diff --git a/apps/gateway/Gateway.API.Tests/Data/MigrationGateMiddlewareTests.cs b/apps/gateway/Gateway.API.Tests/Data/MigrationGateMiddlewareTests.cs
new file mode 100644
index 0000000..c8ae475
--- /dev/null
+++ b/apps/gateway/Gateway.API.Tests/Data/MigrationGateMiddlewareTests.cs
@@ -0,0 +1,143 @@
+// =============================================================================
+// <copyright file="MigrationGateMiddlewareTests.cs" company="Levelup Software">
+// Copyright (c) Levelup Software. All rights reserved.
+// </copyright>
+// =============================================================================
+
+namespace Gateway.API.Tests.Data;
+
+using Gateway.API.Data;
+using Microsoft.AspNetCore.Http;
+
+/// <summary>
+/// Tests for <see cref="MigrationGateMiddleware"/>.
+/// </summary>
+[NotInParallel("MigrationHealthCheck")]
+public class MigrationGateMiddlewareTests
+{
+    [Before(Test)]
+    public void Setup()
+    {
+        MigrationHealthCheck.Reset();
+    }
+
+    [After(Test)]
+    public void Cleanup()
+    {
+        MigrationHealthCheck.Reset();
+    }
+
+    [Test]
+    public async Task InvokeAsync_NotReady_Returns503()
+    {
+        // Arrange — register but don't complete
+        MigrationHealthCheck.RegisterExpected("TestContext");
+        var nextCalled = false;
+        var middleware = new MigrationGateMiddleware(_ =>
+        {
+            nextCalled = true;
+            return Task.CompletedTask;
+        });
+        var context = CreateHttpContext("/api/graphql");
+
+        // Act
+        await middleware.InvokeAsync(context);
+
+        // Assert
+        await Assert.That(context.Response.StatusCode).IsEqualTo(503);
+        await Assert.That(nextCalled).IsFalse();
+    }
+
+    [Test]
+    public async Task InvokeAsync_NotReady_SetsRetryAfterHeader()
+    {
+        MigrationHealthCheck.RegisterExpected("TestContext");
+        var middleware = new MigrationGateMiddleware(_ => Task.CompletedTask);
+        var context = CreateHttpContext("/api/graphql");
+
+        await middleware.InvokeAsync(context);
+
+        await Assert.That(context.Response.Headers["Retry-After"].ToString()).IsEqualTo("5");
+    }
+
+    [Test]
+    public async Task InvokeAsync_Ready_CallsNext()
+    {
+        // Arrange — complete migration
+        MigrationHealthCheck.RegisterExpected("TestContext");
+        MigrationHealthCheck.MarkComplete("TestContext");
+        var nextCalled = false;
+        var middleware = new MigrationGateMiddleware(_ =>
+        {
+            nextCalled = true;
+            return Task.CompletedTask;
+        });
+        var context = CreateHttpContext("/api/graphql");
+
+        // Act
+        await middleware.InvokeAsync(context);
+
+        // Assert
+        await Assert.That(nextCalled).IsTrue();
+    }
+
+    [Test]
+    public async Task InvokeAsync_NotReady_HealthEndpoint_CallsNext()
+    {
+        MigrationHealthCheck.RegisterExpected("TestContext");
+        var nextCalled = false;
+        var middleware = new MigrationGateMiddleware(_ =>
+        {
+            nextCalled = true;
+            return Task.CompletedTask;
+        });
+        var context = CreateHttpContext("/health");
+
+        await middleware.InvokeAsync(context);
+
+        await Assert.That(nextCalled).IsTrue();
+    }
+
+    [Test]
+    public async Task InvokeAsync_NotReady_AliveEndpoint_CallsNext()
+    {
+        MigrationHealthCheck.RegisterExpected("TestContext");
+        var nextCalled = false;
+        var middleware = new MigrationGateMiddleware(_ =>
+        {
+            nextCalled = true;
+            return Task.CompletedTask;
+        });
+        var context = CreateHttpContext("/alive");
+
+        await middleware.InvokeAsync(context);
+
+        await Assert.That(nextCalled).IsTrue();
+    }
+
+    [Test]
+    public async Task InvokeAsync_NoRegistrations_Returns503()
+    {
+        // No registrations means IsReady is false (Count == 0)
+        var nextCalled = false;
+        var middleware = new MigrationGateMiddleware(_ =>
+        {
+            nextCalled = true;
+            return Task.CompletedTask;
+        });
+        var context = CreateHttpContext("/api/data");
+
+        await middleware.InvokeAsync(context);
+
+        await Assert.That(context.Response.StatusCode).IsEqualTo(503);
+        await Assert.That(nextCalled).IsFalse();
+    }
+
+    private static DefaultHttpContext CreateHttpContext(string path)
+    {
+        var context = new DefaultHttpContext();
+        context.Request.Path = path;
+        context.Response.Body = new MemoryStream();
+        return context;
+    }
+}
diff --git a/apps/gateway/Gateway.API.Tests/Data/MigrationHealthCheckTests.cs b/apps/gateway/Gateway.API.Tests/Data/MigrationHealthCheckTests.cs
new file mode 100644
index 0000000..cd6a66a
--- /dev/null
+++ b/apps/gateway/Gateway.API.Tests/Data/MigrationHealthCheckTests.cs
@@ -0,0 +1,109 @@
+// =============================================================================
+// <copyright file="MigrationHealthCheckTests.cs" company="Levelup Software">
+// Copyright (c) Levelup Software. All rights reserved.
