chat: stop reassigning a live session to fable

three defects, one shape: the code has two names for "which session is this",
and every guard checked the one no caller uses.

`fable` is the FIRST entry in the claude-code catalogue, so it is what
ModelSelector picks whenever it decides a conversation needs a default. On a
key without fable entitlement that lands as a 429, after a real mid-session
switch — the reported header read OPUSPLAN → FABLE.

1. the settings default was applied to RESUMED chats. the guard was
   `!initialSessionId`, and no chat caller passes that when resuming — the id
   arrives as `options.resumeSessionId`, and ChatPanelWrapper goes as far as
   `resume ? undefined : sessionKey`. so a user's default model was written
   over sessions that already had one. both names are checked now.

2. `hasStarted` was `!!preloadedMessages?.length`, which is FALSE for the
   first frames of a resumed chat — before its messages arrive. ModelSelector
   reads "not started" as permission to pick a default, so a reload reassigned
   live sessions during that window. a session id is known on the first render
   and a chat that has one is by definition not new.

3. harness lock. `crossesHarness` returned false when the current harness was
   unknown, which reads as "nothing crosses" and is the opposite of what is
   known — with no current harness, everything might. so a started session
   offered a switch to opencode while its model was still resolving, a switch
   that cannot work: it hands a `ses_…` id to `claude --resume` and dispatches
   to the wrong sidecar. unknown is not permission; the strip locks until the
   answer arrives.

the settings themselves were never wrong — chat.defaultModel reads
claude-code/opus on the affected server. nothing was ever reading it for the
sessions that got reassigned.

src/workspaces/officerdev/src/apps/Chat/components/ModelSelector.test.tsx

@@ -1,4 +1,4 @@
-import { describe, expect, test } from 'bun:test';
+import { describe, expect, mock, test } from 'bun:test';
 import { render, screen } from '@testing-library/react';
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
 import type { ModelOption } from 'state/useModels';
@@ -127,3 +127,30 @@ describe('ModelSelector display of an unknown model', () => {
     expect(screen.queryByTitle('claude-code/opus')).toBeNull();
   });
 });
+
+describe('a live session whose model has not resolved yet', () => {
+  // Reported 2026-08-19 from a fresh server: a chat running `opusplan` came back as `fable` after a
+  // reload, and the header showed a real mid-session switch (`OPUSPLAN → FABLE`) followed by a 429 on a
+  // key with no fable entitlement. `fable` is simply the FIRST entry in the claude-code catalogue, so it
+  // is whatever this component picks when it decides a conversation needs a default.
+  test('does not invent a model for a conversation that has started', () => {
+    const onModelChange = mock(() => {});
+    mount({ selectedModel: null, model: null, hasStarted: true, onModelChange });
+    // The old behaviour was `onModelChange('claude-code/fable')` — a confident wrong answer about a
+    // session it knows nothing about. An unset picker is the honest one.
+    expect(onModelChange).not.toHaveBeenCalled();
+  });
+
+  test('still picks a default for a composer that has NOT started', () => {
+    const onModelChange = mock(() => {});
+    mount({ selectedModel: null, model: null, hasStarted: false, onModelChange });
+    expect(onModelChange).toHaveBeenCalled();
+  });
+
+  // Unknown harness is not permission to switch harness: handing a `ses_…` id to `claude --resume`
+  // dispatches to the wrong sidecar, which is why this is pinned rather than left to read nicely.
+  test('offers no harness switch while the harness is unknown', () => {
+    mount({ selectedModel: null, model: null, hasStarted: true });
+    expect(screen.queryByRole('button', { name: /^openai$/i })).toBeNull();
+  });
+});

src/workspaces/officerdev/src/apps/Chat/components/ModelSelector.tsx

@@ -113,9 +113,22 @@ export function ModelSelector({
   const harnessOf = (id: string) => (id.startsWith('claude-code') ? 'claude-code' : 'opencode');
   const currentHarness = displayModel ? harnessOf(displayModel) : null;
 
