diff --git a/SampleWebAppDotNet48/Controllers/BillingController.cs b/SampleWebAppDotNet48/Controllers/BillingController.cs
index 755bd4b..c89ebfc 100644
--- a/SampleWebAppDotNet48/Controllers/BillingController.cs
+++ b/SampleWebAppDotNet48/Controllers/BillingController.cs
@@ -6,6 +6,7 @@
 using System.Net;
 using System.Threading.Tasks;
 using System.Web.Http;
+using Newtonsoft.Json;
 using SampleWebAppDotNet48.Helpers;
 
 using authapi.Api;
@@ -290,8 +291,8 @@ public async Task<IHttpActionResult> GetBillingDashboard(
             }
             catch (Exception ex)
             {
-                var handled = SaasusApiHelpers.HandleApiException(ex);
-                return Content(handled.Item1, handled.Item2);
+                System.Diagnostics.Debug.WriteLine($"Failed to get billing dashboard: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get billing dashboard" });
             }
         }
 
@@ -397,8 +398,8 @@ string Label(DateTime start, DateTime end) =>
             }
             catch (Exception ex)
             {
-                var handled = SaasusApiHelpers.HandleApiException(ex);
-                return Content(handled.Item1, handled.Item2);
+                System.Diagnostics.Debug.WriteLine($"Failed to get plan periods: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get plan periods" });
             }
         }
 
@@ -478,8 +479,8 @@ public async Task<IHttpActionResult> UpdateMeteringCount(
             }
             catch (Exception ex)
             {
-                var handled = SaasusApiHelpers.HandleApiException(ex);
-                return Content(handled.Item1, handled.Item2);
+                System.Diagnostics.Debug.WriteLine($"Failed to update metering count: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to update metering count" });
             }
         }
 