+// </copyright>
+// =============================================================================
+
+namespace Gateway.API.Tests.Data;
+
+using Gateway.API.Data;
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+
+/// <summary>
+/// Tests for <see cref="MigrationHealthCheck"/>.
+/// </summary>
+[NotInParallel("MigrationHealthCheck")]
+public class MigrationHealthCheckTests
+{
+    [Before(Test)]
+    public void Setup()
+    {
+        MigrationHealthCheck.Reset();
+    }
+
+    [After(Test)]
+    public void Cleanup()
+    {
+        MigrationHealthCheck.Reset();
+    }
+
+    [Test]
+    public async Task IsReady_NoRegistrations_ReturnsFalse()
+    {
+        await Assert.That(MigrationHealthCheck.IsReady).IsFalse();
+    }
+
+    [Test]
+    public async Task IsReady_RegisteredButNotComplete_ReturnsFalse()
+    {
+        MigrationHealthCheck.RegisterExpected("TestContext");
+
+        await Assert.That(MigrationHealthCheck.IsReady).IsFalse();
+    }
+
+    [Test]
+    public async Task IsReady_AllComplete_ReturnsTrue()
+    {
+        MigrationHealthCheck.RegisterExpected("TestContext");
+        MigrationHealthCheck.MarkComplete("TestContext");
+
+        await Assert.That(MigrationHealthCheck.IsReady).IsTrue();
+    }
+
+    [Test]
+    public async Task IsReady_MultipleContexts_OneIncomplete_ReturnsFalse()
+    {
+        MigrationHealthCheck.RegisterExpected("Context1");
+        MigrationHealthCheck.RegisterExpected("Context2");
+        MigrationHealthCheck.MarkComplete("Context1");
+
+        await Assert.That(MigrationHealthCheck.IsReady).IsFalse();
+    }
+
+    [Test]
+    public async Task IsReady_MultipleContexts_AllComplete_ReturnsTrue()
+    {
+        MigrationHealthCheck.RegisterExpected("Context1");
+        MigrationHealthCheck.RegisterExpected("Context2");
+        MigrationHealthCheck.MarkComplete("Context1");
+        MigrationHealthCheck.MarkComplete("Context2");
+
+        await Assert.That(MigrationHealthCheck.IsReady).IsTrue();
+    }
+
+    [Test]
+    public async Task CheckHealthAsync_Pending_ReturnsUnhealthy()
+    {
+        MigrationHealthCheck.RegisterExpected("TestContext");
+        var check = new MigrationHealthCheck();
+
+        var result = await check.CheckHealthAsync(new HealthCheckContext());
+
+        await Assert.That(result.Status).IsEqualTo(HealthStatus.Unhealthy);
+    }
+
+    [Test]
+    public async Task CheckHealthAsync_AllComplete_ReturnsHealthy()
+    {
+        MigrationHealthCheck.RegisterExpected("TestContext");
+        MigrationHealthCheck.MarkComplete("TestContext");
+        var check = new MigrationHealthCheck();
+
+        var result = await check.CheckHealthAsync(new HealthCheckContext());
+
+        await Assert.That(result.Status).IsEqualTo(HealthStatus.Healthy);
+    }
+
+    [Test]
+    public async Task Reset_ClearsAllState()
+    {
+        MigrationHealthCheck.RegisterExpected("TestContext");
+        MigrationHealthCheck.MarkComplete("TestContext");
+
+        await Assert.That(MigrationHealthCheck.IsReady).IsTrue();
+
+        MigrationHealthCheck.Reset();
+
+        await Assert.That(MigrationHealthCheck.IsReady).IsFalse();
+    }
+}
diff --git a/apps/gateway/Gateway.API.Tests/Data/PARequestDataSeederTests.cs b/apps/gateway/Gateway.API.Tests/Data/PARequestDataSeederTests.cs
new file mode 100644
index 0000000..8f52097
--- /dev/null
+++ b/apps/gateway/Gateway.API.Tests/Data/PARequestDataSeederTests.cs
@@ -0,0 +1,128 @@
+// =============================================================================
+// <copyright file="PARequestDataSeederTests.cs" company="Levelup Software">
+// Copyright (c) Levelup Software. All rights reserved.
+// </copyright>
+// =============================================================================
+
+namespace Gateway.API.Tests.Data;
+
+using Gateway.API.Data;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using NSubstitute;
+
+/// <summary>
+/// Tests for <see cref="PARequestDataSeeder"/>.
