diff --git a/api/tests/ProposalSystem.Tests/Common/RowVersionCodecTests.cs b/api/tests/ProposalSystem.Tests/Common/RowVersionCodecTests.cs
new file mode 100644
index 0000000..808b08d
--- /dev/null
+++ b/api/tests/ProposalSystem.Tests/Common/RowVersionCodecTests.cs
@@ -0,0 +1,37 @@
+using FluentAssertions;
+using ProposalSystem.Application.Common;
+using Xunit;
+
+namespace ProposalSystem.Tests.Common;
+
+public class RowVersionCodecTests
+{
+    [Theory(DisplayName = "Encode/TryDecode round-trips")]
+    [InlineData(1L)]
+    [InlineData(2L)]
+    [InlineData(long.MaxValue)]
+    public void RoundTrips(long version)
+    {
+        var token = RowVersionCodec.Encode(version);
+
+        RowVersionCodec.TryDecode(token, out var decoded).Should().BeTrue();
+        decoded.Should().Be(version);
+    }
+
+    [Fact(DisplayName = "Version 1 encodes to the SHOC-style opaque token")]
+    public void EncodesOpaqueBase64()
+    {
+        RowVersionCodec.Encode(1).Should().Be("AAAAAAAAAAE=");
+    }
+
+    [Theory(DisplayName = "TryDecode rejects missing/malformed/wrong-length tokens")]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("not base64!!")]
+    [InlineData("AAA=")] // valid base64 but not 8 bytes
+    [InlineData("AAAAAAAAAAAAAAAAAAAAAA==")] // 16 bytes
+    public void RejectsInvalid(string? token)
+    {
+        RowVersionCodec.TryDecode(token, out _).Should().BeFalse();
+    }
+}
diff --git a/api/tests/ProposalSystem.Tests/Helpers/SqliteDbContextFactory.cs b/api/tests/ProposalSystem.Tests/Helpers/SqliteDbContextFactory.cs
index 6177b97..edeaa8f 100644
--- a/api/tests/ProposalSystem.Tests/Helpers/SqliteDbContextFactory.cs
+++ b/api/tests/ProposalSystem.Tests/Helpers/SqliteDbContextFactory.cs
@@ -37,6 +37,29 @@ public static class SqliteDbContextFactory
         return context;
     }
 
+    /// <summary>
+    /// Opens a shared in-memory database and returns the connection plus a factory
+    /// for additional contexts over the SAME database — needed by optimistic-
+    /// concurrency tests that simulate two competing writers. Callers dispose the
+    /// connection after the contexts.
+    /// </summary>
+    public static (SqliteConnection Connection, Func<ProposalDbContext> ContextFactory) CreateShared()
+    {
+        var connection = new SqliteConnection("DataSource=:memory:");
+        connection.Open();
+        RegisterPostgresStubs(connection);
+
+        var options = new DbContextOptionsBuilder<ProposalDbContext>()
+            .UseSqlite(connection)
+            .Options;
+
+        var first = new ProposalDbContext(options);
+        first.Database.EnsureCreated();
+        first.Dispose();
+
+        return (connection, () => new ProposalDbContext(options));
+    }
+
     private static void RegisterPostgresStubs(SqliteConnection connection)
     {
         // hashtext(text) -> integer — returns a constant; only needed so SQL parses
diff --git a/api/tests/ProposalSystem.Tests/Middleware/GlobalExceptionHandlerTests.cs b/api/tests/ProposalSystem.Tests/Middleware/GlobalExceptionHandlerTests.cs
index 78c5c04..33ea6c1 100644
--- a/api/tests/ProposalSystem.Tests/Middleware/GlobalExceptionHandlerTests.cs
+++ b/api/tests/ProposalSystem.Tests/Middleware/GlobalExceptionHandlerTests.cs
@@ -93,4 +93,47 @@ public class GlobalExceptionHandlerTests
 
