diff --git a/src/ClaudeDo.Worker/Findings/FindingsStore.cs b/src/ClaudeDo.Worker/Findings/FindingsStore.cs
new file mode 100644
index 00000000..7354777c
--- /dev/null
+++ b/src/ClaudeDo.Worker/Findings/FindingsStore.cs
@@ -0,0 +1,116 @@
+using System.Text;
+using System.Text.RegularExpressions;
+
+namespace ClaudeDo.Worker.Findings;
+
+/// <summary>
+/// Owns &lt;working-dir&gt;/.claudedo/ — one markdown file per finding plus a rebuilt INDEX.md.
+/// Pure filesystem: the caller supplies the head commit and source task id.
+/// INDEX.md is always regenerated from the files on disk, never patched, so findings the user
+/// deleted or renamed by hand disappear from the index on the next write.
+/// </summary>
+public sealed class FindingsStore : IFindingsStore
+{
+    /// <summary>INDEX.md is read by every run; past this many entries it stops paying for itself.</summary>
+    public const int WarnThreshold = 80;
+
+    private static readonly Regex SlugPattern = new("^[a-z0-9]+(-[a-z0-9]+)*$", RegexOptions.Compiled);
+    private readonly SemaphoreSlim _indexLock = new(1, 1);
+
+    public static string StoreDir(string workingDir) => Path.Combine(workingDir, ".claudedo");
+    public static string TrapsDir(string workingDir) => Path.Combine(StoreDir(workingDir), "traps");
+    public static string IndexPath(string workingDir) => Path.Combine(StoreDir(workingDir), "INDEX.md");
+
+    public async Task<SaveFindingOutcome> SaveAsync(string workingDir, FindingInput input, CancellationToken ct)
+    {
+        if (string.IsNullOrWhiteSpace(input.Slug) || input.Slug.Length > 60 || !SlugPattern.IsMatch(input.Slug))
+            throw new ArgumentException(
+                $"Slug '{input.Slug}' is invalid. Use lowercase kebab-case, max 60 chars (e.g. 'conpty-arg-quoting').",
+                nameof(input));
+        if (string.IsNullOrWhiteSpace(input.Title))
+            throw new ArgumentException("Title must not be empty.", nameof(input));
+        if (string.IsNullOrWhiteSpace(input.Body))
+            throw new ArgumentException("Body must not be empty.", nameof(input));
+
+        var trapsDir = TrapsDir(workingDir);
+        Directory.CreateDirectory(trapsDir);
+
+        var path = Path.Combine(trapsDir, input.Slug + ".md");
+        var created = !File.Exists(path);
+
+        await File.WriteAllTextAsync(path, Render(input), ct);
+
+        int total;
+        await _indexLock.WaitAsync(ct);
+        try
+        {
+            total = await RebuildIndexAsync(workingDir, ct);
+        }
+        finally
+        {
+            _indexLock.Release();
+        }
+
+        return new SaveFindingOutcome(input.Slug, path, created, total, total >= WarnThreshold);
+    }
+
+    private static string Render(FindingInput input)
+    {
+        var sb = new StringBuilder();
+        sb.Append("---\n");
+        sb.Append($"slug: {input.Slug}\n");
+        sb.Append($"scope: {input.Scope}\n");
+        sb.Append($"source-task: {input.SourceTaskId}\n");
+        sb.Append($"verified-against: {input.VerifiedAgainst}\n");
+        sb.Append("---\n\n");
+        sb.Append($"# {input.Title.Trim()}\n\n");
+        sb.Append(input.Body.Trim());
+        sb.Append('\n');
+        return sb.ToString();
+    }
+
+    private static async Task<int> RebuildIndexAsync(string workingDir, CancellationToken ct)
+    {
+        var trapsDir = TrapsDir(workingDir);
+        var slugs = Directory.Exists(trapsDir)
+            ? Directory.EnumerateFiles(trapsDir, "*.md")
+                .Select(Path.GetFileNameWithoutExtension)
+                .Where(s => !string.IsNullOrEmpty(s))
+                .Select(s => s!)
