fix(mcp): make set_task_config/set_list_config patch instead of overwrite

Omitted fields on set_task_config/set_list_config used to be indistinguishable
from an explicit clear, so any single-field update silently wiped every other
override (including SessionSkills, which wasn't even a tool parameter). Now
only the fields you pass are changed; clearing an override requires naming it
in the new clearFields array (a sentinel string doesn't work uniformly since
maxTurns is an int). set_list_config's "all four null deletes the config" case
is preserved but only reachable via clearFields. Return values now reflect the
full resulting config.

src/ClaudeDo.Worker/External/ConfigMcpTools.cs

@@ -55,33 +55,64 @@ public sealed class ConfigMcpTools
             : new TaskConfigResult(true, new TaskConfigDto(cfg.Model, cfg.SystemPrompt, cfg.AgentPath, cfg.MaxTurns));
     }
 
+    // clearFields (not a per-field sentinel like UpdateList's "" for workingDir) because maxTurns is an
+    // int? — there's no blank value to overload as "clear" for a number the way there is for a string.
+    private static readonly string[] ConfigFieldNames = { "model", "systemPrompt", "agentPath", "maxTurns" };
+
+    private static HashSet<string> NormalizeClearFields(IReadOnlyList<string>? clearFields)
+    {
+        var set = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+        foreach (var field in clearFields ?? Array.Empty<string>())
+        {
+            if (!ConfigFieldNames.Contains(field, StringComparer.OrdinalIgnoreCase))
+                throw new InvalidOperationException(
+                    $"Unknown clearFields entry '{field}'. Valid values: {string.Join(", ", ConfigFieldNames)}.");
+            set.Add(field);
+        }
+        return set;
+    }
+
     [McpServerTool, Description(
         "Set a list's default model/system prompt/agent path/max turns — the fallback for tasks in this list " +
-        "that don't override them. Passing all four as null clears the list config instead of setting one.")]
+        "that don't override them. Only the fields you pass are changed; omitted fields keep their current " +
+        "value. To clear a field instead, name it in clearFields; clearing all four deletes the list's config.")]
     public async Task<SetListConfigResult> SetListConfig(
         string listId, string? model = null, string? systemPrompt = null, string? agentPath = null,
-        int? maxTurns = null, CancellationToken cancellationToken = default)
+        int? maxTurns = null,
+        [Description("Field names to explicitly clear: 'model', 'systemPrompt', 'agentPath', 'maxTurns'. " +
+            "A field named here is cleared even if a value was also passed for it.")]
+        IReadOnlyList<string>? clearFields = null,
+        CancellationToken cancellationToken = default)
     {
         _ = await _lists.GetByIdAsync(listId, cancellationToken)
             ?? throw new InvalidOperationException($"List {listId} not found.");
 
-        var m = model.NullIfBlank();
+        var clear = NormalizeClearFields(clearFields);
-        var sp = systemPrompt.NullIfBlank();
+        var existing = await _lists.GetConfigAsync(listId, cancellationToken);
-        var ap = agentPath.NullIfBlank();
+
+        var m = clear.Contains("model") ? null : model.NullIfBlank() ?? existing?.Model;
+        var sp = clear.Contains("systemPrompt") ? null : systemPrompt.NullIfBlank() ?? existing?.SystemPrompt;
+        var ap = clear.Contains("agentPath") ? null : agentPath.NullIfBlank() ?? existing?.AgentPath;
+        var mt = clear.Contains("maxTurns") ? null : maxTurns ?? existing?.MaxTurns;
 
