diff --git a/src/SyncTrayzor.Tests/ChecksumFileUtilitiesTests.cs b/src/SyncTrayzor.Tests/ChecksumFileUtilitiesTests.cs
new file mode 100644
index 00000000..3fc40b59
--- /dev/null
+++ b/src/SyncTrayzor.Tests/ChecksumFileUtilitiesTests.cs
@@ -0,0 +1,133 @@
+using System;
+using System.IO;
+using System.Security.Cryptography;
+using System.Text;
+using SyncTrayzor.Utils;
+using Xunit;
+
+namespace SyncTrayzor.Tests
+{
+    public class ChecksumFileUtilitiesTests
+    {
+        private static MemoryStream ToStream(string text) =>
+            new(Encoding.UTF8.GetBytes(text));
+
+        private static MemoryStream ToStream(byte[] bytes) => new(bytes);
+
+        // ── WriteChecksumToFile ──────────────────────────────────────────────
+
+        [Fact]
+        public void WriteChecksumToFile_ProducesValidChecksumLine()
+        {
+            var content = Encoding.UTF8.GetBytes("hello world");
+            using var fileStream = ToStream(content);
+            using var checksumStream = new MemoryStream();
+
+            using var sha256 = SHA256.Create();
+            ChecksumFileUtilities.WriteChecksumToFile(sha256, checksumStream, "test.txt", fileStream);
+
+            checksumStream.Position = 0;
+            var line = new StreamReader(checksumStream, Encoding.ASCII).ReadLine();
+
+            // Expected: "<hex>  test.txt"
+            Assert.NotNull(line);
+            var parts = line!.Split("  ", 2);
+            Assert.Equal(2, parts.Length);
+            Assert.Equal("test.txt", parts[1].Trim());
+            Assert.Equal(64, parts[0].Length); // SHA-256 hex is 64 chars
+        }
+
+        // ── ValidateChecksum – positive case ────────────────────────────────
+
+        [Fact]
+        public void ValidateChecksum_ReturnsTrueForMatchingContent()
+        {
+            var content = Encoding.UTF8.GetBytes("hello world");
+            var checksumLine = ComputeChecksumLine(content, "test.txt");
+
+            using var checksumStream = ToStream(checksumLine);
+            using var fileStream = ToStream(content);
+
+            using var sha256 = SHA256.Create();
+            Assert.True(ChecksumFileUtilities.ValidateChecksum(sha256, checksumStream, "test.txt", fileStream));
+        }
+
+        // ── ValidateChecksum – negative case ────────────────────────────────
+
+        [Fact]
+        public void ValidateChecksum_ReturnsFalseForModifiedContent()
+        {
+            var originalContent = Encoding.UTF8.GetBytes("hello world");
+            var modifiedContent = Encoding.UTF8.GetBytes("hello WORLD");
+            var checksumLine = ComputeChecksumLine(originalContent, "test.txt");
+
+            using var checksumStream = ToStream(checksumLine);
+            using var fileStream = ToStream(modifiedContent);
+
+            using var sha256 = SHA256.Create();
+            Assert.False(ChecksumFileUtilities.ValidateChecksum(sha256, checksumStream, "test.txt", fileStream));
+        }
+
+        // ── ValidateChecksum – multi-entry checksum file ─────────────────────
+
+        [Fact]
+        public void ValidateChecksum_FindsCorrectEntryInMultiLineFile()
+        {
+            var content = Encoding.UTF8.GetBytes("data");
+            var otherContent = Encoding.UTF8.GetBytes("other");
+
+            var lines = ComputeChecksumLine(otherContent, "other.txt")
+                      + ComputeChecksumLine(content, "data.txt");
+
+            using var checksumStream = ToStream(lines);
+            using var fileStream = ToStream(content);
+
+            using var sha256 = SHA256.Create();
+            Assert.True(ChecksumFileUtilities.ValidateChecksum(sha256, checksumStream, "data.txt", fileStream));
+        }
+
+        // ── ValidateChecksum – missing filename ──────────────────────────────
+
+        [Fact]
+        public void ValidateChecksum_ThrowsWhenFilenameNotFound()
+        {
+            var content = Encoding.UTF8.GetBytes("hello");
+            var checksumLine = ComputeChecksumLine(content, "other.txt");
+
+            using var checksumStream = ToStream(checksumLine);
