diff --git a/app/Tests/Chat/IContentExtensionsTests.cs b/app/Tests/Chat/IContentExtensionsTests.cs
new file mode 100644
index 00000000..873ecf53
--- /dev/null
+++ b/app/Tests/Chat/IContentExtensionsTests.cs
@@ -0,0 +1,205 @@
+using AIStudio.Chat;
+using AIStudio.Tools;
+
+using Markdig.Syntax;
+
+namespace AIStudio.Tests.Chat;
+
+/// <summary>
+/// Checks that an answer leaves AI Studio together with the sources it rests on.
+/// </summary>
+/// <remarks>
+/// The sources under an answer come from AI Studio, not from the model, so they are not part of the
+/// text a file writer reads. With RAG and web search in v26.9.1 that is most of what makes an answer
+/// checkable: a document which says a page was read, without saying which one, is worth little to
+/// whoever receives it. The chat renders the answer and the sources as two texts, which hides every
+/// way the one can run into the other -- an open code fence above all. A document has no such seam.
+/// </remarks>
+[TestFixture]
+public sealed class IContentExtensionsTests
+{
+    private static readonly Source TOOL_SOURCE = new("Search result", "https://example.org/search", SourceOrigin.TOOL);
+
+    [Test]
+    public void TheSourcesFollowTheAnswer()
+    {
+        var content = TextWith("The answer of the model ends here.", TOOL_SOURCE);
+
+        var found = content.TryGetExportMarkdown(out var markdown);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(found, Is.True);
+            Assert.That(markdown, Does.EndWith(content.Sources.ToExportMarkdown()), "What the chat shows below the answer is what the file holds below it.");
+            Assert.That(TopLevelBlocksOf(markdown), Is.EqualTo(new[] { "ParagraphBlock", "h1", "h2", "ListBlock" }), "The answer stays a paragraph of its own; the source list starts under its own heading.");
+        });
+    }
+
+    [Test]
+    public void AnAnswerEndingInATableKeepsIt()
+    {
+        var content = TextWith(Lines(
+            "Here are the numbers:",
+            string.Empty,
+            "| Quarter | Revenue |",
+            "|---|---|",
+            "| Q1 | 100 |"), TOOL_SOURCE);
+
+        var found = content.TryGetExportMarkdown(out var markdown);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(found, Is.True);
+            Assert.That(TopLevelBlocksOf(markdown), Is.EqualTo(new[] { "ParagraphBlock", "Table", "h1", "h2", "ListBlock" }), "The table ends where it ended; the headings below it are not two more rows.");
+        });
+    }
+
+    [Test]
+    public void AnAnswerEndingInAListKeepsIt()
+    {
+        var content = TextWith(Lines(
+            "Three points:",
+            string.Empty,
+            "- one",
+            "- two",
+            "- three"), TOOL_SOURCE);
+
+        var found = content.TryGetExportMarkdown(out var markdown);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(found, Is.True);
+            Assert.That(TopLevelBlocksOf(markdown), Is.EqualTo(new[] { "ParagraphBlock", "ListBlock", "h1", "h2", "ListBlock" }), "Two lists, not one: the sources do not become the fourth point of the answer.");
+        });
+    }
+
+    [Test]
+    public void AnOpenCodeFenceDoesNotSwallowTheSources()
+    {
+        // Either the model forgot the closing fence, or the answer was cut short. Both happen, and
+        // in a document both would turn everything below into code:
+        var content = TextWith(Lines(
+            "Here is the code:",
+            string.Empty,
+            "```csharp",
+            "var answer = 42;"), TOOL_SOURCE);
+
+        var found = content.TryGetExportMarkdown(out var markdown);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(found, Is.True);
+            Assert.That(TopLevelBlocksOf(markdown), Is.EqualTo(new[] { "ParagraphBlock", "FencedCodeBlock", "h1", "h2", "ListBlock" }), "The code block is closed for the model, so the sources stand below it instead of inside it.");
+        });
+    }
+
+    [Test]
+    public void WithoutSourcesNothingIsAdded()
+    {
+        const string ANSWER = "  An answer nobody had to look anything up for.  ";
+        var content = TextWith(ANSWER);
+
+        var found = content.TryGetExportMarkdown(out var markdown);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(found, Is.True);
