fix(desktop): stop the create-agent provider config probe from erasing keystrokes (#4411)

## What

Fixes the create-agent dialog's "Run on" provider config fields eating
keystrokes — reported by Tyler in buzz-remote-agents (channel
`29414326`, thread `db76677a`): the Kubernetes **Kubeconfig context**
field would not accept typing.

## Why it happened (the Typewriter Eraser, shipped in #4289)

`WhereToRunSection`'s probe `useEffect` depended on the whole `draft`:

1. every keystroke changed the draft → effect re-fired → provider binary
re-probed;
2. each probe result is a fresh object written into the draft → the
effect re-triggered **itself**, respawning the provider binary in a loop
for as long as the dialog sat on a provider;
3. every probe resolution reset `providerConfig` to schema defaults —
erasing whatever was typed. A field with no schema default (`context`)
snapped back to empty, i.e. "won't let me type". Unrelated to how many
kubeconfig contexts you have.

## Fix

- **Probe once per provider selection**, keyed on the provider's stable
`binaryPath` — not the draft, not the provider object (a
`useBackendProvidersQuery` refresh must not reprobe an unchanged
selection).
- **Latest-state resolution** via `React.useEffectEvent` + a new pure
`applyProbeResult` helper: schema defaults merge **beneath** the current
`providerConfig`, so a probe landing after the user typed can never
clobber in-flight input (per Wren's pre-patch red-team: changing deps
alone leaves a stale closure).

Existing `cancelled` cleanup keeps provider-switch/unmount safe;
selection reset (`emptyWhereToRunDraft`) and the fail-closed probe-error
path are unchanged.

## Tests

- **Unit** (`whereToRunIntent.test.mjs`): `applyProbeResult` merge
semantics — defaults under typed values, user-cleared fields stay
cleared, schema-less results, unrelated fields preserved.
- **E2E** (new `where-to-run-config.spec.ts`, added to the smoke
project, **red-first verified**: all 3 fail against the unfixed
component):
- typing into a defaultless provider field sticks, and
`probe_backend_provider` fires exactly once per selection;
- the config form is gated on probe resolution (slow probe: no
half-rendered form, defaults prefill once);
  - provider → local → provider re-probes and resets cleanly.
- Mock bridge gains `backendProviders` / `backendProviderProbeResult` /
`backendProviderProbeDelayMs` seams (defaults preserve prior behavior).

## Verification at 8eb7680

- `pnpm check` + `tsc` clean, `pnpm test` 3926/3926;
- new spec 3/3 green (and 3/3 red on the unfixed component);
- pre-push lefthook: desktop-test, desktop-check, desktop-tauri-checks,
rust-tests, mobile-test all green.

---------

Signed-off-by: npub1qyvc0c5kl4gqv2fd97fsk46tu378sqgy35vc83rvgfwne90sel7s0ed67d <011987e296fd5006292d2f930b574be47c7801048d1983c46c425d3c95f0cffd@buzz.block.builderlab.xyz>
Co-authored-by: npub1qyvc0c5kl4gqv2fd97fsk46tu378sqgy35vc83rvgfwne90sel7s0ed67d <011987e296fd5006292d2f930b574be47c7801048d1983c46c425d3c95f0cffd@buzz.block.builderlab.xyz>

desktop/playwright.config.ts

@@ -132,6 +132,7 @@ export default defineConfig({
         "**/harness-management.spec.ts",
         "**/harness-catalog-screenshots.spec.ts",
         "**/inline-custom-harness.spec.ts",
+        "**/where-to-run-config.spec.ts",
         "**/huddle-transcription.spec.ts",
       ],
       use: {

desktop/src/features/agents/ui/WhereToRunSection.tsx

@@ -5,7 +5,11 @@ import { useBackendProvidersQuery } from "@/features/agents/hooks";
 import { probeBackendProvider } from "@/shared/api/tauri";
 
 import { ProviderConfigFields } from "./ProviderConfigFields";
-import { emptyWhereToRunDraft, type WhereToRunDraft } from "./whereToRunIntent";
+import {
+  applyProbeResult,
+  emptyWhereToRunDraft,
+  type WhereToRunDraft,
+} from "./whereToRunIntent";
 