+            using var fileStream = ToStream(content);
+
+            using var sha256 = SHA256.Create();
+            Assert.Throws<ArgumentException>(() =>
+                ChecksumFileUtilities.ValidateChecksum(sha256, checksumStream, "missing.txt", fileStream));
+        }
+
+        // ── Round-trip: WriteChecksumToFile then ValidateChecksum ────────────
+
+        [Fact]
+        public void RoundTrip_WriteAndValidate()
+        {
+            var content = Encoding.UTF8.GetBytes("round-trip test content");
+            using var fileStreamWrite = ToStream(content);
+            using var checksumStream = new MemoryStream();
+
+            using var sha256Write = SHA256.Create();
+            ChecksumFileUtilities.WriteChecksumToFile(sha256Write, checksumStream, "file.bin", fileStreamWrite);
+
+            checksumStream.Position = 0;
+            using var fileStreamValidate = ToStream(content);
+            using var sha256Validate = SHA256.Create();
+            Assert.True(ChecksumFileUtilities.ValidateChecksum(sha256Validate, checksumStream, "file.bin", fileStreamValidate));
+        }
+
+        // ── Helpers ──────────────────────────────────────────────────────────
+
+        private static string ComputeChecksumLine(byte[] content, string filename)
+        {
+            using var sha256 = SHA256.Create();
+            var hash = sha256.ComputeHash(content);
+            var hex = BitConverter.ToString(hash).Replace("-", "").ToLowerInvariant();
+            return $"{hex}  {filename}\n";
+        }
+    }
+}
diff --git a/src/SyncTrayzor.Tests/FormatUtilsTests.cs b/src/SyncTrayzor.Tests/FormatUtilsTests.cs
new file mode 100644
index 00000000..6502bc9a
--- /dev/null
+++ b/src/SyncTrayzor.Tests/FormatUtilsTests.cs
@@ -0,0 +1,48 @@
+using SyncTrayzor.Utils;
+using Xunit;
+
+namespace SyncTrayzor.Tests
+{
+    public class FormatUtilsTests
+    {
+        // ── BytesToHuman ────────────────────────────────────────────────────
+
+        [Theory]
+        [InlineData(0,    0, "0B")]
+        [InlineData(1,    0, "1B")]
+        [InlineData(1023, 0, "1023B")]
+        [InlineData(1024, 0, "1KiB")]
+        [InlineData(1536, 0, "2KiB")]          // 1.5 KiB rounds to 2 with 0 decimal places
+        [InlineData(1048576, 0, "1MiB")]        // 1 MiB
+        [InlineData(1073741824, 0, "1GiB")]     // 1 GiB
+        public void BytesToHuman_DefaultDecimalPlaces(double bytes, int decimalPlaces, string expected)
+        {
+            Assert.Equal(expected, FormatUtils.BytesToHuman(bytes, decimalPlaces));
+        }
+
+        [Theory]
+        [InlineData(1536,    1, "1.5KiB")]      // exactly 1.5 KiB
+        [InlineData(1024,    2, "1.00KiB")]
+        [InlineData(1048576, 1, "1.0MiB")]
+        public void BytesToHuman_WithDecimalPlaces(double bytes, int decimalPlaces, string expected)
+        {
+            Assert.Equal(expected, FormatUtils.BytesToHuman(bytes, decimalPlaces));
+        }
+
+        [Fact]
+        public void BytesToHuman_ByteRangeNeverShowsDecimalPlaces()
+        {
+            // Bytes (order == 0) should never have decimal places regardless of the argument
+            var result = FormatUtils.BytesToHuman(512, decimalPlaces: 3);
+            Assert.Equal("512B", result);
+        }
+
+        [Fact]
+        public void BytesToHuman_CapsAtGiB()
+        {
+            // Even a very large value should be expressed in GiB, not some higher unit
+            var result = FormatUtils.BytesToHuman(1024.0 * 1024 * 1024 * 10, decimalPlaces: 0);
+            Assert.EndsWith("GiB", result);
+        }
+    }
+}
diff --git a/src/SyncTrayzor.Tests/StringExtensionsTests.cs b/src/SyncTrayzor.Tests/StringExtensionsTests.cs
new file mode 100644
index 00000000..bb158d39
--- /dev/null
+++ b/src/SyncTrayzor.Tests/StringExtensionsTests.cs
@@ -0,0 +1,144 @@
+using System.Linq;
+using SyncTrayzor.Utils;
+using Xunit;
+
+namespace SyncTrayzor.Tests
+{
+    public class StringExtensionsTests
+    {
+        // ── TrimStart(string prefix) ────────────────────────────────────────
+
+        [Fact]