+/// </summary>
+public class PARequestDataSeederTests
+{
+    private GatewayDbContext CreateDbContext()
+    {
+        var options = new DbContextOptionsBuilder<GatewayDbContext>()
+            .UseInMemoryDatabase(Guid.NewGuid().ToString())
+            .Options;
+        return new GatewayDbContext(options);
+    }
+
+    [Test]
+    public async Task SeedAsync_EmptyDatabase_SeedsData()
+    {
+        // Arrange
+        using var context = CreateDbContext();
+        var logger = Substitute.For<ILogger<PARequestDataSeeder>>();
+        var seeder = new PARequestDataSeeder(logger);
+
+        // Act
+        await seeder.SeedAsync(context, CancellationToken.None);
+
+        // Assert
+        var count = await context.PriorAuthRequests.CountAsync();
+        await Assert.That(count).IsEqualTo(4);
+    }
+
+    [Test]
+    public async Task SeedAsync_ExistingData_SkipsSeeding()
+    {
+        // Arrange
+        using var context = CreateDbContext();
+        var logger = Substitute.For<ILogger<PARequestDataSeeder>>();
+        var seeder = new PARequestDataSeeder(logger);
+
+        // Seed once
+        await seeder.SeedAsync(context, CancellationToken.None);
+
+        // Act — seed again
+        await seeder.SeedAsync(context, CancellationToken.None);
+
+        // Assert — still 4, not 8
+        var count = await context.PriorAuthRequests.CountAsync();
+        await Assert.That(count).IsEqualTo(4);
+    }
+
+    [Test]
+    public async Task SeedAsync_CreatesVariousStatuses()
+    {
+        // Arrange
+        using var context = CreateDbContext();
+        var logger = Substitute.For<ILogger<PARequestDataSeeder>>();
+        var seeder = new PARequestDataSeeder(logger);
+
+        // Act
+        await seeder.SeedAsync(context, CancellationToken.None);
+
+        // Assert — should have draft, ready, and submitted statuses
+        var statuses = await context.PriorAuthRequests
+            .Select(e => e.Status)
+            .Distinct()
+            .ToListAsync();
+
+        await Assert.That(statuses).Contains("draft");
+        await Assert.That(statuses).Contains("ready");
+        await Assert.That(statuses).Contains("submitted");
+    }
+
+    [Test]
+    public async Task SeedAsync_SetsRequiredFields()
+    {
+        // Arrange
+        using var context = CreateDbContext();
+        var logger = Substitute.For<ILogger<PARequestDataSeeder>>();
+        var seeder = new PARequestDataSeeder(logger);
+
+        // Act
+        await seeder.SeedAsync(context, CancellationToken.None);
+
+        // Assert — all entities have required fields
+        var entities = await context.PriorAuthRequests.ToListAsync();
+        foreach (var entity in entities)
+        {
+            await Assert.That(entity.Id).IsNotNull().And.IsNotEqualTo(string.Empty);
+            await Assert.That(entity.PatientId).IsNotNull().And.IsNotEqualTo(string.Empty);
+            await Assert.That(entity.PatientName).IsNotNull().And.IsNotEqualTo(string.Empty);
+            await Assert.That(entity.ProcedureCode).IsNotNull().And.IsNotEqualTo(string.Empty);
+            await Assert.That(entity.ProcedureName).IsNotNull().And.IsNotEqualTo(string.Empty);
+        }
+    }
+
+    [Test]
+    public async Task CreateDemoEntities_ReturnsExpectedCount()
+    {
+        var entities = PARequestDataSeeder.CreateDemoEntities(DateTimeOffset.UtcNow);
+
+        await Assert.That(entities.Length).IsEqualTo(4);
+    }
+
+    [Test]
+    public async Task CreateDemoEntities_ReadyEntities_HaveCriteria()
+    {
+        var entities = PARequestDataSeeder.CreateDemoEntities(DateTimeOffset.UtcNow);
+        var readyEntities = entities.Where(e => e.Status == "ready").ToList();
+
+        foreach (var entity in readyEntities)
+        {
+            await Assert.That(entity.CriteriaJson).IsNotNull().And.IsNotEqualTo(string.Empty);
+            await Assert.That(entity.Confidence).IsGreaterThan(0);
+            await Assert.That(entity.ReadyAt).IsNotNull();
+        }
+    }
+}
diff --git a/apps/gateway/Gateway.API.Tests/DependencyExtensionsTests.cs b/apps/gateway/Gateway.API.Tests/DependencyExtensionsTests.cs
index defa4d6..734b84b 100644
--- a/apps/gateway/Gateway.API.Tests/DependencyExtensionsTests.cs
+++ b/apps/gateway/Gateway.API.Tests/DependencyExtensionsTests.cs
@@ -135,6 +135,27 @@ public async Task AddGatewayServices_DoesNotRegisterMockDataService()
         await Assert.That(mockRegistrations.Count()).IsEqualTo(0);
     }
 
+    [Test]
+    public async Task AddGatewayPersistence_RegistersDataSeeder()
+    {
+        // Arrange
+        var services = new ServiceCollection();
+        services.AddLogging();
+        services.AddDbContext<GatewayDbContext>(options =>
+            options.UseInMemoryDatabase(Guid.NewGuid().ToString()));
+
+        services.AddGatewayPersistence();
+        var provider = services.BuildServiceProvider();
+
+        // Act
+        using var scope = provider.CreateScope();
+        var seeder = scope.ServiceProvider.GetService<IDataSeeder<GatewayDbContext>>();
+
+        // Assert
+        await Assert.That(seeder).IsNotNull();
+        await Assert.That(seeder).IsTypeOf<PARequestDataSeeder>();
+    }
+
     private static IConfiguration CreateTestConfiguration()
     {
