diff --git a/frontend/src/__tests__/client-store.test.ts b/frontend/src/__tests__/client-store.test.ts
new file mode 100644
index 00000000..138c1fc7
--- /dev/null
+++ b/frontend/src/__tests__/client-store.test.ts
@@ -0,0 +1,19 @@
+import { describe, it, expect, beforeAll } from 'vitest';
+import { readFileSync } from 'fs';
+import { join } from 'path';
+
+// Structural test: client-store uses model-preference sync at module scope,
+// which accesses localStorage. The `typeof window` guard prevents crashes
+// when the module is imported in a non-browser context (e.g. Vitest server tests).
+describe('client-store window guard', () => {
+  let source: string;
+
+  beforeAll(() => {
+    source = readFileSync(join(import.meta.dirname, '..', 'client-store.ts'), 'utf-8');
+  });
+
+  it('guards model preference sync with typeof window check', () => {
+    expect(source).toContain("typeof window !== 'undefined'");
+    expect(source).toContain('getPreferredModel()');
+  });
+});
diff --git a/frontend/src/client-store.ts b/frontend/src/client-store.ts
index 4dcfaffd..7b22781d 100644
--- a/frontend/src/client-store.ts
+++ b/frontend/src/client-store.ts
@@ -28,7 +28,9 @@ export const clientStore = createMitzoStore({
 });
 
 // Sync localStorage model preference into the store so sendMessage() includes it
-clientStore.getState().setModel(getPreferredModel());
+if (typeof window !== 'undefined') {
+  clientStore.getState().setModel(getPreferredModel());
+}
 
 // Wire Capacitor app lifecycle → force WS reconnect on resume, send suspend on background
 registerCapacitorLifecycle(
diff --git a/server/__tests__/chat.test.ts b/server/__tests__/chat.test.ts
index 1d3401e1..b4ee95ec 100644
--- a/server/__tests__/chat.test.ts
+++ b/server/__tests__/chat.test.ts
@@ -332,6 +332,44 @@ describe('startChat stores user message for resumed sessions', () => {
   });
 });
 
+// Structural tests: resume resolution requires the Agent SDK query() pipeline,
+// so we assert against the source rather than invoking startChat() directly.
+describe('resume resolves SDK session ID', () => {
+  let chatSource: string;
+
+  beforeAll(async () => {
+    const { readFileSync } = await import('fs');
+    const { join } = await import('path');
+    chatSource = readFileSync(join(import.meta.dirname, '..', 'chat.ts'), 'utf-8');
+  });
+
+  it('calls getSessionSdkId before passing resume to query()', () => {
+    expect(chatSource).toContain('getSessionSdkId(BASE_REPO, options.resume)');
+  });
+
+  it('guards BASE_REPO with ternary to avoid empty-string falsy bug', () => {
+    // Must use ternary (BASE_REPO ? ...) not && (which returns '' for empty string)
+    expect(chatSource).toContain('BASE_REPO ? getSessionSdkId(BASE_REPO');
+  });
+
+  it('falls back to raw options.resume when lookup returns undefined', () => {
+    expect(chatSource).toContain('?? options.resume');
+  });
+
+  it('resolvedResume is computed before the query() call', () => {
+    const resolveIdx = chatSource.indexOf('let resolvedResume');
+    const queryIdx = chatSource.indexOf('const q = query(');
+    expect(resolveIdx).toBeGreaterThan(-1);
+    expect(queryIdx).toBeGreaterThan(resolveIdx);
+  });
+
+  it('warns when REPO_PATH is unset during resume', () => {
+    expect(chatSource).toContain(
+      'REPO_PATH unset — resume will use raw worktree ID, SDK may reject it',
+    );
+  });
+});
+
 describe('isIsolationEnabled', () => {
   let originalEnv: string | undefined;
 
diff --git a/server/__tests__/session-index.test.ts b/server/__tests__/session-index.test.ts
index 4420532c..a278a291 100644
--- a/server/__tests__/session-index.test.ts
+++ b/server/__tests__/session-index.test.ts
@@ -8,6 +8,7 @@ import {
   registerSession,
   updateSessionTitle,
   updateSessionSdkId,
+  getSessionSdkId,
   finalizeCloseout,
 } from '../session-index.js';
 
