Resolve Tests-001 and Tests-002 code-review findings

Tests-001: FakeSessionManager.TryGetSession unconditionally synthesized a
session, so Invoke_WhenSessionMissing_ThrowsNotFound did not actually
verify the missing-session path. Added ResolveOnlySeededSessions/SeedSession
to the fake, rewrote the missing-session test, and added seeded-resolution
and alarm-RPC missing-session coverage.

Tests-002: re-triaged. GalaxyRepository issues only constant SQL; filters
are applied in-memory by GalaxyHierarchyProjector/GalaxyGlobMatcher. Kept
as a valid coverage gap and added GalaxyFilterInputSafetyTests exercising
filter/glob input safety directly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

code-reviews/Tests/findings.md

@@ -7,7 +7,7 @@
 | Review date | 2026-05-18 |
 | Commit reviewed | `6c64030` |
 | Status | Reviewed |
-| Open findings | 12 |
+| Open findings | 10 |
 
 ## Checklist coverage
 
@@ -33,13 +33,13 @@
 | Severity | High |
 | Category | Testing coverage |
 | Location | `src/MxGateway.Tests/Gateway/Grpc/MxAccessGatewayServiceTests.cs:483-489` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** `FakeSessionManager.TryGetSession` unconditionally returns `true` and synthesizes a session for any id. As a result, `Invoke_WhenSessionMissing_ThrowsNotFound` (line 52) only passes because `InvokeException` is pre-seeded — it does not verify that the gateway service maps a genuinely missing session to `NotFound`. No test exercises the real gateway path where `TryGetSession` returns `false` (for `StreamEvents`, `CloseSession`, alarm RPCs). A regression dropping the missing-session check would not be caught.
 
 **Recommendation:** Make `FakeSessionManager.TryGetSession` return `false` for unknown ids (return only seeded sessions), then assert `NotFound`/`InvalidArgument` is produced by the service's own lookup logic rather than an injected exception.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-18: confirmed root cause — added `ResolveOnlySeededSessions`/`SeedSession` to `FakeSessionManager` so `TryGetSession` returns `false` for unseeded ids, rewrote `Invoke_WhenSessionMissing_ThrowsNotFound` to drop the injected `InvokeException` and exercise the service's own `ResolveSession` lookup (asserts `InvokeCount == 0`), and added `Invoke_WhenSessionSeeded_ResolvesAndInvokes`, `AcknowledgeAlarm_WhenSessionMissing_ThrowsNotFound`, and `QueryActiveAlarms_WhenSessionMissing_ThrowsNotFound`.
 
 ### Tests-002
 
@@ -48,13 +48,15 @@
 | Severity | High |
 | Category | Security |
 | Location | `src/MxGateway.Tests/Gateway/Grpc/GalaxyRepositoryGrpcServiceTests.cs:198-210` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** The Galaxy Repository RPCs browse a SQL Server database (`ZB`). Every test injects a `StubGalaxyHierarchyCache`, so actual SQL query construction, parameterization, and filter/glob translation are never exercised. No test demonstrates that `TagNameGlob`, `RootTagName`, `AlarmFilterPrefix`, etc. are passed as parameters rather than concatenated into SQL. SQL-injection resistance of the Galaxy layer has zero coverage.
 
 **Recommendation:** Add tests for the `GalaxyRepository` query-building layer (against SQLite or an in-memory abstraction, or by asserting parameter objects), covering glob/prefix inputs containing `'`, `%`, `_`, and `;`. At minimum add a unit test over the SQL `LIKE`-pattern escaping helper.
 
-**Resolution:** _(open)_
+**Re-triage note:** The finding's premise is partly misframed. `GalaxyRepository` issues only four *constant* SQL statements (`HierarchySql`, `AttributesSql`, `SELECT 1`, `SELECT time_of_last_deploy FROM galaxy`) — no `DiscoverHierarchyRequest` field is ever concatenated into SQL, so there is no dynamic SQL-injection surface and no `LIKE`-escaping helper to test. `AlarmFilterPrefix` belongs to the worker alarm path, not the Galaxy SQL layer. All filters (`TagNameGlob`, `RootTagName`, template-chain, category, contained-path) are applied **in memory** by `GalaxyHierarchyProjector`/`GalaxyGlobMatcher` against the cached snapshot. The genuine, testable concern — that adversarial filter strings are treated as opaque literals (no wildcard behaviour, no ReDoS, no exceptions) — remains valid and was previously uncovered. Severity left at High: an unsafe in-memory filter would still be a real security gap.
+
+**Resolution:** Resolved 2026-05-18: added `src/MxGateway.Tests/Galaxy/GalaxyFilterInputSafetyTests.cs` (10 test methods, mostly `[Theory]` over adversarial inputs `'`, `' OR '1'='1`, `'; DROP TABLE gobject;--`, `%`, `_`, `100%_off`, `[abc]`, `Pump'001`) covering `GalaxyGlobMatcher` literal-treatment / `LIKE`-wildcard / pathological-input (ReDoS) behaviour and `GalaxyHierarchyProjector` + `DiscoverHierarchy` RPC handling of adversarial `TagNameGlob`, `RootTagName`, and `TemplateChainContains`. No product bug found — the in-memory filter layer treats all metacharacters as literals; the passing tests resolve the coverage gap.
 