         TaskConfigDto? config;
-        if (m is null && sp is null && ap is null && maxTurns is null)
+        if (m is null && sp is null && ap is null && mt is null && clear.Count > 0)
         {
             await _lists.DeleteConfigAsync(listId, cancellationToken);
             config = null;
         }
+        else if (m is null && sp is null && ap is null && mt is null && existing is null)
+        {
+            config = null;
+        }
         else
         {
             await _lists.SetConfigAsync(new ListConfigEntity
             {
-                ListId = listId, Model = m, SystemPrompt = sp, AgentPath = ap, MaxTurns = maxTurns,
+                ListId = listId, Model = m, SystemPrompt = sp, AgentPath = ap, MaxTurns = mt,
+                SessionSkills = existing?.SessionSkills, VerifyCommand = existing?.VerifyCommand,
             }, cancellationToken);
-            config = new TaskConfigDto(m, sp, ap, maxTurns);
+            config = new TaskConfigDto(m, sp, ap, mt);
         }
 
         await _broadcaster.ListUpdated(listId);
@@ -90,21 +121,29 @@ public sealed class ConfigMcpTools
 
     [McpServerTool, Description(
         "Set per-task overrides for model/system prompt/agent path/max turns; these take precedence over the " +
-        "list's default config for this one task. Pass null for any field to clear that override.")]
+        "list's default config for this one task. Only the fields you pass are changed; omitted fields keep " +
+        "their current override. To clear a field instead, name it in clearFields.")]
     public async Task<SetTaskConfigResult> SetTaskConfig(
         string taskId, string? model = null, string? systemPrompt = null, string? agentPath = null,
-        int? maxTurns = null, CancellationToken cancellationToken = default)
+        int? maxTurns = null,
+        [Description("Field names to explicitly clear: 'model', 'systemPrompt', 'agentPath', 'maxTurns'. " +
+            "A field named here is cleared even if a value was also passed for it.")]
+        IReadOnlyList<string>? clearFields = null,
+        CancellationToken cancellationToken = default)
     {
-        _ = await _tasks.GetByIdAsync(taskId, cancellationToken)
+        var task = await _tasks.GetByIdAsync(taskId, cancellationToken)
             ?? throw new InvalidOperationException($"Task {taskId} not found.");
 
-        var m = model.NullIfBlank();
+        var clear = NormalizeClearFields(clearFields);
-        var sp = systemPrompt.NullIfBlank();
-        var ap = agentPath.NullIfBlank();
 
-        await _tasks.UpdateAgentSettingsAsync(taskId, m, sp, ap, maxTurns, ct: cancellationToken);
+        var m = clear.Contains("model") ? null : model.NullIfBlank() ?? task.Model;
+        var sp = clear.Contains("systemPrompt") ? null : systemPrompt.NullIfBlank() ?? task.SystemPrompt;
+        var ap = clear.Contains("agentPath") ? null : agentPath.NullIfBlank() ?? task.AgentPath;
+        var mt = clear.Contains("maxTurns") ? null : maxTurns ?? task.MaxTurns;
+
+        await _tasks.UpdateAgentSettingsAsync(taskId, m, sp, ap, mt, task.SessionSkills, cancellationToken);
         await _broadcaster.TaskUpdated(taskId);
-        return new SetTaskConfigResult(true, taskId, new TaskConfigDto(m, sp, ap, maxTurns));
+        return new SetTaskConfigResult(true, taskId, new TaskConfigDto(m, sp, ap, mt));
     }
 
     [McpServerTool, Description("Read this task's per-task overrides (model/system prompt/agent path/max turns), which take precedence over the list's default config. Returns { found: false, config: null } if none is set.")]

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

@@ -37,7 +37,7 @@ public sealed class ConfigMcpToolsTests : IDisposable
     {
         var listId = await SeedListAsync();
 
-        var setResult = await _sut.SetListConfig(listId, "sonnet", "be terse", null, 42, CancellationToken.None);
+        var setResult = await _sut.SetListConfig(listId, "sonnet", "be terse", null, 42, cancellationToken: CancellationToken.None);
         Assert.True(setResult.Ok);
         Assert.Equal(listId, setResult.ListId);
         Assert.NotNull(setResult.Config);
@@ -65,12 +65,41 @@ public sealed class ConfigMcpToolsTests : IDisposable
     }
 
     [Fact]
-    public async Task SetListConfig_AllNull_ClearsConfig()
+    public async Task SetListConfig_PartialUpdate_LeavesOtherFieldsUntouched()
     {
         var listId = await SeedListAsync();
-        await _sut.SetListConfig(listId, "sonnet", null, null, null, CancellationToken.None);
+        await _sut.SetListConfig(listId, "opus", "be terse", "agent.md", 30, cancellationToken: CancellationToken.None);
 