@@ -535,8 +536,194 @@ public async Task<IHttpActionResult> UpdateMeteringCountNow(
             }
             catch (Exception ex)
             {
-                var handled = SaasusApiHelpers.HandleApiException(ex);
-                return Content(handled.Item1, handled.Item2);
+                System.Diagnostics.Debug.WriteLine($"Failed to update metering count now: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to update metering count now" });
+            }
+        }
+
+        // ──────────────────────────── GET /pricing_plans ────────────────────────────
+        [HttpGet, Route("~/pricing_plans")]
+        public async Task<IHttpActionResult> GetPricingPlans()
+        {
+            try
+            {
+                // 1. 認証チェック
+                var authCfg = SaasusApiHelpers.CreateClientConfiguration(c => c.GetAuthApiClientConfig(), Request);
+                var userInfoApi = new UserInfoApi(authCfg);
+                var idToken = SaasusApiHelpers.GetBearerToken(Request);
+                if (string.IsNullOrEmpty(idToken))
+                    return Content(HttpStatusCode.Unauthorized, new { error = "Unauthorized" });
+                var userInfo = await userInfoApi.GetUserInfoAsync(idToken);
+
+                // 2. 料金プラン一覧を取得
+                var pricingCfg = SaasusApiHelpers.CreateClientConfiguration(c => c.GetPricingApiClientConfig(), Request);
+                var plansApi = new PricingPlansApi(pricingCfg);
+                var plans = await plansApi.GetPricingPlansAsync();
+
+                // JSON形式でレスポンスを返す
+                return Ok(plans.VarPricingPlans);
+            }
+            catch (Exception ex)
+            {
+                System.Diagnostics.Debug.WriteLine($"Failed to get pricing plans: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get pricing plans" });
+            }
+        }
+
+        // ──────────────────────────── GET /tax_rates ────────────────────────────
+        [HttpGet, Route("~/tax_rates")]
+        public async Task<IHttpActionResult> GetTaxRates()
+        {
+            try
+            {
+                // 1. 認証チェック
+                var authCfg = SaasusApiHelpers.CreateClientConfiguration(c => c.GetAuthApiClientConfig(), Request);
+                var userInfoApi = new UserInfoApi(authCfg);
+                var idToken = SaasusApiHelpers.GetBearerToken(Request);
+                if (string.IsNullOrEmpty(idToken))
+                    return Content(HttpStatusCode.Unauthorized, new { error = "Unauthorized" });
+                var userInfo = await userInfoApi.GetUserInfoAsync(idToken);
+
+                // 2. 税率一覧を取得
+                var pricingCfg = SaasusApiHelpers.CreateClientConfiguration(c => c.GetPricingApiClientConfig(), Request);
+                var taxRatesApi = new TaxRateApi(pricingCfg);
+                var taxRates = await taxRatesApi.GetTaxRatesAsync();
+
+                // JSON形式でレスポンスを返す
+                return Ok(taxRates.VarTaxRates);
+            }
+            catch (Exception ex)
+            {
+                System.Diagnostics.Debug.WriteLine($"Failed to get tax rates: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get tax rates" });
+            }
+        }
+
+        // ──────────────────────────── GET /tenants/{tenantId}/plan ────────────────────────────
+        [HttpGet, Route("~/tenants/{tenantId}/plan")]
+        public async Task<IHttpActionResult> GetTenantPlanInfo(string tenantId)
+        {
+            try
+            {
+                if (string.IsNullOrWhiteSpace(tenantId))
+                {
+                    return Content(HttpStatusCode.BadRequest, new { error = "tenant_id is required" });
+                }
+
+                // 1. 認証・認可チェック
+                var authCfg = SaasusApiHelpers.CreateClientConfiguration(c => c.GetAuthApiClientConfig(), Request);
+                var userInfoApi = new UserInfoApi(authCfg);
+                var idToken = SaasusApiHelpers.GetBearerToken(Request);
+                if (string.IsNullOrEmpty(idToken))
+                    return Content(HttpStatusCode.Unauthorized, new { error = "Unauthorized" });
+                var userInfo = await userInfoApi.GetUserInfoAsync(idToken);
+
+                if (!HasBillingAccess(userInfo, tenantId))
+                {
+                    return Content(HttpStatusCode.Forbidden, new { error = "Insufficient permissions" });
+                }
+
+                // 2. テナント詳細情報を取得
+                var tenantApi = new TenantApi(authCfg);
+                var tenant = await tenantApi.GetTenantAsync(tenantId);
+
+                // 3. 現在のプランの税率情報を取得（プラン履歴の最新エントリから）
+                string currentTaxRateId = null;
+                if (tenant.PlanHistories != null && tenant.PlanHistories.Any())
+                {
+                    var latestPlanHistory = tenant.PlanHistories.Last();
+                    if (!string.IsNullOrEmpty(latestPlanHistory.TaxRateId))
