feat(mcp): expose the conflict resolver's hunk API over MCP

get_merge_conflicts and resolve_conflict_hunk let an MCP caller resolve a
leaveConflictsInTree merge hunk-by-hunk (ours/theirs/base/literal text)
instead of grep-parsing diff3 markers itself. Reuses the existing
TaskMergeService/ConflictMarkerParser machinery (adds MergeSegment.StartLine
and a non-staging TaskMergeService.WriteConflictFileAsync) rather than a
second parser. A partially resolved file is written without git add so
continue_merge keeps refusing while any hunk still has markers.

src/ClaudeDo.Data/Git/ConflictMarkerParser.cs

@@ -25,10 +25,13 @@ public sealed record MergeSegment
     /// <summary>"Theirs" side (the incoming branch) when <see cref="IsConflict"/> is true.</summary>
     public string Theirs { get; init; } = "";
 
-    public static MergeSegment Stable(string text) => new() { Text = text };
+    /// <summary>1-based line number in the file where this segment starts (the marker line for a conflict).</summary>
+    public int StartLine { get; init; } = 1;
 
-    public static MergeSegment Conflict(string ours, string? @base, string theirs) =>
-        new() { IsConflict = true, Ours = ours, Base = @base, Theirs = theirs };
+    public static MergeSegment Stable(string text, int startLine = 1) => new() { Text = text, StartLine = startLine };
+
+    public static MergeSegment Conflict(string ours, string? @base, string theirs, int startLine = 1) =>
+        new() { IsConflict = true, Ours = ours, Base = @base, Theirs = theirs, StartLine = startLine };
 }
 
 /// <summary>
@@ -49,22 +52,25 @@ public static class ConflictMarkerParser
         var segments = new List<MergeSegment>();
         var lines = SplitKeepLineEndings(fileText);
         var stable = new StringBuilder();
+        var stableStartLine = 1;
         var i = 0;
 
         while (i < lines.Count)
         {
             if (!IsMarker(lines[i], OursMarker))
             {
+                if (stable.Length == 0) stableStartLine = i + 1;
                 stable.Append(lines[i++]);
                 continue;
             }
 
             if (stable.Length > 0)
             {
-                segments.Add(MergeSegment.Stable(stable.ToString()));
+                segments.Add(MergeSegment.Stable(stable.ToString(), stableStartLine));
                 stable.Clear();
             }
 
+            var conflictStartLine = i + 1;
             i++; // consume "<<<<<<<"
             var ours = new StringBuilder();
             while (i < lines.Count && !IsMarker(lines[i], BaseMarker) && !IsMarker(lines[i], SepMarker))
@@ -88,11 +94,11 @@ public static class ConflictMarkerParser
 
             if (i < lines.Count && IsMarker(lines[i], TheirsMarker)) i++; // consume ">>>>>>>"
 
-            segments.Add(MergeSegment.Conflict(ours.ToString(), @base, theirs.ToString()));
+            segments.Add(MergeSegment.Conflict(ours.ToString(), @base, theirs.ToString(), conflictStartLine));
         }
 
         if (stable.Length > 0)
-            segments.Add(MergeSegment.Stable(stable.ToString()));
+            segments.Add(MergeSegment.Stable(stable.ToString(), stableStartLine));
 
         return segments;
     }

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -88,6 +88,18 @@ public sealed record MergeContinuationResultDto(
     bool Merged, string TaskStatus, IReadOnlyList<string> Conflicts,
     string? RepoPath, string? Message);
 
