diff --git a/src/integrations/harness/providers/codex/codexProtocol.test.ts b/src/integrations/harness/providers/codex/codexProtocol.test.ts
index 4602559cc..6aacb3e3f 100644
--- a/src/integrations/harness/providers/codex/codexProtocol.test.ts
+++ b/src/integrations/harness/providers/codex/codexProtocol.test.ts
@@ -157,6 +157,28 @@ describe("buildThreadStartParams / buildTurnStartParams", () => {
     });
   });
 
+  it("leaves the model to Codex before one has been selected", () => {
+    const params = buildTurnStartParams({
+      threadId: "t",
+      runtimeMode: "auto",
+      model: "",
+    });
+    expect(params).not.toHaveProperty("model");
+    expect(params).not.toHaveProperty("collaborationMode");
+  });
+
+  it("does not send a null collaboration model for a plan turn either", () => {
+    const params = buildTurnStartParams({
+      threadId: "t",
+      runtimeMode: "auto",
+      model: "",
+      intent: "plan",
+    });
+    expect(params).not.toHaveProperty("collaborationMode");
+    expect(params.sandboxPolicy).toEqual({ type: "readOnly" });
+    expect(params.approvalPolicy).toBe("never");
+  });
+
   it("uses native plan mode with a non-escalating read-only sandbox", () => {
     const turn = buildTurnStartParams({
       threadId: "thr_1",
diff --git a/src/integrations/harness/providers/codex/codexProtocol.ts b/src/integrations/harness/providers/codex/codexProtocol.ts
index b46f69c9c..691e178c2 100644
--- a/src/integrations/harness/providers/codex/codexProtocol.ts
+++ b/src/integrations/harness/providers/codex/codexProtocol.ts
@@ -158,21 +158,29 @@ export function buildTurnStartParams(input: {
           input.controlsAgents,
         )
       : runtimeConfig;
+  // settings.model is a required string. Null is rejected
+  // ("invalid type: null, expected a string") and omitting it is
+  // "missing field `model`". Skip the override until a model is known
+  // so Codex keeps the one it chose when the thread started.
+  const model = input.model?.trim() ?? "";
+  const collaborationMode = model
+    ? {
+        mode: input.intent === "plan" ? "plan" : "default",
+        settings: {
+          model,
+          reasoning_effort: input.effort ?? null,
+          developer_instructions: null,
+        },
+      }
+    : undefined;
   return {
     threadId: input.threadId,
     input: codexInput(input.prompt, input.attachments),
     approvalPolicy: config.approvalPolicy,
     approvalsReviewer: config.approvalsReviewer,
     sandboxPolicy: config.sandboxPolicy,
-    collaborationMode: {
-      mode: input.intent === "plan" ? "plan" : "default",
-      settings: {
-        model: input.model ?? null,
-        reasoning_effort: input.effort ?? null,
-        developer_instructions: null,
-      },
-    },
-    ...(input.model ? { model: input.model } : {}),
+    ...(collaborationMode ? { collaborationMode } : {}),
+    ...(model ? { model } : {}),
     ...(input.effort ? { effort: input.effort } : {}),
     ...(input.serviceTier && input.serviceTier !== "default"
       ? { serviceTier: input.serviceTier }