         context.Response.ContentType.Should().Be("application/problem+json");
     }
+
+    // ── Concurrency 409 envelopes (ADR 0004) ─────────────────────────────────
+    // These are SHOC's shapes verbatim, NOT ProblemDetails. Both bodies are wire
+    // contract: web/mobile branch on message + currentState / status + code.
+
+    [Fact(DisplayName = "ProposalConcurrencyException maps to 409 { message, currentState } (SHOC envelope)")]
+    public async Task ConcurrencyException_Maps409WithCurrentState()
+    {
+        var (status, body) = await InvokeWith(
+            new ProposalConcurrencyException(new { Id = "p1", Notes = "winner" }));
+
+        status.Should().Be(409);
+        body.GetProperty("message").GetString()
+            .Should().Be("The record was modified by another user. Refresh and retry.");
+        body.GetProperty("currentState").GetProperty("id").GetString().Should().Be("p1");
+        body.GetProperty("currentState").GetProperty("notes").GetString().Should().Be("winner");
+        // The guarded envelope is NOT the fallback shape.
+        body.TryGetProperty("status", out _).Should().BeFalse();
+        body.TryGetProperty("code", out _).Should().BeFalse();
+    }
+
+    [Fact(DisplayName = "ProposalConcurrencyException with no reloadable state carries currentState: null")]
+    public async Task ConcurrencyException_NullState_SerializesNull()
+    {
+        var (status, body) = await InvokeWith(new ProposalConcurrencyException(null));
+
+        status.Should().Be(409);
+        body.GetProperty("currentState").ValueKind.Should().Be(JsonValueKind.Null);
+    }
+
+    [Fact(DisplayName = "Bare DbUpdateConcurrencyException maps to the 409 fallback { status, message, code }")]
+    public async Task DbUpdateConcurrencyException_Maps409Fallback()
+    {
+        var (status, body) = await InvokeWith(
+            new Microsoft.EntityFrameworkCore.DbUpdateConcurrencyException("boom"));
+
+        status.Should().Be(409);
+        body.GetProperty("status").GetString().Should().Be("Conflict");
+        body.GetProperty("message").GetString()
+            .Should().Be("The record was modified by another user. Refresh and retry.");
+        body.GetProperty("code").GetInt32().Should().Be(409);
+        body.TryGetProperty("currentState", out _).Should().BeFalse();
+    }
 }
diff --git a/api/tests/ProposalSystem.Tests/Services/LineItemServiceStateGuardTests.cs b/api/tests/ProposalSystem.Tests/Services/LineItemServiceStateGuardTests.cs
index c3e068d..715eb30 100644
--- a/api/tests/ProposalSystem.Tests/Services/LineItemServiceStateGuardTests.cs
+++ b/api/tests/ProposalSystem.Tests/Services/LineItemServiceStateGuardTests.cs
@@ -121,7 +121,7 @@ public class LineItemServiceStateGuardTests : IDisposable
         });
 
         // Act
-        var act = () => _sut.BulkUpdateAsync(proposal.Id, request);
+        var act = () => _sut.BulkUpdateAsync(proposal.Id, request with { ProposalVersion = Ver(proposal.Id) });
 
         // Assert
         await act.Should().ThrowAsync<InvalidOperationException>()
@@ -143,7 +143,7 @@ public class LineItemServiceStateGuardTests : IDisposable
         });
 
         // Act
-        var result = await _sut.BulkUpdateAsync(proposal.Id, request);
+        var result = await _sut.BulkUpdateAsync(proposal.Id, request with { ProposalVersion = Ver(proposal.Id) });
 
         // Assert
         result.Should().HaveCount(2);
@@ -165,7 +165,7 @@ public class LineItemServiceStateGuardTests : IDisposable
         });
 
         // Act
-        await _sut.BulkUpdateAsync(proposal.Id, request);
+        await _sut.BulkUpdateAsync(proposal.Id, request with { ProposalVersion = Ver(proposal.Id) });
 
         // Assert
         var updated = await _db.Proposals.FindAsync(proposal.Id);
@@ -199,7 +199,7 @@ public class LineItemServiceStateGuardTests : IDisposable
         });
 
         // Act
-        var result = await _sut.BulkUpdateAsync(proposal.Id, request);
+        var result = await _sut.BulkUpdateAsync(proposal.Id, request with { ProposalVersion = Ver(proposal.Id) });
 
         // Assert
         result.Should().HaveCount(1);
@@ -214,7 +214,7 @@ public class LineItemServiceStateGuardTests : IDisposable
     public async Task BulkUpdateAsync_NonexistentProposal_ThrowsKeyNotFound()
     {
         var request = new BulkUpdateLineItemsRequest(new List<UpdateLineItemEntry>());
-        var act = () => _sut.BulkUpdateAsync(Guid.NewGuid(), request);
+        var act = () => _sut.BulkUpdateAsync(Guid.NewGuid(), request with { ProposalVersion = Ver(Guid.NewGuid()) });
         await act.Should().ThrowAsync<KeyNotFoundException>();
     }
 