 ### Tests-003
 

src/MxGateway.Tests/Galaxy/GalaxyFilterInputSafetyTests.cs

@@ -0,0 +1,331 @@
+using System.Diagnostics;
+using Grpc.Core;
+using Microsoft.Extensions.Logging.Abstractions;
+using MxGateway.Contracts.Proto.Galaxy;
+using MxGateway.Server.Dashboard;
+using MxGateway.Server.Galaxy;
+using MxGateway.Server.Grpc;
+using MxGateway.Server.Security.Authorization;
+
+namespace MxGateway.Tests.Galaxy;
+
+/// <summary>
+///     Adversarial-input coverage for the Galaxy Repository browse filter layer.
+///     <para>
+///         Re-triage note (finding Tests-002): the Galaxy Repository's SQL surface
+///         (<c>HierarchySql</c>, <c>AttributesSql</c>, <c>SELECT 1</c>,
+///         <c>SELECT time_of_last_deploy FROM galaxy</c>) is entirely constant — no
+///         <see cref="DiscoverHierarchyRequest"/> field is ever concatenated into a SQL
+///         string. All filters (<c>TagNameGlob</c>, <c>RootTagName</c>, category ids,
+///         template-chain filters, contained-path roots) are applied in memory by
+///         <see cref="GalaxyHierarchyProjector"/> against the cached snapshot, so there is
+///         no SQL-injection surface and no <c>LIKE</c>-escaping helper to test.
+///     </para>
+///     <para>
+///         The genuine, testable concern is that adversarial filter strings — SQL
+///         metacharacters (<c>'</c>, <c>;</c>) and <c>LIKE</c>-wildcards (<c>%</c>,
+///         <c>_</c>) — are treated as opaque literals by the in-memory filter layer:
+///         they must never act as wildcards, never throw, and never trigger catastrophic
+///         regex backtracking in <see cref="GalaxyGlobMatcher"/>.
+///     </para>
+/// </summary>
+public sealed class GalaxyFilterInputSafetyTests
+{
+    private static readonly string[] AdversarialInputs =
+    [
+        "'",
+        "' OR '1'='1",
+        "'; DROP TABLE gobject;--",
+        "%",
+        "_",
+        "100%_off",
+        "[abc]",
+        "Pump'001",
+    ];
+
+    public static TheoryData<string> AdversarialInputCases()
+    {
+        TheoryData<string> data = [];
+        foreach (string input in AdversarialInputs)
+        {
+            data.Add(input);
+        }
+
+        return data;
+    }
+
+    /// <summary>
+    ///     Verifies <see cref="GalaxyGlobMatcher"/> treats SQL metacharacters and
+    ///     <c>LIKE</c>-wildcards as literals — a glob equal to the literal value matches,
+    ///     and the same glob does not spuriously match an unrelated value.
+    /// </summary>
+    [Theory]
+    [MemberData(nameof(AdversarialInputCases))]
+    public void GlobMatcher_TreatsSqlMetacharactersAsLiterals(string input)
+    {
+        Assert.True(
+            GalaxyGlobMatcher.IsMatch(input, input),
+            $"A glob equal to the literal value should match: {input}");
+        Assert.False(
+            GalaxyGlobMatcher.IsMatch("UnrelatedTagName", input),
+            $"Adversarial glob must not behave as a wildcard against unrelated text: {input}");
+    }
+
+    /// <summary>
+    ///     Verifies the SQL <c>LIKE</c> wildcards <c>%</c> and <c>_</c> are NOT treated as
+    ///     wildcards by the glob matcher; only <c>*</c> and <c>?</c> are glob wildcards.
+    /// </summary>
+    [Fact]
+    public void GlobMatcher_DoesNotTreatLikeWildcardsAsWildcards()
+    {
+        // '%' would match anything if interpreted as a SQL LIKE wildcard.
+        Assert.False(GalaxyGlobMatcher.IsMatch("Pump_001", "%"));
+        // '_' would match a single character if interpreted as a SQL LIKE wildcard.
+        Assert.False(GalaxyGlobMatcher.IsMatch("A", "_"));
+        Assert.True(GalaxyGlobMatcher.IsMatch("_", "_"));
+        // '*' and '?' remain glob wildcards.
+        Assert.True(GalaxyGlobMatcher.IsMatch("Pump_001", "Pump*"));
+        Assert.True(GalaxyGlobMatcher.IsMatch("Pump_001", "Pump_00?"));
+    }
+
+    /// <summary>
+    ///     Verifies a pathological glob does not cause catastrophic regex backtracking —
+    ///     <see cref="GalaxyGlobMatcher"/> escapes every literal character and applies a
+    ///     100 ms regex timeout, so a long adversarial input completes promptly.
+    /// </summary>
+    [Fact]
+    public void GlobMatcher_WithPathologicalInput_DoesNotHang()
+    {
+        string pathologicalGlob = new string('a', 5000) + "!";
+        string pathologicalValue = new string('a', 5000);
+
+        Stopwatch stopwatch = Stopwatch.StartNew();
+        bool matched = GalaxyGlobMatcher.IsMatch(pathologicalValue, pathologicalGlob);