 /** Optional remote-backend selector. Buzz shared compute is an LLM provider, not a run destination. */
 export function WhereToRunSection({
@@ -26,32 +30,37 @@ export function WhereToRunSection({
     [backendProviders, draft.runOn],
   );
 
+  // Latest-state seam for probe resolution: an Effect Event always sees the
+  // draft as it is *now*. Without this, the probe promise closes over the
+  // draft from probe start, and anything typed while the probe was in flight
+  // gets thrown away when it resolves (a second, subtler Typewriter Eraser).
+  const applyProbe = React.useEffectEvent(
+    (result: Awaited<ReturnType<typeof probeBackendProvider>>) => {
+      onDraftChange(applyProbeResult(draft, result));
+    },
+  );
+
+  // Probe once per provider *selection*, keyed on the provider's stable
+  // path — never on the draft. Depending on the draft made every keystroke
+  // refire the probe, and each resolution reset providerConfig to schema
+  // defaults, which erased what the user was typing (the Typewriter Eraser)
+  // and spawned the provider binary in a loop for as long as the dialog was
+  // open. Keying on the path (not the provider object) also keeps a
+  // providers-query refresh from reprobing an unchanged selection.
+  const selectedBinaryPath = isProviderMode
+    ? (selectedBackendProvider?.binaryPath ?? null)
+    : null;
   React.useEffect(() => {
-    if (!isProviderMode || !selectedBackendProvider) {
+    if (!selectedBinaryPath) {
       setProbeError(null);
       return;
     }
     let cancelled = false;
     setProbeError(null);
-    void probeBackendProvider(selectedBackendProvider.binaryPath)
+    void probeBackendProvider(selectedBinaryPath)
       .then((result) => {
         if (cancelled) return;
-        const defaults: Record<string, string> = {};
-        const properties =
-          (result.config_schema as Record<string, unknown> | undefined)
-            ?.properties ?? {};
-        for (const [key, property] of Object.entries(properties) as [
-          string,
-          Record<string, unknown>,
-        ][]) {
-          if (property.default != null)
-            defaults[key] = String(property.default);
-        }
-        onDraftChange({
-          ...draft,
-          probedProvider: result,
-          providerConfig: defaults,
-        });
+        applyProbe(result);
       })
       .catch((error: unknown) => {
         if (!cancelled) {
@@ -61,7 +70,7 @@ export function WhereToRunSection({
     return () => {
       cancelled = true;
     };
-  }, [draft, isProviderMode, onDraftChange, selectedBackendProvider]);
+  }, [selectedBinaryPath]);
 
   if (backendProviders.length === 0) return null;
 

desktop/src/features/agents/ui/whereToRunIntent.test.mjs

@@ -2,6 +2,7 @@ import assert from "node:assert/strict";
 import test from "node:test";
 
