feat: Support overriding built-in tools (Issue #411)

Auto-add user-registered tool names to excludedTools in session.create/resume
RPC payloads so that SDK-registered tools override CLI built-in tools.

- Node.js: mergeExcludedTools() helper + createSession/resumeSession updates
- Python: inline merge logic in create_session/resume_session
- Go: mergeExcludedTools() helper + CreateSession/ResumeSessionWithOptions updates
- .NET: MergeExcludedTools() helper + CreateSessionAsync/ResumeSessionAsync updates
- Tests added for all 4 SDKs
- All 4 READMEs updated with "Overriding Built-in Tools" documentation

Co-authored-by: patniko <26906478+patniko@users.noreply.github.com>

Author: Copilot

dotnet/README.md

@@ -415,6 +415,19 @@ var session = await client.CreateSessionAsync(new SessionConfig
 
 When Copilot invokes `lookup_issue`, the client automatically runs your handler and responds to the CLI. Handlers can return any JSON-serializable value (automatically wrapped), or a `ToolResultAIContent` wrapping a `ToolResultObject` for full control over result metadata.
 
+#### Overriding Built-in Tools
+
+If you register a tool with the same name as a built-in CLI tool (e.g. `edit_file`, `read_file`), your tool takes precedence. The SDK automatically adds the tool name to `ExcludedTools`, so the built-in is disabled and your handler is called instead. This is useful when you need custom behavior for built-in operations.
+
+```csharp
+AIFunctionFactory.Create(
+    async ([Description("File path")] string path, [Description("New content")] string content) => {
+        // your logic
+    },
+    "edit_file",
+    "Custom file editor with project-specific validation")
+```
+
 ### System Message Customization
 
 Control the system prompt using `SystemMessage` in session config:

dotnet/src/Client.cs

@@ -382,7 +382,7 @@ public async Task<CopilotSession> CreateSessionAsync(SessionConfig? config = nul
             config?.Tools?.Select(ToolDefinition.FromAIFunction).ToList(),
             config?.SystemMessage,
             config?.AvailableTools,
-            config?.ExcludedTools,
+            MergeExcludedTools(config?.ExcludedTools, config?.Tools),
             config?.Provider,
             (bool?)true,
             config?.OnUserInputRequest != null ? true : null,
@@ -467,7 +467,7 @@ public async Task<CopilotSession> ResumeSessionAsync(string sessionId, ResumeSes
             config?.Tools?.Select(ToolDefinition.FromAIFunction).ToList(),
             config?.SystemMessage,
             config?.AvailableTools,
-            config?.ExcludedTools,
+            MergeExcludedTools(config?.ExcludedTools, config?.Tools),
             config?.Provider,
             (bool?)true,
             config?.OnUserInputRequest != null ? true : null,
@@ -852,6 +852,14 @@ private void DispatchLifecycleEvent(SessionLifecycleEvent evt)
         }
     }
 
+    internal static List<string>? MergeExcludedTools(List<string>? excludedTools, ICollection<AIFunction>? tools)
+    {
+        var toolNames = tools?.Select(t => t.Name).ToList();
+        if (toolNames is null or { Count: 0 }) return excludedTools;
+        if (excludedTools is null or { Count: 0 }) return toolNames;
+        return excludedTools.Union(toolNames).ToList();
+    }
+
     internal static async Task<T> InvokeRpcAsync<T>(JsonRpc rpc, string method, object?[]? args, CancellationToken cancellationToken)
     {
         return await InvokeRpcAsync<T>(rpc, method, args, null, cancellationToken);

dotnet/src/GitHub.Copilot.SDK.csproj

@@ -17,6 +17,10 @@
         <IsAotCompatible>true</IsAotCompatible>
     </PropertyGroup>
 