+public sealed record ConflictHunkDto(
+    int Index, int StartLine, string Ours, string? Base, string Theirs);
+
+public sealed record ConflictFileHunksDto(
+    string Path, bool IsBinary, IReadOnlyList<ConflictHunkDto> Hunks);
+
+public sealed record GetMergeConflictsResultDto(
+    IReadOnlyList<ConflictFileHunksDto> Files, int RemainingHunks, string ConflictStyleNote = McpToolDocs.Diff3Note);
+
+public sealed record ResolveConflictHunkResultDto(
+    bool Resolved, int RemainingHunks, string ConflictStyleNote = McpToolDocs.Diff3Note);
+
 // IsEmpty = the review range contributed nothing (worktree ahead-of-base is empty, or a
 // worktree-less handler task's HandlerBaseCommit == HandlerHeadCommit) -- distinguishable from
 // a merge that is merely small, so an empty branch can't be misread as "changedFileCount: 0
@@ -927,6 +939,107 @@ public sealed class ExternalMcpService
         return ToRefDto((await _tasks.GetByIdAsync(taskId, cancellationToken))!);
     }
 
+    [McpServerTool, Description(
+        "List every conflict hunk left in a paused merge (started via merge_task/review_task with " +
+        "leaveConflictsInTree=true), so you can resolve them with resolve_conflict_hunk instead of parsing marker " +
+        "text yourself. Each hunk carries the file's path, its index within that file (what resolve_conflict_hunk " +
+        "expects back), and its startLine. remainingHunks is the total across every file — call continue_merge once " +
+        "it reaches 0, or abort_merge to cancel. Throws if the task has no in-progress merge." + " " + McpToolDocs.Diff3Note)]
+    public async Task<GetMergeConflictsResultDto> GetMergeConflicts(string taskId, CancellationToken cancellationToken = default)
+    {
+        _ = await _tasks.GetByIdAsync(taskId, cancellationToken)
+            ?? throw new InvalidOperationException($"Task {taskId} not found.");
+
+        var docs = await _merge.GetConflictDocumentsAsync(taskId, cancellationToken);
+        var files = docs.Files.Select(ToConflictFileHunksDto).ToList();
+        var remaining = files.Sum(f => f.Hunks.Count);
+        return new GetMergeConflictsResultDto(files, remaining);
+    }
+
+    [McpServerTool, Description(
+        "Resolve exactly one conflict hunk from get_merge_conflicts, identified by file path and its index within " +
+        "that file. Writes only that hunk — every other hunk (in this file or another) is left exactly as it was, " +
+        "so call get_merge_conflicts again afterward: resolved hunks disappear and every remaining hunk's index " +
+        "shifts down to stay zero-based. This never decides FOR you which side wins — resolution must be 'ours', " +
+        "'theirs', 'base' (only valid when that hunk has a diff3 base), or literal replacement text (used exactly " +
+        "as given, including any trailing newline the surrounding file needs). remainingHunks is the total across " +
+        "every file; call continue_merge once it reaches 0. Throws if the file/index don't match a current hunk." +
+        " " + McpToolDocs.Diff3Note)]
+    public async Task<ResolveConflictHunkResultDto> ResolveConflictHunk(
+        string taskId,
+        [Description("File path exactly as returned by get_merge_conflicts, relative to the repo root.")]
+        string file,
+        [Description("Zero-based hunk index within that file, as returned by get_merge_conflicts.")]
+        int index,
+        [Description("'ours', 'theirs', 'base', or literal text to use for this hunk.")]
+        string resolution,
+        CancellationToken cancellationToken = default)
+    {
+        if (string.IsNullOrEmpty(resolution))
+            throw new InvalidOperationException("resolution is required.");
+
+        var docs = await _merge.GetConflictDocumentsAsync(taskId, cancellationToken);
+        var doc = docs.Files.FirstOrDefault(f => string.Equals(f.Path, file, StringComparison.Ordinal))
+            ?? throw new InvalidOperationException($"File '{file}' has no conflict for task {taskId}.");
+        if (doc.IsBinary)
+            throw new InvalidOperationException($"File '{file}' is binary; resolve it directly in the working tree.");
+
+        var hunkCount = doc.Segments.Count(s => s.IsConflict);
+        if (index < 0 || index >= hunkCount)
+            throw new InvalidOperationException(
+                $"Hunk index {index} is out of range for '{file}' ({hunkCount} hunk(s) remaining).");
+
+        var counter = -1;
+        var composed = ConflictMarkerParser.Compose(doc.Segments, seg =>
+        {
+            counter++;
+            if (counter != index) return ReconstructConflictMarkers(seg);
+
+            var choice = resolution.Trim().ToLowerInvariant();
+            return choice switch
+            {
+                "ours" => seg.Ours,
+                "theirs" => seg.Theirs,
+                "base" => seg.Base ?? throw new InvalidOperationException(
+                    $"Hunk {index} in '{file}' has no diff3 base to resolve to."),
+                _ => resolution,
+            };
+        });
+
+        await _merge.WriteConflictFileAsync(taskId, file, composed, cancellationToken);
+
+        var refreshed = await _merge.GetConflictDocumentsAsync(taskId, cancellationToken);
+        var remaining = refreshed.Files.Sum(f => f.Segments.Count(s => s.IsConflict));
+        return new ResolveConflictHunkResultDto(true, remaining);
+    }
+
+    private static ConflictFileHunksDto ToConflictFileHunksDto(ConflictDocumentContent f)
+    {
+        var hunks = new List<ConflictHunkDto>();
+        foreach (var seg in f.Segments)
+        {
+            if (!seg.IsConflict) continue;
+            hunks.Add(new ConflictHunkDto(hunks.Count, seg.StartLine, seg.Ours, seg.Base, seg.Theirs));
+        }
+        return new ConflictFileHunksDto(f.Path, f.IsBinary, hunks);
+    }
+
+    // Re-renders a still-unresolved hunk as valid (unlabeled) conflict markers so it round-trips through
+    // ConflictMarkerParser.Parse again untouched, and git still refuses continue_merge while it stands. The
+    // original marker labels (e.g. "<<<<<<< HEAD") aren't recoverable from a parsed MergeSegment and aren't
+    // needed for either check -- ConflictMarkerParser.IsMarker and ContinueMergeAsync's rescan only match the
+    // marker prefixes.
+    private static string ReconstructConflictMarkers(MergeSegment seg)
+    {
+        var sb = new StringBuilder();
+        sb.Append("<<<<<<<\n").Append(seg.Ours);
+        if (seg.Base is not null)
+            sb.Append("|||||||\n").Append(seg.Base);
+        sb.Append("=======\n").Append(seg.Theirs);
+        sb.Append(">>>>>>>\n");
+        return sb.ToString();
+    }
+
     [McpServerTool, Description(
         "Check whether a task would merge cleanly before touching anything — `git merge-tree --write-tree`, so the " +
         "working tree, index and HEAD are untouched. status is 'clean' or 'conflict' (conflictFiles lists where git " +

