diff --git a/tests/FSharp.Data.Core.Tests/CsvRuntimeOperations.fs b/tests/FSharp.Data.Core.Tests/CsvRuntimeOperations.fs
new file mode 100644
index 000000000..9ecd8af79
--- /dev/null
+++ b/tests/FSharp.Data.Core.Tests/CsvRuntimeOperations.fs
@@ -0,0 +1,261 @@
+module FSharp.Data.Tests.CsvRuntimeOperations
+
+open NUnit.Framework
+open FsUnit
+open System
+open System.IO
+open FSharp.Data
+open FSharp.Data.CsvExtensions
+
+let private sampleCsv =
+    "Name,Age,City\r\nAlice,30,London\r\nBob,25,Paris\r\nCarol,35,Berlin\r\nDave,22,Tokyo\r\nEve,28,Sydney"
+
+// ============================================================
+// Filter
+// ============================================================
+
+[<Test>]
+let ``Filter keeps only rows matching the predicate`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let filtered = csv.Filter(fun row -> row.["Age"].AsInteger() >= 28)
+    filtered.Rows |> Seq.length |> should equal 3
+
+[<Test>]
+let ``Filter returns empty rows when no rows match`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let filtered = csv.Filter(fun row -> row.["Age"].AsInteger() > 100)
+    filtered.Rows |> Seq.length |> should equal 0
+
+[<Test>]
+let ``Filter preserves header information`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let filtered = csv.Filter(fun row -> row.["City"] = "London")
+    filtered.Headers |> should equal (Some [| "Name"; "Age"; "City" |])
+
+[<Test>]
+let ``Filter returns all rows when all match`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let filtered = csv.Filter(fun row -> row.["Age"].AsInteger() > 0)
+    filtered.Rows |> Seq.length |> should equal 5
+
+// ============================================================
+// Take
+// ============================================================
+
+[<Test>]
+let ``Take returns the first N rows`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let taken = csv.Take 3
+    let rows = taken.Rows |> Seq.toArray
+    rows |> should haveLength 3
+    rows.[0].["Name"] |> should equal "Alice"
+    rows.[2].["Name"] |> should equal "Carol"
+
+[<Test>]
+let ``Take 0 returns empty rows`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    csv.Take(0).Rows |> Seq.length |> should equal 0
+
+[<Test>]
+let ``Take preserves headers`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let taken = csv.Take 2
+    taken.Headers |> should equal (Some [| "Name"; "Age"; "City" |])
+
+// ============================================================
+// TakeWhile
+// ============================================================
+
+[<Test>]
+let ``TakeWhile yields rows while predicate is true`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    // Ages: 30, 25, 35, 22, 28 — take while Age > 26; Bob=25 fails first
+    let taken = csv.TakeWhile(fun row -> row.["Age"].AsInteger() > 26)
+    let rows = taken.Rows |> Seq.toArray
+    rows |> should haveLength 1
+    rows.[0].["Name"] |> should equal "Alice"
+
+[<Test>]
+let ``TakeWhile returns all rows when predicate always true`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let taken = csv.TakeWhile(fun row -> row.["Age"].AsInteger() > 0)
+    taken.Rows |> Seq.length |> should equal 5
+
+[<Test>]
+let ``TakeWhile returns empty when first row fails predicate`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let taken = csv.TakeWhile(fun row -> row.["Age"].AsInteger() > 100)
+    taken.Rows |> Seq.length |> should equal 0
+
+// ============================================================
+// Skip
+// ============================================================
+
+[<Test>]
+let ``Skip omits the first N rows`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let skipped = csv.Skip 3
+    let rows = skipped.Rows |> Seq.toArray
+    rows |> should haveLength 2
+    rows.[0].["Name"] |> should equal "Dave"
+    rows.[1].["Name"] |> should equal "Eve"
+
+[<Test>]
+let ``Skip 0 returns all rows`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    csv.Skip(0).Rows |> Seq.length |> should equal 5
+
+[<Test>]
+let ``Skip preserves headers`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    csv.Skip(2).Headers |> should equal (Some [| "Name"; "Age"; "City" |])
+
+// ============================================================
+// SkipWhile
+// ============================================================
+
+[<Test>]
+let ``SkipWhile skips rows while predicate is true then yields rest`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    // Ages: 30, 25, 35, 22, 28 — skip while Age >= 25 (Alice=30, Bob=25, Carol=35, then Dave=22 stops)
+    let skipped = csv.SkipWhile(fun row -> row.["Age"].AsInteger() >= 25)
+    let rows = skipped.Rows |> Seq.toArray
+    rows |> should haveLength 2
+    rows.[0].["Name"] |> should equal "Dave"
+
+[<Test>]
+let ``SkipWhile returns all rows when first row fails predicate`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let skipped = csv.SkipWhile(fun row -> row.["Age"].AsInteger() > 100)
+    skipped.Rows |> Seq.length |> should equal 5
+
+// ============================================================
+// Truncate