+                    {
+                        currentTaxRateId = latestPlanHistory.TaxRateId;
+                    }
+                }
+
+                // 4. レスポンスを構築
+                object planReservation = null;
+                if (tenant.UsingNextPlanFrom > 0)
+                {
+                    planReservation = new
+                    {
+                        next_plan_id = tenant.NextPlanId,
+                        using_next_plan_from = tenant.UsingNextPlanFrom,
+                        next_plan_tax_rate_id = tenant.NextPlanTaxRateId
+                    };
+                }
+
+                var response = new
+                {
+                    id = tenant.Id,
+                    name = tenant.Name,
+                    plan_id = tenant.PlanId,
+                    tax_rate_id = currentTaxRateId,
+                    plan_reservation = planReservation
+                };
+
+                return Ok(response);
+            }
+            catch (Exception ex)
+            {
+                System.Diagnostics.Debug.WriteLine($"Failed to retrieve tenant detail: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to retrieve tenant detail" });
+            }
+        }
+
+        // ──────────────────────────── PUT /tenants/{tenantId}/plan ────────────────────────────
+        [HttpPut, Route("~/tenants/{tenantId}/plan")]
+        public async Task<IHttpActionResult> UpdateTenantPlan(
+            string tenantId,
+            [FromBody] UpdateTenantPlanRequest request)
+        {
+            try
+            {
+                if (string.IsNullOrWhiteSpace(tenantId))
+                {
+                    return Content(HttpStatusCode.BadRequest, new { error = "tenant_id is required" });
+                }
+
+                // 1. 認証・認可チェック
+                var authCfg = SaasusApiHelpers.CreateClientConfiguration(c => c.GetAuthApiClientConfig(), Request);
+                var userInfoApi = new UserInfoApi(authCfg);
+                var idToken = SaasusApiHelpers.GetBearerToken(Request);
+                if (string.IsNullOrEmpty(idToken))
+                    return Content(HttpStatusCode.Unauthorized, new { error = "Unauthorized" });
+                var userInfo = await userInfoApi.GetUserInfoAsync(idToken);
+
+                if (!HasBillingAccess(userInfo, tenantId))
+                {
+                    return Content(HttpStatusCode.Forbidden, new { error = "Insufficient permissions" });
+                }
+
+                // 2. テナントプランを更新
+                var tenantApi = new TenantApi(authCfg);
+                var updateParam = new authapi.Model.PlanReservation();
+
+                // NextPlanIdが指定されている場合のみ設定（プラン解除時は空文字列またはnull）
+                if (!string.IsNullOrEmpty(request.NextPlanId))
+                {
+                    updateParam.NextPlanId = request.NextPlanId;
+                }
+
+                // 税率IDが指定されている場合のみ設定
+                if (!string.IsNullOrEmpty(request.TaxRateId))
+                {
+                    updateParam.NextPlanTaxRateId = request.TaxRateId;
+                }
+
+                // using_next_plan_fromが指定されている場合のみ設定
+                if (request.UsingNextPlanFrom.HasValue && request.UsingNextPlanFrom.Value > 0)
+                {
+                    updateParam.UsingNextPlanFrom = (int)request.UsingNextPlanFrom.Value;
+                }
+
+                await tenantApi.UpdateTenantPlanAsync(tenantId, updateParam);
+
+                return Ok(new { message = "Tenant plan updated successfully" });
+            }
+            catch (Exception ex)
+            {
+                System.Diagnostics.Debug.WriteLine($"Failed to update tenant plan: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to update tenant plan" });
             }
         }
     }
@@ -552,4 +739,19 @@ public sealed class UpdateMeteringCountRequest
         [Range(0, int.MaxValue)]
         public int Count { get; set; }
     }
+
+    /// <summary>
+    /// テナントプラン更新リクエスト
+    /// </summary>
+    public class UpdateTenantPlanRequest
+    {
+        [JsonProperty("next_plan_id")]
+        public string NextPlanId { get; set; }
+
+        [JsonProperty("tax_rate_id")]
+        public string TaxRateId { get; set; }
+
+        [JsonProperty("using_next_plan_from")]
+        public long? UsingNextPlanFrom { get; set; }
+    }
 }
\ No newline at end of file
diff --git a/SampleWebAppDotNet48/Controllers/MainController.cs b/SampleWebAppDotNet48/Controllers/MainController.cs
index 84c27ec..e6ab211 100644
--- a/SampleWebAppDotNet48/Controllers/MainController.cs
+++ b/SampleWebAppDotNet48/Controllers/MainController.cs
@@ -47,8 +47,8 @@ public async Task<IHttpActionResult> GetCredentials(string code)
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to get credentials: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get credentials" });
             }
         }
 