-        var clearResult = await _sut.SetListConfig(listId, null, null, null, null, CancellationToken.None);
+        var result = await _sut.SetListConfig(listId, maxTurns: 40, cancellationToken: CancellationToken.None);
+
+        Assert.Equal("opus", result.Config!.Model);
+        Assert.Equal("be terse", result.Config.SystemPrompt);
+        Assert.Equal("agent.md", result.Config.AgentPath);
+        Assert.Equal(40, result.Config.MaxTurns);
+    }
+
+    [Fact]
+    public async Task SetListConfig_ClearFields_ClearsOnlyNamedField()
+    {
+        var listId = await SeedListAsync();
+        await _sut.SetListConfig(listId, "opus", "be terse", "agent.md", 30, cancellationToken: CancellationToken.None);
+
+        var result = await _sut.SetListConfig(listId, clearFields: new[] { "model" }, cancellationToken: CancellationToken.None);
+
+        Assert.Null(result.Config!.Model);
+        Assert.Equal("be terse", result.Config.SystemPrompt);
+        Assert.Equal("agent.md", result.Config.AgentPath);
+        Assert.Equal(30, result.Config.MaxTurns);
+    }
+
+    [Fact]
+    public async Task SetListConfig_AllFieldsCleared_DeletesConfig()
+    {
+        var listId = await SeedListAsync();
+        await _sut.SetListConfig(listId, "sonnet", null, null, null, cancellationToken: CancellationToken.None);
+
+        var clearResult = await _sut.SetListConfig(
+            listId, clearFields: new[] { "model", "systemPrompt", "agentPath", "maxTurns" }, cancellationToken: CancellationToken.None);
 
         Assert.True(clearResult.Ok);
         Assert.Null(clearResult.Config);
@@ -78,6 +107,15 @@ public sealed class ConfigMcpToolsTests : IDisposable
         Assert.False(cfg.Found);
     }
 
+    [Fact]
+    public async Task SetListConfig_UnknownClearField_Throws()
+    {
+        var listId = await SeedListAsync();
+
+        await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            _sut.SetListConfig(listId, clearFields: new[] { "bogus" }, cancellationToken: CancellationToken.None));
+    }
+
     [Fact]
     public async Task SetTaskConfig_PersistsOverrides()
     {
@@ -93,7 +131,7 @@ public sealed class ConfigMcpToolsTests : IDisposable
         };
         await _tasks.AddAsync(task);
 
-        var result = await _sut.SetTaskConfig(task.Id, "opus", null, null, 15, CancellationToken.None);
+        var result = await _sut.SetTaskConfig(task.Id, "opus", null, null, 15, cancellationToken: CancellationToken.None);
 
         Assert.True(result.Ok);
         Assert.Equal(task.Id, result.TaskId);
@@ -106,7 +144,7 @@ public sealed class ConfigMcpToolsTests : IDisposable
     }
 
     [Fact]
-    public async Task SetTaskConfig_NullField_ClearsThatOverride()
+    public async Task SetTaskConfig_PartialUpdate_LeavesOtherOverridesUntouched()
     {
         var listId = await SeedListAsync();
         var task = new TaskEntity
@@ -119,19 +157,70 @@ public sealed class ConfigMcpToolsTests : IDisposable
             CommitType = "chore",
         };
         await _tasks.AddAsync(task);
-        await _sut.SetTaskConfig(task.Id, "opus", "be terse", "agent.md", 15, CancellationToken.None);
+        await _sut.SetTaskConfig(task.Id, "opus", "be terse", "agent.md", 15, cancellationToken: CancellationToken.None);
 