+        public void TrimStart_RemovesPrefixWhenPresent()
+        {
+            Assert.Equal("world", "helloworld".TrimStart("hello"));
+        }
+
+        [Fact]
+        public void TrimStart_ReturnsOriginalWhenPrefixAbsent()
+        {
+            Assert.Equal("world", "world".TrimStart("hello"));
+        }
+
+        [Fact]
+        public void TrimStart_EmptyPrefixReturnsOriginal()
+        {
+            Assert.Equal("hello", "hello".TrimStart(""));
+        }
+
+        [Fact]
+        public void TrimStart_OnlyRemovesLeadingOccurrence()
+        {
+            // "abab" starts with "ab", so the result should be "ab" (second occurrence)
+            Assert.Equal("ab", "abab".TrimStart("ab"));
+        }
+
+        // ── TrimMatchingQuotes(char quote) ─────────────────────────────────
+
+        [Fact]
+        public void TrimMatchingQuotes_RemovesWrappingDoubleQuotes()
+        {
+            Assert.Equal("hello world", "\"hello world\"".TrimMatchingQuotes('"'));
+        }
+
+        [Fact]
+        public void TrimMatchingQuotes_ReturnsOriginalWhenNotWrapped()
+        {
+            Assert.Equal("hello", "hello".TrimMatchingQuotes('"'));
+        }
+
+        [Fact]
+        public void TrimMatchingQuotes_ReturnsOriginalWhenOnlyLeadingQuote()
+        {
+            Assert.Equal("\"hello", "\"hello".TrimMatchingQuotes('"'));
+        }
+
+        [Fact]
+        public void TrimMatchingQuotes_ReturnsOriginalWhenOnlyTrailingQuote()
+        {
+            Assert.Equal("hello\"", "hello\"".TrimMatchingQuotes('"'));
+        }
+
+        [Fact]
+        public void TrimMatchingQuotes_ReturnsEmptyStringForTwoQuotes()
+        {
+            Assert.Equal("", "\"\"".TrimMatchingQuotes('"'));
+        }
+
+        // ── SplitCommandLine ────────────────────────────────────────────────
+
+        [Fact]
+        public void SplitCommandLine_SplitsSimpleArgs()
+        {
+            var result = StringExtensions.SplitCommandLine("foo bar baz").ToList();
+            Assert.Equal(new[] { "foo", "bar", "baz" }, result);
+        }
+
+        [Fact]
+        public void SplitCommandLine_KeepsQuotedSpaces()
+        {
+            var result = StringExtensions.SplitCommandLine("\"hello world\" foo").ToList();
+            Assert.Equal(new[] { "hello world", "foo" }, result);
+        }
+
+        [Fact]
+        public void SplitCommandLine_EmptyStringReturnsNoElements()
+        {
+            var result = StringExtensions.SplitCommandLine("").ToList();
+            Assert.Empty(result);
+        }
+
+        [Fact]
+        public void SplitCommandLine_SingleArgReturnsOneElement()
+        {
+            var result = StringExtensions.SplitCommandLine("only").ToList();
+            Assert.Equal(new[] { "only" }, result);
+        }
+
+        [Fact]
+        public void SplitCommandLine_SkipsExtraSpaces()
+        {
+            var result = StringExtensions.SplitCommandLine("foo  bar").ToList();
+            // extra space creates an empty token which is filtered out
+            Assert.Equal(new[] { "foo", "bar" }, result);
+        }
+
+        // ── JoinCommandLine ─────────────────────────────────────────────────
+
+        [Fact]
+        public void JoinCommandLine_SimpleArgsNoSpaces()
+        {
+            Assert.Equal("foo bar", StringExtensions.JoinCommandLine(new[] { "foo", "bar" }));
+        }
+
+        [Fact]
+        public void JoinCommandLine_QuotesArgWithSpace()
+        {
+            Assert.Equal("\"hello world\" foo", StringExtensions.JoinCommandLine(new[] { "hello world", "foo" }));
+        }
+
+        [Fact]
+        public void JoinCommandLine_EscapesEmbeddedQuotes()
+        {
+            // An arg that itself contains a double-quote should be quoted and the inner quote escaped
+            Assert.Equal("\"say \\\"hi\\\"\"", StringExtensions.JoinCommandLine(new[] { "say \"hi\"" }));
+        }
+
+        [Fact]
+        public void JoinCommandLine_EmptyInputReturnsEmptyString()
+        {
+            Assert.Equal("", StringExtensions.JoinCommandLine(new string[] { }));
