diff --git a/src/dotnet/QuantEngine.Infrastructure/Services/KisApiClient.cs b/src/dotnet/QuantEngine.Infrastructure/Services/KisApiClient.cs
index 678b3569..ed4227b5 100644
--- a/src/dotnet/QuantEngine.Infrastructure/Services/KisApiClient.cs
+++ b/src/dotnet/QuantEngine.Infrastructure/Services/KisApiClient.cs
@@ -1,5 +1,7 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.Concurrent;
+using System.Globalization;
 using System.Linq;
 using System.Net.Http;
 using System.Net.Http.Json;
@@ -29,6 +31,7 @@ public class KisApiClient : IKisApiClient
     private readonly HttpClient _httpClient;
     private readonly ITokenCache _tokenCache;
     private readonly ILogger<KisApiClient> _logger;
+    private static readonly ConcurrentDictionary<string, SemaphoreSlim> TokenLocks = new();
 
     public KisApiClient(HttpClient httpClient, ITokenCache tokenCache, ILogger<KisApiClient> logger)
     {
@@ -159,40 +162,54 @@ public class KisApiClient : IKisApiClient
 
     private async Task<string> GetOrRefreshTokenAsync(KisCredentials creds)
     {
-        var cachedToken = await _tokenCache.GetCachedTokenAsync(creds.Account);
-        if (!string.IsNullOrEmpty(cachedToken))
-            return cachedToken;
-
-        var tokenRequest = new { grant_type = "client_credentials", appkey = creds.AppKey, appsecret = creds.AppSecret };
-
+        var tokenLock = TokenLocks.GetOrAdd(creds.Account, _ => new SemaphoreSlim(1, 1));
+        await tokenLock.WaitAsync();
         try
         {
-            var response = await _httpClient.PostAsJsonAsync(
-                $"{creds.Domain}/oauth2/tokenP",
-                tokenRequest
-            );
-            response.EnsureSuccessStatusCode();
+            // Re-check after acquiring the account lock. Another request may
+            // have refreshed the shared cache while this request was waiting.
+            var cachedToken = await _tokenCache.GetCachedTokenAsync(creds.Account);
+            if (!string.IsNullOrEmpty(cachedToken))
+                return cachedToken;
 
-            var tokenData = await response.Content.ReadFromJsonAsync<Dictionary<string, object>>();
-            if (tokenData == null) throw new InvalidOperationException("Token response body is empty");
+            var tokenRequest = new { grant_type = "client_credentials", appkey = creds.AppKey, appsecret = creds.AppSecret };
 
-            if (!tokenData.TryGetValue("access_token", out var tokenObj) || tokenObj == null)
-                throw new InvalidOperationException("No access_token in response");
-            var accessToken = tokenObj.ToString()!;
+            try
+            {
+                var response = await _httpClient.PostAsJsonAsync(
+                    $"{creds.Domain}/oauth2/tokenP",
+                    tokenRequest
+                );
+                response.EnsureSuccessStatusCode();
 
-            var expiresInStr = tokenData.TryGetValue("expires_in", out var expiresObj) && expiresObj != null
-                ? expiresObj.ToString()
-                : "86400";
-            var expiresInSec = int.TryParse(expiresInStr, out var seconds) ? seconds : 86400;
-            var expiresAt = DateTime.UtcNow.AddSeconds(expiresInSec);
+                var tokenData = await response.Content.ReadFromJsonAsync<Dictionary<string, object>>();
+                if (tokenData == null) throw new InvalidOperationException("Token response body is empty");
 
-            await _tokenCache.SaveTokenAsync(creds.Account, accessToken, expiresAt);
-            return accessToken;
+                if (!tokenData.TryGetValue("access_token", out var tokenObj) || tokenObj == null)
+                    throw new InvalidOperationException("No access_token in response");
+                var accessToken = tokenObj.ToString()!;
+
+                var expiresInStr = tokenData.TryGetValue("expires_in", out var expiresObj) && expiresObj != null
+                    ? expiresObj.ToString()
+                    : "86400";
+                var expiresInSec = int.TryParse(expiresInStr, NumberStyles.Integer, CultureInfo.InvariantCulture, out var seconds)
+                    ? seconds
+                    : 86400;
+                var expiresAt = DateTime.UtcNow.AddSeconds(expiresInSec);
+
+                await _tokenCache.SaveTokenAsync(creds.Account, accessToken, expiresAt);
+                _logger.LogInformation("KIS token refreshed for {Account}; expires at {ExpiresAtUtc}", creds.Account, expiresAt);
+                return accessToken;
+            }
+            catch (Exception ex)
+            {
+                _logger.LogError(ex, "KIS token refresh failed");
+                throw new InvalidOperationException("KIS token refresh failed; check credentials and API availability.", ex);
+            }
         }
-        catch (Exception ex)
+        finally
         {
-            _logger.LogError(ex, "KIS token refresh failed");
-            throw new InvalidOperationException("KIS token refresh failed; check credentials and API availability.", ex);
+            tokenLock.Release();
         }
     }
 
diff --git a/src/dotnet/QuantEngine.Infrastructure/Services/PostgresTokenCache.cs b/src/dotnet/QuantEngine.Infrastructure/Services/PostgresTokenCache.cs
index 392eeb41..320acc9d 100644
--- a/src/dotnet/QuantEngine.Infrastructure/Services/PostgresTokenCache.cs
+++ b/src/dotnet/QuantEngine.Infrastructure/Services/PostgresTokenCache.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Globalization;
 using System.Threading.Tasks;
 using Dapper;
 using QuantEngine.Core.Interfaces;
@@ -31,7 +32,11 @@ namespace QuantEngine.Infrastructure.Services
             if (token == null)
                 return null;
 
-            var expiresAt = DateTime.Parse(token.ExpiresAt);
+            DateTime expiresAt;
+            if (!DateTime.TryParse((string)token.ExpiresAt, CultureInfo.InvariantCulture,
+                    DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal,
+                    out expiresAt))
+                return null;
             var now = DateTime.UtcNow;
             var refreshSkew = TimeSpan.FromMinutes(TokenRefreshSkewMinutes);