+        stopwatch.Stop();
+
+        Assert.False(matched);
+        Assert.True(
+            stopwatch.Elapsed < TimeSpan.FromSeconds(2),
+            $"Glob matching took {stopwatch.ElapsedMilliseconds} ms — expected sub-second.");
+    }
+
+    /// <summary>
+    ///     Verifies the <see cref="GalaxyHierarchyProjector"/> <c>TagNameGlob</c> filter
+    ///     treats an adversarial glob as a literal: it never wildcard-matches the whole
+    ///     hierarchy and never throws.
+    /// </summary>
+    [Theory]
+    [MemberData(nameof(AdversarialInputCases))]
+    public void Projector_TagNameGlob_WithAdversarialInput_DoesNotMatchEverything(string glob)
+    {
+        GalaxyHierarchyCacheEntry entry = CreateEntry(CreateObjects());
+
+        GalaxyHierarchyQueryResult result = GalaxyHierarchyProjector.Project(
+            entry,
+            new DiscoverHierarchyRequest { TagNameGlob = glob });
+
+        // None of the seeded tag names equal an adversarial string, so a correctly
+        // literal filter returns zero matches rather than the whole hierarchy.
+        Assert.Equal(0, result.TotalObjectCount);
+        Assert.Empty(result.Objects);
+    }
+
+    /// <summary>
+    ///     Verifies an adversarial <c>RootTagName</c> resolves through the projector as a
+    ///     literal — an exact-match lookup that finds nothing and surfaces NotFound,
+    ///     never matching unrelated objects or throwing an unexpected exception.
+    /// </summary>
+    [Theory]
+    [MemberData(nameof(AdversarialInputCases))]
+    public void Projector_RootTagName_WithAdversarialInput_ThrowsNotFound(string rootTagName)
+    {
+        GalaxyHierarchyCacheEntry entry = CreateEntry(CreateObjects());
+
+        RpcException exception = Assert.Throws<RpcException>(
+            () => GalaxyHierarchyProjector.Project(
+                entry,
+                new DiscoverHierarchyRequest { RootTagName = rootTagName }));
+
+        Assert.Equal(StatusCode.NotFound, exception.StatusCode);
+    }
+
+    /// <summary>
+    ///     Verifies an adversarial <c>TemplateChainContains</c> filter is a literal
+    ///     substring test — it never matches unrelated template chains and never throws.
+    /// </summary>
+    [Theory]
+    [MemberData(nameof(AdversarialInputCases))]
+    public void Projector_TemplateChainContains_WithAdversarialInput_MatchesNothing(string filter)
+    {
+        GalaxyHierarchyCacheEntry entry = CreateEntry(CreateObjects());
+        DiscoverHierarchyRequest request = new();
+        request.TemplateChainContains.Add(filter);
+
+        GalaxyHierarchyQueryResult result = GalaxyHierarchyProjector.Project(entry, request);
+
+        Assert.Equal(0, result.TotalObjectCount);
+    }
+
+    /// <summary>
+    ///     Verifies the <see cref="GalaxyRepositoryGrpcService.DiscoverHierarchy"/> RPC
+    ///     handles an adversarial <c>TagNameGlob</c> end-to-end: the request succeeds with
+    ///     zero matches rather than returning the whole hierarchy or faulting.
+    /// </summary>
+    [Theory]
+    [MemberData(nameof(AdversarialInputCases))]
+    public async Task DiscoverHierarchy_WithAdversarialTagNameGlob_ReturnsZeroMatches(string glob)
+    {
+        GalaxyRepositoryGrpcService service = CreateService(CreateEntry(CreateObjects()));
+
+        DiscoverHierarchyReply reply = await service.DiscoverHierarchy(
+            new DiscoverHierarchyRequest { TagNameGlob = glob, PageSize = 100 },
+            new TestServerCallContext());
+
+        Assert.Equal(0, reply.TotalObjectCount);
+        Assert.Empty(reply.Objects);
+    }
+
+    /// <summary>
+    ///     Verifies the <see cref="GalaxyRepositoryGrpcService.DiscoverHierarchy"/> RPC
+    ///     maps an adversarial <c>RootTagName</c> to NotFound rather than executing it as
+    ///     a query fragment or matching unrelated objects.
+    /// </summary>
+    [Theory]
+    [MemberData(nameof(AdversarialInputCases))]
+    public async Task DiscoverHierarchy_WithAdversarialRootTagName_ReturnsNotFound(string rootTagName)
+    {
+        GalaxyRepositoryGrpcService service = CreateService(CreateEntry(CreateObjects()));
+
+        RpcException exception = await Assert.ThrowsAsync<RpcException>(
+            async () => await service.DiscoverHierarchy(
+                new DiscoverHierarchyRequest { RootTagName = rootTagName, PageSize = 100 },
+                new TestServerCallContext()));
+
+        Assert.Equal(StatusCode.NotFound, exception.StatusCode);
+    }
+