@@ -317,11 +317,9 @@ public class LineItemServiceStateGuardTests : IDisposable
         await _sut.CreateAsync(proposal.Id, request);
 
         // Assert
-        await _audit.Received(1).LogAsync(
-            AuditAction.EditLineItem,
+        _audit.Received(1).Stage(AuditAction.EditLineItem,
             proposal.Id,
-            Arg.Is<string?>(s => s != null && s.Contains(request.Description)),
-            Arg.Any<CancellationToken>());
+            Arg.Is<string?>(s => s != null && s.Contains(request.Description)));
     }
 
     [Fact(DisplayName = "QA-C2: BulkUpdateAsync logs audit event on success")]
@@ -338,14 +336,12 @@ public class LineItemServiceStateGuardTests : IDisposable
         });
 
         // Act
-        await _sut.BulkUpdateAsync(proposal.Id, request);
+        await _sut.BulkUpdateAsync(proposal.Id, request with { ProposalVersion = Ver(proposal.Id) });
 
         // Assert — audit detail may be plain text ("Bulk update: N items") or JSON
-        await _audit.Received(1).LogAsync(
-            AuditAction.EditLineItem,
+        _audit.Received(1).Stage(AuditAction.EditLineItem,
             proposal.Id,
-            Arg.Is<string?>(s => s != null),
-            Arg.Any<CancellationToken>());
+            Arg.Is<string?>(s => s != null));
     }
 
     [Fact(DisplayName = "QA-C2: DeleteAsync logs audit event on success")]
@@ -374,11 +370,9 @@ public class LineItemServiceStateGuardTests : IDisposable
         await _sut.DeleteAsync(proposal.Id, lineItem.Id);
 
         // Assert
-        await _audit.Received(1).LogAsync(
-            AuditAction.EditLineItem,
+        _audit.Received(1).Stage(AuditAction.EditLineItem,
             proposal.Id,
-            Arg.Is<string?>(s => s != null && s.Contains("Audit test item")),
-            Arg.Any<CancellationToken>());
+            Arg.Is<string?>(s => s != null && s.Contains("Audit test item")));
     }
 