src/ClaudeDo.Worker/External/McpToolDocs.cs

@@ -25,4 +25,15 @@ internal static class McpToolDocs
 
     /// <summary>Mutations that refuse to touch a task while its agent is running.</summary>
     public const string NotWhileRunning = " Refused while the task is Running — cancel it first.";
+
+    /// <summary>
+    /// Explains this project's diff3 conflict-marker layout so a caller grepping only for
+    /// "&lt;&lt;&lt;&lt;&lt;&lt;&lt;/=======/&gt;&gt;&gt;&gt;&gt;&gt;&gt;" doesn't mistake the "|||||||" base section for one of the two sides.
+    /// No leading space (unlike the other consts here) since it also doubles as a DTO field default value.
+    /// </summary>
+    public const string Diff3Note =
+        "Conflicts are parsed in git's diff3 style: each hunk has an 'ours' side (the target branch) and a " +
+        "'theirs' side (the branch being merged in), plus an optional 'base' — the common-ancestor text for that " +
+        "hunk, shown between a third '|||||||' marker and the '=======' separator. base is null when git recorded " +
+        "none for that hunk.";
 }

src/ClaudeDo.Worker/Lifecycle/TaskMergeService.cs

@@ -488,6 +488,24 @@ public sealed class TaskMergeService
         await _git.AddPathAsync(list.WorkingDir, path, ct);
     }
 
