diff --git a/apps/desktop/src/renderer/src/shell/desktop-shell.tsx b/apps/desktop/src/renderer/src/shell/desktop-shell.tsx
index b13f082a..b51ca143 100644
--- a/apps/desktop/src/renderer/src/shell/desktop-shell.tsx
+++ b/apps/desktop/src/renderer/src/shell/desktop-shell.tsx
@@ -79,6 +79,7 @@ export function DesktopShell({
   runtimeCues,
   attachmentSupport,
   agentCatalogs,
+  selectableHarnesses,
   accountModels,
   newSessionPreferredEfforts,
   newSessionPreferredBranches,
@@ -428,6 +429,7 @@ export function DesktopShell({
           runtimeCues={runtimeCues}
           attachmentSupport={attachmentSupport}
           agentCatalogs={agentCatalogs}
+          selectableHarnesses={selectableHarnesses}
           accountModels={accountModels}
           preferredEfforts={newSessionPreferredEfforts}
           preferredBranches={newSessionPreferredBranches}
diff --git a/packages/client/workbench/src/settings/providers/__tests__/model-options.test.ts b/packages/client/workbench/src/settings/providers/__tests__/model-options.test.ts
index 542f8aa2..2d7730cd 100644
--- a/packages/client/workbench/src/settings/providers/__tests__/model-options.test.ts
+++ b/packages/client/workbench/src/settings/providers/__tests__/model-options.test.ts
@@ -5,7 +5,11 @@ import { getProviderConfig } from '@linkcode/sdk';
 import { modelChoiceKey } from '@linkcode/ui';
 import { cleanup, renderHook } from '@testing-library/react';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
-import { accountModelOptions, useAccountModelOptions } from '../model-options';
+import {
+  accountModelOptions,
+  selectableHarnessKinds,
+  useAccountModelOptions,
+} from '../model-options';
 
 const { useDataMock } = vi.hoisted(() => ({ useDataMock: vi.fn() }));
 
@@ -127,3 +131,14 @@ describe('accountModelOptions', () => {
     expect(result.current).toEqual({});
   });
 });
+
+describe('selectableHarnessKinds', () => {
+  it('includes unconfigured harnesses by default and removes disabled ones', () => {
+    expect(selectableHarnessKinds({ opencode: { enabled: false } })).toEqual([
+      'claude-code',
+      'codex',
+      'pi',
+      'grok-build',
+    ]);
+  });
+});
diff --git a/packages/client/workbench/src/settings/providers/model-options.ts b/packages/client/workbench/src/settings/providers/model-options.ts
index d243dd10..f9d91fa3 100644
--- a/packages/client/workbench/src/settings/providers/model-options.ts
+++ b/packages/client/workbench/src/settings/providers/model-options.ts
@@ -38,6 +38,11 @@ export function accountModelOptions(
   return options;
 }
 