+// ============================================================
+
+[<Test>]
+let ``Truncate returns at most N rows`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let truncated = csv.Truncate 2
+    truncated.Rows |> Seq.length |> should equal 2
+
+[<Test>]
+let ``Truncate with count larger than row count returns all rows`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let truncated = csv.Truncate 100
+    truncated.Rows |> Seq.length |> should equal 5
+
+[<Test>]
+let ``Truncate 0 returns empty rows`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    csv.Truncate(0).Rows |> Seq.length |> should equal 0
+
+// ============================================================
+// Append
+// ============================================================
+
+[<Test>]
+let ``Append adds rows from another sequence`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let first2 = csv.Take 2
+    let last3 = csv.Skip 2
+    let combined = first2.Append(last3.Rows)
+    combined.Rows |> Seq.length |> should equal 5
+
+[<Test>]
+let ``Append preserves existing rows`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let first1 = csv.Take 1
+    let extra = csv.Take 1
+    let combined = first1.Append(extra.Rows)
+    let rows = combined.Rows |> Seq.toArray
+    rows |> should haveLength 2
+    rows.[0].["Name"] |> should equal "Alice"
+    rows.[1].["Name"] |> should equal "Alice"
+
+// ============================================================
+// Cache
+// ============================================================
+
+[<Test>]
+let ``Cache preserves all rows`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let cached = csv.Cache()
+    cached.Rows |> Seq.length |> should equal 5
+
+[<Test>]
+let ``Cache can be enumerated multiple times`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let cached = csv.Cache()
+    cached.Rows |> Seq.length |> should equal 5
+    cached.Rows |> Seq.length |> should equal 5
+
+// ============================================================
+// SaveToString
+// ============================================================
+
+[<Test>]
+let ``SaveToString produces header row and all data rows`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let result = csv.SaveToString()
+    result |> should contain "Name,Age,City"
+    result |> should contain "Alice,30,London"
+    result |> should contain "Eve,28,Sydney"
+
+[<Test>]
+let ``SaveToString round-trips CSV correctly`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let serialised = csv.SaveToString()
+    let reparsed = CsvFile.Parse(serialised)
+    reparsed.Rows |> Seq.length |> should equal 5
+    reparsed.Rows |> Seq.head |> fun r -> r.["Name"] |> should equal "Alice"
+
+[<Test>]
+let ``SaveToString uses CRLF line endings per RFC 4180`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let result = csv.SaveToString()
+    result |> should contain "\r\n"
+
+[<Test>]
+let ``SaveToString quotes fields containing the separator`` () =
+    let csv = CsvFile.Parse("Name,Note\r\nAlice,\"hello, world\"")
+    let result = csv.SaveToString()
+    result |> should contain "\"hello, world\""
+
+[<Test>]
+let ``SaveToString with custom separator uses it in output`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let result = csv.SaveToString(separator = ';')
+    result |> should startWith "Name;Age;City"
+
+// ============================================================
+// Save to TextWriter
+// ============================================================
+
+[<Test>]
+let ``Save to TextWriter produces the same output as SaveToString`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    use writer = new StringWriter()
+    csv.Save(writer)
+    let fromSave = writer.ToString()
+    let fromSaveToString = csv.SaveToString()
+    fromSave |> should equal fromSaveToString
+
+// ============================================================
+// Chaining operations
+// ============================================================
+
+[<Test>]
+let ``Chaining Filter and Take works correctly`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let result = csv.Filter(fun row -> row.["Age"].AsInteger() >= 28).Take(2)
+    let rows = result.Rows |> Seq.toArray
+    rows |> should haveLength 2
+
+[<Test>]
+let ``Chaining Skip and Truncate works correctly`` () =
+    let csv = CsvFile.Parse(sampleCsv)
+    let result = csv.Skip(1).Truncate(2)
+    let rows = result.Rows |> Seq.toArray
+    rows |> should haveLength 2
+    rows.[0].["Name"] |> should equal "Bob"
diff --git a/tests/FSharp.Data.Core.Tests/FSharp.Data.Core.Tests.fsproj b/tests/FSharp.Data.Core.Tests/FSharp.Data.Core.Tests.fsproj
index 8947b3d4f..e4197fd8e 100644
--- a/tests/FSharp.Data.Core.Tests/FSharp.Data.Core.Tests.fsproj
+++ b/tests/FSharp.Data.Core.Tests/FSharp.Data.Core.Tests.fsproj
@@ -37,6 +37,7 @@
     <Compile Include="JsonSchema.fs" />
     <Compile Include="CsvReader.fs" />
     <Compile Include="CsvFile.fs" />
+    <Compile Include="CsvRuntimeOperations.fs" />
     <Compile Include="CsvParserProperties.fs" />
     <Compile Include="HtmlCharRefs.fs" />
     <Compile Include="HtmlParser.fs" />