-        var result = await _sut.SetTaskConfig(task.Id, "opus", null, "agent.md", null, CancellationToken.None);
+        var result = await _sut.SetTaskConfig(task.Id, maxTurns: 40, cancellationToken: CancellationToken.None);
+
+        Assert.True(result.Ok);
+        Assert.Equal("opus", result.Config!.Model);
+        Assert.Equal("be terse", result.Config.SystemPrompt);
+        Assert.Equal("agent.md", result.Config.AgentPath);
+        Assert.Equal(40, result.Config.MaxTurns);
+
+        var loaded = await _tasks.GetByIdAsync(task.Id);
+        Assert.Equal("opus", loaded!.Model);
+        Assert.Equal("be terse", loaded.SystemPrompt);
+        Assert.Equal(40, loaded.MaxTurns);
+    }
+
+    [Fact]
+    public async Task SetTaskConfig_ClearFields_ClearsOnlyNamedOverride()
+    {
+        var listId = await SeedListAsync();
+        var task = new TaskEntity
+        {
+            Id = Guid.NewGuid().ToString(),
+            ListId = listId,
+            Title = "t",
+            Status = ClaudeDo.Data.Models.TaskStatus.Idle,
+            CreatedAt = DateTime.UtcNow,
+            CommitType = "chore",
+        };
+        await _tasks.AddAsync(task);
+        await _sut.SetTaskConfig(task.Id, "opus", "be terse", "agent.md", 15, cancellationToken: CancellationToken.None);
+
+        var result = await _sut.SetTaskConfig(task.Id, clearFields: new[] { "systemPrompt" }, cancellationToken: CancellationToken.None);
 
         Assert.True(result.Ok);
         Assert.Equal("opus", result.Config!.Model);
         Assert.Null(result.Config.SystemPrompt);
         Assert.Equal("agent.md", result.Config.AgentPath);
-        Assert.Null(result.Config.MaxTurns);
+        Assert.Equal(15, result.Config.MaxTurns);
 
         var loaded = await _tasks.GetByIdAsync(task.Id);
-        Assert.Null(loaded!.SystemPrompt);
+        Assert.Equal("opus", loaded!.Model);
-        Assert.Null(loaded.MaxTurns);
+        Assert.Null(loaded.SystemPrompt);
+        Assert.Equal("agent.md", loaded.AgentPath);
+        Assert.Equal(15, loaded.MaxTurns);
+    }
+
+    [Fact]
+    public async Task SetTaskConfig_UnknownClearField_Throws()
+    {
+        var listId = await SeedListAsync();
+        var task = new TaskEntity
+        {
+            Id = Guid.NewGuid().ToString(),
+            ListId = listId,
+            Title = "t",
+            Status = ClaudeDo.Data.Models.TaskStatus.Idle,
+            CreatedAt = DateTime.UtcNow,
+            CommitType = "chore",
+        };
+        await _tasks.AddAsync(task);
+
+        await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            _sut.SetTaskConfig(task.Id, clearFields: new[] { "bogus" }, cancellationToken: CancellationToken.None));
     }
 
     [Fact]

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

@@ -81,7 +81,7 @@ public sealed class EffectiveRunConfigTests : IDisposable
     public async Task Task_override_beats_list_override_beats_preset()
     {
         var listId = await SeedListAsync();
-        await _sut.SetListConfig(listId, "opus", null, "list-agent.md", 50, CancellationToken.None);
+        await _sut.SetListConfig(listId, "opus", null, "list-agent.md", 50, cancellationToken: CancellationToken.None);
         var task = await SeedTaskAsync(listId, t => { t.Model = "haiku"; t.MaxTurns = 12; });
 
         var result = await _sut.GetEffectiveRunConfig(task.Id, CancellationToken.None);
@@ -98,7 +98,7 @@ public sealed class EffectiveRunConfigTests : IDisposable
     public async Task List_override_wins_when_no_task_override()
     {
         var listId = await SeedListAsync();
-        await _sut.SetListConfig(listId, "opus", null, null, 50, CancellationToken.None);
+        await _sut.SetListConfig(listId, "opus", null, null, 50, cancellationToken: CancellationToken.None);
         var task = await SeedTaskAsync(listId);
 
         var result = await _sut.GetEffectiveRunConfig(task.Id, CancellationToken.None);