 import {
+  applyProbeResult,
   canSubmitWhereToRun,
   emptyWhereToRunDraft,
   providerConfigComplete,
@@ -59,3 +60,72 @@ test("provider draft resolves with coerced config values", () => {
     config: { region: "us", size: 3 },
   });
 });
+
+// ── applyProbeResult: probe resolution must merge, not overwrite ─────────────
+//
+// Pins the seam that fixed the "Typewriter Eraser" (agent-create dialog's
+// provider config fields losing keystrokes): a probe resolution prefills
+// schema defaults *beneath* the user's in-flight config, never over it. The
+// effect in WhereToRunSection keys probing on the provider's binary path, so
+// the only probe writes that reach providerConfig are the ones pinned here.
+
+const probeWithDefaults = {
+  ok: true,
+  config_schema: {
+    properties: {
+      context: { type: "string", title: "Kubeconfig context" },
+      namespace: { type: "string", default: "buzz-agents-x1y2z3" },
+      inactivity_seconds: { type: "number", default: 1800 },
+    },
+    required: ["namespace"],
+  },
+};
+
+const unprobedDraft = {
+  ...emptyWhereToRunDraft,
+  runOn: "kubernetes",
+};
+
+test("probe resolution prefills schema defaults on a fresh draft", () => {
+  const next = applyProbeResult(unprobedDraft, probeWithDefaults);
+  assert.equal(next.probedProvider, probeWithDefaults);
+  assert.deepEqual(next.providerConfig, {
+    namespace: "buzz-agents-x1y2z3",
+    inactivity_seconds: "1800",
+  });
+});
+
+test("probe resolution keeps user-typed values over schema defaults", () => {
+  const typed = {
+    ...unprobedDraft,
+    providerConfig: { context: "prod-us-west", namespace: "my-ns" },
+  };
+  const next = applyProbeResult(typed, probeWithDefaults);
+  assert.deepEqual(next.providerConfig, {
+    context: "prod-us-west",
+    namespace: "my-ns",
+    inactivity_seconds: "1800",
+  });
+});
+
+test("probe resolution keeps a user-cleared field cleared", () => {
+  // "" is a deliberate user state — coerceConfigValues drops empty numerics
+  // and required-gating treats "" as incomplete; the probe must not undo it.
+  const cleared = { ...unprobedDraft, providerConfig: { namespace: "" } };
+  const next = applyProbeResult(cleared, probeWithDefaults);
+  assert.equal(next.providerConfig.namespace, "");
+});
+
+test("a schema-less probe result records the probe without touching config", () => {
+  const typed = { ...unprobedDraft, providerConfig: { context: "abc" } };
+  const next = applyProbeResult(typed, { ok: true });
+  assert.deepEqual(next.providerConfig, { context: "abc" });
+  assert.deepEqual(next.probedProvider, { ok: true });
+});
+
+test("probe resolution preserves unrelated draft fields", () => {
+  assert.equal(
+    applyProbeResult(unprobedDraft, probeWithDefaults).runOn,
+    "kubernetes",
+  );
+});

desktop/src/features/agents/ui/whereToRunIntent.ts

@@ -15,6 +15,35 @@ export const emptyWhereToRunDraft: WhereToRunDraft = {
   probedProvider: null,
 };
 
+/**
+ * Fold a completed probe into the draft the user has *now* — not the draft
+ * that existed when the probe started. Schema defaults prefill only the keys
+ * the user has not touched: anything already in `providerConfig` (typed while
+ * the probe was in flight) wins over the default. Overwriting instead of
+ * merging is the "Typewriter Eraser" bug — every probe resolution silently
+ * erased in-flight keystrokes.
+ */
+export function applyProbeResult(
+  current: WhereToRunDraft,
+  result: BackendProviderProbeResult,
+): WhereToRunDraft {
+  const defaults: Record<string, string> = {};
+  const properties =
+    (result.config_schema as Record<string, unknown> | undefined)?.properties ??
+    {};
+  for (const [key, property] of Object.entries(properties) as [
+    string,
+    Record<string, unknown>,
+  ][]) {
+    if (property.default != null) defaults[key] = String(property.default);
+  }
+  return {
+    ...current,
+    probedProvider: result,
+    providerConfig: { ...defaults, ...current.providerConfig },
+  };
+}
+
 export function providerConfigComplete(draft: WhereToRunDraft): boolean {
   if (draft.runOn === "local") return true;
   if (!draft.probedProvider) return false;

desktop/src/testing/e2eBridge.ts

@@ -507,6 +507,11 @@ type E2eConfig = {
      * returning a catalog.
      */
     discoverAgentModelsError?: string;
+    // Backend provider mocks for the create-agent "Run on" section. See
+    // tests/helpers/bridge.ts:MockBridgeOptions for semantics.
+    backendProviders?: Array<{ id: string; binaryPath: string }>;
+    backendProviderProbeResult?: Record<string, unknown>;
+    backendProviderProbeDelayMs?: number;
   };
   relayHttpUrl?: string;
   relayWsUrl?: string;
@@ -11064,9 +11069,22 @@ export function maybeInstallE2eTauriMocks() {
           activeConfig,
         );
       case "discover_backend_providers":
-        return [];
-      case "probe_backend_provider":
-        return { ok: false, error: "mock: no providers available" };
+        return activeConfig?.mock?.backendProviders ?? [];
+      case "probe_backend_provider": {
+        const probeDelayMs =
+          activeConfig?.mock?.backendProviderProbeDelayMs ?? 0;
+        if (probeDelayMs > 0) {
+          await new Promise((resolve) =>
+            window.setTimeout(resolve, probeDelayMs),
+          );
+        }
+        return (
+          activeConfig?.mock?.backendProviderProbeResult ?? {
+            ok: false,
+            error: "mock: no providers available",
+          }
+        );
+      }
       case "discover_managed_agent_prereqs":
         return handleDiscoverManagedAgentPrereqs(
           payload as Parameters<typeof handleDiscoverManagedAgentPrereqs>[0],