+    private static GalaxyRepositoryGrpcService CreateService(GalaxyHierarchyCacheEntry entry)
+    {
+        GalaxyRepositoryOptions options = new()
+        {
+            ConnectionString = "Server=localhost;Database=ZB;Integrated Security=True;Encrypt=False;",
+        };
+        return new GalaxyRepositoryGrpcService(
+            new MxGateway.Server.Galaxy.GalaxyRepository(options),
+            new StubGalaxyHierarchyCache(entry),
+            new GalaxyDeployNotifier(),
+            new GatewayRequestIdentityAccessor(),
+            NullLogger<GalaxyRepositoryGrpcService>.Instance);
+    }
+
+    private static GalaxyHierarchyCacheEntry CreateEntry(IReadOnlyList<GalaxyObject> objects)
+    {
+        return GalaxyHierarchyCacheEntry.Empty with
+        {
+            Status = GalaxyCacheStatus.Healthy,
+            Sequence = 1,
+            LastSuccessAt = DateTimeOffset.UtcNow,
+            Objects = objects,
+            Index = GalaxyHierarchyIndex.Build(objects),
+            DashboardSummary = DashboardGalaxySummary.Unknown with
+            {
+                Status = DashboardGalaxyStatus.Healthy,
+                ObjectCount = objects.Count,
+            },
+            ObjectCount = objects.Count,
+        };
+    }
+
+    private static IReadOnlyList<GalaxyObject> CreateObjects()
+    {
+        return
+        [
+            new GalaxyObject
+            {
+                GobjectId = 1,
+                TagName = "Area1",
+                ContainedName = "Area1",
+                BrowseName = "Area1",
+                IsArea = true,
+                CategoryId = 13,
+            },
+            new GalaxyObject
+            {
+                GobjectId = 2,
+                TagName = "Pump_001",
+                ContainedName = "Pump",
+                BrowseName = "Pump_001",
+                ParentGobjectId = 1,
+                CategoryId = 10,
+                TemplateChain = { "$Pump", "$Base" },
+            },
+            new GalaxyObject
+            {
+                GobjectId = 3,
+                TagName = "Valve_001",
+                ContainedName = "Valve",
+                BrowseName = "Valve_001",
+                ParentGobjectId = 1,
+                CategoryId = 11,
+                TemplateChain = { "$Valve" },
+            },
+        ];
+    }
+
+    private sealed class StubGalaxyHierarchyCache(GalaxyHierarchyCacheEntry current) : IGalaxyHierarchyCache
+    {
+        public GalaxyHierarchyCacheEntry Current { get; } = current;
+
+        public Task RefreshAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+
+        public Task WaitForFirstLoadAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+    }
+
+    private sealed class TestServerCallContext(CancellationToken cancellationToken = default) : ServerCallContext
+    {
+        private readonly Metadata requestHeaders = [];
+        private readonly Metadata responseTrailers = [];
+        private readonly Dictionary<object, object> userState = [];
+        private Status status;
+        private WriteOptions? writeOptions;
+
+        protected override string MethodCore => "/galaxy_repository.v1.GalaxyRepository/DiscoverHierarchy";
+
+        protected override string HostCore => "localhost";
+
+        protected override string PeerCore => "ipv4:127.0.0.1:5000";
+
+        protected override DateTime DeadlineCore => DateTime.UtcNow.AddMinutes(1);
+
+        protected override Metadata RequestHeadersCore => requestHeaders;
+
+        protected override CancellationToken CancellationTokenCore => cancellationToken;
+
+        protected override Metadata ResponseTrailersCore => responseTrailers;
+
+        protected override Status StatusCore
+        {
+            get => status;
+            set => status = value;
+        }
+
+        protected override WriteOptions? WriteOptionsCore
+        {
+            get => writeOptions;
+            set => writeOptions = value;
+        }
+
+        protected override AuthContext AuthContextCore { get; } = new(
+            string.Empty,
+            new Dictionary<string, List<AuthProperty>>(StringComparer.Ordinal));
+
+        protected override IDictionary<object, object> UserStateCore => userState;
+
+        protected override Task WriteResponseHeadersAsyncCore(Metadata responseHeaders) => Task.CompletedTask;
+
+        protected override ContextPropagationToken CreatePropagationTokenCore(ContextPropagationOptions? options)
+        {
+            throw new NotSupportedException();
+        }
+    }
+}

