diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDriver.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDriver.cs
index d8324306..1a2e8285 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDriver.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDriver.cs
@@ -895,8 +895,16 @@ public sealed class MTConnectDriver
     ///         happily, making the corruption invisible until someone reads the observation back.
     ///         TIME_SERIES / DATA_SET writes are out of scope for v1.
     ///     </para>
+    ///     <para>
+    ///         <b>Every formatting throw is caught, not just the documented conversion ones.</b> The
+    ///         rendering runs the value's own <see cref="object.ToString"/> /
+    ///         <see cref="IFormattable"/>, which is arbitrary code — and this runs OUTSIDE
+    ///         <see cref="WriteAsync"/>'s per-slot <c>try</c>, so anything that escapes here escapes
+    ///         the whole batch and breaks its never-throws contract. It costs its own slot a
+    ///         <c>BadTypeMismatch</c> and is logged.
+    ///     </para>
     /// </remarks>
-    private static bool TryFormatValue(object? value, out string text)
+    private bool TryFormatValue(object? value, out string text)
     {
         text = string.Empty;
 
@@ -912,8 +920,20 @@ public sealed class MTConnectDriver
         {
             rendered = Convert.ToString(value, CultureInfo.InvariantCulture);
         }
-        catch (Exception ex) when (ex is FormatException or InvalidCastException or OverflowException)
+        catch (Exception ex)
         {
+            // Catch-all, and deliberately so. Convert.ToString falls back to the value's OWN
+            // ToString/IFormattable, which is arbitrary caller code that may throw ANYTHING — and this
+            // call sits outside WriteAsync's per-slot try, so a narrower filter (FormatException /
+            // InvalidCastException / OverflowException) let every other exception escape WriteAsync
+            // uncaught and break the documented never-throws contract for the whole batch. One
+            // unformattable value must cost its own slot, nothing more.
+            _logger.LogWarning(
+                ex,
+                "MTConnect driver {DriverInstanceId} could not render a write value of type {ValueType} to its invariant string form; the write is refused as a type mismatch.",
+                _driverInstanceId,
+                value.GetType());
+
             return false;
         }
 
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.Tests/MTConnectWriteTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.Tests/MTConnectWriteTests.cs
index 89b30f14..936f3b6b 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.Tests/MTConnectWriteTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.Tests/MTConnectWriteTests.cs
@@ -191,6 +191,36 @@ public sealed class MTConnectWriteTests
         client.Writes.ShouldBeEmpty();
     }
 
+    [Fact]
+    public async Task A_value_whose_own_formatter_throws_costs_its_slot_and_nothing_more()
+    {
+        // Convert.ToString runs the value's OWN ToString — arbitrary code that may throw anything —
+        // and it runs OUTSIDE WriteAsync's per-slot try. A narrow catch filter let every exception
+        // other than Format/InvalidCast/Overflow escape WriteAsync and fail the WHOLE batch, breaking
+        // the never-throws contract. The load-bearing half of this test is the second slot: the good
+        // write still happens.
+        var (driver, client) = await WritableDriverAsync();
+
+        var results = await driver.WriteAsync(
+            [
+                new WriteRequest(PositionRawPath, new ThrowingFormatter()),
+                new WriteRequest(PositionRawPath, 42.5),
+            ],
+            Ct);
+
+        results.Count.ShouldBe(2);
+        results[0].StatusCode.ShouldBe(BadTypeMismatch);
+        results[1].StatusCode.ShouldBe(Good);
+        client.Writes.ShouldHaveSingleItem().Value.ShouldBe("42.5");
+    }
+
+    /// <summary>A value whose <see cref="ToString"/> throws something no conversion filter would name.</summary>
+    private sealed class ThrowingFormatter
+    {
+        public override string ToString() =>
+            throw new InvalidOperationException("this formatter is broken");
+    }
+
     // ---- what actually reaches the seam ----
 
     [Fact]
@@ -322,6 +352,45 @@ public sealed class MTConnectWriteTests
         client.Writes.ShouldBeEmpty();
     }
 
+    [Fact]
+    public async Task Cancellation_mid_batch_leaves_the_slots_already_written_with_their_real_outcomes()
+    {
+        // The fill must start at the cancelled slot, not overwrite the batch. Slot 0 completed against
+        // the Agent before the token was cancelled, and reporting it BadCommunicationError would tell
+        // an operator a write failed that actually landed — the one thing worse than reporting a
+        // failure. Cancellation is injected from inside slot 1's own formatting, so the ordering is a
+        // fact of the run rather than a timing hope.
+        var (driver, client) = await WritableDriverAsync();
+        using var cts = new CancellationTokenSource();
+
+        var results = await driver.WriteAsync(
+            [
+                new WriteRequest(PositionRawPath, 42.5),
+                new WriteRequest(PositionRawPath, new CancellingFormatter(cts)),
+                new WriteRequest(PositionRawPath, 7.5),
+            ],
+            cts.Token);
+
+        results.Count.ShouldBe(3);
+        results[0].StatusCode.ShouldBe(Good);
+        results[1].StatusCode.ShouldBe(BadCommunicationError);
+        results[2].StatusCode.ShouldBe(BadCommunicationError);
+
+        // Exactly the pre-cancellation slot reached the Agent.
+        client.Writes.ShouldHaveSingleItem().Value.ShouldBe("42.5");
+    }
+
+    /// <summary>Cancels the batch from inside its own rendering, then renders normally.</summary>
+    private sealed class CancellingFormatter(CancellationTokenSource cts)
+    {
+        public override string ToString()
+        {
+            cts.Cancel();
+
+            return "99";
+        }
+    }
+
     [Fact]
     public async Task One_bad_request_does_not_fail_the_others_in_the_batch()
     {