diff --git a/ui/src/components/AgentConfigForm.render.test.tsx b/ui/src/components/AgentConfigForm.render.test.tsx
index 2c4d24cc6f..1005860c20 100644
--- a/ui/src/components/AgentConfigForm.render.test.tsx
+++ b/ui/src/components/AgentConfigForm.render.test.tsx
@@ -7,6 +7,7 @@ import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import type { Agent, Environment } from "@paperclipai/shared";
 import { TooltipProvider } from "@/components/ui/tooltip";
 import { AgentConfigForm } from "./AgentConfigForm";
+import { defaultCreateValues } from "./agent-config-defaults";
 
 const mockAgentsApi = vi.hoisted(() => ({
   adapterModelProfiles: vi.fn(),
@@ -68,7 +69,9 @@ vi.mock("../adapters", () => ({
       adapterType === "hermes_gateway"
         ? <div data-testid="hermes-gateway-config-fields">Hermes Gateway fields</div>
         : null,
-    buildAdapterConfig: () => ({}),
+    buildAdapterConfig: (values: { model?: string }) => ({
+      model: values.model || undefined,
+    }),
     parseStdoutLine: () => [],
   }),
 }));
@@ -223,6 +226,51 @@ async function renderForm(
   return { container, root };
 }
 
+async function renderCreateForm(
+  environments: Environment[],
+  valueOverrides: Partial<typeof defaultCreateValues> = {},
+  options: { showAdapterTestEnvironmentButton?: boolean } = {},
+) {
+  mockEnvironmentsApi.list.mockResolvedValue(environments);
+
+  const container = document.createElement("div");
+  document.body.appendChild(container);
+  const root = createRoot(container);
+  const queryClient = new QueryClient({
+    defaultOptions: {
+      queries: { retry: false },
+      mutations: { retry: false },
+    },
+  });
+
+  const values = {
+    ...defaultCreateValues,
+    adapterType: "codex_local",
+    ...valueOverrides,
+  };
+  const onChange = vi.fn();
+
+  await act(async () => {
+    root.render(
+      <QueryClientProvider client={queryClient}>
+        <TooltipProvider>
+          <AgentConfigForm
+            mode="create"
+            values={values}
+            onChange={onChange}
+            hidePromptTemplate
+            showAdapterTypeField={false}
+            showAdapterTestEnvironmentButton={options.showAdapterTestEnvironmentButton ?? false}
+          />
+        </TooltipProvider>
+      </QueryClientProvider>,
+    );
+  });
+
+  await flushReact();
+  return { container, root, onChange };
+}
+
 describe("AgentConfigForm environment selector", () => {
   let roots: Root[] = [];
 
@@ -395,6 +443,87 @@ describe("AgentConfigForm environment selector", () => {
     });
   });
 
