diff --git a/lambda/qbo-lookup/index.ts b/lambda/qbo-lookup/index.ts
index 025d3b5..f3f3221 100644
--- a/lambda/qbo-lookup/index.ts
+++ b/lambda/qbo-lookup/index.ts
@@ -200,7 +200,7 @@ export const handler = async (event: BedrockActionEvent): Promise<BedrockActionR
     const vendors = await queryVendors(accessToken, secret.realmId, trade, name);
     return makeResponse(event, formatVendors(vendors));
   } catch (err) {
-    console.error('QBO lookup error:', err);
+    console.error('QBO lookup error:', (err as Error).message);
     return makeResponse(event, `QuickBooks lookup failed: ${(err as Error).message}`);
   }
 };
diff --git a/lambda/qbo-oauth/index.ts b/lambda/qbo-oauth/index.ts
index 498dd5e..67311be 100644
--- a/lambda/qbo-oauth/index.ts
+++ b/lambda/qbo-oauth/index.ts
@@ -19,26 +19,39 @@ const REVOKE_URL = 'https://developer.api.intuit.com/v2/oauth2/tokens/revoke';
 // Scopes needed for vendor queries
 const SCOPES = 'com.intuit.quickbooks.accounting';
 
+// Security headers applied to every response (Intuit requires no-cache, no-store)
+const SECURITY_HEADERS: Record<string, string> = {
+  'Cache-Control': 'no-cache, no-store',
+  'Pragma': 'no-cache',
+};
+
 interface APIGatewayEvent {
   requestContext: { http: { method: string; path: string } };
   queryStringParameters?: Record<string, string>;
   headers: Record<string, string>;
+  cookies?: string[];
 }
 
 interface APIGatewayResponse {
   statusCode: number;
   headers?: Record<string, string>;
+  cookies?: string[];
   body: string;
 }
 
-function redirect(url: string): APIGatewayResponse {
-  return { statusCode: 302, headers: { Location: url }, body: '' };
+function redirect(url: string, cookies?: string[]): APIGatewayResponse {
+  return {
+    statusCode: 302,
+    headers: { ...SECURITY_HEADERS, Location: url },
+    ...(cookies && { cookies }),
+    body: '',
+  };
 }
 
 function html(title: string, message: string): APIGatewayResponse {
   return {
     statusCode: 200,
-    headers: { 'Content-Type': 'text/html' },
+    headers: { ...SECURITY_HEADERS, 'Content-Type': 'text/html' },
     body: `<!DOCTYPE html>
 <html><head><meta charset="UTF-8"><meta name="viewport" content="width=device-width,initial-scale=1">
 <title>${title} — Sea Haven Industries</title>
@@ -52,10 +65,30 @@ function html(title: string, message: string): APIGatewayResponse {
   };
 }
 
+// ── Cookie helpers for CSRF state ────────────────────────────────────────────
+
+function parseCookies(cookieHeaders: string[] | undefined): Record<string, string> {
+  const cookies: Record<string, string> = {};
+  if (!cookieHeaders) return cookies;
+  for (const header of cookieHeaders) {
+    const [name, ...rest] = header.split('=');
+    if (name) cookies[name.trim()] = rest.join('=').trim();
+  }
+  return cookies;
+}
+
+function makeStateCookie(state: string): string {
+  // 10-minute expiry, Secure + HttpOnly per Intuit cookie requirements
+  return `qbo_oauth_state=${state}; Max-Age=600; Path=/qbo; Secure; HttpOnly; SameSite=Lax`;
+}
+
+function clearStateCookie(): string {
+  return 'qbo_oauth_state=; Max-Age=0; Path=/qbo; Secure; HttpOnly; SameSite=Lax';
+}
+
 // ── /qbo/connect — redirect to Intuit OAuth ──────────────────────────────────
 
 function handleConnect(): APIGatewayResponse {
-  // Generate a simple state parameter for CSRF protection
   const state = crypto.randomUUID();
 
   const params = new URLSearchParams({
@@ -66,18 +99,34 @@ function handleConnect(): APIGatewayResponse {
     state,
   });
 
-  return redirect(`${AUTHORIZE_URL}?${params.toString()}`);
+  return redirect(
+    `${AUTHORIZE_URL}?${params.toString()}`,
+    [makeStateCookie(state)],
+  );
 }
 