+            Assert.That(markdown, Is.EqualTo(ANSWER.Trim()), "The everyday case: no heading, no empty line, nothing anybody has to explain.");
+        });
+    }
+
+    [Test]
+    public void WithoutAnAnswerTheSourcesStandAlone()
+    {
+        var content = TextWith(string.Empty, TOOL_SOURCE);
+
+        var found = content.TryGetExportMarkdown(out var markdown);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(found, Is.True);
+            Assert.That(markdown, Is.EqualTo(content.Sources.ToExportMarkdown()), "Nothing above the heading means no empty line above it either.");
+        });
+    }
+
+    [Test]
+    public void APictureHasNothingToExport()
+    {
+        IContent picture = new ContentImage
+        {
+            SourceType = ContentImageSource.URL,
+            Source = "https://example.org/picture.png",
+            Sources = [TOOL_SOURCE],
+        };
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(picture.TryGetExportMarkdown(out var markdown), Is.False, "There is no text document for a picture, so the caller hears no and says so.");
+            Assert.That(markdown, Is.Empty, "A file writer must not put an excuse into the file it writes.");
+        });
+    }
+
+    [Test]
+    public void TheTableReadingStaysTheTextOfTheModel()
+    {
+        const string ANSWER = "  An answer with a source hanging on it.  ";
+        var content = TextWith(ANSWER, TOOL_SOURCE);
+
+        var found = content.TryGetMarkdownText(out var markdown);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(found, Is.True);
+            Assert.That(markdown, Is.EqualTo(ANSWER), "Neither trimmed nor extended: whoever reads a table out of a message wants what the model wrote and nothing else.");
+        });
+    }
+
+    [Test]
+    public void ATableExportCarriesNoSources()
+    {
+        var content = TextWith(Lines(
+            "| Quarter | Revenue |",
+            "|---|---|",
+            "| Q1 | 100 |"), TOOL_SOURCE);
+
+        content.TryGetMarkdownText(out var markdown);
+        var tables = PlainFileExport.ExtractTables(markdown, ',');
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(tables, Has.Count.EqualTo(1), "One table in the message, one table offered for it.");
+            Assert.That(tables[0].Content, Does.Not.Contain("example.org"), "A data table has no column a link list would fit into.");
+        });
+    }
+
+    /// <summary>
+    /// A text message with the given sources hanging on it.
+    /// </summary>
+    /// <param name="text">The text the model wrote.</param>
+    /// <param name="sources">The sources AI Studio collected for it.</param>
+    /// <returns>The content.</returns>
+    private static ContentText TextWith(string text, params Source[] sources) => new()
+    {
+        Text = text,
+        Sources = [..sources],
+    };
+
+    /// <summary>
+    /// Names the blocks a Markdown text is made of, headings by their level.
+    /// </summary>
+    /// <remarks>
+    /// Only the blocks of the document itself, not the ones nested in them: whether the source list
+    /// stands on the document or inside the last block of the answer is the whole question here.
+    /// Markdig hangs a group for link reference definitions at the end of every document, which
+    /// carries no text and is left out.
+    /// </remarks>
+    /// <param name="markdown">The Markdown text to read.</param>
+    /// <returns>The names, in the order the blocks stand in.</returns>
+    private static IReadOnlyList<string> TopLevelBlocksOf(string markdown) => Markdig.Markdown
+        .Parse(markdown, Markdown.SAFE_MARKDOWN_PIPELINE)
+        .Where(block => block is not LinkReferenceDefinitionGroup)
+        .Select(block => block is HeadingBlock heading ? $"h{heading.Level}" : block.GetType().Name)
+        .ToList();
+
+    private static string Lines(params string[] lines) => string.Join(Environment.NewLine, lines);
+}
\ No newline at end of file
diff --git a/app/Tests/Tools/MarkdownTests.cs b/app/Tests/Tools/MarkdownTests.cs
new file mode 100644
index 00000000..9344de8a
--- /dev/null
+++ b/app/Tests/Tools/MarkdownTests.cs
@@ -0,0 +1,66 @@
+using AIStudio.Tools;
+
+namespace AIStudio.Tests.Tools;
+
+/// <summary>
+/// Checks that an answer which opens a code fence without closing it ends where it ends.