+                .OrderBy(s => s, StringComparer.Ordinal)
+                .ToList()
+            : new List<string>();
+
+        var sb = new StringBuilder();
+        sb.Append("# Findings\n\n");
+        sb.Append("Traps and invariants for this project. Read this index first, then open only the\n");
+        sb.Append("findings that touch your task. Generated by ClaudeDo — edits to entries belong in\n");
+        sb.Append("`traps/`, this file is rewritten on every save.\n\n");
+
+        if (slugs.Count == 0)
+        {
+            sb.Append("_No findings yet._\n");
+        }
+        else
+        {
+            foreach (var slug in slugs)
+            {
+                var title = await ReadTitleAsync(Path.Combine(trapsDir, slug + ".md"), slug, ct);
+                sb.Append($"- [{slug}](traps/{slug}.md) — {title}\n");
+            }
+        }
+
+        await File.WriteAllTextAsync(IndexPath(workingDir), sb.ToString(), ct);
+        return slugs.Count;
+    }
+
+    private static async Task<string> ReadTitleAsync(string path, string fallback, CancellationToken ct)
+    {
+        foreach (var line in await File.ReadAllLinesAsync(path, ct))
+        {
+            if (line.StartsWith("# ", StringComparison.Ordinal))
+                return line[2..].Trim();
+        }
+        return fallback;
+    }
+}
diff --git a/src/ClaudeDo.Worker/Findings/Interfaces/IFindingsStore.cs b/src/ClaudeDo.Worker/Findings/Interfaces/IFindingsStore.cs
new file mode 100644
index 00000000..eb015b52
--- /dev/null
+++ b/src/ClaudeDo.Worker/Findings/Interfaces/IFindingsStore.cs
@@ -0,0 +1,22 @@
+namespace ClaudeDo.Worker.Findings;
+
+/// <summary>One durable finding, as handed in by a caller. Git and DB lookups happen above this layer.</summary>
+public sealed record FindingInput(
+    string Slug,
+    string Title,
+    string Body,
+    string Scope,
+    string SourceTaskId,
+    string VerifiedAgainst);
+
+public sealed record SaveFindingOutcome(
+    string Slug,
+    string Path,
+    bool Created,
+    int TotalFindings,
+    bool NearCapacity);
+
+public interface IFindingsStore
+{
+    Task<SaveFindingOutcome> SaveAsync(string workingDir, FindingInput input, CancellationToken ct);
+}
diff --git a/tests/ClaudeDo.Worker.Tests/Findings/FindingsStoreTests.cs b/tests/ClaudeDo.Worker.Tests/Findings/FindingsStoreTests.cs
new file mode 100644
index 00000000..843aa492
--- /dev/null
+++ b/tests/ClaudeDo.Worker.Tests/Findings/FindingsStoreTests.cs
@@ -0,0 +1,127 @@
+using ClaudeDo.Worker.Findings;
+
+namespace ClaudeDo.Worker.Tests.Findings;
+
+public sealed class FindingsStoreTests : IDisposable
+{
+    private readonly string _root;
+
+    public FindingsStoreTests()
+    {
+        _root = Path.Combine(Path.GetTempPath(), "cdo-findings-" + Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(_root);
+    }
+
+    public void Dispose()
+    {
+        try { Directory.Delete(_root, recursive: true); } catch { /* best effort */ }
+    }
+
+    private static FindingInput Input(string slug, string title = "Something is not what it looks like")
+        => new(slug, title, "Body line one.\n\nBody line two.", "src/ClaudeDo.Worker", "task-1", "abc1234");
+
+    [Fact]
+    public async Task SaveAsync_WritesFindingFileWithFrontmatter()
+    {
+        var store = new FindingsStore();
+
+        var outcome = await store.SaveAsync(_root, Input("conpty-arg-quoting"), CancellationToken.None);
+
+        Assert.True(outcome.Created);
+        Assert.Equal("conpty-arg-quoting", outcome.Slug);
+
+        var text = await File.ReadAllTextAsync(Path.Combine(_root, ".claudedo", "traps", "conpty-arg-quoting.md"));
+        Assert.StartsWith("---\n", text.Replace("\r\n", "\n"));