@@ -305,4 +306,23 @@ describe('session-index', () => {
       expect(readIndex(repoPath)).toEqual([]);
     });
   });
+
+  describe('getSessionSdkId', () => {
+    it('returns the stored SDK session ID', () => {
+      upsertEntry(repoPath, { id: 'sess-1', status: 'active' });
+      updateSessionSdkId(repoPath, 'sess-1', 'f2fd42cf-5c9f-4524-be08-9b019c1cc3a2');
+
+      expect(getSessionSdkId(repoPath, 'sess-1')).toBe('f2fd42cf-5c9f-4524-be08-9b019c1cc3a2');
+    });
+
+    it('returns undefined when entry does not exist', () => {
+      expect(getSessionSdkId(repoPath, 'nonexistent')).toBeUndefined();
+    });
+
+    it('returns undefined when sdk_session_id is not set', () => {
+      upsertEntry(repoPath, { id: 'sess-1', status: 'active' });
+
+      expect(getSessionSdkId(repoPath, 'sess-1')).toBeUndefined();
+    });
+  });
 });
diff --git a/server/chat.ts b/server/chat.ts
index 819914f9..e970bbb0 100644
--- a/server/chat.ts
+++ b/server/chat.ts
@@ -63,7 +63,12 @@ export function setSessionChangeCallback(cb: SessionChangeCallback): void {
 import { EventStore } from './event-store.js';
 import { capturePromptComparison } from './prompt-compare.js';
 import { shouldAutoRename, extractRecentPrompts, generateSessionName } from './auto-rename.js';
-import { registerSession, updateSessionTitle, finalizeCloseout } from './session-index.js';
+import {
+  registerSession,
+  updateSessionTitle,
+  finalizeCloseout,
+  getSessionSdkId,
+} from './session-index.js';
 import { createLogger } from './logger.js';
 import { withSpan, withSpanAsync } from './tracing.js';
 
@@ -886,6 +891,16 @@ async function _startChatInner(
   })();
   capturePromptComparison(wtId, cwd, systemPromptAppend, repoWorktrees).catch(() => {});
 
+  // Resolve SDK session UUID for resume — worktree IDs are not valid SDK session IDs
+  let resolvedResume: string | undefined;
+  if (options.resume) {
+    if (!BASE_REPO) {
+      log.warn('REPO_PATH unset — resume will use raw worktree ID, SDK may reject it');
+    }
+    resolvedResume =
+      (BASE_REPO ? getSessionSdkId(BASE_REPO, options.resume) : undefined) ?? options.resume;
+  }
+
   try {
     const q = query({
       prompt: inputQueue as AsyncIterable<SDKUserMessage>,
@@ -904,7 +919,7 @@ async function _startChatInner(
         allowedTools: [...modeAllowed, ...mcpAllowed, ...extraTools],
         thinking: resolveThinking(options.model),
         ...(options.model ? { model: parseModelSpec(options.model).model } : {}),
-        ...(options.resume ? { resume: options.resume } : {}),
+        ...(resolvedResume ? { resume: resolvedResume } : {}),
         ...(Object.keys(allMcpServers).length > 0 ? { mcpServers: allMcpServers } : {}),
         ...(hooks ? { hooks } : {}),
         canUseTool: buildPermissionHandler(clientId, registry, {
diff --git a/server/session-index.ts b/server/session-index.ts
index 61083e09..d37cf140 100644
--- a/server/session-index.ts
+++ b/server/session-index.ts
@@ -155,3 +155,10 @@ export function updateSessionSdkId(repoPath: string, wtId: string, sdkSessionId:
 
   upsertEntry(repoPath, { id: wtId, sdk_session_id: sdkSessionId });
 }
+
+/** Look up the SDK session UUID stored for a worktree ID. */
+export function getSessionSdkId(repoPath: string, wtId: string): string | undefined {
+  const entries = readIndex(repoPath);
+  const entry = entries.find((e) => e.id === wtId);
+  return entry?.sdk_session_id;
+}