@@ -73,8 +73,8 @@ public async Task<IHttpActionResult> Refresh()
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to refresh credentials: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to refresh credentials" });
             }
         }
 
@@ -94,8 +94,8 @@ public async Task<IHttpActionResult> GetUserInfo()
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to get user info: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get user info" });
             }
         }
 
@@ -143,8 +143,8 @@ public async Task<IHttpActionResult> GetUsers(string tenant_id)
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to get users: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get users" });
             }
         }
 
@@ -207,8 +207,8 @@ public async Task<IHttpActionResult> GetTenantAttributes(string tenant_id)
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to get tenant attributes: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get tenant attributes" });
             }
         }
 
@@ -228,8 +228,8 @@ public async Task<IHttpActionResult> GetUserAttributes()
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to get user attributes: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get user attributes" });
             }
         }
 
@@ -262,8 +262,8 @@ public async Task<IHttpActionResult> GetPricingPlan(string plan_id)
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to get pricing plan: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get pricing plan" });
             }
         }
 
@@ -354,8 +354,8 @@ public async Task<IHttpActionResult> RegisterUser([FromBody] UserRegisterRequest
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to register user: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to register user" });
             }
         }
 
@@ -416,8 +416,8 @@ public async Task<IHttpActionResult> DeleteUser([FromBody] UserDeleteRequest req
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to delete user: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to delete user" });
             }
         }
 