+    /// <summary>
+    /// Writes a conflicted file's content without staging it. Unlike <see cref="WriteResolutionAsync"/> this must
+    /// NOT run `git add` — that marks the path resolved in git's index regardless of its content, so a partially
+    /// resolved hunk (markers still present elsewhere in the file) would drop out of
+    /// <see cref="GetConflictDocumentsAsync"/>'s conflicted-file list and <see cref="ContinueMergeAsync"/>'s
+    /// pre-stage marker scan, letting a still-conflicted file slip into a commit. Staging happens once, for
+    /// everything, inside <see cref="ContinueMergeAsync"/>.
+    /// </summary>
+    public async Task WriteConflictFileAsync(string taskId, string path, string content, CancellationToken ct)
+    {
+        var (_, list, _, _) = await LoadMergeContextAsync(taskId, ct);
+        if (string.IsNullOrWhiteSpace(list.WorkingDir))
+            throw new InvalidOperationException("list has no working directory");
+
+        var full = Path.Combine(list.WorkingDir, path.Replace('/', Path.DirectorySeparatorChar));
+        await File.WriteAllTextAsync(full, content, ct);
+    }
+
     public async Task<MergeTargets> GetTargetsAsync(string taskId, CancellationToken ct)
     {
         var (_, list, _, _) = await LoadMergeContextAsync(taskId, ct);

tests/ClaudeDo.Data.Tests/ConflictMarkerParserTests.cs

@@ -68,6 +68,20 @@ public class ConflictMarkerParserTests
         Assert.Equal("Y\n", segments[1].Theirs);
     }
 
+    [Fact]
+    public void Segments_ExposeStartLine()
+    {
+        const string text =
+            "line1\nline2\n<<<<<<< HEAD\nours\n=======\ntheirs\n>>>>>>> branch\nline3\n";
+
+        var segments = ConflictMarkerParser.Parse(text);
+
+        Assert.Equal(3, segments.Count);
+        Assert.Equal(1, segments[0].StartLine);   // "line1\nline2\n"
+        Assert.Equal(3, segments[1].StartLine);   // the "<<<<<<<" line
+        Assert.Equal(8, segments[2].StartLine);   // "line3\n"
+    }
+
     [Fact]
     public void MultipleConflicts_AreEachCaptured()
     {

tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs

@@ -1124,6 +1124,49 @@ public sealed class ExternalMcpServiceTests : IDisposable
         return (task, list, wt);
     }
 