+    <ItemGroup>
+        <InternalsVisibleTo Include="GitHub.Copilot.SDK.Test" />
+    </ItemGroup>
+
     <ItemGroup>
         <None Include="../README.md" Pack="true" PackagePath="/" />
     </ItemGroup>

dotnet/test/MergeExcludedToolsTests.cs

@@ -0,0 +1,79 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *--------------------------------------------------------------------------------------------*/
+
+using Microsoft.Extensions.AI;
+using System.ComponentModel;
+using Xunit;
+
+namespace GitHub.Copilot.SDK.Test;
+
+public class MergeExcludedToolsTests
+{
+    [Fact]
+    public void Tool_Names_Are_Added_To_ExcludedTools()
+    {
+        var tools = new List<AIFunction>
+        {
+            AIFunctionFactory.Create(Noop, "my_tool"),
+        };
+
+        var result = CopilotClient.MergeExcludedTools(null, tools);
+
+        Assert.NotNull(result);
+        Assert.Contains("my_tool", result!);
+    }
+
+    [Fact]
+    public void Merges_With_Existing_ExcludedTools_And_Deduplicates()
+    {
+        var existing = new List<string> { "view", "my_tool" };
+        var tools = new List<AIFunction>
+        {
+            AIFunctionFactory.Create(Noop, "my_tool"),
+            AIFunctionFactory.Create(Noop, "another_tool"),
+        };
+
+        var result = CopilotClient.MergeExcludedTools(existing, tools);
+
+        Assert.NotNull(result);
+        Assert.Equal(3, result!.Count);
+        Assert.Contains("view", result);
+        Assert.Contains("my_tool", result);
+        Assert.Contains("another_tool", result);
+    }
+
+    [Fact]
+    public void Returns_Null_When_No_Tools_Provided()
+    {
+        var result = CopilotClient.MergeExcludedTools(null, null);
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void Returns_ExcludedTools_Unchanged_When_Tools_Empty()
+    {
+        var existing = new List<string> { "view" };
+        var result = CopilotClient.MergeExcludedTools(existing, new List<AIFunction>());
+
+        Assert.Same(existing, result);
+    }
+
+    [Fact]
+    public void Returns_Tool_Names_When_ExcludedTools_Null()
+    {
+        var tools = new List<AIFunction>
+        {
+            AIFunctionFactory.Create(Noop, "my_tool"),
+        };
+
+        var result = CopilotClient.MergeExcludedTools(null, tools);
+
+        Assert.NotNull(result);
+        Assert.Single(result!);
+        Assert.Equal("my_tool", result[0]);
+    }
+
+    [Description("No-op")]
+    static string Noop() => "";
+}

go/README.md

@@ -266,6 +266,17 @@ session, _ := client.CreateSession(context.Background(), &copilot.SessionConfig{
 
 When the model selects a tool, the SDK automatically runs your handler (in parallel with other calls) and responds to the CLI's `tool.call` with the handler's result.
 
+#### Overriding Built-in Tools
+
+If you register a tool with the same name as a built-in CLI tool (e.g. `edit_file`, `read_file`), your tool takes precedence. The SDK automatically adds the tool name to `ExcludedTools`, so the built-in is disabled and your handler is called instead. This is useful when you need custom behavior for built-in operations.
+
+```go
+editFile := copilot.DefineTool("edit_file", "Custom file editor with project-specific validation",
+    func(params EditFileParams, inv copilot.ToolInvocation) (any, error) {
+        // your logic
+    })
+```
+
 ## Streaming
 
 Enable streaming to receive assistant response chunks as they're generated:

go/client.go

@@ -461,7 +461,7 @@ func (c *Client) CreateSession(ctx context.Context, config *SessionConfig) (*Ses
 		req.Tools = config.Tools
 		req.SystemMessage = config.SystemMessage
 		req.AvailableTools = config.AvailableTools
-		req.ExcludedTools = config.ExcludedTools
+		req.ExcludedTools = mergeExcludedTools(config.ExcludedTools, config.Tools)
 		req.Provider = config.Provider
 		req.WorkingDirectory = config.WorkingDirectory
 		req.MCPServers = config.MCPServers
@@ -558,7 +558,7 @@ func (c *Client) ResumeSessionWithOptions(ctx context.Context, sessionID string,
 		req.Tools = config.Tools
 		req.Provider = config.Provider
 		req.AvailableTools = config.AvailableTools
-		req.ExcludedTools = config.ExcludedTools
+		req.ExcludedTools = mergeExcludedTools(config.ExcludedTools, config.Tools)
 		if config.Streaming {
 			req.Streaming = Bool(true)
 		}
@@ -1352,6 +1352,29 @@ func buildFailedToolResult(internalError string) ToolResult {
 }
 