+/** Harnesses offered when creating a thread; missing config entries retain the enabled default. */
+export function selectableHarnessKinds(providers: ProvidersConfig): AgentKind[] {
+  return AgentKindSchema.options.filter((kind) => providers[kind]?.enabled ?? true);
+}
+
 /** `null` until both daemon-owned sources have loaded, so a picker never briefly offers a set that
  * is not actually available — or one the enabled list would have narrowed. */
 export function useAccountModelOptions(): Partial<Record<AgentKind, ModelOption[]>> | null {
diff --git a/packages/client/workbench/src/surface/workbench.tsx b/packages/client/workbench/src/surface/workbench.tsx
index 31347d12..74ac1e89 100644
--- a/packages/client/workbench/src/surface/workbench.tsx
+++ b/packages/client/workbench/src/surface/workbench.tsx
@@ -13,6 +13,7 @@ import { MessageIdSchema, workspaceKind } from '@linkcode/schema';
 import {
   archiveWorkspace,
   cancelTurn,
+  getProviderConfig,
   hostArtifact,
   hostWorkspaceFile,
   readWorkspaceFile,
@@ -57,8 +58,11 @@ import { RuntimeNewSessionBranchPicker } from '../git/new-session-branch-picker'
 import { WorkbenchCommandPalette } from '../palette/command-palette';
 import { openCommandPalette } from '../palette/store';
 import { useWorkbenchSdkClient } from '../runtime/provider';
-import { useMutation } from '../runtime/tayori';
-import { useAccountModelOptions } from '../settings/providers/model-options';
+import { useData, useMutation } from '../runtime/tayori';
+import {
+  selectableHarnessKinds,
+  useAccountModelOptions,
+} from '../settings/providers/model-options';
 import { RuntimeBranchStatus } from '../sidebar/branch-status';
 import { useSidebarGroupCollapseStore } from '../sidebar/collapse-store';
 import { useSidebarOrderStore } from '../sidebar/order-store';
@@ -242,6 +246,8 @@ function WorkbenchSessionSurface({
   const currentPlan: CurrentPlan | null = selectCurrentPlan(conversation);
   const { mentionItems, onMentionQueryChange } = useFileMentionSource();
   const accountModels = useAccountModelOptions();
+  const { data: providers } = useData(getProviderConfig, {});
+  const selectableHarnesses = providers === undefined ? null : selectableHarnessKinds(providers);
   const sdkClient = useWorkbenchSdkClient();
   const activeSessionId = sessions.activeId;
   // Announce observation of the focused session so the daemon replays buffered per-session state
@@ -660,6 +666,7 @@ function WorkbenchSessionSurface({
       newSessionWorkspaceId={newSessionWorkspaceId}
       onNewSessionWorkspaceChange={handleNewSessionWorkspaceChange}
       accountModels={accountModels}
+      selectableHarnesses={selectableHarnesses}
       agentCatalogs={agentCatalogs}
       newSessionPreferredEfforts={newSessionPreferredEfforts}
       newSessionPreferredBranches={newSessionPreferredBranches}
diff --git a/packages/presentation/ui/src/shell/__tests__/new-session-surface.test.tsx b/packages/presentation/ui/src/shell/__tests__/new-session-surface.test.tsx
index 1fba8111..b9e3b938 100644
--- a/packages/presentation/ui/src/shell/__tests__/new-session-surface.test.tsx
+++ b/packages/presentation/ui/src/shell/__tests__/new-session-surface.test.tsx
@@ -56,6 +56,7 @@ const RE_PI_WIDE = /Pi Wide/;
 const RE_HIGH_EFFORT = /High/;
 const RE_LOW_EFFORT = /Low/;
 const RE_GPT_56_SOL = /GPT-5.6-Sol/;
+const RE_HARNESS_CLAUDE_CODE_BUTTON = /Claude Code/;
 const RE_HARNESS_CLAUDE_CODE_MENU = /harness.*Claude Code/;
 const RE_MODEL_SONNET_5_MENU = /model.*Sonnet 5/;
 const RE_OPUS_5 = /Opus 5/;
@@ -291,6 +292,60 @@ describe('NewSessionSurface', () => {
     },
   );
 