 // ── /qbo/callback — exchange code for tokens, store in Secrets Manager ───────
+// Per Intuit sensitive-info requirement: this endpoint receives tokens in URL
+// params, so it must NEVER return HTML — always 302 redirect.
 
 async function handleCallback(
   query: Record<string, string>,
+  cookies: Record<string, string>,
 ): Promise<APIGatewayResponse> {
-  const { code, realmId } = query;
+  const clearCookie = [clearStateCookie()];
+
+  // Validate CSRF state
+  const { state, code, realmId } = query;
+  const savedState = cookies['qbo_oauth_state'];
+
+  if (!state || !savedState || state !== savedState) {
+    console.error('OAuth callback: state mismatch');
+    return redirect('/qbo/launch?error=csrf', clearCookie);
+  }
 
   if (!code || !realmId) {
-    return html('Connection Failed', 'Missing authorization code or company ID from Intuit. Please try connecting again.');
+    console.error('OAuth callback: missing code or realmId');
+    return redirect('/qbo/launch?error=missing_params', clearCookie);
   }
 
   const credentials = Buffer.from(`${QBO_CLIENT_ID}:${QBO_CLIENT_SECRET}`).toString('base64');
@@ -97,9 +146,8 @@ async function handleCallback(
   });
 
   if (!tokenRes.ok) {
-    const err = await tokenRes.text();
-    console.error('Token exchange failed:', err);
-    return html('Connection Failed', 'Could not exchange authorization code for tokens. Please try again.');
+    console.error('OAuth callback: token exchange failed with status', tokenRes.status);
+    return redirect('/qbo/launch?error=token_exchange', clearCookie);
   }
 
   const tokens = (await tokenRes.json()) as {
@@ -122,8 +170,8 @@ async function handleCallback(
     }),
   );
 
-  console.log('QBO OAuth tokens stored successfully for realmId:', realmId);
-  return redirect('/qbo/launch');
+  console.log('QBO OAuth: tokens stored successfully');
+  return redirect('/qbo/launch', clearCookie);
 }
 
 // ── /qbo/disconnect — revoke token and clear secret ──────────────────────────
@@ -155,8 +203,8 @@ async function handleDisconnect(): Promise<APIGatewayResponse> {
         },
         body: JSON.stringify({ token: refreshToken }),
       });
-    } catch (err) {
-      console.error('Token revocation failed (non-fatal):', err);
+    } catch {
+      console.error('OAuth disconnect: token revocation failed (non-fatal)');
     }
 
     // Clear the stored secret
@@ -179,9 +227,20 @@ async function handleDisconnect(): Promise<APIGatewayResponse> {
   );
 }
 
-// ── /qbo/launch — success landing page ───────────────────────────────────────
+// ── /qbo/launch — success/error landing page ────────────────────────────────
+
+function handleLaunch(query: Record<string, string>): APIGatewayResponse {
+  const error = query['error'];
+
+  if (error) {
+    const messages: Record<string, string> = {
+      csrf: 'The connection request could not be verified. Please try again.',
+      missing_params: 'Missing authorization details from Intuit. Please try connecting again.',
+      token_exchange: 'Could not complete the connection to QuickBooks. Please try again.',
+    };
+    return html('Connection Failed', messages[error] ?? 'An unexpected error occurred. Please try again.');
+  }
 
-function handleLaunch(): APIGatewayResponse {
   return html(
     'Connected',
     'Your QuickBooks account is connected to Sea Haven Industries. You can close this window.',
@@ -193,17 +252,18 @@ function handleLaunch(): APIGatewayResponse {
 export const handler = async (event: APIGatewayEvent): Promise<APIGatewayResponse> => {
   const path = event.requestContext.http.path;
   const query = event.queryStringParameters ?? {};
+  const cookies = parseCookies(event.cookies);
 
   switch (path) {
     case '/qbo/connect':
       return handleConnect();
     case '/qbo/callback':
-      return handleCallback(query);
+      return handleCallback(query, cookies);
     case '/qbo/disconnect':
       return handleDisconnect();
     case '/qbo/launch':
-      return handleLaunch();
+      return handleLaunch(query);
     default:
-      return { statusCode: 404, body: 'Not found' };
+      return { statusCode: 404, headers: SECURITY_HEADERS, body: 'Not found' };
   }
 };