+        }
+
+        // ── Round-trip: SplitCommandLine ∘ JoinCommandLine ─────────────────
+
+        [Fact]
+        public void RoundTrip_SplitThenJoin()
+        {
+            var original = new[] { "path to/file", "simple", "--flag=value" };
+            var joined = StringExtensions.JoinCommandLine(original);
+            var split = StringExtensions.SplitCommandLine(joined).ToArray();
+            Assert.Equal(original, split);
+        }
+    }
+}
diff --git a/src/SyncTrayzor.Tests/SyncthingCapabilitiesTests.cs b/src/SyncTrayzor.Tests/SyncthingCapabilitiesTests.cs
new file mode 100644
index 00000000..25c3f93b
--- /dev/null
+++ b/src/SyncTrayzor.Tests/SyncthingCapabilitiesTests.cs
@@ -0,0 +1,65 @@
+using System;
+using SyncTrayzor.Syncthing;
+using Xunit;
+
+namespace SyncTrayzor.Tests
+{
+    public class SyncthingCapabilitiesTests
+    {
+        // ── SupportsDebugFacilities ──────────────────────────────────────────
+
+        [Fact]
+        public void SupportsDebugFacilities_FalseBeforeIntroducedVersion()
+        {
+            var caps = new SyncthingCapabilities { SyncthingVersion = new Version(0, 11, 99) };
+            Assert.False(caps.SupportsDebugFacilities);
+        }
+
+        [Fact]
+        public void SupportsDebugFacilities_TrueAtIntroducedVersion()
+        {
+            var caps = new SyncthingCapabilities { SyncthingVersion = new Version(0, 12, 0) };
+            Assert.True(caps.SupportsDebugFacilities);
+        }
+
+        [Fact]
+        public void SupportsDebugFacilities_TrueAfterIntroducedVersion()
+        {
+            var caps = new SyncthingCapabilities { SyncthingVersion = new Version(1, 0, 0) };
+            Assert.True(caps.SupportsDebugFacilities);
+        }
+
+        // ── SupportsDevicePauseResume ────────────────────────────────────────
+
+        [Fact]
+        public void SupportsDevicePauseResume_FalseBeforeIntroducedVersion()
+        {
+            var caps = new SyncthingCapabilities { SyncthingVersion = new Version(0, 11, 0) };
+            Assert.False(caps.SupportsDevicePauseResume);
+        }
+
+        [Fact]
+        public void SupportsDevicePauseResume_TrueAtIntroducedVersion()
+        {
+            var caps = new SyncthingCapabilities { SyncthingVersion = new Version(0, 12, 0) };
+            Assert.True(caps.SupportsDevicePauseResume);
+        }
+
+        [Fact]
+        public void SupportsDevicePauseResume_TrueAfterIntroducedVersion()
+        {
+            var caps = new SyncthingCapabilities { SyncthingVersion = new Version(2, 0, 0) };
+            Assert.True(caps.SupportsDevicePauseResume);
+        }
+
+        // ── Default version (0.0.0) ──────────────────────────────────────────
+
+        [Fact]
+        public void DefaultVersion_NoCapabilitiesSupported()
+        {
+            var caps = new SyncthingCapabilities(); // default version is 0.0.0
+            Assert.False(caps.SupportsDebugFacilities);
+            Assert.False(caps.SupportsDevicePauseResume);
+        }
+    }
+}
diff --git a/src/SyncTrayzor.Tests/SyncthingVersionInformationTests.cs b/src/SyncTrayzor.Tests/SyncthingVersionInformationTests.cs
new file mode 100644
index 00000000..77654c56
--- /dev/null
+++ b/src/SyncTrayzor.Tests/SyncthingVersionInformationTests.cs
@@ -0,0 +1,55 @@
+using System;
+using SyncTrayzor.Syncthing;
+using Xunit;
+
+namespace SyncTrayzor.Tests
+{
+    public class SyncthingVersionInformationTests
+    {
+        [Theory]
+        [InlineData("v1.23.4",  "1.23.4")]
+        [InlineData("v0.14.28", "0.14.28")]
+        [InlineData("v0.12.0",  "0.12.0")]
+        public void ParsedVersion_ExtractsVersionFromShortVersionString(string shortVersion, string expectedVersion)
+        {
+            var info = new SyncthingVersionInformation(shortVersion, "");
+            Assert.Equal(Version.Parse(expectedVersion), info.ParsedVersion);
+        }
+
+        [Fact]
+        public void ParsedVersion_HandlesLongVersionString()
+        {
+            // syncthing sometimes reports "syncthing v1.2.3 (go1.21 linux-amd64) ..."