+  it('omits disabled harnesses and falls back from a disabled remembered harness', async () => {
+    const user = userEvent.setup();
+    const onSubmit = vi.fn().mockResolvedValue(undefined);
+    render(
+      <NewSessionSurface
+        chatWorkspace={CHAT_WORKSPACE}
+        draft={{ initialHarness: 'opencode', initialWorkspaceId: CHAT_WORKSPACE.workspaceId }}
+        mentionItems={[]}
+        onMentionQueryChange={vi.fn()}
+        onRegisterWorkspace={vi.fn().mockResolvedValue(CHAT_WORKSPACE)}
+        onSubmit={onSubmit}
+        selectableHarnesses={['claude-code', 'codex']}
+        workspaces={[]}
+      />,
+    );
+
+    await user.click(screen.getByRole('button', { name: RE_HARNESS_CLAUDE_CODE_BUTTON }));
+    const harnessItem = await screen.findByRole('menuitem', {
+      name: RE_HARNESS_CLAUDE_CODE_MENU,
+    });
+    harnessItem.focus();
+    await user.keyboard('{ArrowRight}');
+    expect(await screen.findByRole('menuitemradio', { name: 'Claude Code' })).toBeTruthy();
+    expect(screen.getByRole('menuitemradio', { name: 'Codex' })).toBeTruthy();
+    expect(screen.queryByRole('menuitemradio', { name: 'OpenCode' })).toBeNull();
+    await user.keyboard('{ArrowLeft}{Escape}');
+
+    typeInComposer('use enabled harness');
+    await pressInComposer('Enter');
+    await waitFor(() =>
+      expect(onSubmit).toHaveBeenCalledWith(expect.objectContaining({ kind: 'claude-code' })),
+    );
+  });
+
+  it('blocks submission when every harness is disabled', async () => {
+    const onSubmit = vi.fn().mockResolvedValue(undefined);
+    render(
+      <NewSessionSurface
+        chatWorkspace={CHAT_WORKSPACE}
+        draft={{ initialHarness: 'opencode', initialWorkspaceId: CHAT_WORKSPACE.workspaceId }}
+        mentionItems={[]}
+        onMentionQueryChange={vi.fn()}
+        onRegisterWorkspace={vi.fn().mockResolvedValue(CHAT_WORKSPACE)}
+        onSubmit={onSubmit}
+        selectableHarnesses={[]}
+        workspaces={[]}
+      />,
+    );
+
+    typeInComposer('no harness can run this');
+    await pressInComposer('Enter');
+    expect(onSubmit).not.toHaveBeenCalled();
+  });
+
   it('keeps permission mode visible while the agent catalog is unavailable', () => {
     render(
       <NewSessionSurface
diff --git a/packages/presentation/ui/src/shell/new-session-surface.tsx b/packages/presentation/ui/src/shell/new-session-surface.tsx
index 0d0387e9..f97d9d8f 100644
--- a/packages/presentation/ui/src/shell/new-session-surface.tsx
+++ b/packages/presentation/ui/src/shell/new-session-surface.tsx
@@ -89,6 +89,8 @@ export interface NewSessionSurfaceProps {
   /** Frontend capability stub used until attachment support is advertised by sessions. */
   attachmentSupport?: AttachmentSupportByAgent;
   agentCatalogs?: AgentStartCatalogs;
+  /** Harnesses enabled for new threads; null while configuration is loading. */
+  selectableHarnesses?: AgentKind[] | null;
   /** The models each agent offers, from every account enabled for it. They lead the picker and their
    * head is the agent's default; the agent's own catalog follows for a run on its own login. */
   accountModels?: Readonly<Partial<Record<AgentKind, ModelOption[]>>> | null;
@@ -144,6 +146,7 @@ export function NewSessionSurface({
   runtimeCues,
   attachmentSupport,
   agentCatalogs,
+  selectableHarnesses,
   accountModels,
   preferredEfforts,
   preferredBranches,
@@ -159,7 +162,12 @@ export function NewSessionSurface({
   onPickAttachmentFiles,
 }: NewSessionSurfaceProps): React.ReactNode {
   const t = useTranslations('workbench.newSession');
-  const [harness, setHarness] = useState(draft.initialHarness);
+  const availableHarnesses =
+    selectableHarnesses === undefined ? SELECTABLE_HARNESSES : (selectableHarnesses ?? []);
+  const [preferredHarness, setPreferredHarness] = useState(draft.initialHarness);
+  const harness = availableHarnesses.includes(preferredHarness)
+    ? preferredHarness
+    : availableHarnesses.at(0);
   const [selectedModels, setSelectedModels] = useState<Partial<Record<AgentKind, string | null>>>(
     {},
   );
@@ -182,17 +190,20 @@ export function NewSessionSurface({
     ? (selectedBranches[selected.workspaceId] ?? preferredBranches?.[selected.workspaceId])
     : undefined;
   const branchMode = selectedBranch?.mode ?? 'local';
-  const catalog = agentCatalogs?.[harness];
-  const localModel = selectedModels[harness];
+  const catalog = harness === undefined ? undefined : agentCatalogs?.[harness];
+  const localModel = harness === undefined ? undefined : selectedModels[harness];
   const selectedModel = localModel === undefined ? null : localModel;
-  const localEffort = selectedEfforts[harness];
-  const effort = localEffort === undefined ? (preferredEfforts?.[harness] ?? null) : localEffort;
+  const localEffort = harness === undefined ? undefined : selectedEfforts[harness];
+  const effort =
+    localEffort === undefined && harness !== undefined
+      ? (preferredEfforts?.[harness] ?? null)
+      : (localEffort ?? null);
   // Every offered model comes from an account enabled for this agent, and its head is what an
   // untouched draft runs on — the same entry the daemon derives. The agent's own catalog is
   // deliberately absent: a model nobody enabled an account for is not on offer, so the account
   // switches govern this menu completely rather than sitting beside a list they cannot reach.
-  const pickable: ModelOption[] = accountModels?.[harness] ?? [];
-  const localAccount = selectedAccounts[harness];
+  const pickable: ModelOption[] = harness === undefined ? [] : (accountModels?.[harness] ?? []);
+  const localAccount = harness === undefined ? undefined : selectedAccounts[harness];
   // `null` is "the accounts have not loaded", so there is no head yet.
   const modelOption =
     selectedModel === null
@@ -223,7 +234,7 @@ export function NewSessionSurface({
   // Only a real pick travels to the adapter. Every catalog default — policy, model, effort — is a
   // display value: submitting one would read as an explicit choice and override the agent's own
   // startup resolution — claude's `permissions.defaultMode`, codex's configured `config.toml`.
-  const pickedPolicyId = selectedPolicies[harness];
+  const pickedPolicyId = harness === undefined ? undefined : selectedPolicies[harness];
   const currentPolicyId =
     pickedPolicyId ?? catalog?.defaultPolicyId ?? catalog?.policies[0]?.policyId;
   const approvalPolicy =
@@ -233,6 +244,7 @@ export function NewSessionSurface({
 
