eh_matrix.py
@@ -123,7 +123,16 @@ def test_gnat_passes_gcc_flags_through_cargs(self) -> None:
         self.assertEqual(flags[0], "-q")
         self.assertIn("-cargs", flags)
         self.assertGreater(flags.index("-cargs"), flags.index("-gnat2022"))
-        self.assertGreater(flags.index("-fexceptions"), flags.index("-cargs"))
+        self.assertGreater(flags.index("-cargs"), flags.index("-O0"))
+        self.assertEqual(
+            flags[flags.index("-cargs") :],
+            (
+                "-cargs",
+                "-fexceptions",
+                "-g0",
+                "-ffile-prefix-map=/checkout=/testbins",
+            ),
+        )
 
 
 class CommandTests(unittest.TestCase):
diff --git a/scripts/tests/test_build_language_eh_corpus.py b/scripts/tests/test_build_language_eh_corpus.py
index a30099d..e3eb650 100644
--- a/scripts/tests/test_build_language_eh_corpus.py
+++ b/scripts/tests/test_build_language_eh_corpus.py
@@ -12,6 +12,7 @@
     sys.path.insert(0, str(SCRIPTS_ROOT))
 
 import ada_d_eh_matrix as matrix  # noqa: E402
+import build_language_eh_corpus as builder  # noqa: E402
 
 BUILD_SCRIPT = SCRIPTS_ROOT / "build_language_eh_corpus.py"
 
@@ -64,5 +65,27 @@ def test_each_cell_describes_exactly_the_contract_the_matrix_defines(self) -> No
                     self.assertEqual(entry["neverd"], matrix.neverd_contract(cell))
 
 
+class GnatmakeCommandTests(unittest.TestCase):
+    def test_places_source_and_output_before_cargs(self) -> None:
+        cell = matrix.validate_cell("gnat", "x86_64-linux-gnu")
+        flags = list(
+            matrix.compiler_flags(cell, matrix.validate_variant("o0"), "/checkout")
+        )
+        command = builder.gnatmake_command(
+            "gnatmake-13",
+            flags,
+            "/checkout/sources/ada-d-eh/ada_eh_probe.adb",
+            "/out/ada_eh_probe",
+        )
+
+        self.assertLess(
+            command.index("/checkout/sources/ada-d-eh/ada_eh_probe.adb"),
+            command.index("-cargs"),
+        )
+        self.assertLess(command.index("-o"), command.index("-cargs"))
+        self.assertLess(command.index("/out/ada_eh_probe"), command.index("-cargs"))
+        self.assertEqual(command[command.index("-cargs") + 1], "-fexceptions")
+
+
 if __name__ == "__main__":
     unittest.main()