@@ -471,8 +471,8 @@ public async Task<IHttpActionResult> GetDeleteUserLogs(string tenant_id)
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to get delete user log: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get delete user log" });
             }
         }
 
@@ -492,8 +492,8 @@ public async Task<IHttpActionResult> GetTenantAttributes()
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to get tenant attributes list: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get tenant attributes list" });
             }
         }
 
@@ -595,8 +595,8 @@ public async Task<IHttpActionResult> SelfSignUp([FromBody] SelfSignUpRequest req
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to self sign up: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to self sign up" });
             }
         }
 
@@ -655,8 +655,8 @@ public async Task<IHttpActionResult> GetInvitations(string tenant_id)
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to get invitations: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get invitations" });
             }
         }
 
@@ -703,8 +703,8 @@ public async Task<IHttpActionResult> UserInvitation([FromBody] UserInvitationReq
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to invite user: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to invite user" });
             }
         }
 
@@ -734,8 +734,8 @@ public async Task<IHttpActionResult> GetMfaStatus()
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to get MFA status: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to get MFA status" });
             }
         }
 
@@ -776,8 +776,8 @@ public async Task<IHttpActionResult> SetupMfa()
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to setup MFA: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to setup MFA" });
             }
         }
 
@@ -814,8 +814,8 @@ await saasUserApi.UpdateSoftwareTokenAsync(
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to verify MFA: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to verify MFA" });
             }
         }
 
@@ -839,8 +839,8 @@ public async Task<IHttpActionResult> EnableMfa()
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to enable MFA: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to enable MFA" });
             }
         }
 
@@ -864,8 +864,8 @@ public async Task<IHttpActionResult> DisableMfa()
             }
             catch (Exception ex)
             {
-                var (status, body) = SaasusApiHelpers.HandleApiException(ex);
-                return Content(status, body);
+                System.Diagnostics.Debug.WriteLine($"Failed to disable MFA: {ex.Message}");
+                return Content(HttpStatusCode.InternalServerError, new { error = "Failed to disable MFA" });
             }
         }
 
diff --git a/SampleWebAppDotNet48/Helpers/SaasusApiHelpers.cs b/SampleWebAppDotNet48/Helpers/SaasusApiHelpers.cs
index f03e3ef..4d7293e 100644
--- a/SampleWebAppDotNet48/Helpers/SaasusApiHelpers.cs
+++ b/SampleWebAppDotNet48/Helpers/SaasusApiHelpers.cs
@@ -67,16 +67,16 @@ public static (HttpStatusCode, object) HandleApiException(Exception ex)
       {
         case authapi.Client.ApiException a:
           Debug.WriteLine($"Auth API Error: {a.ErrorCode} - {a.Message}");
-          return ((HttpStatusCode)a.ErrorCode, new { error = a.Message });
+          return ((HttpStatusCode)a.ErrorCode, new { detail = a.ErrorContent ?? a.Message });
 
         case pricingapi.Client.ApiException p:
           Debug.WriteLine($"Pricing API Error: {p.ErrorCode} - {p.Message}");
-          return ((HttpStatusCode)p.ErrorCode, new { error = p.Message });
+          return ((HttpStatusCode)p.ErrorCode, new { detail = p.ErrorContent ?? p.Message });
 
         default:
           Debug.WriteLine($"Unhandled Error: {ex.Message}");
-          return (HttpStatusCode.InternalServerError, new { error = ex.Message });
+          return (HttpStatusCode.InternalServerError, new { detail = ex.Message });
       }
     }
   }
-}
+}
\ No newline at end of file
diff --git a/SampleWebAppDotNet8/Controllers/BillingController.cs b/SampleWebAppDotNet8/Controllers/BillingController.cs
index 61655d8..3b06beb 100644
--- a/SampleWebAppDotNet8/Controllers/BillingController.cs
+++ b/SampleWebAppDotNet8/Controllers/BillingController.cs
@@ -1,4 +1,6 @@
 using Microsoft.AspNetCore.Mvc;
+using Newtonsoft.Json;
+using System.Text.Json.Serialization;
 using System.Collections.Concurrent;
 using System.ComponentModel.DataAnnotations;
 using SampleWebAppDotNet8.Helpers;
@@ -334,7 +336,8 @@ public async Task<IActionResult> GetBillingDashboard(
       }
       catch (Exception ex)
       {
-        return SaasusApiHelpers.HandleApiExceptionMvc(ex, this);
+        _logger.LogError(ex, "Failed to get billing dashboard");
+        return StatusCode(500, new { error = "Failed to get billing dashboard" });
       }
     }
 