+        Assert.Contains("slug: conpty-arg-quoting", text);
+        Assert.Contains("scope: src/ClaudeDo.Worker", text);
+        Assert.Contains("source-task: task-1", text);
+        Assert.Contains("verified-against: abc1234", text);
+        Assert.Contains("# Something is not what it looks like", text);
+        Assert.Contains("Body line one.", text);
+    }
+
+    [Fact]
+    public async Task SaveAsync_SameSlugOverwritesInsteadOfCreatingASecondFile()
+    {
+        var store = new FindingsStore();
+        await store.SaveAsync(_root, Input("dup", "First title"), CancellationToken.None);
+
+        var outcome = await store.SaveAsync(_root, Input("dup", "Second title"), CancellationToken.None);
+
+        Assert.False(outcome.Created);
+        Assert.Equal(1, outcome.TotalFindings);
+        var files = Directory.GetFiles(Path.Combine(_root, ".claudedo", "traps"));
+        Assert.Single(files);
+        var text = await File.ReadAllTextAsync(files[0]);
+        Assert.Contains("# Second title", text);
+        Assert.DoesNotContain("# First title", text);
+    }
+
+    [Fact]
+    public async Task SaveAsync_RebuildsIndexFromDiskSortedBySlug()
+    {
+        var store = new FindingsStore();
+        await store.SaveAsync(_root, Input("zulu", "Zulu trap"), CancellationToken.None);
+        await store.SaveAsync(_root, Input("alpha", "Alpha trap"), CancellationToken.None);
+
+        var index = await File.ReadAllTextAsync(Path.Combine(_root, ".claudedo", "INDEX.md"));
+        var lines = index.Replace("\r\n", "\n").Split('\n');
+
+        var alphaAt = Array.FindIndex(lines, l => l.Contains("alpha"));
+        var zuluAt = Array.FindIndex(lines, l => l.Contains("zulu"));
+        Assert.True(alphaAt >= 0 && zuluAt >= 0);
+        Assert.True(alphaAt < zuluAt);
+        Assert.Contains("- [alpha](traps/alpha.md) — Alpha trap", index);
+    }
+
+    [Fact]
+    public async Task SaveAsync_IndexDropsFindingsDeletedOnDisk()
+    {
+        var store = new FindingsStore();
+        await store.SaveAsync(_root, Input("gone", "Gone trap"), CancellationToken.None);
+        await store.SaveAsync(_root, Input("stays", "Stays trap"), CancellationToken.None);
+
+        File.Delete(Path.Combine(_root, ".claudedo", "traps", "gone.md"));
+        await store.SaveAsync(_root, Input("stays", "Stays trap"), CancellationToken.None);
+
+        var index = await File.ReadAllTextAsync(Path.Combine(_root, ".claudedo", "INDEX.md"));
+        Assert.DoesNotContain("gone", index);
+        Assert.Contains("stays", index);
+    }
+
+    [Theory]
+    [InlineData("../escape")]
+    [InlineData("has space")]
+    [InlineData("UPPER")]
+    [InlineData("trailing-")]
+    [InlineData("")]
+    public async Task SaveAsync_RejectsInvalidSlug(string slug)
+    {
+        var store = new FindingsStore();
+
+        await Assert.ThrowsAsync<ArgumentException>(
+            () => store.SaveAsync(_root, Input(slug), CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task SaveAsync_FlagsNearCapacityAtWarnThreshold()
+    {
+        var store = new FindingsStore();
+        SaveFindingOutcome outcome = null!;
+        for (var i = 0; i < FindingsStore.WarnThreshold; i++)
+            outcome = await store.SaveAsync(_root, Input($"trap-{i:000}"), CancellationToken.None);
+
+        Assert.Equal(FindingsStore.WarnThreshold, outcome.TotalFindings);
+        Assert.True(outcome.NearCapacity);
+    }
+
+    [Fact]
+    public async Task SaveAsync_DoesNotFlagNearCapacityBelowThreshold()
+    {
+        var store = new FindingsStore();
+
+        var outcome = await store.SaveAsync(_root, Input("only-one"), CancellationToken.None);
+
+        Assert.False(outcome.NearCapacity);
+    }
+}