         var configValues = new Dictionary<string, string?>
diff --git a/apps/gateway/Gateway.API.Tests/Integration/EncounterProcessingAlbaBootstrap.cs b/apps/gateway/Gateway.API.Tests/Integration/EncounterProcessingAlbaBootstrap.cs
index 44300c3..3c2fbea 100644
--- a/apps/gateway/Gateway.API.Tests/Integration/EncounterProcessingAlbaBootstrap.cs
+++ b/apps/gateway/Gateway.API.Tests/Integration/EncounterProcessingAlbaBootstrap.cs
@@ -91,6 +91,20 @@ public async Task InitializeAsync()
                 services.AddDbContext<GatewayDbContext>(options =>
                     options.UseInMemoryDatabase(databaseName));
 
+                // Remove MigrationService (in-memory DB doesn't need migration)
+                var migrationServiceDescriptor = services.FirstOrDefault(d =>
+                    d.ServiceType == typeof(IHostedService) &&
+                    d.ImplementationType?.IsGenericType == true &&
+                    d.ImplementationType.GetGenericTypeDefinition() == typeof(MigrationService<>));
+                if (migrationServiceDescriptor != null)
+                {
+                    services.Remove(migrationServiceDescriptor);
+                }
+
+                // Mark migrations as complete so MigrationGateMiddleware passes requests through
+                MigrationHealthCheck.RegisterExpected(nameof(GatewayDbContext));
+                MigrationHealthCheck.MarkComplete(nameof(GatewayDbContext));
+
                 // Remove AthenaPollingService to avoid scoped dependency validation issue
                 // The singleton background service cannot consume scoped IPatientRegistry
                 services.RemoveAll<AthenaPollingService>();
diff --git a/apps/gateway/Gateway.API.Tests/Integration/GatewayAlbaBootstrap.cs b/apps/gateway/Gateway.API.Tests/Integration/GatewayAlbaBootstrap.cs
index a441532..d1c37a7 100644
--- a/apps/gateway/Gateway.API.Tests/Integration/GatewayAlbaBootstrap.cs
+++ b/apps/gateway/Gateway.API.Tests/Integration/GatewayAlbaBootstrap.cs
@@ -90,6 +90,20 @@ public async Task InitializeAsync()
                 services.AddDbContext<GatewayDbContext>(options =>
                     options.UseInMemoryDatabase(databaseName));
 
+                // Remove MigrationService (in-memory DB doesn't need migration)
+                var migrationServiceDescriptor = services.FirstOrDefault(d =>
+                    d.ServiceType == typeof(IHostedService) &&
+                    d.ImplementationType?.IsGenericType == true &&
+                    d.ImplementationType.GetGenericTypeDefinition() == typeof(MigrationService<>));
+                if (migrationServiceDescriptor != null)
+                {
+                    services.Remove(migrationServiceDescriptor);
+                }
+
+                // Mark migrations as complete so MigrationGateMiddleware passes requests through
+                MigrationHealthCheck.RegisterExpected(nameof(GatewayDbContext));
+                MigrationHealthCheck.MarkComplete(nameof(GatewayDbContext));
+
                 // Remove AthenaPollingService to avoid scoped dependency validation issue
                 // The singleton background service cannot consume scoped IPatientRegistry
                 services.RemoveAll<AthenaPollingService>();
diff --git a/apps/gateway/Gateway.API.Tests/Integration/PARequestLifecycleAlbaBootstrap.cs b/apps/gateway/Gateway.API.Tests/Integration/PARequestLifecycleAlbaBootstrap.cs
index 8b2db53..f896375 100644
--- a/apps/gateway/Gateway.API.Tests/Integration/PARequestLifecycleAlbaBootstrap.cs
+++ b/apps/gateway/Gateway.API.Tests/Integration/PARequestLifecycleAlbaBootstrap.cs
@@ -79,6 +79,20 @@ public async Task InitializeAsync()
                 services.AddDbContext<GatewayDbContext>(options =>
                     options.UseInMemoryDatabase(databaseName));
 
+                // Remove MigrationService (in-memory DB doesn't need migration)
+                var migrationServiceDescriptor = services.FirstOrDefault(d =>
+                    d.ServiceType == typeof(IHostedService) &&
+                    d.ImplementationType?.IsGenericType == true &&
+                    d.ImplementationType.GetGenericTypeDefinition() == typeof(MigrationService<>));
+                if (migrationServiceDescriptor != null)
+                {
+                    services.Remove(migrationServiceDescriptor);
+                }
+
+                // Mark migrations as complete so MigrationGateMiddleware passes requests through
+                MigrationHealthCheck.RegisterExpected(nameof(GatewayDbContext));
+                MigrationHealthCheck.MarkComplete(nameof(GatewayDbContext));
+
                 // Remove AthenaPollingService to avoid scoped dependency validation issue
                 services.RemoveAll<AthenaPollingService>();
                 var hostedServiceDescriptor = services.FirstOrDefault(d =>
diff --git a/apps/gateway/Gateway.API/Data/IDataSeeder.cs b/apps/gateway/Gateway.API/Data/IDataSeeder.cs
new file mode 100644
index 0000000..7245823
--- /dev/null
+++ b/apps/gateway/Gateway.API/Data/IDataSeeder.cs
@@ -0,0 +1,26 @@
+// =============================================================================
+// <copyright file="IDataSeeder.cs" company="Levelup Software">
+// Copyright (c) Levelup Software. All rights reserved.