     #endregion
@@ -418,4 +412,7 @@ public class LineItemServiceStateGuardTests : IDisposable
             Source: LineItemSource.Manual
         );
     }
+    private string Ver(Guid id) =>
+        ProposalSystem.Application.Common.RowVersionCodec.Encode(_db.Proposals.Find(id)?.Version ?? 1);
+
 }
diff --git a/api/tests/ProposalSystem.Tests/Services/ProposalConcurrencyTests.cs b/api/tests/ProposalSystem.Tests/Services/ProposalConcurrencyTests.cs
new file mode 100644
index 0000000..39a92fd
--- /dev/null
+++ b/api/tests/ProposalSystem.Tests/Services/ProposalConcurrencyTests.cs
@@ -0,0 +1,249 @@
+using FluentAssertions;
+using Microsoft.Data.Sqlite;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.Logging;
+using NSubstitute;
+using ProposalSystem.Application.Common;
+using ProposalSystem.Application.DTOs;
+using ProposalSystem.Application.Interfaces;
+using ProposalSystem.Domain.Entities;
+using ProposalSystem.Infrastructure.Data;
+using ProposalSystem.Infrastructure.Services;
+using ProposalSystem.Tests.Helpers;
+using Xunit;
+
+namespace ProposalSystem.Tests.Services;
+
+/// <summary>
+/// Optimistic-concurrency tests for the proposal aggregate (ADR 0004, SHOC
+/// double-guard). Covers the pre-check path, both 422 token codes, the
+/// DB-level race window (two writers over one SQLite database), version
+/// bumping, and audit atomicity (staged rows must not survive a failed save).
+/// </summary>
+public class ProposalConcurrencyTests : IDisposable
+{
+    private readonly SqliteConnection _connection;
+    private readonly Func<ProposalDbContext> _contextFactory;
+    private readonly ProposalDbContext _db;
+    private readonly ICurrentUserService _currentUser;
+    private readonly ProposalService _sut;
+    private readonly LineItemService _lineItems;
+    private readonly Guid _userId = Guid.NewGuid();
+
+    public ProposalConcurrencyTests()
+    {
+        (_connection, _contextFactory) = SqliteDbContextFactory.CreateShared();
+        _db = _contextFactory();
+
+        _currentUser = Substitute.For<ICurrentUserService>();
+        _currentUser.UserId.Returns(_userId);
+        _currentUser.Role.Returns(UserRole.Admin);
+
+        _db.Users.Add(new User
+        {
+            Id = _userId,
+            CognitoSub = $"sub-{_userId}",
+            Email = "admin@test.com",
+            DisplayName = "Test Admin",
+            Role = UserRole.Admin,
+            CreatedAt = DateTime.UtcNow,
+            UpdatedAt = DateTime.UtcNow,
+        });
+        _db.SaveChanges();
+
+        var config = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?> { { "GENERATED_BUCKET", "test-bucket" } })
+            .Build();
+
+        // Real AuditService on the SAME context so staged-audit atomicity is exercised.
+        var audit = new AuditService(_db, _currentUser);
+        _sut = new ProposalService(_db, _currentUser, Substitute.For<IProposalNumberGenerator>(), audit,
+            Substitute.For<IJobPublisher>(), Substitute.For<IEmailService>(), Substitute.For<IS3Service>(),
+            config, Substitute.For<ILogger<ProposalService>>());
+        _lineItems = new LineItemService(_db, audit, Substitute.For<ILogger<LineItemService>>());
+    }
+
+    public void Dispose()
+    {
+        _db.Dispose();
+        _connection.Dispose();
+    }
+
+    private Proposal SeedProposal(ProposalStatus status = ProposalStatus.InReview)
+    {
+        var proposal = new Proposal
+        {
+            Id = Guid.NewGuid(),
+            ProposalNumber = $"P-{Guid.NewGuid():N}"[..12],
+            WorkOrderNumber = "WO-001",
+            CustomerName = "Test Customer",
+            CustomerAddress = "123 Test St",
+            ScopeOfWork = "Test scope of work",
+            ServiceCategory = ServiceCategory.HVAC,
+            Priority = Priority.Standard,
+            Status = status,
+            Notes = "",
+            SubmittedById = _userId,
+            SubmittedAt = DateTime.UtcNow.AddDays(-1),
+            CurrentRevision = 1,
+            CreatedAt = DateTime.UtcNow.AddDays(-1),
+            UpdatedAt = DateTime.UtcNow.AddDays(-1),
+        };
+        _db.Proposals.Add(proposal);
+        _db.SaveChanges();
+        return proposal;
+    }
+
+    private static UpdateProposalRequest UpdateNotes(string notes, string? version) =>
+        new(null, notes, null, null, null, version);
+
+    [Fact(DisplayName = "Missing proposalVersion -> 422 ProposalVersionRequired")]
+    public async Task MissingVersion_ThrowsVersionRequired()
+    {
+        var proposal = SeedProposal();
+
+        var act = () => _sut.UpdateAsync(proposal.Id, UpdateNotes("x", null));
+
+        var ex = await act.Should().ThrowAsync<BusinessRuleException>();
+        ex.Which.Code.Should().Be("ProposalVersionRequired");
+    }
+
+    [Fact(DisplayName = "Malformed proposalVersion -> 422 InvalidRowVersion")]
+    public async Task MalformedVersion_ThrowsInvalidRowVersion()
+    {
+        var proposal = SeedProposal();
+
+        var act = () => _sut.UpdateAsync(proposal.Id, UpdateNotes("x", "not-a-token!"));
+
+        var ex = await act.Should().ThrowAsync<BusinessRuleException>();
+        ex.Which.Code.Should().Be("InvalidRowVersion");
+    }
+
+    [Fact(DisplayName = "Stale token fails the pre-check and carries currentState")]
+    public async Task StaleToken_PreCheck_ThrowsWithCurrentState()
+    {
+        var proposal = SeedProposal();
+
+        var act = () => _sut.UpdateAsync(proposal.Id, UpdateNotes("x", RowVersionCodec.Encode(99)));
+
+        var ex = await act.Should().ThrowAsync<ProposalConcurrencyException>();