@@ -448,7 +451,8 @@ public async Task<IActionResult> GetPlanPeriods([FromQuery(Name = "tenant_id")]
       }
       catch (Exception ex)
       {
-        return SaasusApiHelpers.HandleApiExceptionMvc(ex, this);
+        _logger.LogError(ex, "Failed to get plan periods");
+        return StatusCode(500, new { error = "Failed to get plan periods" });
       }
     }
 
@@ -548,7 +552,8 @@ public async Task<IActionResult> UpdateMeteringCount(
       }
       catch (Exception ex)
       {
-        return SaasusApiHelpers.HandleApiExceptionMvc(ex, this);
+        _logger.LogError(ex, "Failed to update metering count");
+        return StatusCode(500, new { error = "Failed to update metering count" });
       }
     }
 
@@ -609,7 +614,205 @@ public async Task<IActionResult> UpdateMeteringCountNow(
       }
       catch (Exception ex)
       {
-        return SaasusApiHelpers.HandleApiExceptionMvc(ex, this);
+        _logger.LogError(ex, "Failed to update metering count now");
+        return StatusCode(500, new { error = "Failed to update metering count now" });
+      }
+    }
+
+    /// <summary>
+    /// 料金プラン一覧取得
+    /// </summary>
+    [HttpGet("/pricing_plans")]
+    public async Task<IActionResult> GetPricingPlans()
+    {
+      try
+      {
+        // 1. 認証チェック
+        var authApiClientConfig = SaasusApiHelpers.CreateClientConfiguration(c => c.GetAuthApiClientConfig(), HttpContext);
+        var userInfoApi = new UserInfoApi(authApiClientConfig);
+        var idToken = SaasusApiHelpers.GetBearerToken(HttpContext);
+        if (string.IsNullOrEmpty(idToken))
+          return Unauthorized();
+        var userInfo = await userInfoApi.GetUserInfoAsync(idToken);
+
+        // 2. 料金プラン一覧を取得
+        var pricingConfig = SaasusApiHelpers.CreateClientConfiguration(c => c.GetPricingApiClientConfig(), HttpContext);
+        var plansApi = new PricingPlansApi(pricingConfig);
+        var plans = await plansApi.GetPricingPlansAsync();
+
+        // JSON形式でレスポンスを返す
+        var jsonResponse = JsonConvert.SerializeObject(plans.VarPricingPlans);
+        return Content(jsonResponse, "application/json");
+      }
+      catch (Exception ex)
+      {
+        _logger.LogError(ex, "Failed to get pricing plans");
+        return StatusCode(500, new { detail = "Internal server error" });
+      }
+    }
+
+    /// <summary>
+    /// 税率一覧取得
+    /// </summary>
+    [HttpGet("/tax_rates")]
+    public async Task<IActionResult> GetTaxRates()
+    {
+      try
+      {
+        // 1. 認証チェック
+        var authApiClientConfig = SaasusApiHelpers.CreateClientConfiguration(c => c.GetAuthApiClientConfig(), HttpContext);
+        var userInfoApi = new UserInfoApi(authApiClientConfig);
+        var idToken = SaasusApiHelpers.GetBearerToken(HttpContext);
+        if (string.IsNullOrEmpty(idToken))
+          return Unauthorized();
+        var userInfo = await userInfoApi.GetUserInfoAsync(idToken);
+
+        // 2. 税率一覧を取得
+        var pricingConfig = SaasusApiHelpers.CreateClientConfiguration(c => c.GetPricingApiClientConfig(), HttpContext);
+        var taxRatesApi = new TaxRateApi(pricingConfig);
+        var taxRates = await taxRatesApi.GetTaxRatesAsync();
+
+        // JSON形式でレスポンスを返す
+        var jsonResponse = JsonConvert.SerializeObject(taxRates.VarTaxRates);
+        return Content(jsonResponse, "application/json");
+      }
+      catch (Exception ex)
+      {
+        _logger.LogError(ex, "Failed to get tax rates");
+        return StatusCode(500, new { detail = "Internal server error" });
+      }
+    }
+
+    /// <summary>
+    /// テナントプラン情報取得
+    /// </summary>
+    [HttpGet("/tenants/{tenantId}/plan")]
+    public async Task<IActionResult> GetTenantPlanInfo([FromRoute] string tenantId)
+    {
+      try
+      {
+        if (string.IsNullOrWhiteSpace(tenantId))
+        {
+          return BadRequest(new { error = "tenant_id is required" });
+        }
+
+        // 1. 認証・認可チェック
+        var authApiClientConfig = SaasusApiHelpers.CreateClientConfiguration(c => c.GetAuthApiClientConfig(), HttpContext);
+        var userInfoApi = new UserInfoApi(authApiClientConfig);
+        var idToken = SaasusApiHelpers.GetBearerToken(HttpContext);
+        if (string.IsNullOrEmpty(idToken))
+          return Unauthorized();
+        var userInfo = await userInfoApi.GetUserInfoAsync(idToken);
+
+        if (!HasBillingAccess(userInfo, tenantId))
+        {
+          return StatusCode(403, new { error = "Insufficient permissions" });
+        }
+
+        // 2. テナント詳細情報を取得
+        var tenantApi = new TenantApi(authApiClientConfig);
+        var tenant = await tenantApi.GetTenantAsync(tenantId);
+
+        // 3. 現在のプランの税率情報を取得（プラン履歴の最新エントリから）
+        string? currentTaxRateId = null;
+        if (tenant.PlanHistories != null && tenant.PlanHistories.Any())
+        {
+          var latestPlanHistory = tenant.PlanHistories.Last();
+          if (!string.IsNullOrEmpty(latestPlanHistory.TaxRateId))
+          {
+            currentTaxRateId = latestPlanHistory.TaxRateId;
+          }
+        }
+
+        // 4. レスポンスを構築
+        var response = new Dictionary<string, object?>
+        {
+          ["id"] = tenant.Id,
+          ["name"] = tenant.Name,
+          ["plan_id"] = tenant.PlanId,
+          ["tax_rate_id"] = currentTaxRateId,
+          ["plan_reservation"] = null
+        };
+
+        // 5. 予約情報がある場合は追加
+        if (tenant.UsingNextPlanFrom > 0)
+        {
+          var planReservation = new Dictionary<string, object?>
+          {
+            ["next_plan_id"] = tenant.NextPlanId,
+            ["using_next_plan_from"] = tenant.UsingNextPlanFrom,
+            ["next_plan_tax_rate_id"] = tenant.NextPlanTaxRateId
+          };
+          response["plan_reservation"] = planReservation;
+        }
+
+        return Ok(response);
+      }
+      catch (Exception ex)
+      {
+        _logger.LogError(ex, "Failed to retrieve tenant detail");
+        return StatusCode(500, new { error = "Failed to retrieve tenant detail" });
+      }
+    }
+
+    /// <summary>
+    /// テナントプラン更新
+    /// </summary>
+    [HttpPut("/tenants/{tenantId}/plan")]
+    public async Task<IActionResult> UpdateTenantPlan(
+        [FromRoute] string tenantId,
+        [FromBody] UpdateTenantPlanRequest request)
+    {
+      try
+      {
+        if (string.IsNullOrWhiteSpace(tenantId))
+        {
+          return BadRequest(new { error = "tenant_id is required" });
+        }
+
+        // 1. 認証・認可チェック
+        var authApiClientConfig = SaasusApiHelpers.CreateClientConfiguration(c => c.GetAuthApiClientConfig(), HttpContext);
+        var userInfoApi = new UserInfoApi(authApiClientConfig);
+        var idToken = SaasusApiHelpers.GetBearerToken(HttpContext);
+        if (string.IsNullOrEmpty(idToken))
+          return Unauthorized();
+        var userInfo = await userInfoApi.GetUserInfoAsync(idToken);
+
+        if (!HasBillingAccess(userInfo, tenantId))
+        {
+          return StatusCode(403, new { error = "Insufficient permissions" });
+        }
+
+        // 2. テナントプランを更新
+        var tenantApi = new TenantApi(authApiClientConfig);
+        var updateParam = new authapi.Model.PlanReservation();
+
+        // NextPlanIdが指定されている場合のみ設定（プラン解除時は空文字列またはnull）
+        if (!string.IsNullOrEmpty(request.NextPlanId))
+        {
+          updateParam.NextPlanId = request.NextPlanId;
+        }
+
+        // 税率IDが指定されている場合のみ設定
+        if (!string.IsNullOrEmpty(request.TaxRateId))
+        {
+          updateParam.NextPlanTaxRateId = request.TaxRateId;
+        }
+
+        // using_next_plan_fromが指定されている場合のみ設定
+        if (request.UsingNextPlanFrom.HasValue && request.UsingNextPlanFrom.Value > 0)
+        {
+          updateParam.UsingNextPlanFrom = (int)request.UsingNextPlanFrom.Value;
+        }
+
+        await tenantApi.UpdateTenantPlanAsync(tenantId, updateParam);
+
+        return Ok(new { message = "Tenant plan updated successfully" });
+      }
+      catch (Exception ex)
+      {
+        _logger.LogError(ex, "Failed to update tenant plan");
+        return StatusCode(500, new { error = "Failed to update tenant plan" });
       }
     }
 