+// </copyright>
+// =============================================================================
+
+using Microsoft.EntityFrameworkCore;
+
+namespace Gateway.API.Data;
+
+/// <summary>
+/// Interface for seeding initial data into a database context after migration.
+/// Implementations should be idempotent (safe to run multiple times).
+/// </summary>
+/// <typeparam name="TContext">The DbContext type this seeder targets.</typeparam>
+public interface IDataSeeder<in TContext>
+    where TContext : DbContext
+{
+    /// <summary>
+    /// Seeds data into the database. Must be idempotent.
+    /// </summary>
+    /// <param name="context">The database context to seed data into.</param>
+    /// <param name="cancellationToken">The cancellation token.</param>
+    /// <returns>A task representing the asynchronous operation.</returns>
+    Task SeedAsync(TContext context, CancellationToken cancellationToken);
+}
diff --git a/apps/gateway/Gateway.API/Data/MigrationGateMiddleware.cs b/apps/gateway/Gateway.API/Data/MigrationGateMiddleware.cs
new file mode 100644
index 0000000..4055955
--- /dev/null
+++ b/apps/gateway/Gateway.API/Data/MigrationGateMiddleware.cs
@@ -0,0 +1,61 @@
+// =============================================================================
+// <copyright file="MigrationGateMiddleware.cs" company="Levelup Software">
+// Copyright (c) Levelup Software. All rights reserved.
+// </copyright>
+// =============================================================================
+
+using System.Net;
+
+namespace Gateway.API.Data;
+
+/// <summary>
+/// Middleware that returns 503 Service Unavailable while database migrations are in progress.
+/// Always allows health check endpoints through so orchestrators can monitor readiness.
+/// </summary>
+public sealed class MigrationGateMiddleware
+{
+    private static readonly string[] AllowedPaths = ["/health", "/alive"];
+    private readonly RequestDelegate _next;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="MigrationGateMiddleware"/> class.
+    /// </summary>
+    /// <param name="next">The next middleware in the pipeline.</param>
+    public MigrationGateMiddleware(RequestDelegate next)
+    {
+        _next = next;
+    }
+
+    /// <summary>
+    /// Invokes the middleware. Returns 503 if migrations are not yet complete,
+    /// unless the request is to an allowed path (health checks).
+    /// </summary>
+    /// <param name="context">The HTTP context.</param>
+    /// <returns>A task representing the asynchronous operation.</returns>
+    public async Task InvokeAsync(HttpContext context)
+    {
+        if (!MigrationHealthCheck.IsReady && !IsAllowedPath(context.Request.Path))
+        {
+            context.Response.StatusCode = (int)HttpStatusCode.ServiceUnavailable;
+            context.Response.Headers["Retry-After"] = "5";
+            context.Response.ContentType = "text/plain";
+            await context.Response.WriteAsync("Service is starting up. Database migrations in progress.", context.RequestAborted);
+            return;
+        }
+
+        await _next(context);
+    }
+
+    private static bool IsAllowedPath(PathString path)
+    {
+        foreach (var allowed in AllowedPaths)
+        {
+            if (path.StartsWithSegments(allowed, StringComparison.OrdinalIgnoreCase))
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+}
diff --git a/apps/gateway/Gateway.API/Data/MigrationHealthCheck.cs b/apps/gateway/Gateway.API/Data/MigrationHealthCheck.cs
index 61aaa47..21b2244 100644
--- a/apps/gateway/Gateway.API/Data/MigrationHealthCheck.cs
+++ b/apps/gateway/Gateway.API/Data/MigrationHealthCheck.cs
@@ -30,6 +30,13 @@ public static void MarkComplete(string contextName) =>
     public static void RegisterExpected(string contextName) =>
         s_completedMigrations[contextName] = false;
 
+    /// <summary>
+    /// Gets a value indicating whether all registered migrations have completed.