+/// </summary>
+/// <remarks>
+/// A fence without its counterpart runs to the end of the document, so whatever is appended below an
+/// answer is read as code instead of as Markdown. The chat never shows this, because it renders the
+/// answer and the sources below it as two texts. A document is one text, and there an answer which
+/// ends in an open fence takes the source list with it into a grey box.
+/// </remarks>
+[TestFixture]
+public sealed class MarkdownTests
+{
+    [Test]
+    public void AnOpenFenceGetsItsCounterpart()
+    {
+        var answer = Lines("Here is the code:", string.Empty, "```csharp", "var answer = 42;");
+
+        Assert.That(Markdown.CloseOpenCodeFence(answer), Is.EqualTo(Lines(answer, "```")), "The fence is closed with the same three backticks which opened it.");
+    }
+
+    [Test]
+    public void ALongerFenceIsClosedAtItsOwnLength()
+    {
+        // Four backticks are what a model writes when the block itself holds Markdown with code in
+        // it. The three backticks inside are content then, not the end of the block:
+        var answer = Lines("````markdown", "```csharp", "var answer = 42;", "```");
+
+        Assert.That(Markdown.CloseOpenCodeFence(answer), Is.EqualTo(Lines(answer, "````")), "Only a fence of at least the opening length closes the block.");
+    }
+
+    [Test]
+    public void ATildeFenceIsClosedWithTildes()
+    {
+        var answer = Lines("~~~", "var answer = 42;");
+
+        Assert.That(Markdown.CloseOpenCodeFence(answer), Is.EqualTo(Lines(answer, "~~~")), "A block opened with tildes cannot be closed with backticks.");
+    }
+
+    [Test]
+    public void AClosedFenceIsLeftAlone()
+    {
+        var answer = Lines("Here is the code:", string.Empty, "```csharp", "var answer = 42;", "```");
+
+        Assert.That(Markdown.CloseOpenCodeFence(answer), Is.EqualTo(answer), "The usual case: the model closed its block itself.");
+    }
+
+    [Test]
+    public void TextWithoutAnyFenceIsLeftAlone()
+    {
+        const string ANSWER = "Nothing in this answer opens a code block.";
+
+        Assert.That(Markdown.CloseOpenCodeFence(ANSWER), Is.EqualTo(ANSWER));
+    }
+
+    [Test]
+    public void AnEmptyTextIsLeftAlone()
+    {
+        Assert.That(Markdown.CloseOpenCodeFence(string.Empty), Is.Empty);
+    }
+
+    private static string Lines(params string[] lines) => string.Join(Environment.NewLine, lines);
+}
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diff --git a/app/Tests/Tools/SourceExtensionsTests.cs b/app/Tests/Tools/SourceExtensionsTests.cs
new file mode 100644
index 00000000..af1ebf56
--- /dev/null
+++ b/app/Tests/Tools/SourceExtensionsTests.cs
@@ -0,0 +1,97 @@
+using AIStudio.Tools;
+
+using Markdig.Syntax;
+
+namespace AIStudio.Tests.Tools;
+
+/// <summary>
+/// Checks how the list of sources reads once it is written out as Markdown.
+/// </summary>
+/// <remarks>
+/// The sources are the one part of an answer which AI Studio writes itself, and since v26.9.1 they
+/// no longer stay in the chat: they travel into every exported document and into the clipboard. A
+/// number which starts over per group, or a title which breaks out of the link it sits in, is then
+/// in a file somebody sends on. Nothing here asserts on the wording of a heading: I18N is
+/// process-wide state without a reset, so an Init somewhere else would decide whether these pass.
