diff --git a/extensions/codex/harness.test.ts b/extensions/codex/harness.test.ts
index 2dbb0737886d..c387732d9ebf 100644
--- a/extensions/codex/harness.test.ts
+++ b/extensions/codex/harness.test.ts
@@ -248,6 +248,51 @@ describe("Codex agent harness supports()", () => {
     expect(!result.supported ? result.reason : undefined).toContain("not declared");
   });
 
+  it("lets explicitly selected Codex discover unlisted models with its own account", () => {
+    expect(
+      harness.supports({
+        provider: "openai",
+        modelId: "gpt-future",
+        requestedRuntime: "codex",
+        modelProvider: {
+          requestTransportOverrides: "none",
+          preparedAuth: { source: "harness" },
+        },
+      }),
+    ).toEqual({ supported: true, priority: 100 });
+  });
+
+  it.each([
+    {
+      label: "automatic runtime selection",
+      requestedRuntime: "auto" as const,
+      modelProvider: { preparedAuth: { source: "harness" as const } },
+    },
+    {
+      label: "an authored endpoint",
+      requestedRuntime: "codex" as const,
+      modelProvider: {
+        baseUrl: "https://relay.example.test/v1",
+        preparedAuth: { source: "harness" as const },
+      },
+    },
+    {
+      label: "an owner-selected credential",
+      requestedRuntime: "codex" as const,
+      modelProvider: { preparedAuth: { source: "profile" as const, mode: "api-key" } },
+    },
+  ])("does not infer native model access for $label", ({ requestedRuntime, modelProvider }) => {
+    const result = harness.supports({
+      provider: "openai",
+      modelId: "gpt-future",
+      requestedRuntime,
+      modelProvider: { requestTransportOverrides: "none", ...modelProvider },
+    });
+
+    expect(result.supported).toBe(false);
+    expect(!result.supported ? result.reason : undefined).toContain("not declared");
+  });
+
   it.each([
     {
       label: "forwarded OAuth subscription",
diff --git a/extensions/codex/harness.ts b/extensions/codex/harness.ts
index b1ccb94fe5bc..8fdb1698da98 100644
--- a/extensions/codex/harness.ts
+++ b/extensions/codex/harness.ts
@@ -204,6 +204,19 @@ export function createCodexAppServerAgentHarness(
       }
       const preparedAuth = ctx.modelProvider?.preparedAuth;
       const runtimePolicy = ctx.modelProvider?.runtimePolicy;
+      // Codex owns discovery and auth for new first-party models. Only trust that
+      // native account when no authored transport or host credential is involved.
+      const nativeAccountOwnsUnobservedModel =
+        provider === "openai" &&
+        ctx.requestedRuntime === "codex" &&
+        Boolean(ctx.modelId?.trim()) &&
+        preparedAuth?.source === "harness" &&
+        preparedAuth.mode === undefined &&
+        preparedAuth.requirement === undefined &&
+        ctx.modelProvider?.api === undefined &&
+        ctx.modelProvider?.baseUrl === undefined &&
+        ctx.modelProvider?.azureApiVersion === undefined &&
+        ctx.modelProvider?.request === undefined;
       if (runtimePolicy) {
         const compatible = runtimePolicy.compatibleIds.some(
           (id) => id.trim().toLowerCase() === normalizedHarnessRuntimeId,
@@ -214,7 +227,7 @@ export function createCodexAppServerAgentHarness(
             reason: "Codex cannot reproduce the prepared provider route",
           };
         }
-      } else if (ctx.modelProvider && provider !== "codex") {
+      } else if (ctx.modelProvider && provider !== "codex" && !nativeAccountOwnsUnobservedModel) {
         return {
           supported: false,
           reason: "provider route compatibility with Codex is not declared",