   async function submit(input: NewSessionSubmission['input']): Promise<void> {
     if (!selected) throw new Error('Cannot start a session without a workspace');
+    if (!harness) throw new Error('Cannot start a session without an enabled harness');
     setPending(true);
     try {
       await onSubmit({
@@ -270,11 +282,12 @@ export function NewSessionSurface({
   }
 
   function handleHarnessChange(nextHarness: AgentKind): Promise<void> {
-    setHarness(nextHarness);
+    setPreferredHarness(nextHarness);
     return Promise.resolve();
   }
 
   function handleModelChange(next: ModelOption): Promise<void> {
+    if (!harness) return Promise.resolve();
     setSelectedModels((current) => ({ ...current, [harness]: next.id }));
     // The account is part of the pick: two accounts can serve the same id, and the session must
     // start on the one whose entry was chosen.
@@ -283,16 +296,19 @@ export function NewSessionSurface({
   }
 
   function handleEffortChange(nextEffort: EffortLevel): Promise<void> {
+    if (!harness) return Promise.resolve();
     setSelectedEfforts((current) => ({ ...current, [harness]: nextEffort }));
     return Promise.resolve();
   }
 
   function handleResetModel(): void {
+    if (!harness) return;
     setSelectedModels((current) => ({ ...current, [harness]: null }));
     setSelectedAccounts((current) => ({ ...current, [harness]: null }));
   }
 
   function handleResetEffort(): void {
+    if (!harness) return;
     setSelectedEfforts((current) => ({ ...current, [harness]: null }));
   }
 
