diff --git a/src/ZB.MOM.WW.ScadaBridge.InboundAPI/ApiMethodCache.cs b/src/ZB.MOM.WW.ScadaBridge.InboundAPI/ApiMethodCache.cs
index 0d4c4a14..dab26a3e 100644
--- a/src/ZB.MOM.WW.ScadaBridge.InboundAPI/ApiMethodCache.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.InboundAPI/ApiMethodCache.cs
@@ -34,6 +34,8 @@ public sealed class ApiMethodCache
     private sealed record Entry(ApiMethod? Method, DateTimeOffset ExpiresAtUtc);
 
     private readonly ConcurrentDictionary<string, Entry> _entries = new(StringComparer.Ordinal);
+    private const int PruneThreshold = 256;
+    private const int HardCap = 4096;
     private readonly TimeSpan _ttl;
     private readonly TimeProvider _timeProvider;
 
@@ -73,10 +75,34 @@ public sealed class ApiMethodCache
         }
 
         var method = await fetch(cancellationToken).ConfigureAwait(false);
+        if (_entries.Count >= PruneThreshold)
+            PruneExpired(now);
         _entries[methodName] = new Entry(method, now + _ttl);
         return method;
     }
 
+    /// <summary>
+    /// Opportunistically removes expired entries on a miss once the dictionary is large, and
+    /// clears it outright at the hard cap. Growth is bounded today by the invariant that
+    /// <c>EndpointExtensions</c> reaches this cache only after the in-memory scope check, and
+    /// a key's scopes are EXACT method names (no wildcard/prefix scope exists in
+    /// <c>ZB.MOM.WW.Auth.ApiKeys</c>; pinned by <c>EndpointExtensionsTests</c>). Should a
+    /// wildcard scope ever be added, every probed path would otherwise be a permanent
+    /// negative entry — this prune is the safety net.
+    /// </summary>
+    private void PruneExpired(DateTimeOffset now)
+    {
+        foreach (var kv in _entries)
+        {
+            if (kv.Value.ExpiresAtUtc <= now)
+                _entries.TryRemove(kv);
+        }
+
+        // Still at the cap with nothing expired: drop everything (entries are re-fetchable).
+        if (_entries.Count >= HardCap)
+            _entries.Clear();
+    }
+
     /// <summary>Drops the cached entry for one method name. Safe to call for an unknown name.</summary>
     /// <param name="methodName">The method name to invalidate.</param>
     public void Invalidate(string methodName) => _entries.TryRemove(methodName, out _);
diff --git a/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/ApiMethodCacheTests.cs b/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/ApiMethodCacheTests.cs
index b4980ce9..dbeb0188 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/ApiMethodCacheTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/ApiMethodCacheTests.cs
@@ -122,4 +122,24 @@ public class ApiMethodCacheTests
         /// <inheritdoc />
         public override DateTimeOffset GetUtcNow() => _now;
     }
+
+    /// <summary>Verifies that expired entries are pruned on a miss once the cache is large.</summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task GetOrFetchAsync_Miss_PrunesExpiredEntries()
+    {
+        // Review 2026-10-21 05/M3: expired entries for methods no longer called were never evicted.
+        var time = new FakeTimeProvider(DateTimeOffset.UtcNow);
+        var cache = new ApiMethodCache(TimeSpan.FromSeconds(1), time);
+        Task<ApiMethod?> Fetch(CancellationToken _) => Task.FromResult<ApiMethod?>(null);
+
+        for (var i = 0; i < 300; i++)
+            await cache.GetOrFetchAsync("m" + i, Fetch, CancellationToken.None);
+        Assert.Equal(300, cache.Count);
+
+        time.Advance(TimeSpan.FromSeconds(2));
+        await cache.GetOrFetchAsync("fresh", Fetch, CancellationToken.None);
+
+        Assert.Equal(1, cache.Count);
+    }
 }
diff --git a/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/EndpointExtensionsTests.cs b/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/EndpointExtensionsTests.cs
index b17e6237..00f2ac98 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/EndpointExtensionsTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/EndpointExtensionsTests.cs
@@ -327,6 +327,27 @@ public sealed class EndpointExtensionsTests : IDisposable
         Assert.Equal(NotApprovedBodyJson, body);
     }
 
+    /// <summary>Verifies that scopes are exact method names, not prefixes or wildcards.</summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Theory]
+    [InlineData("*")]
+    [InlineData("sec*")]
+    [InlineData("sec")]
+    public async Task ValidKey_WildcardOrPrefixScope_DoesNotGrantMethod(string scope)
+    {
+        // Review 2026-10-21 05/M3: ApiMethodCache bounds its growth on the invariant that scopes
+        // are exact names; pin it so a future wildcard scope has to revisit the cache.
+        var method = SeedMethod(1, "secured", "return 1;");
+
+        using var host = await BuildHostAsync(method);
+        var token = await SeedKeyAsync(host, "key1", "caller", new[] { scope });
+        var client = host.GetTestClient();
+
+        var response = await client.SendAsync(BuildPost("secured", "{}", token));
+
+        Assert.Equal(HttpStatusCode.Forbidden, response.StatusCode);
+    }
+
     /// <summary>Verifies that an unknown method returns 403 with a body identical to the not-in-scope body.</summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]