desktop/tests/e2e/where-to-run-config.spec.ts

@@ -0,0 +1,154 @@
+/**
+ * E2E spec for the create-agent "Run on" provider config fields.
+ *
+ * Pins the fix for the "Typewriter Eraser": WhereToRunSection's probe effect
+ * used to depend on the whole draft, so every keystroke re-probed the
+ * provider and every probe resolution reset providerConfig to schema
+ * defaults — typing into a defaultless field (the k8s "Kubeconfig context")
+ * looked completely dead, and the provider binary respawned in a loop.
+ *
+ * Covers:
+ *  - typing into a defaultless provider field sticks, and the provider is
+ *    probed exactly once for the selection (not once per keystroke)
+ *  - the config form is gated on probe resolution (no half-rendered form),
+ *    and defaults prefill exactly once when a slow probe lands
+ *  - switching provider → local → provider re-probes and resets cleanly
+ *
+ * The stale-closure merge on probe resolution (defaults beneath in-flight
+ * typing) is unreachable through this UI because the fields render only
+ * after the probe resolves; it is pinned at the unit level in
+ * whereToRunIntent.test.mjs (applyProbeResult).
+ */
+import { expect, test } from "@playwright/test";
+
+import { installMockBridge } from "../helpers/bridge";
+
+type Page = import("@playwright/test").Page;
+
+const PROVIDER = {
+  id: "kubernetes",
+  binaryPath: "/mock/buzz-backend-kubernetes",
+};
+
+const PROBE_RESULT = {
+  ok: true,
+  name: "kubernetes",
+  version: "0.0.0-mock",
+  config_schema: {
+    type: "object",
+    properties: {
+      context: {
+        type: "string",
+        title: "Kubeconfig context",
+        description: "Context from your kubeconfig.",
+      },
+      namespace: {
+        type: "string",
+        title: "Namespace",
+        default: "buzz-agents-mock01",
+      },
+    },
+    required: ["namespace"],
+  },
+};
+
+async function probeInvocations(page: Page): Promise<number> {
+  return page.evaluate(
+    () =>
+      (
+        window as Window & { __BUZZ_E2E_COMMANDS__?: string[] }
+      ).__BUZZ_E2E_COMMANDS__?.filter(
+        (command) => command === "probe_backend_provider",
+      ).length ?? 0,
+  );
+}
+
+/** Open the create-agent dialog and select the mocked provider in "Run on". */
+async function openCreateDialogOnProvider(page: Page) {
+  await page.goto("/", { waitUntil: "domcontentloaded" });
+  await page.getByTestId("open-agents-view").click();
+  await page.getByTestId("new-agent-card").click();
+  await page.getByRole("menuitem", { name: "Create agent" }).click();
+  const dialog = page.getByTestId("persona-dialog");
+  await expect(dialog).toBeVisible({ timeout: 10_000 });
+  await dialog.locator("#agent-run-on").selectOption(PROVIDER.id);
+  return dialog;
+}
+
+test("typing into a defaultless provider field sticks and probes only once", async ({
+  page,
+}) => {
+  await installMockBridge(page, {
+    backendProviders: [PROVIDER],
+    backendProviderProbeResult: PROBE_RESULT,
+  });
+  const dialog = await openCreateDialogOnProvider(page);
+
+  const contextField = dialog.locator("#provider-cfg-context");
+  await expect(contextField).toBeVisible({ timeout: 10_000 });
+  // Defaults prefilled from the schema; context has none.
+  await expect(dialog.locator("#provider-cfg-namespace")).toHaveValue(
+    "buzz-agents-mock01",
+  );
+  await expect(contextField).toHaveValue("");
+
+  await contextField.pressSequentially("prod-us-west", { delay: 20 });
