feat: support session-scoped model overrides

Model selection was acting like a provider-level preference.

That made resumed sessions drift back to a default or request-time model.

Users expect /models changes made inside a conversation to affect that session.

Store explicit session choices in app-owned ~/.cloudcli state.

This avoids editing provider transcripts or native provider config.

Resolve the effective model before launching each provider runtime.

Claude, Cursor, Codex, Gemini, and OpenCode now honor stored resume choices.

Expose a backend active-model change endpoint for existing sessions.

The models modal can now distinguish default changes from session overrides.

It also shows when a selected model will apply on the next response.

For Claude, stop probing active model state by resuming with a dummy prompt.

Read the indexed JSONL transcript from the end instead.

This preserves provider history while honoring /model stdout or model fields.

Add service tests for adapter delegation and resume-model precedence.

The tests keep cache state, override state, and requested fallback separate.

server/shared/utils.ts

@@ -22,9 +22,12 @@ import type {
   AnyRecord,
   ApiSuccessShape,
   AppErrorOptions,
+  LLMProvider,
   NormalizedMessage,
+  ProviderChangeActiveModelInput,
   ProviderCurrentActiveModel,
   ProviderModelsDefinition,
+  ProviderSessionActiveModelChange,
   ProviderSkillSource,
   WorkspacePathValidationResult,
 } from '@/shared/types.js';
@@ -434,6 +437,213 @@ export function buildDefaultProviderCurrentActiveModel(
   };
 }
 
