diff --git a/web/test-browser-wasm-smoke-opfs-parameter-sections.js b/web/test-browser-wasm-smoke-opfs-parameter-sections.js
index 1d99974..b3e0300 100644
--- a/web/test-browser-wasm-smoke-opfs-parameter-sections.js
+++ b/web/test-browser-wasm-smoke-opfs-parameter-sections.js
@@ -14,13 +14,14 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
     runSection,
     withSmokeSimulator,
   } = context;
+  const backend = { backend: "linuxcnc-rs274" };
 
   await runSection("opfs parameter save", async () => {
     await simulator.parseWithParameterFile(
       "G21 G90\nG0 X1\nG28.1\nM30\n",
       "parameters/rs274ngc.var",
       "linuxcnc",
-      { backend: "linuxcnc-rs274" },
+      backend,
     );
     const opfsParameterFile = await readOpfsText(simulator, "parameters/rs274ngc.var");
     expectIncludes(opfsParameterFile, "5161\t1.000000", "LinuxCNC parameter file was not persisted into OPFS");
@@ -34,7 +35,7 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
         "G21 G90\nF100\nO10 if [#5161 EQ 1]\nG1 X8\nO10 endif\n",
         "parameters/rs274ngc.var",
         "linuxcnc",
-        { backend: "linuxcnc-rs274" },
+        backend,
       );
       expectEvent(restoredWorkspaceEvents, (event) => event.type === "linear-feed" && near(event.end.x, 8),
         "LinuxCNC OPFS parameter file was not restored across WASM simulator instances");
@@ -48,11 +49,10 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
       "G21 G90\nM30\n",
       "parameters/rs274ngc.var",
       "linuxcnc",
-      { backend: "linuxcnc-rs274" },
+      backend,
     );
     const defaultParameterBackup = await readOpfsText(simulator, "parameters/rs274ngc.var.bak");
-    expectIncludes(defaultParameterBackup, "5161\t1.000000",
-      "LinuxCNC default OPFS parameter backup did not preserve the previous file");
+    expectIncludes(defaultParameterBackup, "5161\t1.000000", "LinuxCNC default OPFS parameter backup did not preserve the previous file");
   });
 
   await runSection("opfs parameter temporary cleanup", async () => {
@@ -64,7 +64,7 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
       "G21 G90\nF100\nO10 if [#5161 EQ 1]\nG1 X9\nO10 endif\n",
       "parameters/rs274ngc.var",
       "linuxcnc",
-      { backend: "linuxcnc-rs274" },
+      backend,
     );
     expectEvent(restoredParameterEvents, (event) => event.type === "linear-feed" && near(event.end.x, 9),
       "LinuxCNC parameter file was not restored from OPFS");
@@ -100,7 +100,7 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
       "G21 G90\nG0 X4\nG28.1\nM30\n",
       "parameters/existing-save.var",
       "linuxcnc",
-      { backend: "linuxcnc-rs274" },
+      backend,
     );
     const existingSaveParameterFile = await readOpfsText(simulator, "parameters/existing-save.var");
     expectIncludes(existingSaveParameterFile, "5161\t4.000000",
@@ -125,7 +125,7 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
       "G21 G90\nM30\n",
       "parameters/stale-backup-cleanup.var",
       "linuxcnc",
-      { backend: "linuxcnc-rs274" },
+      backend,
     );
     await expectMissingOpfsFile(
       () => simulator.fs.opfs.readFile("parameters/stale-backup-cleanup.var.bak"),
@@ -142,7 +142,7 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
       "programs/parameter-file-smoke.ngc",
       "parameters/rs274ngc.var",
       "linuxcnc",
-      { backend: "linuxcnc-rs274" },
+      backend,
     );
     expectEvent(fileParameterEvents, (event) => event.type === "program-end",
       "OPFS program file with LinuxCNC parameter persistence did not parse");
@@ -163,7 +163,7 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
       "G21 G90\nM30\n",
       "parameters/missing-main.var",
       "linuxcnc",
-      { backend: "linuxcnc-rs274" },
+      backend,
     );
     const missingMainParameterFile = await readOpfsText(simulator, "parameters/missing-main.var");
     expectIncludes(missingMainParameterFile, "5220\t1.000000",
@@ -178,7 +178,7 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
       "G21 G90\nF100\nO10 if [#5220 EQ 1]\nG1 X6\nO10 endif\n",
       "parameters/missing-main.var",
       "linuxcnc",
-      { backend: "linuxcnc-rs274" },
+      backend,
     );
     expectEvent(missingMainRestoreEvents, (event) => event.type === "linear-feed" && near(event.end.x, 6),
       "LinuxCNC-created missing OPFS parameter file was not restored on the next parse");
@@ -206,7 +206,7 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
         "G21 G90\nM30\n",
         "parameters/out-of-order.var",
         "linuxcnc",
-        { backend: "linuxcnc-rs274" },
+        backend,
       ),
       "Parameter file out of order",
       "LinuxCNC accepted an out-of-order OPFS parameter file",
@@ -224,7 +224,7 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
         "G21 G90\nM30\n",
         "parameters/out-of-range.var",
         "linuxcnc",
-        { backend: "linuxcnc-rs274" },
+        backend,
       ),
       "Parameter number out of range",
       "LinuxCNC accepted an out-of-range OPFS parameter file",
@@ -242,7 +242,7 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
         "G21 G90\nG28.1\n#0 = 1\n",
         "parameters/failed-parse.var",
         "linuxcnc",
-        { backend: "linuxcnc-rs274" },
+        backend,
       ),
       "",
       "LinuxCNC accepted a failing parse that should not persist parameters",
@@ -251,6 +251,15 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
     if (failedParseParameterFile.includes("5161\t1.000000")) {
       throw new Error("LinuxCNC persisted failed parse parameter updates into OPFS");
     }
+    // LinuxCNC source basis: save_parameters() runs only after successful interpretation.
+    await simulator.fs.opfs.writeFile("parameters/failed-backup.var", "5161\t5.000000\n");
+    await simulator.fs.opfs.writeFile("parameters/failed-backup.var.bak", "5161\t4.000000\n");
+    await expectErrorContaining(
+      () => simulator.parseWithParameterFile("G21 G90\n#0 = 1\n", "parameters/failed-backup.var", "linuxcnc", backend),
+      "",
+      "LinuxCNC accepted a failing parse that should not replace the parameter backup",
+    );
+    expectText(await readOpfsText(simulator, "parameters/failed-backup.var.bak"), "5161\t4.000000\n", "LinuxCNC replaced the OPFS parameter backup after a failed parse");
   });
 
   await runSection("opfs stale parameter state cleanup", async () => {
@@ -259,7 +268,7 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
       "G21 G90\nF100\nO10 if [#5161 EQ 1]\nG1 X9\nO10 endif\n",
       "parameters/stale-state.var",
       "linuxcnc",
-      { backend: "linuxcnc-rs274" },
+      backend,
     );
     expectEvent(staleStateEvents, (event) => event.type === "linear-feed" && near(event.end.x, 9),
       "LinuxCNC did not honor the loaded OPFS parameter state");
@@ -268,7 +277,7 @@ export async function runBrowserOpfsParameterSections(context, simulator) {
       "G21 G90\nF100\nO10 if [#5161 EQ 1]\nG1 X9\nO10 endif\n",
       "parameters/stale-state.var",
       "linuxcnc",
-      { backend: "linuxcnc-rs274" },
+      backend,
     );
     expectNoEvent(missingStateEvents, (event) => event.type === "linear-feed" && near(event.end.x, 9),
       "LinuxCNC stale OPFS parameter state was not cleared when the file was missing");