diff --git a/scripts/test-layout/layout.json b/scripts/test-layout/layout.json
index 99af70edae..30181da1b6 100644
--- a/scripts/test-layout/layout.json
+++ b/scripts/test-layout/layout.json
@@ -920,6 +920,7 @@
     "management-integration-journal-delete.test.ts": "server",
     "management-integration-routes.test.ts": "server",
     "management-origin-tls.test.ts": "server",
+    "management-provider-proto-override.test.ts": "server",
     "management-provider-validation.test.ts": "server",
     "management-route-registry.test.ts": "server",
     "management-workflow-budget-routes.test.ts": "server",
diff --git a/tests/fixtures/test-layout-expected.json b/tests/fixtures/test-layout-expected.json
index b091096f80..3fbfdc5dc6 100644
--- a/tests/fixtures/test-layout-expected.json
+++ b/tests/fixtures/test-layout-expected.json
@@ -746,6 +746,7 @@
   "management-integration-journal-delete.test.ts": "server",
   "management-integration-routes.test.ts": "server",
   "management-origin-tls.test.ts": "server",
+  "management-provider-proto-override.test.ts": "server",
   "management-provider-validation.test.ts": "server",
   "management-route-registry.test.ts": "server",
   "management-workflow-budget-routes.test.ts": "server",
diff --git a/tests/server/management-provider-proto-override.test.ts b/tests/server/management-provider-proto-override.test.ts
new file mode 100644
index 0000000000..a25b6bda85
--- /dev/null
+++ b/tests/server/management-provider-proto-override.test.ts
@@ -0,0 +1,69 @@
+/**
+ * The prototype-named model context override, held in a sibling file.
+ *
+ * Split out of management-provider-validation.test.ts for the reason recorded in
+ * d3ca5522db and #4908: that file sits at its file-size ratchet cap, and the cap only
+ * ever moves downward, so a test added to it after the cap was set fails the ratchet
+ * for every later pull request rather than only its own. The case is unchanged.
+ */
+import { describe, expect, setDefaultTimeout, spyOn, test } from "bun:test";
+import { managementFetch as fetch } from "../helpers/management-auth";
+import { existsSync, mkdirSync, mkdtempSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { loadConfig, saveConfig } from "../../src/config";
+import { startServer } from "../../src/server";
+import * as destinationPolicy from "../../src/lib/destination-policy";
+import { removeTreeWithRetry } from "../helpers/remove-tree";
+import type { OcxConfig } from "../../src/types";
+
+setDefaultTimeout(60_000);
+
+const TEST_DIR = mkdtempSync(join(tmpdir(), "ocx-management-provider-proto-"));
+
+const canonicalDirect = {
+  adapter: "openai-responses",
+  baseUrl: "https://chatgpt.com/backend-api/codex",
+  authMode: "forward",
+  codexAccountMode: "direct",
+} as const;
+
+describe("provider management validation", () => {
+  // A "__proto__" model id is a legitimate override key once the GUI can draft it.
+  // The merge target must be a null-prototype map: on an ordinary object the
+  // assignment windows["__proto__"] = n invokes the inherited setter, so the
+  // PATCH would return success while silently dropping the override.
+  test("PATCH modelContextWindows persists a __proto__-named model override", async () => {
+    if (existsSync(TEST_DIR)) removeTreeWithRetry(TEST_DIR);
+    mkdirSync(TEST_DIR, { recursive: true });
+    process.env.OPENCODEX_HOME = TEST_DIR;
+    saveConfig({
+      port: 0,
+      openaiProviderTierVersion: 2,
+      defaultProvider: "openai",
+      providers: {
+        openai: { ...canonicalDirect },
+      },
+    } as OcxConfig);
+    const resolvedError = spyOn(destinationPolicy, "providerDestinationResolvedError").mockResolvedValue(null);
+
+    const server = startServer(0);
+    try {
+      const patch = await fetch(new URL("/api/providers?name=openai", server.url), {
+        method: "PATCH",
+        headers: { "content-type": "application/json" },
+        // Written as a raw body: an object literal "__proto__" key would set the
+        // prototype instead of creating the own property under test.
+        body: '{"modelContextWindows":{"__proto__":128000}}',
+      });
+      expect(patch.status).toBe(200);
+      const windows = loadConfig().providers.openai?.modelContextWindows ?? {};
+      expect(Object.hasOwn(windows, "__proto__")).toBe(true);
+      expect(Object.getOwnPropertyDescriptor(windows, "__proto__")?.value).toBe(128000);
+    } finally {
+      resolvedError.mockRestore();
+      await server.stop(true);
+      removeTreeWithRetry(TEST_DIR);
+    }
+  });
+});
diff --git a/tests/server/management-provider-validation.test.ts b/tests/server/management-provider-validation.test.ts
index 7b07a8eff0..c474870126 100644
--- a/tests/server/management-provider-validation.test.ts
+++ b/tests/server/management-provider-validation.test.ts
@@ -1749,43 +1749,6 @@ describe("provider management validation", () => {
     }
   });
 
-  // A "__proto__" model id is a legitimate override key once the GUI can draft it.
-  // The merge target must be a null-prototype map: on an ordinary object the
-  // assignment windows["__proto__"] = n invokes the inherited setter, so the
-  // PATCH would return success while silently dropping the override.
-  test("PATCH modelContextWindows persists a __proto__-named model override", async () => {
-    if (existsSync(TEST_DIR)) removeTreeWithRetry(TEST_DIR);
-    mkdirSync(TEST_DIR, { recursive: true });
-    process.env.OPENCODEX_HOME = TEST_DIR;
-    saveConfig({
-      port: 0,
-      openaiProviderTierVersion: 2,
-      defaultProvider: "openai",
-      providers: {
-        openai: { ...canonicalDirect },
-      },
-    } as OcxConfig);
-    const resolvedError = spyOn(destinationPolicy, "providerDestinationResolvedError").mockResolvedValue(null);
-
-    const server = startServer(0);
-    try {
-      const patch = await fetch(new URL("/api/providers?name=openai", server.url), {
-        method: "PATCH",
-        headers: { "content-type": "application/json" },
-        // Written as a raw body: an object literal "__proto__" key would set the
-        // prototype instead of creating the own property under test.
-        body: '{"modelContextWindows":{"__proto__":128000}}',
-      });
-      expect(patch.status).toBe(200);
-      const windows = loadConfig().providers.openai?.modelContextWindows ?? {};
-      expect(Object.hasOwn(windows, "__proto__")).toBe(true);
-      expect(Object.getOwnPropertyDescriptor(windows, "__proto__")?.value).toBe(128000);
-    } finally {
-      resolvedError.mockRestore();
-      await server.stop(true);
-    }
-  });
-
   test("canonical OpenAI with selectedModels still rejects transport tampering", async () => {
     if (existsSync(TEST_DIR)) removeTreeWithRetry(TEST_DIR);
     mkdirSync(TEST_DIR, { recursive: true });