+    // Three edits spread far enough apart in a 20-line file that git's 3-way merge keeps them as three
+    // separate, non-adjacent hunks instead of folding them into one.
+    private static readonly int[] MultiHunkEditLines = [3, 9, 15];
+
+    private async Task<(TaskEntity task, ListEntity list, WorktreeContext wt)> SeedMultiHunkConflictingWorktreeAsync()
+    {
+        var repo = new GitRepoFixture();
+        _repos.Add(repo);
+
+        var baseLines = Enumerable.Range(1, 20).Select(n => $"line{n}").ToArray();
+        File.WriteAllText(Path.Combine(repo.RepoDir, "README.md"), string.Join('\n', baseLines) + "\n");
+        GitRepoFixture.RunGit(repo.RepoDir, "add", "README.md");
+        GitRepoFixture.RunGit(repo.RepoDir, "commit", "-m", "multi-line base");
+
+        var listId = Guid.NewGuid().ToString();
+        var list = new ListEntity { Id = listId, Name = "L", WorkingDir = repo.RepoDir, CreatedAt = DateTime.UtcNow };
+        await _lists.AddAsync(list);
+        var task = await SeedTaskAsync(listId, status: TaskStatus.WaitingForReview);
+
+        var cfg = new WorkerConfig { WorktreeRootStrategy = "sibling" };
+        var mgr = new WorktreeManager(new GitService(), _db.CreateFactory(), cfg, NullLogger<WorktreeManager>.Instance);
+        var wt = await mgr.CreateAsync(task, list, CancellationToken.None);
+        _worktreeCleanups.Add((repo.RepoDir, wt.WorktreePath));
+
+        WriteMultiHunkVariant(wt.WorktreePath, baseLines, "WT");
+        GitRepoFixture.RunGit(wt.WorktreePath, "add", "README.md");
+        GitRepoFixture.RunGit(wt.WorktreePath, "commit", "-m", "worktree edit");
+
+        WriteMultiHunkVariant(list.WorkingDir!, baseLines, "MAIN");
+        GitRepoFixture.RunGit(list.WorkingDir!, "add", "README.md");
+        GitRepoFixture.RunGit(list.WorkingDir!, "commit", "-m", "main edit");
+
+        return (task, list, wt);
+    }
+
+    private static void WriteMultiHunkVariant(string dir, string[] baseLines, string tag)
+    {
+        var lines = (string[])baseLines.Clone();
+        foreach (var lineNum in MultiHunkEditLines)
+            lines[lineNum - 1] = $"{tag}-{lineNum}";
+        File.WriteAllText(Path.Combine(dir, "README.md"), string.Join('\n', lines) + "\n");
+    }
+
     [Fact]
     public async Task MergeTask_LeaveConflictsInTree_LeavesMarkersAndKeepsRepoMidMerge()
     {
@@ -1296,6 +1339,130 @@ public sealed class ExternalMcpServiceTests : IDisposable
         Assert.Contains("mid-merge", ex.Message);
     }
 