+  it("tests a Codex agent after clearing the primary model to the adapter default", async () => {
+    const result = await renderForm([
+      makeEnvironment({ id: "local-1", name: "Local", driver: "local" }),
+    ], {
+      adapterConfig: { model: "gpt-5.4" },
+    }, {
+      showAdapterTestEnvironmentButton: true,
+    });
+    roots.push(result.root);
+
+    const modelButton = Array.from(result.container.querySelectorAll("button")).find(
+      (button) => button.textContent?.trim() === "gpt-5.4",
+    );
+    expect(modelButton).toBeTruthy();
+
+    await act(async () => {
+      modelButton?.dispatchEvent(new MouseEvent("click", { bubbles: true }));
+    });
+    await flushReact();
+
+    const defaultButton = Array.from(document.body.querySelectorAll("button")).find(
+      (button) => button.textContent?.trim() === "Default",
+    );
+    expect(defaultButton).toBeTruthy();
+
+    await act(async () => {
+      defaultButton?.dispatchEvent(new MouseEvent("click", { bubbles: true }));
+    });
+    await flushReact();
+
+    const testButton = Array.from(result.container.querySelectorAll("button")).find(
+      (button) => button.textContent?.trim() === "Test",
+    );
+    expect(testButton).toBeTruthy();
+
+    await act(async () => {
+      testButton?.dispatchEvent(new MouseEvent("click", { bubbles: true }));
+    });
+    await flushReact();
+
+    expect(mockAgentsApi.testEnvironment).toHaveBeenCalledTimes(1);
+    expect(mockAgentsApi.testEnvironment.mock.calls[0]?.[2]).toMatchObject({
+      adapterConfig: {},
+    });
+    const adapterConfig = (mockAgentsApi.testEnvironment.mock.calls[0]?.[2] as {
+      adapterConfig: Record<string, unknown>;
+    }).adapterConfig;
+    expect(adapterConfig).not.toHaveProperty("model");
+    expect(result.container.textContent).not.toContain("Cannot read properties of undefined");
+  });
+
+  it("omits undefined adapter config entries when testing a create form with the default model", async () => {
+    const result = await renderCreateForm([
+      makeEnvironment({ id: "local-1", name: "Local", driver: "local" }),
+    ], {
+      model: "",
+    }, {
+      showAdapterTestEnvironmentButton: true,
+    });
+    roots.push(result.root);
+
+    const testButton = Array.from(result.container.querySelectorAll("button")).find(
+      (button) => button.textContent?.trim() === "Test",
+    );
+    expect(testButton).toBeTruthy();
+
+    await act(async () => {
+      testButton?.dispatchEvent(new MouseEvent("click", { bubbles: true }));
+    });
+    await flushReact();
+
+    expect(mockAgentsApi.testEnvironment).toHaveBeenCalledTimes(1);
+    expect(mockAgentsApi.testEnvironment.mock.calls[0]?.[2]).toMatchObject({
+      adapterConfig: {},
+    });
+    const adapterConfig = (mockAgentsApi.testEnvironment.mock.calls[0]?.[2] as {
+      adapterConfig: Record<string, unknown>;
+    }).adapterConfig;
+    expect(adapterConfig).not.toHaveProperty("model");
+  });
+
   it("flushes pending environment variable edits before testing adapter config", async () => {
     const result = await renderForm([
       makeEnvironment({ id: "local-1", name: "Local", driver: "local" }),
diff --git a/ui/src/components/AgentConfigForm.tsx b/ui/src/components/AgentConfigForm.tsx
index 5b00190ff7..d30af2b4a8 100644
--- a/ui/src/components/AgentConfigForm.tsx
+++ b/ui/src/components/AgentConfigForm.tsx
@@ -55,7 +55,7 @@ import { shouldShowLegacyWorkingDirectoryField } from "../lib/legacy-agent-confi
 import { listAdapterOptions, listVisibleAdapterTypes } from "../adapters/metadata";
 import { getAdapterDisplay, getAdapterLabel } from "../adapters/adapter-display-registry";
 import { useDisabledAdaptersSync } from "../adapters/use-disabled-adapters";
-import { buildAgentUpdatePatch, type AgentConfigOverlay } from "../lib/agent-config-patch";
+import { buildAgentUpdatePatch, omitUndefinedEntries, type AgentConfigOverlay } from "../lib/agent-config-patch";
 import { useAdapterCapabilities } from "../adapters/use-adapter-capabilities";
 import { resolveForcedKubernetesEnvironment } from "../lib/forced-kubernetes-environment";
 
@@ -195,7 +195,6 @@ function clampDelayMsFromSeconds(value: number) {
   return clampInteger(value, 0, MAX_TURN_CONTINUATION_MAX_DELAY_SEC) * 1000;
 }
 
-
 /* ---- Form ---- */
 
 export function AgentConfigForm(props: AgentConfigFormProps) {
@@ -538,14 +537,14 @@ export function AgentConfigForm(props: AgentConfigFormProps) {
       if (adapterConfigPatch) {
         Object.assign(next, adapterConfigPatch);
       }
-      return next;
+      return omitUndefinedEntries(next);
     }
     const base = config as Record<string, unknown>;
     const next = { ...base, ...overlay.adapterConfig };
     if (adapterConfigPatch) {
       Object.assign(next, adapterConfigPatch);
     }
-    return next;
+    return omitUndefinedEntries(next);
   }
 
   function buildCheapAdapterConfigForTest(adapterConfigPatch?: Record<string, unknown>): Record<string, unknown> {
@@ -747,9 +746,10 @@ export function AgentConfigForm(props: AgentConfigFormProps) {
   }, [props.onTestFeedbackChange, testActionError, testEnvironment.data, testEnvironment.error]);
 
   // Current model for display
-  const currentModelId = isCreate
-    ? val!.model
+  const currentModelValue = isCreate
+    ? val!.model ?? ""
     : eff("adapterConfig", "model", String(config.model ?? ""));
+  const currentModelId = typeof currentModelValue === "string" ? currentModelValue : "";
 
   async function handleRefreshModels() {
     if (!selectedCompanyId) return;
diff --git a/ui/src/lib/agent-config-patch.ts b/ui/src/lib/agent-config-patch.ts
index 9817650738..cc4d4f1e30 100644
--- a/ui/src/lib/agent-config-patch.ts
+++ b/ui/src/lib/agent-config-patch.ts
@@ -19,7 +19,7 @@ export interface AgentConfigOverlay {
   modelProfiles?: { cheap?: AgentModelProfileOverlay };
 }
 
-function omitUndefinedEntries(value: Record<string, unknown>) {
+export function omitUndefinedEntries(value: Record<string, unknown>) {
   return Object.fromEntries(
     Object.entries(value).filter(([, entryValue]) => entryValue !== undefined),
   );