feat(mcp): an agent can finally read back what it recorded to the SD card

README.md

@@ -23,7 +23,7 @@ DAQiFi builds wireless data acquisition hardware designed to get out of the way
 
 Prefer a ready-made GUI? Check out [DAQiFi Desktop](https://github.com/daqifi/daqifi-desktop), which is built on top of this library.
 
-Want to drive a device from an AI assistant? The repo also ships an **[MCP server](src/Daqifi.Mcp)** — point Claude, Cursor, Codex, or any MCP-aware client at it to discover, configure channels, drive digital I/O and PWM outputs, set the sample rate, and run SD-card logging through plain conversation.
+Want to drive a device from an AI assistant? The repo also ships an **[MCP server](src/Daqifi.Mcp)** — point Claude, Cursor, Codex, or any MCP-aware client at it to discover, configure channels, drive digital I/O and PWM outputs, set the sample rate, and run SD-card logging — then list, download, and CSV the recorded data back — through plain conversation.
 
 ## See it in 30 seconds
 
@@ -70,7 +70,7 @@ More examples at [daqifi.com](https://daqifi.com).
 | Hardware | Nyquist 1 / Nyquist 3 — wireless DAQ devices (and their on-device firmware) |
 | **SDK** | **DAQiFi Core — this library** |
 | App | [DAQiFi Desktop](https://github.com/daqifi/daqifi-desktop) — GUI built on this SDK |
-| Agent | [MCP server](src/Daqifi.Mcp) — drive a device from Claude / Cursor / any MCP client: discover, configure channels, DIO/PWM, and SD logging |
+| Agent | [MCP server](src/Daqifi.Mcp) — drive a device from Claude / Cursor / any MCP client: discover, configure channels, DIO/PWM, SD logging, and SD data retrieval |
 | Your code | Custom apps, dashboards, pipelines, test rigs |
 
 ## What you can do

src/Daqifi.Core.Tests/Device/SdCard/SdCardDeviceConfigurationTests.cs

@@ -0,0 +1,84 @@
+using System;
+using System.Diagnostics;
+using System.Threading;
+using System.Threading.Tasks;
+using Daqifi.Core.Device;
+using Daqifi.Core.Device.SdCard;
+using Xunit;
+
+namespace Daqifi.Core.Tests.Device.SdCard;
+
+/// <summary>
+/// Tests for building an <see cref="SdCardDeviceConfiguration"/> from a live device.
+/// </summary>
+public class SdCardDeviceConfigurationTests
+{
+    [Fact]
+    public void FromDevice_NullDevice_Throws()
+    {
+        Assert.Throws<ArgumentNullException>(() => SdCardDeviceConfiguration.FromDevice(null!));
+    }
+
+    [Fact]
+    public void FromDevice_ReportsAnalogAndDigitalCounts()
+    {
+        var device = new DaqifiDevice("TestDevice");
+        device.PopulateChannelsFromStatus(new DaqifiOutMessage { AnalogInPortNum = 4, DigitalPortNum = 2 });
+
+        var config = SdCardDeviceConfiguration.FromDevice(device);
+
+        Assert.NotNull(config);
+        Assert.Equal(4, config.AnalogPortCount);
+        Assert.Equal(2, config.DigitalPortCount);
+    }
+
+    /// <summary>
+    /// The natural moment to build one of these is right before parsing a download — on whichever
+    /// thread the caller is on, while the device's consumer thread is still decoding status
+    /// messages and repopulating the channel collection. Folding the live <c>Channels</c> view
+    /// there throws "Collection was modified"; the snapshot exists so it cannot.
+    /// </summary>
+    [Fact]
+    public async Task FromDevice_WhileStatusMessagesRepopulateChannels_DoesNotThrow()
+    {
+        var device = new DaqifiDevice("TestDevice");
+        var status = new DaqifiOutMessage { AnalogInPortNum = 16, DigitalPortNum = 16, TimestampFreq = 42_000_000 };
+        device.PopulateChannelsFromStatus(status);
+
+        using var stop = new CancellationTokenSource();
+        Exception? failure = null;
+
+        // Stands in for the consumer thread: repopulating clears and refills the backing list, so
+        // an enumeration of the live view that spans it observes a half-built collection.
+        var repopulate = Task.Run(() =>
+        {
+            while (!stop.IsCancellationRequested)
+            {
+                device.PopulateChannelsFromStatus(status);
+            }
+        });
+
+        try
+        {
+            var watch = Stopwatch.StartNew();
+            for (var i = 0; i < 5_000 && failure is null && watch.Elapsed < TimeSpan.FromSeconds(5); i++)
+            {
+                try
+                {
+                    Assert.NotNull(SdCardDeviceConfiguration.FromDevice(device));
+                }
+                catch (Exception ex)
+                {
+                    failure = ex;
+                }
+            }
+        }
+        finally
+        {
+            stop.Cancel();
+            await repopulate;
+        }
+
+        Assert.Null(failure);
+    }
+}

src/Daqifi.Core/Device/SdCard/SdCardDeviceConfiguration.cs

@@ -47,13 +47,22 @@ public sealed record SdCardDeviceConfiguration(
     /// <returns>A configuration snapshot, or <c>null</c> if the device has no analog channels.</returns>
     public static SdCardDeviceConfiguration? FromDevice(DaqifiDevice device)
     {
-        var analogChannels = device.Channels.OfType<IAnalogChannel>().ToList();
+        ArgumentNullException.ThrowIfNull(device);
+
+        // Taken through GetChannelsSnapshot rather than the live Channels view: this runs on
+        // whatever thread the caller is on (a download about to be parsed, an MCP tool dispatch)
+        // while the device's consumer thread can be repopulating the collection from a status
+        // message. Folding the live view there is exactly what DaqifiDevice.Channels documents as
+        // unsafe — it throws "Collection was modified" mid-enumeration. One snapshot also makes
+        // the analog and digital counts below describe the same instant instead of two.
+        var channels = device.GetChannelsSnapshot();
+        var analogChannels = channels.OfType<IAnalogChannel>().ToList();
         if (analogChannels.Count == 0)
         {
             return null;
         }
 
-        var digitalCount = device.Channels.Count(c => c.Type == ChannelType.Digital);
+        var digitalCount = channels.Count(c => c.Type == ChannelType.Digital);
         var firstAnalog = analogChannels[0];
 
         return new SdCardDeviceConfiguration(