+    // ── get_merge_conflicts / resolve_conflict_hunk ─────────────────────────────
+
+    [Fact]
+    public async Task GetMergeConflicts_ReportsHunksWithStartLineAndDiff3Base()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var (task, list, _) = await SeedConflictingWorktreeAsync(TaskStatus.WaitingForReview);
+        var sut = BuildSut(CreateQueue());
+        await sut.ReviewTask(task.Id, "approve", null, null, leaveConflictsInTree: true, CancellationToken.None);
+
+        var result = await sut.GetMergeConflicts(task.Id, CancellationToken.None);
+
+        Assert.Equal(1, result.RemainingHunks);
+        var file = Assert.Single(result.Files);
+        Assert.Equal("README.md", file.Path);
+        Assert.False(file.IsBinary);
+        var hunk = Assert.Single(file.Hunks);
+        Assert.Equal(0, hunk.Index);
+        Assert.True(hunk.StartLine >= 1);
+        Assert.Equal("# from main", hunk.Ours.TrimEnd('\r', '\n'));
+        Assert.Equal("# from worktree", hunk.Theirs.TrimEnd('\r', '\n'));
+        Assert.NotEmpty(result.ConflictStyleNote);
+
+        GitRepoFixture.RunGit(list.WorkingDir!, "merge", "--abort");
+    }
+
+    [Fact]
+    public async Task GetMergeConflicts_NoMergeInProgress_ReturnsEmpty()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var (task, _, _) = await SeedWorktreeAsync(TaskStatus.WaitingForReview);
+        var sut = BuildSut(CreateQueue());
+
+        var result = await sut.GetMergeConflicts(task.Id, CancellationToken.None);
+
+        Assert.Equal(0, result.RemainingHunks);
+        Assert.Empty(result.Files);
+    }
+
+    [Fact]
+    public async Task ResolveConflictHunk_OursTheirsAndText_ResolveEachHunkAndTrackRemainingCount()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var (task, list, _) = await SeedMultiHunkConflictingWorktreeAsync();
+        var target = await new GitService().GetCurrentBranchAsync(list.WorkingDir!);
+        var sut = BuildSut(CreateQueue());
+
+        var merge = await sut.MergeTask(task.Id, target, true, dryRun: false,
+            allowWaitingForReview: true, leaveConflictsInTree: true, CancellationToken.None);
+        Assert.True(merge.ConflictsInTree);
+
+        var before = await sut.GetMergeConflicts(task.Id, CancellationToken.None);
+        Assert.Equal(3, before.RemainingHunks);
+        Assert.Equal(3, Assert.Single(before.Files).Hunks.Count);
+
+        // Resolve hunk 0 three times in a row: each resolution removes a hunk from the front, so the
+        // next still-open hunk always slides into index 0.
+        var r1 = await sut.ResolveConflictHunk(task.Id, "README.md", 0, "ours", CancellationToken.None);
+        Assert.Equal(2, r1.RemainingHunks);
+
+        var r2 = await sut.ResolveConflictHunk(task.Id, "README.md", 0, "theirs", CancellationToken.None);
+        Assert.Equal(1, r2.RemainingHunks);
+
+        var r3 = await sut.ResolveConflictHunk(task.Id, "README.md", 0, "CUSTOM-15\n", CancellationToken.None);
+        Assert.Equal(0, r3.RemainingHunks);
+
+        var finalConflicts = await sut.GetMergeConflicts(task.Id, CancellationToken.None);
+        Assert.Equal(0, finalConflicts.RemainingHunks);
+
+        var continueResult = await sut.ContinueMerge(task.Id, CancellationToken.None);
+        Assert.True(continueResult.Merged);
+        Assert.Equal("Done", continueResult.TaskStatus);
+
+        var mergedText = File.ReadAllText(Path.Combine(list.WorkingDir!, "README.md"));
+        Assert.Contains("MAIN-3", mergedText);   // hunk 1 resolved "ours" (the target branch)
+        Assert.Contains("WT-9", mergedText);     // hunk 2 resolved "theirs" (the incoming branch)
+        Assert.Contains("CUSTOM-15", mergedText); // hunk 3 resolved to literal text
+        Assert.DoesNotContain("<<<<<<<", mergedText);
+    }
+
+    [Fact]
+    public async Task ResolveConflictHunk_LeavesOtherHunksUntouchedUntilContinueMerge()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var (task, list, _) = await SeedMultiHunkConflictingWorktreeAsync();
+        var target = await new GitService().GetCurrentBranchAsync(list.WorkingDir!);
+        var sut = BuildSut(CreateQueue());
+        await sut.MergeTask(task.Id, target, true, dryRun: false,
+            allowWaitingForReview: true, leaveConflictsInTree: true, CancellationToken.None);
+
+        // Resolve only one of the three hunks.
+        await sut.ResolveConflictHunk(task.Id, "README.md", 0, "ours", CancellationToken.None);
+
+        // continue_merge must still refuse: writing one hunk must not have staged the file as resolved.
+        var result = await sut.ContinueMerge(task.Id, CancellationToken.None);
+
+        Assert.False(result.Merged);
+        Assert.Contains("README.md", result.Conflicts);
+        Assert.True(await new GitService().IsMidMergeAsync(list.WorkingDir!));
+
+        GitRepoFixture.RunGit(list.WorkingDir!, "merge", "--abort");
+    }
+
+    [Fact]
+    public async Task ResolveConflictHunk_UnknownFileOrOutOfRangeIndex_Throws()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var (task, list, _) = await SeedConflictingWorktreeAsync(TaskStatus.WaitingForReview);
+        var sut = BuildSut(CreateQueue());
+        await sut.ReviewTask(task.Id, "approve", null, null, leaveConflictsInTree: true, CancellationToken.None);
+
+        await Assert.ThrowsAsync<InvalidOperationException>(
+            () => sut.ResolveConflictHunk(task.Id, "not-a-real-file.md", 0, "ours", CancellationToken.None));
+        await Assert.ThrowsAsync<InvalidOperationException>(
+            () => sut.ResolveConflictHunk(task.Id, "README.md", 5, "ours", CancellationToken.None));
+
+        GitRepoFixture.RunGit(list.WorkingDir!, "merge", "--abort");
+    }
+
     // ── RevertMerge ────────────────────────────────────────────────────────────
 
     [Fact]