+        var state = ex.Which.CurrentState.Should().BeOfType<ProposalResponse>().Subject;
+        state.Id.Should().Be(proposal.Id);
+        state.RowVersion.Should().Be(RowVersionCodec.Encode(1));
+    }
+
+    [Fact(DisplayName = "DB-level race: concurrent writer wins, loser gets 409 state with the winner's values")]
+    public async Task LostRace_ReloadsCurrentState()
+    {
+        var proposal = SeedProposal();
+        var token = RowVersionCodec.Encode(1);
+
+        // Load into the service context first (pre-check will pass against version 1)...
+        var tracked = await _db.Proposals.FirstAsync(p => p.Id == proposal.Id);
+        tracked.Should().NotBeNull();
+
+        // ...then a second writer commits between the pre-check read and our save.
+        await using (var other = _contextFactory())
+        {
+            var competing = await other.Proposals.FirstAsync(p => p.Id == proposal.Id);
+            other.Entry(competing).Property(p => p.Version).OriginalValue = 1L;
+            competing.Notes = "the winner's notes";
+            competing.Version = 2;
+            await other.SaveChangesAsync();
+        }
+
+        var act = () => _sut.UpdateAsync(proposal.Id, UpdateNotes("the loser's notes", token));
+
+        var ex = await act.Should().ThrowAsync<ProposalConcurrencyException>();
+        var state = ex.Which.CurrentState.Should().BeOfType<ProposalResponse>().Subject;
+        state.Notes.Should().Be("the winner's notes");
+        state.RowVersion.Should().Be(RowVersionCodec.Encode(2));
+    }
+
+    [Fact(DisplayName = "Audit atomicity: a save lost to a concurrent writer persists no audit row")]
+    public async Task LostRace_PersistsNoAuditRow()
+    {
+        var proposal = SeedProposal();
+        await _db.Proposals.FirstAsync(p => p.Id == proposal.Id);
+
+        await using (var other = _contextFactory())
+        {
+            var competing = await other.Proposals.FirstAsync(p => p.Id == proposal.Id);
+            competing.Notes = "winner";
+            competing.Version = 2;
+            await other.SaveChangesAsync();
+        }
+
+        var act = () => _sut.UpdateAsync(proposal.Id, UpdateNotes("loser", RowVersionCodec.Encode(1)));
+        await act.Should().ThrowAsync<ProposalConcurrencyException>();
+
+        await using var check = _contextFactory();
+        (await check.AuditLogs.CountAsync()).Should().Be(0);
+    }
+
+    [Fact(DisplayName = "Successful update bumps the version and commits exactly one audit row atomically")]
+    public async Task SuccessfulUpdate_BumpsVersionAndCommitsAudit()
+    {
+        var proposal = SeedProposal();
+
+        var result = await _sut.UpdateAsync(proposal.Id, UpdateNotes("new notes", RowVersionCodec.Encode(1)));
+
+        result.RowVersion.Should().Be(RowVersionCodec.Encode(2));
+        await using var check = _contextFactory();
+        (await check.AuditLogs.CountAsync(a => a.ProposalId == proposal.Id)).Should().Be(1);
+        (await check.Proposals.SingleAsync(p => p.Id == proposal.Id)).Version.Should().Be(2);
+    }
+
+    [Fact(DisplayName = "Bulk line-item update is guarded by the proposal token")]
+    public async Task BulkUpdate_StaleProposalToken_Conflicts()
+    {
+        var proposal = SeedProposal();
+        var request = new BulkUpdateLineItemsRequest(
+            new List<UpdateLineItemEntry>
+            {
+                new(null, "Item", 1, "each", null, 100m, PricingMode.TotalPrice, 0, LineItemSource.Manual),
+            },
+            RowVersionCodec.Encode(42));
+
+        var act = () => _lineItems.BulkUpdateAsync(proposal.Id, request);
+
+        await act.Should().ThrowAsync<ProposalConcurrencyException>();
+    }
+
+    [Fact(DisplayName = "Line-item create bumps the proposal aggregate version")]
+    public async Task LineItemCreate_BumpsProposalVersion()
+    {
+        var proposal = SeedProposal();
+
+        var created = await _lineItems.CreateAsync(proposal.Id,
+            new CreateLineItemRequest("Item", 1, "each", null, 50m, PricingMode.TotalPrice, 0, LineItemSource.Manual));
+
+        created.RowVersion.Should().Be(RowVersionCodec.Encode(1));
+        await using var check = _contextFactory();
+        (await check.Proposals.SingleAsync(p => p.Id == proposal.Id)).Version.Should().Be(2);
+    }
+
+    [Fact(DisplayName = "Approve with the current token succeeds and returns the bumped token")]
+    public async Task Approve_WithCurrentToken_Succeeds()
+    {
+        var proposal = SeedProposal();
+        _db.LineItems.Add(new LineItem
+        {
+            Id = Guid.NewGuid(),
+            ProposalId = proposal.Id,
+            Description = "Priced item",
+            Quantity = 1,
+            Unit = "each",
+            TotalPrice = 500m,
+            PricingMode = PricingMode.TotalPrice,
+            Source = LineItemSource.Manual,
+        });
+        _db.SaveChanges();
+
+        var result = await _sut.ApproveAsync(proposal.Id, RowVersionCodec.Encode(1));
+
+        result.Status.Should().Be(ProposalStatus.Approved);
+        result.RowVersion.Should().Be(RowVersionCodec.Encode(2));
+    }
+}
diff --git a/api/tests/ProposalSystem.Tests/Services/ProposalDeliveryTests.cs b/api/tests/ProposalSystem.Tests/Services/ProposalDeliveryTests.cs
index 157dac2..bd23b61 100644
--- a/api/tests/ProposalSystem.Tests/Services/ProposalDeliveryTests.cs
+++ b/api/tests/ProposalSystem.Tests/Services/ProposalDeliveryTests.cs
@@ -108,7 +108,7 @@ public class ProposalDeliveryTests : IDisposable
             .Throws(new InvalidOperationException("SES is down"));
 