@@ -627,4 +830,19 @@ public class UpdateMeteringCountRequest
     [Range(0, int.MaxValue, ErrorMessage = "Count must be greater than or equal to 0")]
     public int Count { get; set; }
   }
+
+  /// <summary>
+  /// テナントプラン更新リクエスト
+  /// </summary>
+  public class UpdateTenantPlanRequest
+  {
+    [JsonPropertyName("next_plan_id")]
+    public string? NextPlanId { get; set; }
+
+    [JsonPropertyName("tax_rate_id")]
+    public string? TaxRateId { get; set; }
+
+    [JsonPropertyName("using_next_plan_from")]
+    public long? UsingNextPlanFrom { get; set; }
+  }
 }
\ No newline at end of file
diff --git a/SampleWebAppDotNet8/Helpers/SaasusApiHelpers.cs b/SampleWebAppDotNet8/Helpers/SaasusApiHelpers.cs
index 65c2bc0..dae78f2 100644
--- a/SampleWebAppDotNet8/Helpers/SaasusApiHelpers.cs
+++ b/SampleWebAppDotNet8/Helpers/SaasusApiHelpers.cs
@@ -43,37 +43,5 @@ public static T CreateClientConfiguration<T>(
       return null;
     }
 
-    /// <summary>
-    /// SaaSus SDK の ApiException → HTTP レスポンス変換
-    /// Minimal-API 用 IResult を返す版
-    /// </summary>
-    public static IResult HandleApiException(Exception ex)
-    {
-      return ex switch
-      {
-        authapi.Client.ApiException aex => Results.Problem(aex.Message,
-                                                              statusCode: (int)aex.ErrorCode),
-        pricingapi.Client.ApiException pex => Results.Problem(pex.Message,
-                                                              statusCode: (int)pex.ErrorCode),
-        /* 例外型を追加したいときは ↓ にケースを足すだけ */
-        // billingapi.Client.ApiException bex => Results.Problem(bex.Message, (int)bex.ErrorCode),
-
-        _ => Results.Problem(ex.Message, statusCode: 500)
-      };
-    }
-
-    public static IActionResult HandleApiExceptionMvc(Exception ex, ControllerBase ctrl)
-    {
-        return ex switch
-        {
-            authapi.Client.ApiException a =>
-                ctrl.Problem(detail: a.Message, statusCode: (int)a.ErrorCode),
-
-            pricingapi.Client.ApiException p =>
-                ctrl.Problem(detail: p.Message, statusCode: (int)p.ErrorCode),
-
-            _ => ctrl.Problem(detail: ex.Message, statusCode: 500)
-        };
-    }
   }
 }
diff --git a/SampleWebAppDotNet8/Program.cs b/SampleWebAppDotNet8/Program.cs
index a22549f..376fe09 100644
--- a/SampleWebAppDotNet8/Program.cs
+++ b/SampleWebAppDotNet8/Program.cs
@@ -129,7 +129,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to get credentials: {ex.Message}");
+                    return Results.Problem("Failed to get credentials", statusCode: 500);
                 }
             });
 