+/// </remarks>
+[TestFixture]
+public sealed class SourceExtensionsTests
+{
+    [Test]
+    public void TheGroupsKeepTheirOrderAndTheNumbersRunThrough()
+    {
+        // Mixed on purpose, so that the order of the output cannot come from the order of the input:
+        IList<Source> sources =
+        [
+            new("Handbook", "https://example.org/handbook", SourceOrigin.RAG),
+            new("Search result", "https://example.org/search", SourceOrigin.TOOL),
+            new("Cited by the model", "https://example.org/cited", SourceOrigin.LLM),
+        ];
+
+        Assert.That(EntriesOf(sources.ToMarkdown()), Is.EqualTo(new[]
+        {
+            "- [1] [Cited by the model](<https://example.org/cited>)",
+            "- [2] [Search result](<https://example.org/search>)",
+            "- [3] [Handbook](<https://example.org/handbook>)",
+        }), "What the AI cited comes first, then what the tools read, then what the data providers gave -- and a reader can follow the numbers straight down the list.");
+    }
+
+    [Test]
+    public void ATitleCannotBreakOutOfItsLink()
+    {
+        IList<Source> sources = [new("A [strange] title\\with a break\nin it", "https://example.org/", SourceOrigin.TOOL)];
+
+        Assert.That(EntriesOf(sources.ToMarkdown()).Single(), Is.EqualTo(@"- [1] [A \[strange\] title\\with a break in it](<https://example.org/>)"), "Brackets and backslashes are escaped, and the line break becomes a space so the entry stays one line.");
+    }
+
+    [Test]
+    public void ALocalPathKeepsWorkingAsALink()
+    {
+        // This is what a RAG hit on a file of the user looks like, and spaces in file names are the
+        // rule rather than the exception:
+        IList<Source> sources = [new("Handbook (page 12)", "file:///Users/someone/My Documents/handbook.pdf", SourceOrigin.RAG)];
+
+        Assert.That(EntriesOf(sources.ToMarkdown()).Single(), Is.EqualTo("- [1] [Handbook (page 12)](<file:///Users/someone/My%20Documents/handbook.pdf>)"), "A space would end the link destination, so it is escaped.");
+    }
+
+    [Test]
+    public void NoSourcesMeanNoText()
+    {
+        IList<Source> sources = [];
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(sources.ToMarkdown(), Is.Empty);
+            Assert.That(sources.ToExportMarkdown(), Is.Empty, "An answer nobody had to look up gets no heading over an empty list.");
+        });
+    }
+
+    [Test]
+    public void TheExportPutsOneHeadingOfItsOwnAboveTheGroups()
+    {
+        IList<Source> sources =
+        [
+            new("Search result", "https://example.org/search", SourceOrigin.TOOL),
+            new("Handbook", "https://example.org/handbook", SourceOrigin.RAG),
+        ];
+
+        var exported = sources.ToExportMarkdown();
+        var document = Markdig.Markdown.Parse(exported, Markdown.SAFE_MARKDOWN_PIPELINE);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(document.OfType<HeadingBlock>().Select(heading => heading.Level), Is.EqualTo(new[] { 1, 2, 2 }), "One heading of its own stands above the two groups the chat already shows.");
+            Assert.That(exported, Does.EndWith(sources.ToMarkdown()), "Below that heading, the export is what the chat shows, unchanged.");
+        });
+    }
+
+    /// <summary>
+    /// Reads the entries of a source list, without the headings above them.
+    /// </summary>
+    /// <param name="markdown">The Markdown of the sources.</param>
+    /// <returns>The entries, in the order they stand in.</returns>
+    private static IReadOnlyList<string> EntriesOf(string markdown) => markdown
+        .Split(Environment.NewLine, StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries)
+        .Where(line => line.StartsWith("- [", StringComparison.Ordinal))
+        .ToList();
+}
\ No newline at end of file