src/MxGateway.Tests/Gateway/Grpc/MxAccessGatewayServiceTests.cs

@@ -47,16 +47,17 @@ public sealed class MxAccessGatewayServiceTests
         Assert.Equal("operator-session", sessionManager.LastOpenRequest?.ClientSessionName);
     }
 
-    /// <summary>Verifies that Invoke throws NotFound when the session does not exist.</summary>
+    /// <summary>
+    ///     Verifies that Invoke maps a genuinely missing session to NotFound via the
+    ///     service's own <c>ResolveSession</c> lookup. No <c>InvokeException</c> is
+    ///     injected — <see cref="FakeSessionManager.ResolveOnlySeededSessions"/> makes
+    ///     <c>TryGetSession</c> return false, so this test fails if the service drops
+    ///     its missing-session check rather than passing for the wrong reason.
+    /// </summary>
     [Fact]
     public async Task Invoke_WhenSessionMissing_ThrowsNotFound()
     {
-        FakeSessionManager sessionManager = new()
-        {
-            InvokeException = new SessionManagerException(
-                SessionManagerErrorCode.SessionNotFound,
-                "Session session-missing was not found."),
-        };
+        FakeSessionManager sessionManager = new() { ResolveOnlySeededSessions = true };
         MxAccessGatewayService service = CreateService(sessionManager);
 
         RpcException exception = await Assert.ThrowsAsync<RpcException>(
@@ -66,6 +67,76 @@ public sealed class MxAccessGatewayServiceTests
 