         // Act — should NOT throw
-        var result = await _sut.MarkSentAsync(proposal.Id);
+        var result = await _sut.MarkSentAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert — transition completed despite email failure
         result.Status.Should().Be(ProposalStatus.Sent);
@@ -146,7 +146,7 @@ public class ProposalDeliveryTests : IDisposable
             .Returns("https://s3.example.com/acme.pdf");
 
         // Act
-        var result = await _sut.MarkSentAsync(proposal.Id);
+        var result = await _sut.MarkSentAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         result.Status.Should().Be(ProposalStatus.Sent);
@@ -176,7 +176,7 @@ public class ProposalDeliveryTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var result = await _sut.MarkSentAsync(proposal.Id);
+        var result = await _sut.MarkSentAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         result.Status.Should().Be(ProposalStatus.Sent);
@@ -193,7 +193,7 @@ public class ProposalDeliveryTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var result = await _sut.MarkSentAsync(proposal.Id);
+        var result = await _sut.MarkSentAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         result.Status.Should().Be(ProposalStatus.Sent);
@@ -219,7 +219,7 @@ public class ProposalDeliveryTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var result = await _sut.MarkSentAsync(proposal.Id);
+        var result = await _sut.MarkSentAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         result.Status.Should().Be(ProposalStatus.Sent);
@@ -248,4 +248,7 @@ public class ProposalDeliveryTests : IDisposable
             UpdatedAt = DateTime.UtcNow.AddDays(-1),
         };
     }
+    private string Ver(Guid id) =>
+        ProposalSystem.Application.Common.RowVersionCodec.Encode(_db.Proposals.Find(id)?.Version ?? 1);
+
 }
diff --git a/api/tests/ProposalSystem.Tests/Services/ProposalStateMachineTests.cs b/api/tests/ProposalSystem.Tests/Services/ProposalStateMachineTests.cs
index feea3f8..1a3b4c3 100644
--- a/api/tests/ProposalSystem.Tests/Services/ProposalStateMachineTests.cs
+++ b/api/tests/ProposalSystem.Tests/Services/ProposalStateMachineTests.cs
@@ -92,7 +92,7 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var result = await _sut.ApproveAsync(proposal.Id);
+        var result = await _sut.ApproveAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         result.Status.Should().Be(ProposalStatus.Approved);
@@ -109,7 +109,7 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var result = await _sut.MarkSentAsync(proposal.Id);
+        var result = await _sut.MarkSentAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         result.Status.Should().Be(ProposalStatus.Sent);
@@ -137,7 +137,7 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var result = await _sut.ReviseAsync(proposal.Id);
+        var result = await _sut.ReviseAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         result.Status.Should().Be(ProposalStatus.InReview);
@@ -159,7 +159,7 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var result = await _sut.ApproveAsync(proposal.Id);
+        var result = await _sut.ApproveAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         result.Status.Should().Be(ProposalStatus.Approved);
@@ -174,7 +174,7 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var result = await _sut.MarkSentAsync(proposal.Id);
+        var result = await _sut.MarkSentAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         result.Status.Should().Be(ProposalStatus.Sent);
@@ -193,7 +193,7 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var act = () => _sut.MarkSentAsync(proposal.Id);
+        var act = () => _sut.MarkSentAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         await act.Should().ThrowAsync<InvalidOperationException>()
@@ -209,7 +209,7 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var act = () => _sut.ReviseAsync(proposal.Id);
+        var act = () => _sut.ReviseAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         await act.Should().ThrowAsync<InvalidOperationException>()
@@ -236,7 +236,7 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var act = () => _sut.ApproveAsync(proposal.Id);
+        var act = () => _sut.ApproveAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         await act.Should().ThrowAsync<InvalidOperationException>()
@@ -252,7 +252,7 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var act = () => _sut.ReviseAsync(proposal.Id);
+        var act = () => _sut.ReviseAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         await act.Should().ThrowAsync<InvalidOperationException>()
@@ -267,7 +267,7 @@ public class ProposalStateMachineTests : IDisposable
         _db.Proposals.Add(proposal);
         await _db.SaveChangesAsync();
 