 // buildUnsupportedToolResult creates a failure ToolResult for an unsupported tool.
+// mergeExcludedTools returns a deduplicated list combining excludedTools with
+// the names of any SDK-registered tools, so the CLI won't handle them.
+func mergeExcludedTools(excludedTools []string, tools []Tool) []string {
+	if len(tools) == 0 {
+		return excludedTools
+	}
+	seen := make(map[string]bool, len(excludedTools)+len(tools))
+	merged := make([]string, 0, len(excludedTools)+len(tools))
+	for _, name := range excludedTools {
+		if !seen[name] {
+			seen[name] = true
+			merged = append(merged, name)
+		}
+	}
+	for _, t := range tools {
+		if !seen[t.Name] {
+			seen[t.Name] = true
+			merged = append(merged, t.Name)
+		}
+	}
+	return merged
+}
+
 func buildUnsupportedToolResult(toolName string) ToolResult {
 	return ToolResult{
 		TextResultForLLM: fmt.Sprintf("Tool '%s' is not supported by this client instance.", toolName),

go/client_test.go

@@ -444,3 +444,39 @@ func TestResumeSessionRequest_ClientName(t *testing.T) {
 		}
 	})
 }
+
+func TestMergeExcludedTools(t *testing.T) {
+t.Run("adds tool names to excluded tools", func(t *testing.T) {
+tools := []Tool{{Name: "edit_file"}, {Name: "read_file"}}
+got := mergeExcludedTools(nil, tools)
+want := []string{"edit_file", "read_file"}
+if !reflect.DeepEqual(got, want) {
+t.Errorf("got %v, want %v", got, want)
+}
+})
+
+t.Run("deduplicates with existing excluded tools", func(t *testing.T) {
+excluded := []string{"edit_file", "run_shell"}
+tools := []Tool{{Name: "edit_file"}, {Name: "read_file"}}
+got := mergeExcludedTools(excluded, tools)
+want := []string{"edit_file", "run_shell", "read_file"}
+if !reflect.DeepEqual(got, want) {
+t.Errorf("got %v, want %v", got, want)
+}
+})
+
+t.Run("returns original list when no tools provided", func(t *testing.T) {
+excluded := []string{"edit_file"}
+got := mergeExcludedTools(excluded, nil)
+if !reflect.DeepEqual(got, excluded) {
+t.Errorf("got %v, want %v", got, excluded)
+}
+})
+
+t.Run("returns nil when both inputs are empty", func(t *testing.T) {
+got := mergeExcludedTools(nil, nil)
+if got != nil {
+t.Errorf("got %v, want nil", got)
+}
+})
+}

nodejs/README.md

@@ -402,6 +402,18 @@ const session = await client.createSession({
 
 When Copilot invokes `lookup_issue`, the client automatically runs your handler and responds to the CLI. Handlers can return any JSON-serializable value (automatically wrapped), a simple string, or a `ToolResultObject` for full control over result metadata. Raw JSON schemas are also supported if Zod isn't desired.
 
+#### Overriding Built-in Tools
+
+If you register a tool with the same name as a built-in CLI tool (e.g. `edit_file`, `read_file`), your tool takes precedence. The SDK automatically adds the tool name to `excludedTools`, so the built-in is disabled and your handler is called instead. This is useful when you need custom behavior for built-in operations.
+
+```ts
+defineTool("edit_file", {
+    description: "Custom file editor with project-specific validation",
+    parameters: z.object({ path: z.string(), content: z.string() }),
+    handler: async ({ path, content }) => { /* your logic */ },
+})
+```
+
 ### System Message Customization
 
 Control the system prompt using `systemMessage` in session config:

nodejs/src/client.ts

@@ -50,6 +50,19 @@ import type {
     TypedSessionLifecycleHandler,
 } from "./types.js";
 