+    /// Returns false if no migrations have been registered.
+    /// </summary>
+    public static bool IsReady =>
+        s_completedMigrations.Count > 0 && s_completedMigrations.Values.All(v => v);
+
     /// <summary>
     /// Checks if migration is complete for the specified context.
     /// </summary>
@@ -57,6 +64,11 @@ public static async Task WaitForMigrationAsync(
         }
     }
 
+    /// <summary>
+    /// Resets all migration tracking state. For testing only.
+    /// </summary>
+    internal static void Reset() => s_completedMigrations.Clear();
+
     /// <summary>
     /// Checks the health of database migrations.
     /// </summary>
diff --git a/apps/gateway/Gateway.API/Data/MigrationService.cs b/apps/gateway/Gateway.API/Data/MigrationService.cs
index 5585d7f..8060fd5 100644
--- a/apps/gateway/Gateway.API/Data/MigrationService.cs
+++ b/apps/gateway/Gateway.API/Data/MigrationService.cs
@@ -88,6 +88,11 @@ protected override async Task ExecuteAsync(CancellationToken cancellationToken)
                 await RunMigrationAsync(context, cancellationToken).ConfigureAwait(false);
             }
 
+            if (_options.Value.SeedData && _environment.IsDevelopment())
+            {
+                await RunSeedersAsync(scope.ServiceProvider, context, cancellationToken).ConfigureAwait(false);
+            }
+
             MigrationHealthCheck.MarkComplete(contextName);
             _logger.LogInformation("Migration process completed successfully for {ContextName}.", contextName);
         }
@@ -98,6 +103,18 @@ protected override async Task ExecuteAsync(CancellationToken cancellationToken)
         }
     }
 
+    private async Task RunSeedersAsync(IServiceProvider scopedProvider, TContext context, CancellationToken cancellationToken)
+    {
+        var seeders = scopedProvider.GetServices<IDataSeeder<TContext>>();
+        foreach (var seeder in seeders)
+        {
+            var seederName = seeder.GetType().Name;
+            _logger.LogInformation("Running data seeder {SeederName} for {ContextName}...", seederName, typeof(TContext).Name);
+            await seeder.SeedAsync(context, cancellationToken).ConfigureAwait(false);
+            _logger.LogInformation("Data seeder {SeederName} completed for {ContextName}.", seederName, typeof(TContext).Name);
+        }
+    }
+
     private async Task ValidateDatabaseExistsAsync(TContext context, CancellationToken cancellationToken)
     {
         var strategy = context.Database.CreateExecutionStrategy();
diff --git a/apps/gateway/Gateway.API/Data/MigrationServiceOptions.cs b/apps/gateway/Gateway.API/Data/MigrationServiceOptions.cs
index 3d6b9ca..cba6a05 100644
--- a/apps/gateway/Gateway.API/Data/MigrationServiceOptions.cs
+++ b/apps/gateway/Gateway.API/Data/MigrationServiceOptions.cs
@@ -22,4 +22,10 @@ public class MigrationServiceOptions
     /// WARNING: This will delete all existing data. Only use in development environments.
     /// </summary>
     public bool RecreateDatabase { get; set; }
+
+    /// <summary>
+    /// Gets or sets a value indicating whether to run data seeders after migration completes.
+    /// Defaults to true. Seeders are only invoked in Development environment.
+    /// </summary>
+    public bool SeedData { get; set; } = true;
 }
diff --git a/apps/gateway/Gateway.API/Data/PARequestDataSeeder.cs b/apps/gateway/Gateway.API/Data/PARequestDataSeeder.cs
new file mode 100644
index 0000000..058ff82
--- /dev/null
+++ b/apps/gateway/Gateway.API/Data/PARequestDataSeeder.cs
@@ -0,0 +1,196 @@
+// =============================================================================
+// <copyright file="PARequestDataSeeder.cs" company="Levelup Software">
+// Copyright (c) Levelup Software. All rights reserved.
+// </copyright>
+// =============================================================================
+
+using System.Text.Json;
+using Gateway.API.Data.Entities;
+using Gateway.API.GraphQL.Models;
+using Microsoft.EntityFrameworkCore;
+
+namespace Gateway.API.Data;
+
+/// <summary>
+/// Seeds demo prior authorization requests for development environments.
+/// Idempotent: skips seeding if any PA requests already exist.
+/// </summary>
+public sealed class PARequestDataSeeder : IDataSeeder<GatewayDbContext>
+{
+    private static readonly JsonSerializerOptions JsonOptions = new()
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+    };
+
+    private readonly ILogger<PARequestDataSeeder> _logger;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="PARequestDataSeeder"/> class.