-        var act = () => _sut.ApproveAsync(proposal.Id);
+        var act = () => _sut.ApproveAsync(proposal.Id, Ver(proposal.Id));
 
         await act.Should().ThrowAsync<InvalidOperationException>()
             .WithMessage("*in review*");
@@ -282,7 +282,7 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var act = () => _sut.ApproveAsync(proposal.Id);
+        var act = () => _sut.ApproveAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         await act.Should().ThrowAsync<InvalidOperationException>()
@@ -309,7 +309,7 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var act = () => _sut.ApproveAsync(proposal.Id);
+        var act = () => _sut.ApproveAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         await act.Should().ThrowAsync<InvalidOperationException>()
@@ -321,9 +321,9 @@ public class ProposalStateMachineTests : IDisposable
     {
         var missingId = Guid.NewGuid();
 
-        var approveAct = () => _sut.ApproveAsync(missingId);
-        var sendAct = () => _sut.MarkSentAsync(missingId);
-        var reviseAct = () => _sut.ReviseAsync(missingId);
+        var approveAct = () => _sut.ApproveAsync(missingId, Ver(missingId));
+        var sendAct = () => _sut.MarkSentAsync(missingId, Ver(missingId));
+        var reviseAct = () => _sut.ReviseAsync(missingId, Ver(missingId));
 
         await approveAct.Should().ThrowAsync<KeyNotFoundException>();
         await sendAct.Should().ThrowAsync<KeyNotFoundException>();
@@ -368,7 +368,7 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        var result = await _sut.ReviseAsync(proposal.Id);
+        var result = await _sut.ReviseAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         var revision = await _db.Proposals.FindAsync(result.Id);
@@ -398,10 +398,10 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        await _sut.ApproveAsync(proposal.Id);
+        await _sut.ApproveAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
-        await _audit.Received(1).LogAsync(AuditAction.Approve, proposal.Id, Arg.Any<string?>(), Arg.Any<CancellationToken>());
+        _audit.Received(1).Stage(AuditAction.Approve, proposal.Id, Arg.Any<string?>());
     }
 
     [Fact(DisplayName = "QA-C2: MarkSent publishes library-ingest job")]
@@ -413,7 +413,7 @@ public class ProposalStateMachineTests : IDisposable
         await _db.SaveChangesAsync();
 
         // Act
-        await _sut.MarkSentAsync(proposal.Id);
+        await _sut.MarkSentAsync(proposal.Id, Ver(proposal.Id));
 
         // Assert
         await _jobPublisher.Received(1).PublishAsync("library-ingest", Arg.Any<object>(), Arg.Any<CancellationToken>());
@@ -442,4 +442,7 @@ public class ProposalStateMachineTests : IDisposable
             UpdatedAt = DateTime.UtcNow.AddDays(-1),
         };
     }
+    private string Ver(Guid id) =>
+        ProposalSystem.Application.Common.RowVersionCodec.Encode(_db.Proposals.Find(id)?.Version ?? 1);
+
 }