+/**
+ * Merge user-provided excludedTools with tool names from config.tools so that
+ * SDK-registered tools automatically override built-in CLI tools.
+ */
+function mergeExcludedTools(
+    excludedTools: string[] | undefined,
+    tools: Tool[] | undefined
+): string[] | undefined {
+    const toolNames = tools?.map((t) => t.name);
+    if (!excludedTools?.length && !toolNames?.length) return excludedTools;
+    return [...new Set([...(excludedTools ?? []), ...(toolNames ?? [])])];
+}
+
 /**
  * Check if value is a Zod schema (has toJSONSchema method)
  */
@@ -529,7 +542,7 @@ export class CopilotClient {
             })),
             systemMessage: config.systemMessage,
             availableTools: config.availableTools,
-            excludedTools: config.excludedTools,
+            excludedTools: mergeExcludedTools(config.excludedTools, config.tools),
             provider: config.provider,
             requestPermission: true,
             requestUserInput: !!config.onUserInputRequest,
@@ -607,7 +620,7 @@ export class CopilotClient {
             reasoningEffort: config.reasoningEffort,
             systemMessage: config.systemMessage,
             availableTools: config.availableTools,
-            excludedTools: config.excludedTools,
+            excludedTools: mergeExcludedTools(config.excludedTools, config.tools),
             tools: config.tools?.map((tool) => ({
                 name: tool.name,
                 description: tool.description,

nodejs/test/client.test.ts

@@ -243,4 +243,71 @@ describe("CopilotClient", () => {
             }).toThrow(/githubToken and useLoggedInUser cannot be used with cliUrl/);
         });
     });
+
+    describe("excludedTools merging with config.tools", () => {
+        it("adds tool names from config.tools to excludedTools in session.create", async () => {
+            const client = new CopilotClient();
+            await client.start();
+            onTestFinished(() => client.forceStop());
+
+            const spy = vi.spyOn((client as any).connection!, "sendRequest");
+            await client.createSession({
+                tools: [{ name: "edit_file", description: "edit", handler: async () => "ok" }],
+            });
+
+            expect(spy).toHaveBeenCalledWith(
+                "session.create",
+                expect.objectContaining({ excludedTools: ["edit_file"] })
+            );
+        });
+
+        it("merges and deduplicates with existing excludedTools", async () => {
+            const client = new CopilotClient();
+            await client.start();
+            onTestFinished(() => client.forceStop());
+
+            const spy = vi.spyOn((client as any).connection!, "sendRequest");
+            await client.createSession({
+                tools: [{ name: "edit_file", description: "edit", handler: async () => "ok" }],
+                excludedTools: ["edit_file", "run_command"],
+            });
+
+            const payload = spy.mock.calls.find((c) => c[0] === "session.create")![1] as any;
+            expect(payload.excludedTools).toEqual(
+                expect.arrayContaining(["edit_file", "run_command"])
+            );
+            expect(payload.excludedTools).toHaveLength(2);
+        });
+
+        it("leaves excludedTools unchanged when no tools provided", async () => {
+            const client = new CopilotClient();
+            await client.start();
+            onTestFinished(() => client.forceStop());
+
+            const spy = vi.spyOn((client as any).connection!, "sendRequest");
+            await client.createSession({ excludedTools: ["run_command"] });
+
+            expect(spy).toHaveBeenCalledWith(
+                "session.create",
+                expect.objectContaining({ excludedTools: ["run_command"] })
+            );
+        });
+
+        it("adds tool names from config.tools to excludedTools in session.resume", async () => {
+            const client = new CopilotClient();
+            await client.start();
+            onTestFinished(() => client.forceStop());
+
+            const session = await client.createSession();
+            const spy = vi.spyOn((client as any).connection!, "sendRequest");
+            await client.resumeSession(session.sessionId, {
+                tools: [{ name: "edit_file", description: "edit", handler: async () => "ok" }],
+            });
+
+            expect(spy).toHaveBeenCalledWith(
+                "session.resume",
+                expect.objectContaining({ excludedTools: ["edit_file"] })
+            );
+        });
+    });
 });