+  await expect(contextField).toHaveValue("prod-us-west");
+
+  // One selection, one probe — keystrokes must not refire it.
+  expect(await probeInvocations(page)).toBe(1);
+});
+
+test("config fields render only after a slow probe resolves, with defaults", async ({
+  page,
+}) => {
+  // The fields are gated on the probe result (draft.probedProvider), which is
+  // what makes mid-flight typing unreachable through the UI — the stale-probe
+  // merge seam (applyProbeResult) is pinned at the unit level instead. This
+  // spec holds the gate: no half-rendered form before the probe lands, and
+  // defaults appear exactly once when it does.
+  await installMockBridge(page, {
+    backendProviders: [PROVIDER],
+    backendProviderProbeResult: PROBE_RESULT,
+    backendProviderProbeDelayMs: 1_000,
+  });
+  const dialog = await openCreateDialogOnProvider(page);
+
+  // Pre-resolution: the security warning is up, the form is not.
+  await expect(dialog.getByText("will receive your agent")).toBeVisible();
+  await expect(dialog.locator("#provider-cfg-context")).toHaveCount(0);
+
+  // Post-resolution: fields render with schema defaults prefilled.
+  await expect(dialog.locator("#provider-cfg-context")).toBeVisible({
+    timeout: 10_000,
+  });
+  await expect(dialog.locator("#provider-cfg-namespace")).toHaveValue(
+    "buzz-agents-mock01",
+  );
+  expect(await probeInvocations(page)).toBe(1);
+});
+
+test("provider → local → provider re-probes and resets the config", async ({
+  page,
+}) => {
+  await installMockBridge(page, {
+    backendProviders: [PROVIDER],
+    backendProviderProbeResult: PROBE_RESULT,
+  });
+  const dialog = await openCreateDialogOnProvider(page);
+
+  const contextField = dialog.locator("#provider-cfg-context");
+  await expect(contextField).toBeVisible({ timeout: 10_000 });
+  await contextField.fill("stale-value");
+
+  await dialog.locator("#agent-run-on").selectOption("local");
+  await expect(contextField).toHaveCount(0);
+
+  await dialog.locator("#agent-run-on").selectOption(PROVIDER.id);
+  await expect(dialog.locator("#provider-cfg-context")).toBeVisible({
+    timeout: 10_000,
+  });
+  // Fresh selection = fresh draft: the stale value must not leak back.
+  await expect(dialog.locator("#provider-cfg-context")).toHaveValue("");
+  expect(await probeInvocations(page)).toBe(2);
+});

desktop/tests/helpers/bridge.ts

@@ -524,6 +524,25 @@ type MockBridgeOptions = {
    * returning a catalog. Exercises the discovery-failure UI path.
    */
   discoverAgentModelsError?: string;
+  /**
+   * Providers returned by `discover_backend_providers`. Defaults to `[]`
+   * (the "Run on" section stays hidden). Setting this renders the remote
+   * backend selector in the create-agent dialog.
+   */
+  backendProviders?: Array<{ id: string; binaryPath: string }>;
+  /**
+   * Result returned by `probe_backend_provider`. Defaults to
+   * `{ ok: false, error: "mock: no providers available" }`.
+   */
+  backendProviderProbeResult?: Record<string, unknown>;
+  /**
+   * Delay (ms) applied to `probe_backend_provider` so a spec can assert the
+   * pre-resolution state (config fields stay probe-gated until the result
+   * lands). Typing while a probe is in flight is unreachable through the UI
+   * for the same reason; that merge path is pinned at the unit level
+   * (`applyProbeResult` in whereToRunIntent.test.mjs).
+   */
+  backendProviderProbeDelayMs?: number;
 };
 
 type BridgeOptions = {