@@ -153,7 +154,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to refresh credentials: {ex.Message}");
+                    return Results.Problem("Failed to refresh credentials", statusCode: 500);
                 }
             });
 
@@ -177,7 +179,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to get user info: {ex.Message}");
+                    return Results.Problem("Failed to get user info", statusCode: 500);
                 }
             });
 
@@ -232,7 +235,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to get users: {ex.Message}");
+                    return Results.Problem("Failed to get users", statusCode: 500);
                 }
             });
 
@@ -300,7 +304,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to get tenant attributes: {ex.Message}");
+                    return Results.Problem("Failed to get tenant attributes", statusCode: 500);
                 }
             });
 
@@ -324,7 +329,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to get user attributes: {ex.Message}");
+                    return Results.Problem("Failed to get user attributes", statusCode: 500);
                 }
             });
 
@@ -353,7 +359,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to get pricing plan: {ex.Message}");
+                    return Results.Problem("Failed to get pricing plan", statusCode: 500);
                 }
             });
 
@@ -444,8 +451,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    Console.Error.WriteLine($"Error: {ex.Message}");
-                    return Results.Problem(detail: ex.Message, statusCode: 500);
+                    Console.WriteLine($"Failed to register user: {ex.Message}");
+                    return Results.Problem("Failed to register user", statusCode: 500);
                 }
             });
 
@@ -508,7 +515,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to delete user: {ex.Message}");
+                    return Results.Problem("Failed to delete user", statusCode: 500);
                 }
             });
 
@@ -565,8 +573,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    Console.Error.WriteLine($"Error: {ex.Message}");
-                    return Results.Problem(detail: ex.Message, statusCode: 500);
+                    Console.WriteLine($"Failed to get delete user log: {ex.Message}");
+                    return Results.Problem("Failed to get delete user log", statusCode: 500);
                 }
             });
 
@@ -590,7 +598,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to get tenant attributes list: {ex.Message}");
+                    return Results.Problem("Failed to get tenant attributes list", statusCode: 500);
                 }
             });
 
@@ -684,8 +693,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    Console.Error.WriteLine($"Error: {ex.Message}");
-                    return Results.Problem(detail: ex.Message, statusCode: 500);
+                    Console.WriteLine($"Failed to self sign up: {ex.Message}");
+                    return Results.Problem("Failed to self sign up", statusCode: 500);
                 }
             });
 
@@ -742,7 +751,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to get invitations: {ex.Message}");
+                    return Results.Problem("Failed to get invitations", statusCode: 500);
                 }
             });
 
@@ -803,8 +813,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    Console.Error.WriteLine($"Error: {ex.Message}");
-                    return Results.Problem(detail: ex.Message, statusCode: 500);
+                    Console.WriteLine($"Failed to invite user: {ex.Message}");
+                    return Results.Problem("Failed to invite user", statusCode: 500);
                 }
             });
 
@@ -833,7 +843,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to get MFA status: {ex.Message}");
+                    return Results.Problem("Failed to get MFA status", statusCode: 500);
                 }
             });
 
@@ -868,7 +879,8 @@ static object ConvertToExpectedType(object value, string expectedType)
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to setup MFA: {ex.Message}");
+                    return Results.Problem("Failed to setup MFA", statusCode: 500);
                 }
             });
 
@@ -902,7 +914,8 @@ await saasUserApi.UpdateSoftwareTokenAsync(
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to verify MFA: {ex.Message}");
+                    return Results.Problem("Failed to verify MFA", statusCode: 500);
                 }
             });
 
@@ -931,7 +944,8 @@ await saasUserApi.UpdateSoftwareTokenAsync(
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to enable MFA: {ex.Message}");
+                    return Results.Problem("Failed to enable MFA", statusCode: 500);
                 }
             });
 
@@ -960,7 +974,8 @@ await saasUserApi.UpdateSoftwareTokenAsync(
                 }
                 catch (Exception ex)
                 {
-                    return SaasusApiHelpers.HandleApiException(ex);
+                    Console.WriteLine($"Failed to disable MFA: {ex.Message}");
+                    return Results.Problem("Failed to disable MFA", statusCode: 500);
                 }
             });