@@ -302,6 +318,7 @@ export function NewSessionSurface({
   }
 
   function handleApprovalPolicyChange(policyId: string): Promise<void> {
+    if (!harness) return Promise.resolve();
     setSelectedPolicies((current) => ({ ...current, [harness]: policyId }));
     return Promise.resolve();
   }
@@ -310,13 +327,13 @@ export function NewSessionSurface({
     selected && !isChatSelected
       ? t('headingIn', { name: selected.name ?? repositoryLabel(selected.cwd) })
       : t('heading');
-  const cue = runtimeCues?.[harness];
-  const capabilities = AGENT_INPUT_CAPABILITIES[harness];
+  const cue = harness === undefined ? undefined : runtimeCues?.[harness];
+  const capabilities = harness === undefined ? undefined : AGENT_INPUT_CAPABILITIES[harness];
   const directiveControls: ComposerDirectiveControls = {
-    slash: capabilities.slashCommands
+    slash: capabilities?.slashCommands
       ? { state: 'loading', onInvokeCommand: handleInvokeCommand }
       : { state: 'unsupported' },
-    shell: capabilities.shellCommand
+    shell: capabilities?.shellCommand
       ? { state: 'ready', onRunShellCommand: handleRunShellCommand }
       : { state: 'unsupported' },
   };
@@ -350,7 +367,7 @@ export function NewSessionSurface({
           <h1 className="px-4 pb-8 text-center font-semibold text-2xl text-foreground">
             {heading}
           </h1>
-          {cue && (
+          {cue && harness && (
             <div className="px-4 pb-3">
               <div className="mx-auto max-w-3xl">
                 <AgentOnboardingCard
@@ -364,9 +381,9 @@ export function NewSessionSurface({
             </div>
           )}
           <Composer
-            agentLabel={AGENT_LABELS[harness]}
+            agentLabel={harness === undefined ? undefined : AGENT_LABELS[harness]}
             agentKind={harness}
-            attachmentsSupported={Boolean(attachmentSupport?.[harness])}
+            attachmentsSupported={Boolean(harness && attachmentSupport?.[harness])}
             blockDirectivesWithAttachments
             disabled={pending || !selected}
             directiveControls={directiveControls}
@@ -374,10 +391,8 @@ export function NewSessionSurface({
             mentionItems={mentionItems}
             onMentionQueryChange={(query) => onMentionQueryChange(selected?.cwd, query)}
             runtimeCues={runtimeCues}
-            // Only the runtime gates sending. An empty list is not a refusal: an agent with no
-            // account still resolves its own model, and the one case the daemon does refuse — an
-            // account pinned with nothing picked — reports itself rather than greying the button.
-            sendBlocked={cue !== undefined}
+            // A missing runtime or enabled harness blocks sending; account/model failures report themselves.
+            sendBlocked={harness === undefined || cue !== undefined}
             currentModeId={modeId}
             currentModel={displayedModel}
             currentEffort={displayedEffort}
@@ -385,7 +400,7 @@ export function NewSessionSurface({
             currentAccountId={modelOption?.accountId}
             approvalPolicy={approvalPolicy}
             approvalPolicyPlaceholder={t('permissionMode')}
-            selectableHarnesses={SELECTABLE_HARNESSES}
+            selectableHarnesses={availableHarnesses}
             onSend={handleSend}
             onStop={noop}
             onPickAttachmentFiles={onPickAttachmentFiles}
diff --git a/packages/presentation/ui/src/shell/shell-frame.tsx b/packages/presentation/ui/src/shell/shell-frame.tsx
index 77d7ce3c..390a6839 100644
--- a/packages/presentation/ui/src/shell/shell-frame.tsx
+++ b/packages/presentation/ui/src/shell/shell-frame.tsx
@@ -56,6 +56,8 @@ export interface ShellFrameProps
   /** Frontend capability stub used until attachment support is advertised by sessions. */
   attachmentSupport?: AttachmentSupportByAgent;
   agentCatalogs?: AgentStartCatalogs;
+  /** Harnesses enabled for new threads; null while provider configuration is loading. */
+  selectableHarnesses: AgentKind[] | null;
   /** The models each agent may run on, pooled from the accounts enabled for it, in the order the
    * pickers offer them — the head is the agent's default. */
   accountModels: Readonly<Partial<Record<AgentKind, ModelOption[]>>> | null;
@@ -132,6 +134,7 @@ export function ShellFrame({
   runtimeCues,
   attachmentSupport,
   agentCatalogs,
+  selectableHarnesses,
   accountModels,
   newSessionPreferredEfforts,
   newSessionPreferredBranches,
@@ -223,6 +226,7 @@ export function ShellFrame({
             runtimeCues={runtimeCues}
             attachmentSupport={attachmentSupport}
             agentCatalogs={agentCatalogs}
+            selectableHarnesses={selectableHarnesses}
             accountModels={accountModels}
             preferredEfforts={newSessionPreferredEfforts}
             preferredBranches={newSessionPreferredBranches}