diff --git a/tests/codex-prompt-route.test.ts b/tests/codex-prompt-route.test.ts
index bf21e4ed1c..734027ac3a 100644
--- a/tests/codex-prompt-route.test.ts
+++ b/tests/codex-prompt-route.test.ts
@@ -11,7 +11,7 @@ import { existsSync, mkdirSync, mkdtempSync, readFileSync, readdirSync, rmSync,
 import { tmpdir } from "node:os";
 import { dirname, join } from "node:path";
 import { handleManagementAPI } from "../src/server/management-api";
-import { encodeBasicString, LAYER_INVENTORY, readPromptLayers } from "../src/codex/prompt-layers";
+import { LAYER_INVENTORY, readPromptLayers } from "../src/codex/prompt-layers";
 import {
   promptTextProbeSpawnAttemptsForTests,
   resetPromptTextProbeForTests,
@@ -1132,17 +1132,19 @@ describe("020 coverage completions", () => {
    */
   test("34. editing an external base prompt invalidates an in-flight text probe", async () => {
     const fx = fixture("model = \"x\"\n");
-    // A literal backslash makes the Windows path condition reproducible on POSIX:
-    // interpolating this path raw would create an invalid TOML escape and make the
-    // fingerprint silently classify the selection as default.
+    // On Windows the backslash is a separator; on POSIX it is a legal filename
+    // character that deterministically exercises the same TOML escaping boundary.
     const externalPath = join(fx.decoyHome, "external\\base.md");
+    mkdirSync(dirname(externalPath), { recursive: true });
     writeFileSync(externalPath, "old-external", "utf8");
-    await call("PUT", "/api/codex-prompt/base/select", fx, {
-      kind: "external", path: externalPath, revision: await revision(fx),
-    });
-    // Selection through the route is not assumed: the fixture config is what the
-    // fingerprint reads, so assert the state this case depends on.
-    writeFileSync(fx.configPath, `model_instructions_file = ${encodeBasicString(externalPath)}\n`, "utf8");
+    // JSON string encoding is an independent, compatible encoding for this TOML
+    // basic string, so the fixture does not use the production encoder as its oracle.
+    writeFileSync(fx.configPath, `model_instructions_file = ${JSON.stringify(externalPath)}\n`, "utf8");
+    expect(readPromptLayers({
+      configPath: fx.configPath,
+      storePath: fx.storePath,
+      baseVariantDir: fx.baseVariantDir,
+    }).baseSelection).toEqual({ kind: "external", path: externalPath });
 
     const startedPath = join(fx.decoyHome, "external-probe-starts.txt");
     const source = [