+    /// </summary>
+    /// <param name="logger">The logger instance.</param>
+    public PARequestDataSeeder(ILogger<PARequestDataSeeder> logger)
+    {
+        _logger = logger;
+    }
+
+    /// <inheritdoc />
+    public async Task SeedAsync(GatewayDbContext context, CancellationToken cancellationToken)
+    {
+        if (await context.PriorAuthRequests.AnyAsync(cancellationToken))
+        {
+            _logger.LogInformation("PA requests already exist, skipping seed.");
+            return;
+        }
+
+        var now = DateTimeOffset.UtcNow;
+        var entities = CreateDemoEntities(now);
+
+        context.PriorAuthRequests.AddRange(entities);
+        await context.SaveChangesAsync(cancellationToken);
+
+        _logger.LogInformation("Seeded {Count} demo PA requests.", entities.Length);
+    }
+
+    internal static PriorAuthRequestEntity[] CreateDemoEntities(DateTimeOffset now)
+    {
+        return
+        [
+            new PriorAuthRequestEntity
+            {
+                Id = "PA-DEMO-001",
+                PatientId = "60178",
+                FhirPatientId = "a-195900.E-60178",
+                PatientName = "Sarah Johnson",
+                PatientMrn = "MRN-10001",
+                PatientDob = "March 15, 1985",
+                PatientMemberId = "BCBS-100234",
+                PatientPayer = "Blue Cross Blue Shield",
+                PatientAddress = "123 Main St, Springfield, IL 62701",
+                PatientPhone = "(555) 123-4567",
+                ProcedureCode = "72148",
+                ProcedureName = "MRI Lumbar Spine w/o Contrast",
+                DiagnosisCode = "M54.5",
+                DiagnosisName = "Low Back Pain",
+                ProviderId = "DR001",
+                ProviderName = "Dr. Amanda Martinez",
+                ProviderNpi = "1234567890",
+                ServiceDate = now.ToString("MMMM d, yyyy"),
+                PlaceOfService = "Outpatient",
+                ClinicalSummary = "Patient presents with chronic low back pain for 6 months, unresponsive to conservative treatment including physical therapy and NSAIDs. MRI recommended to evaluate for disc herniation or spinal stenosis.",
+                Status = "ready",
+                Confidence = 87,
+                CriteriaJson = SerializeCriteria(
+                [
+                    new CriterionModel { Met = true, Label = "Conservative treatment failed", Reason = "6 months of PT and NSAIDs without improvement" },
+                    new CriterionModel { Met = true, Label = "Clinical indication present", Reason = "Chronic low back pain with radiculopathy symptoms" },
+                    new CriterionModel { Met = true, Label = "Prior imaging reviewed", Reason = "X-ray completed showing degenerative changes" },
+                    new CriterionModel { Met = false, Label = "Specialist referral", Reason = "No specialist referral documented" },
+                ]),
+                CreatedAt = now.AddHours(-2),
+                UpdatedAt = now.AddMinutes(-30),
+                ReadyAt = now.AddMinutes(-30),
+            },
+            new PriorAuthRequestEntity
+            {
+                Id = "PA-DEMO-002",
+                PatientId = "60179",
+                FhirPatientId = "a-195900.E-60179",
+                PatientName = "James Wilson",
+                PatientMrn = "MRN-10002",
+                PatientDob = "July 22, 1958",
+                PatientMemberId = "AET-200567",
+                PatientPayer = "Aetna",
+                PatientAddress = "456 Oak Ave, Chicago, IL 60601",
+                PatientPhone = "(555) 234-5678",
+                ProcedureCode = "27447",
+                ProcedureName = "Total Knee Replacement",
+                DiagnosisCode = "M17.11",
+                DiagnosisName = "Primary Osteoarthritis, Right Knee",
+                ProviderId = "DR002",
+                ProviderName = "Dr. Robert Kim",
+                ProviderNpi = "0987654321",
+                ServiceDate = now.AddDays(14).ToString("MMMM d, yyyy"),
+                PlaceOfService = "Inpatient",
+                ClinicalSummary = "67-year-old male with severe right knee osteoarthritis. BMI 28. Failed 12 months of conservative management including corticosteroid injections, physical therapy, and bracing. Kellgren-Lawrence Grade IV on imaging.",
+                Status = "ready",
+                Confidence = 94,
+                CriteriaJson = SerializeCriteria(
+                [
+                    new CriterionModel { Met = true, Label = "Conservative treatment exhausted", Reason = "12 months of PT, injections, and bracing" },
+                    new CriterionModel { Met = true, Label = "Imaging confirms severity", Reason = "KL Grade IV osteoarthritis on X-ray" },
+                    new CriterionModel { Met = true, Label = "Functional limitation", Reason = "Unable to walk more than 1 block, difficulty with stairs" },
+                    new CriterionModel { Met = true, Label = "BMI within range", Reason = "BMI 28, within surgical criteria" },
+                ]),