+            var info = new SyncthingVersionInformation("syncthing v1.2.3", "syncthing v1.2.3 (go1.21)");
+            Assert.Equal(new Version(1, 2, 3), info.ParsedVersion);
+        }
+
+        [Fact]
+        public void ParsedVersion_DefaultsToZeroWhenNoVersionFound()
+        {
+            var info = new SyncthingVersionInformation("no-version-here", "");
+            Assert.Equal(new Version(0, 0, 0), info.ParsedVersion);
+        }
+
+        [Fact]
+        public void ParsedVersion_DefaultsToZeroForEmptyString()
+        {
+            var info = new SyncthingVersionInformation("", "");
+            Assert.Equal(new Version(0, 0, 0), info.ParsedVersion);
+        }
+
+        [Fact]
+        public void ShortVersion_StoredAsProvided()
+        {
+            var info = new SyncthingVersionInformation("v1.0.0", "long version");
+            Assert.Equal("v1.0.0", info.ShortVersion);
+        }
+
+        [Fact]
+        public void LongVersion_StoredAsProvided()
+        {
+            var info = new SyncthingVersionInformation("v1.0.0", "long version");
+            Assert.Equal("long version", info.LongVersion);
+        }
+    }
+}
diff --git a/src/SyncTrayzor.Tests/UriExtensionsTests.cs b/src/SyncTrayzor.Tests/UriExtensionsTests.cs
new file mode 100644
index 00000000..1eceda21
--- /dev/null
+++ b/src/SyncTrayzor.Tests/UriExtensionsTests.cs
@@ -0,0 +1,49 @@
+using System;
+using SyncTrayzor.Utils;
+using Xunit;
+
+namespace SyncTrayzor.Tests
+{
+    public class UriExtensionsTests
+    {
+        [Fact]
+        public void NormalizeZeroHost_ReplacesZeroHostWith127()
+        {
+            var input = new Uri("http://0.0.0.0:8384/");
+            var result = input.NormalizeZeroHost();
+            Assert.Equal("127.0.0.1", result.Host);
+        }
+
+        [Fact]
+        public void NormalizeZeroHost_PreservesPort()
+        {
+            var input = new Uri("http://0.0.0.0:8384/");
+            var result = input.NormalizeZeroHost();
+            Assert.Equal(8384, result.Port);
+        }
+
+        [Fact]
+        public void NormalizeZeroHost_PreservesPath()
+        {
+            var input = new Uri("http://0.0.0.0:8384/some/path");
+            var result = input.NormalizeZeroHost();
+            Assert.Equal("/some/path", result.AbsolutePath);
+        }
+
+        [Fact]
+        public void NormalizeZeroHost_LeavesNonZeroHostUnchanged()
+        {
+            var input = new Uri("http://192.168.1.1:8384/");
+            var result = input.NormalizeZeroHost();
+            Assert.Equal("192.168.1.1", result.Host);
+        }
+
+        [Fact]
+        public void NormalizeZeroHost_LeavesLocalhostUnchanged()
+        {
+            var input = new Uri("http://localhost:8384/");
+            var result = input.NormalizeZeroHost();
+            Assert.Equal("localhost", result.Host);
+        }
+    }
+}