+// ---------------------------
+//----------------- PROVIDER SESSION MODEL CHANGE UTILITIES ------------
+type ProviderSessionActiveModelChangeCacheEntry = ProviderSessionActiveModelChange & {
+  updatedAt: string;
+};
+
+type ProviderSessionActiveModelChangeCacheFile = {
+  version: number;
+  entries: Record<string, ProviderSessionActiveModelChangeCacheEntry>;
+};
+
+const PROVIDER_SESSION_ACTIVE_MODEL_CHANGE_CACHE_VERSION = 1;
+
+/**
+ * Resolves the backend-owned cache file used for session-scoped resume model
+ * overrides.
+ *
+ * The file lives under `~/.cloudcli` because these overrides are an application
+ * concern rather than a provider-native config file. Providers, routes, and
+ * runtime command launchers should all use this helper instead of re-creating
+ * the path so the storage location stays consistent.
+ */
+export function getProviderSessionActiveModelChangesPath(): string {
+  return path.join(os.homedir(), '.cloudcli', 'provider-session-active-model-changes.json');
+}
+
+const buildProviderSessionActiveModelChangeKey = (
+  provider: LLMProvider,
+  sessionId: string,
+): string => `${provider}:${sessionId}`;
+
+const isProviderSessionActiveModelChangeCacheEntry = (
+  value: unknown,
+): value is ProviderSessionActiveModelChangeCacheEntry => {
+  const record = readObjectRecord(value);
+  return Boolean(
+    record
+    && typeof record.provider === 'string'
+    && typeof record.sessionId === 'string'
+    && typeof record.supported === 'boolean'
+    && typeof record.changed === 'boolean'
+    && (typeof record.model === 'string' || record.model === null)
+    && typeof record.updatedAt === 'string',
+  );
+};
+
+const readProviderSessionActiveModelChangeCacheFile = async (
+  filePath: string,
+): Promise<ProviderSessionActiveModelChangeCacheFile> => {
+  try {
+    const raw = await readFile(filePath, 'utf8');
+    const parsed = readObjectRecord(JSON.parse(raw));
+    if (
+      !parsed
+      || parsed.version !== PROVIDER_SESSION_ACTIVE_MODEL_CHANGE_CACHE_VERSION
+      || !readObjectRecord(parsed.entries)
+    ) {
+      return {
+        version: PROVIDER_SESSION_ACTIVE_MODEL_CHANGE_CACHE_VERSION,
+        entries: {},
+      };
+    }
+
+    const entries = Object.fromEntries(
+      Object.entries(parsed.entries).filter((entry): entry is [string, ProviderSessionActiveModelChangeCacheEntry] =>
+        isProviderSessionActiveModelChangeCacheEntry(entry[1]),
+      ),
+    );
+
+    return {
+      version: PROVIDER_SESSION_ACTIVE_MODEL_CHANGE_CACHE_VERSION,
+      entries,
+    };
+  } catch {
+    return {
+      version: PROVIDER_SESSION_ACTIVE_MODEL_CHANGE_CACHE_VERSION,
+      entries: {},
+    };
+  }
+};
+
+const writeProviderSessionActiveModelChangeCacheFile = async (
+  filePath: string,
+  payload: ProviderSessionActiveModelChangeCacheFile,
+): Promise<void> => {
+  await mkdir(path.dirname(filePath), { recursive: true });
+  await writeFile(filePath, `${JSON.stringify(payload, null, 2)}\n`, 'utf8');
+};
+
+const buildUnsupportedProviderSessionActiveModelChange = (
+  provider: LLMProvider,
+  sessionId: string,
+): ProviderSessionActiveModelChange => ({
+  provider,
+  sessionId,
+  supported: false,
+  changed: false,
+  model: null,
+});
+
+/**
+ * Reads the persisted session model-change state for one provider session.
+ *
+ * Runtime resume paths use this to decide whether they should inject a
+ * provider-specific model argument/thread option for the next resumed turn.
+ * Missing cache entries are normalized to `{ changed: false }` so callers can
+ * treat absence as "use the ordinary model selection flow".
+ */
+export async function readProviderSessionActiveModelChange(
+  provider: LLMProvider,
+  sessionId: string,
+  options: {
+    filePath?: string;
+    supported?: boolean;
+  } = {},
+): Promise<ProviderSessionActiveModelChange> {
+  const normalizedSessionId = sessionId.trim();
+  if (!normalizedSessionId) {
+    return buildUnsupportedProviderSessionActiveModelChange(provider, normalizedSessionId);
+  }
+
+  const supported = options.supported ?? true;
+  if (!supported) {
+    return buildUnsupportedProviderSessionActiveModelChange(provider, normalizedSessionId);
+  }
+
+  const filePath = options.filePath ?? getProviderSessionActiveModelChangesPath();
+  const cacheFile = await readProviderSessionActiveModelChangeCacheFile(filePath);
+  const cacheEntry = cacheFile.entries[
+    buildProviderSessionActiveModelChangeKey(provider, normalizedSessionId)
+  ];
+
+  if (!cacheEntry || !cacheEntry.changed || !cacheEntry.model?.trim()) {
+    return {
+      provider,
+      sessionId: normalizedSessionId,
+      supported: true,
+      changed: false,
+      model: null,
+    };
+  }
+
+  return {
+    provider,
+    sessionId: normalizedSessionId,
+    supported: true,
+    changed: true,
+    model: cacheEntry.model.trim(),
+  };
+}
+
+/**
+ * Persists a session model-change request for one provider.
+ *
+ * Provider adapters call this when the frontend explicitly selects a different
+ * model for an existing session. The stored `changed: true` flag is the single
+ * source of truth used later by resume paths to decide whether they should add
+ * a provider-native model override on the next invocation.
+ */
+export async function writeProviderSessionActiveModelChange(
+  provider: LLMProvider,
+  input: ProviderChangeActiveModelInput,
+  options: {
+    filePath?: string;
+    supported?: boolean;
+  } = {},
+): Promise<ProviderSessionActiveModelChange> {
+  const normalizedSessionId = input.sessionId.trim();
+  const normalizedModel = input.model.trim();
+  const supported = options.supported ?? true;
+
+  if (!supported) {
+    return buildUnsupportedProviderSessionActiveModelChange(provider, normalizedSessionId);
+  }
+
+  if (!normalizedSessionId || !normalizedModel) {
+    return {
+      provider,
+      sessionId: normalizedSessionId,
+      supported: true,
+      changed: false,
+      model: null,
+    };
+  }
+
+  const filePath = options.filePath ?? getProviderSessionActiveModelChangesPath();
+  const cacheFile = await readProviderSessionActiveModelChangeCacheFile(filePath);
+  cacheFile.entries[buildProviderSessionActiveModelChangeKey(provider, normalizedSessionId)] = {
+    provider,
+    sessionId: normalizedSessionId,
+    supported: true,
+    changed: true,
+    model: normalizedModel,
+    updatedAt: new Date().toISOString(),
+  };
+
+  await writeProviderSessionActiveModelChangeCacheFile(filePath, cacheFile);
+
+  return {
+    provider,
+    sessionId: normalizedSessionId,
+    supported: true,
+    changed: true,
+    model: normalizedModel,
+  };
+}
+
 // ---------------------------
 //----------------- WEBSOCKET PAYLOAD PARSING UTILITIES ------------
 /**