+                CreatedAt = now.AddHours(-4),
+                UpdatedAt = now.AddHours(-1),
+                ReadyAt = now.AddHours(-1),
+            },
+            new PriorAuthRequestEntity
+            {
+                Id = "PA-DEMO-003",
+                PatientId = "60180",
+                FhirPatientId = "a-195900.E-60180",
+                PatientName = "Maria Garcia",
+                PatientMrn = "MRN-10003",
+                PatientDob = "November 8, 1972",
+                PatientMemberId = "UHC-300891",
+                PatientPayer = "United Healthcare",
+                PatientAddress = "789 Pine Rd, Austin, TX 78701",
+                PatientPhone = "(555) 345-6789",
+                ProcedureCode = "J1745",
+                ProcedureName = "Infliximab (Remicade)",
+                DiagnosisCode = "M06.9",
+                DiagnosisName = "Rheumatoid Arthritis, Unspecified",
+                ProviderId = "DR001",
+                ProviderName = "Dr. Amanda Martinez",
+                ProviderNpi = "1234567890",
+                ServiceDate = now.AddDays(7).ToString("MMMM d, yyyy"),
+                PlaceOfService = "Outpatient",
+                ClinicalSummary = "Patient with moderate-to-severe rheumatoid arthritis, failed methotrexate and hydroxychloroquine. DAS28 score 5.1 indicating high disease activity. Requesting biologic therapy initiation.",
+                Status = "draft",
+                Confidence = 0,
+                CreatedAt = now.AddMinutes(-45),
+                UpdatedAt = now.AddMinutes(-45),
+            },
+            new PriorAuthRequestEntity
+            {
+                Id = "PA-DEMO-004",
+                PatientId = "60181",
+                FhirPatientId = "a-195900.E-60181",
+                PatientName = "Robert Chen",
+                PatientMrn = "MRN-10004",
+                PatientDob = "February 3, 1990",
+                PatientMemberId = "CIG-400123",
+                PatientPayer = "Cigna",
+                PatientAddress = "321 Elm St, Seattle, WA 98101",
+                PatientPhone = "(555) 456-7890",
+                ProcedureCode = "70553",
+                ProcedureName = "MRI Brain w/ & w/o Contrast",
+                DiagnosisCode = "G43.909",
+                DiagnosisName = "Migraine, Unspecified",
+                ProviderId = "DR003",
+                ProviderName = "Dr. Lisa Thompson",
+                ProviderNpi = "1122334455",
+                ServiceDate = now.AddDays(3).ToString("MMMM d, yyyy"),
+                PlaceOfService = "Outpatient",
+                ClinicalSummary = "35-year-old male with new-onset severe migraines with aura, increasing in frequency over 3 months. Neurological exam notable for mild papilledema. MRI brain indicated to rule out intracranial pathology.",
+                Status = "submitted",
+                Confidence = 91,
+                CriteriaJson = SerializeCriteria(
+                [
+                    new CriterionModel { Met = true, Label = "New neurological symptoms", Reason = "New-onset migraines with aura and papilledema" },
+                    new CriterionModel { Met = true, Label = "Red flag symptoms", Reason = "Papilledema on exam suggests elevated intracranial pressure" },
+                    new CriterionModel { Met = true, Label = "Progressive symptoms", Reason = "Increasing frequency over 3 months" },
+                ]),
+                CreatedAt = now.AddDays(-1),
+                UpdatedAt = now.AddHours(-6),
+                ReadyAt = now.AddHours(-12),
+                SubmittedAt = now.AddHours(-6),
+                ReviewTimeSeconds = 142,
+            },
+        ];
+    }
+
+    private static string SerializeCriteria(List<CriterionModel> criteria) =>
+        JsonSerializer.Serialize(criteria, JsonOptions);
+}
diff --git a/apps/gateway/Gateway.API/DependencyExtensions.cs b/apps/gateway/Gateway.API/DependencyExtensions.cs
index ff1e002..61dcabf 100644
--- a/apps/gateway/Gateway.API/DependencyExtensions.cs
+++ b/apps/gateway/Gateway.API/DependencyExtensions.cs
@@ -152,6 +152,9 @@ public static IServiceCollection AddGatewayPersistence(this IServiceCollection s
         services.AddScoped<IPatientRegistry, PostgresPatientRegistry>();
         services.AddScoped<IPARequestStore, PostgresPARequestStore>();
 
+        // Data seeder for demo data in development
+        services.AddScoped<IDataSeeder<GatewayDbContext>, PARequestDataSeeder>();
+
         return services;
     }
 
diff --git a/apps/gateway/Gateway.API/Program.cs b/apps/gateway/Gateway.API/Program.cs
index 459a930..827ca7d 100644
--- a/apps/gateway/Gateway.API/Program.cs
+++ b/apps/gateway/Gateway.API/Program.cs
@@ -45,6 +45,9 @@
 // ---------------------------------------------------------------------------
 // Middleware Pipeline
 // ---------------------------------------------------------------------------
+// Gate all requests (except health checks) until migrations complete
+app.UseMiddleware<MigrationGateMiddleware>();
+
 app.MapOpenApi();
 app.MapScalarApiReference(options =>
 {