+  /*
+    A started conversation whose model has NOT resolved yet.
+
+    `crossesHarness` used to return false here, which reads as "nothing crosses the harness" and is the
+    opposite of what is known: with no current harness, EVERY provider might cross it. So a live session
+    offered a switch to opencode during the window before its model arrived — a switch that cannot work,
+    because it hands a `ses_…` id to `claude --resume` and dispatches to the wrong sidecar.
+
+    Unknown is not permission. The strip locks until the answer arrives.
+  */
+  const harnessUnknown = hasStarted && !currentHarness;
+
   /** Would picking this provider move the conversation to the other sidecar? */
   const crossesHarness = (provider: string) => {
-    if (!hasStarted || !currentHarness) return false;
+    if (!hasStarted) return false;
+    if (!currentHarness) return true; // unknown — refuse rather than guess
     const target = availableModels.find((m) => m.provider === provider);
     return !!target && harnessOf(target.id) !== currentHarness;
   };
@@ -123,7 +136,7 @@ export function ModelSelector({
   // The provider strip still collapses to a label when there is nothing to choose between — but now that
   // means "one harness on offer", not "you have already sent a message".
   const selectableProviders = providers.filter((p) => !crossesHarness(p));
-  const providerStripLocked = isBusy || selectableProviders.length < 2;
+  const providerStripLocked = isBusy || harnessUnknown || selectableProviders.length < 2;
 
   const handleProviderClick = (provider: string) => {
     if (isBusy || crossesHarness(provider)) return;

src/workspaces/officerdev/src/hooks/useChat.ts

@@ -112,19 +112,42 @@ export function useChat(initialSessionId?: string, initialModel?: string | null,
   const cursorRef = useRef(0);
   const sendRef = useRef<(d: Record<string, unknown>) => void>(() => {});
 
-  // Track if session has started (first message sent)
-  const [hasStarted, setHasStarted] = useState(!!preloadedMessages?.length);
+  /*
+    Has this conversation already happened?
+
+    `!!preloadedMessages?.length` alone was wrong for the window that matters. A resumed chat renders
+    before its messages arrive, so for those first frames `hasStarted` was FALSE on a conversation with a
+    hundred turns behind it — and `ModelSelector` treats "not started" as permission to pick a default.
+    The catalogue's first entry is `fable`, so that is what a reload reassigned live sessions to, and the
+    change propagated to the running session as a real model switch (`OPUSPLAN → FABLE` in the header,
+    followed by a 429 on a key with no fable entitlement).
+
+    A session id is the earlier and more reliable signal: it is known on the first render, and a chat that
+    has one is by definition not new.
+  */
+  const [hasStarted, setHasStarted] = useState(!!preloadedMessages?.length || !!resumeSessionId || !!initialSessionId);
 
   // Get user settings for default model
   const { settings } = useSettings();
 
-  // Set default model from settings when starting a new chat (no initialSessionId, no initialModel)
+  /*
+    The settings default, for a genuinely NEW chat only.
+
+    This guarded on `initialSessionId`, which no chat caller ever passes when resuming — the id arrives as
+    `options.resumeSessionId`, and `ChatPanelWrapper` goes as far as `resume ? undefined : sessionKey`.
+    So the guard was true for every resumed conversation and the user's default was applied on top of a
+    session that already had a model of its own.
+
+    Both names mean "this conversation exists", so both are checked. They are separate parameters for
+    reasons that have nothing to do with this question.
+  */
+  const existingSessionId = initialSessionId ?? resumeSessionId;
+
   useEffect(() => {
-    if (!initialSessionId && !initialModel) {
+    if (existingSessionId || initialModel) return;
     const defaultModel = (projectScoped && settings?.chat?.defaultProjectModel) || settings?.chat?.defaultModel;
     if (defaultModel) setSelectedModel(defaultModel);
-    }
-  }, [initialSessionId, initialModel, settings, projectScoped]);
+  }, [existingSessionId, initialModel, settings, projectScoped]);
 
   // Sync selectedModel when initialModel changes (e.g. resuming a session)
   useEffect(() => {