         Assert.Equal(StatusCode.NotFound, exception.StatusCode);
         Assert.Contains("session-missing", exception.Status.Detail, StringComparison.Ordinal);
+        // The service must reject before delegating to the session manager.
+        Assert.Equal(0, sessionManager.InvokeCount);
+    }
+
+    /// <summary>
+    ///     Verifies that Invoke resolves a session that was seeded into the session
+    ///     manager when <see cref="FakeSessionManager.ResolveOnlySeededSessions"/> is on,
+    ///     confirming the missing-session test above is gated on a real lookup.
+    /// </summary>
+    [Fact]
+    public async Task Invoke_WhenSessionSeeded_ResolvesAndInvokes()
+    {
+        FakeSessionManager sessionManager = new() { ResolveOnlySeededSessions = true };
+        sessionManager.SeedSession(CreateSession("session-1", processId: 1234));
+        MxAccessGatewayService service = CreateService(sessionManager);
+
+        MxCommandReply reply = await service.Invoke(
+            CreatePingRequest("session-1"),
+            new TestServerCallContext());
+
+        Assert.Equal(ProtocolStatusCode.Ok, reply.ProtocolStatus.Code);
+        Assert.Equal(1, sessionManager.InvokeCount);
+    }
+
+    /// <summary>
+    ///     Verifies that AcknowledgeAlarm maps a genuinely missing session to NotFound via
+    ///     the service's own <c>ResolveSession</c> lookup rather than an injected exception.
+    /// </summary>
+    [Fact]
+    public async Task AcknowledgeAlarm_WhenSessionMissing_ThrowsNotFound()
+    {
+        FakeSessionManager sessionManager = new() { ResolveOnlySeededSessions = true };
+        MxAccessGatewayService service = CreateService(sessionManager);
+
+        RpcException exception = await Assert.ThrowsAsync<RpcException>(
+            async () => await service.AcknowledgeAlarm(
+                new AcknowledgeAlarmRequest
+                {
+                    SessionId = "session-missing",
+                    AlarmFullReference = "Tank01.Level.HiHi",
+                    OperatorUser = "alice",
+                },
+                new TestServerCallContext()));
+
+        Assert.Equal(StatusCode.NotFound, exception.StatusCode);
+        Assert.Contains("session-missing", exception.Status.Detail, StringComparison.Ordinal);
+    }
+
+    /// <summary>
+    ///     Verifies that QueryActiveAlarms maps a genuinely missing session to NotFound via
+    ///     the service's own <c>ResolveSession</c> lookup rather than an injected exception.
+    /// </summary>
+    [Fact]
+    public async Task QueryActiveAlarms_WhenSessionMissing_ThrowsNotFound()
+    {
+        FakeSessionManager sessionManager = new() { ResolveOnlySeededSessions = true };
+        MxAccessGatewayService service = CreateService(sessionManager);
+
+        RpcException exception = await Assert.ThrowsAsync<RpcException>(
+            async () => await service.QueryActiveAlarms(
+                new QueryActiveAlarmsRequest
+                {
+                    SessionId = "session-missing",
+                    AlarmFilterPrefix = "Tank01.",
+                },
+                new RecordingStreamWriter<ActiveAlarmSnapshot>(),
+                new TestServerCallContext()));
+
+        Assert.Equal(StatusCode.NotFound, exception.StatusCode);
+        Assert.Contains("session-missing", exception.Status.Detail, StringComparison.Ordinal);
     }
 
     /// <summary>Verifies that Invoke throws InvalidArgument and does not invoke the session manager when payload is mismatched.</summary>
@@ -425,9 +496,26 @@ public sealed class MxAccessGatewayServiceTests
 
     private sealed class FakeSessionManager : ISessionManager
     {
+        private readonly Dictionary<string, GatewaySession> seededSessions = new(StringComparer.Ordinal);
+
         /// <summary>The session to return from OpenSessionAsync.</summary>
         public GatewaySession? OpenSessionResult { get; init; }
 
+        /// <summary>
+        ///     When true, <see cref="TryGetSession"/> only resolves sessions that have been
+        ///     explicitly seeded via <see cref="SeedSession"/> (or <see cref="OpenSessionResult"/>),
+        ///     and returns false for any other id. This exercises the gateway service's own
+        ///     missing-session handling instead of masking it with a synthesized session.
+        /// </summary>
+        public bool ResolveOnlySeededSessions { get; init; }
+
+        /// <summary>Registers a session so <see cref="TryGetSession"/> resolves its id.</summary>
+        /// <param name="session">Session to register by its <see cref="GatewaySession.SessionId"/>.</param>
+        public void SeedSession(GatewaySession session)
+        {
+            seededSessions[session.SessionId] = session;
+        }
+
         /// <summary>The last OpenSessionAsync request captured.</summary>
         public SessionOpenRequest? LastOpenRequest { get; private set; }
 
@@ -484,6 +572,18 @@ public sealed class MxAccessGatewayServiceTests
             string sessionId,
             out GatewaySession session)
         {
+            if (seededSessions.TryGetValue(sessionId, out GatewaySession? seeded))
+            {
+                session = seeded;
+                return true;
+            }
+
+            if (ResolveOnlySeededSessions)
+            {
+                session = null!;
+                return false;
+            }
+
             session = OpenSessionResult ?? CreateSession(sessionId, processId: 1234);
             return true;
         }