Merge pull request #109 from microsoft/ozzafar/issue-15-logpoints

Fix #15: add logpoints via a new add_logpoint tool

Author: ozzafar

README.md

@@ -4,7 +4,7 @@ Let AI agents debug your code inside VS Code - set breakpoints, step through exe
 
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
 [![VS Code](https://img.shields.io/badge/VS%20Code-1.104.0+-blue.svg)](https://code.visualstudio.com/)
-[![Version](https://img.shields.io/badge/version-2.2.3-green.svg)](https://github.com/microsoft/DebugMCP)
+[![Version](https://img.shields.io/badge/version-2.2.4-green.svg)](https://github.com/microsoft/DebugMCP)
 [![VS Marketplace](https://img.shields.io/badge/VS%20Marketplace-Install-blue.svg)](https://marketplace.visualstudio.com/items?itemName=ozzafar.debugmcpextension)
 
 > ⭐ **If you find DebugMCP useful, please [star the repo on GitHub](https://github.com/microsoft/DebugMCP)!** It helps others discover the project and motivates continued development.
@@ -61,6 +61,7 @@ DebugMCP is an MCP server that gives AI coding agents full control over the VS C
 | **pause_execution** | Interrupt a freely-running program and stop at its current location (no breakpoint needed) | None |
 | **restart_debugging** | Restart the current debug session | None |
 | **add_breakpoint** | Add a breakpoint at a specific line (optionally conditional) | `fileFullPath` (required)<br>`line` (required, 1-based)<br>`condition` (optional) |
+| **add_logpoint** | Add a logpoint that logs a message (instead of pausing) when a line is reached | `fileFullPath` (required)<br>`line` (required, 1-based)<br>`logMessage` (required, `{expr}` interpolated)<br>`condition` (optional) |
 | **remove_breakpoint** | Remove a breakpoint from a specific line | `fileFullPath` (required)<br>`line` (required) |
 | **clear_all_breakpoints** | Remove all breakpoints at once | None |
 | **list_breakpoints** | List all active breakpoints | None |

package-lock.json

@@ -2,7 +2,7 @@
   "name": "debugmcpextension",
   "displayName": "DebugMCP — Agentic Debugging for VS Code, Cursor & More",
   "description": "Your AI agent debugs for you — right inside VS Code, Cursor & other VS Code-based editors. Let Copilot, Cline, Cursor, Codex & any MCP agent set breakpoints, step through code, and inspect variables live instead of guessing from logs.",
-  "version": "2.2.3",
+  "version": "2.2.4",
   "publisher": "ozzafar",
   "author": {
     "name": "Oz Zafar",

package.json

@@ -4,6 +4,7 @@ description: Drive an interactive VS Code debugger to investigate bugs, failing
 license: MIT
 allowed-tools:
   - add_breakpoint
+  - add_logpoint
   - remove_breakpoint
   - clear_all_breakpoints
   - list_breakpoints
@@ -169,6 +170,10 @@ Before ending the debug session, confirm you can answer:
   you isolate the issue, add tighter breakpoints around the problematic region.
 - **Use line numbers.** `add_breakpoint` takes a 1-based `line`; re-check the line after
   edits since numbers shift when code changes.
+- **Prefer logpoints for loops/hot paths.** When you want to observe how a value evolves
+  across many iterations without stopping, use `add_logpoint` with `{expression}`
+  interpolation (e.g. `iter {i}: total={total}`) instead of repeatedly continuing from a
+  breakpoint. Logpoints also avoid distorting timing-sensitive code.
 - **Don't overuse breakpoints.** A handful of well-placed pauses beats dozens of noisy
   ones. After each session, `clear_all_breakpoints` to start fresh.
 - **For test debugging,** pass `testName` to `start_debugging`. The server routes through

skills/debug-live/SKILL.md

@@ -97,6 +97,8 @@ export class ControlServer {
 				return this.handler.handleRestart();
 			case 'handleAddBreakpoint':
 				return this.handler.handleAddBreakpoint(args);
+			case 'handleAddLogpoint':
+				return this.handler.handleAddLogpoint(args);
 			case 'handleRemoveBreakpoint':
 				return this.handler.handleRemoveBreakpoint(args);
 			case 'handleClearAllBreakpoints':

src/controlServer.ts

@@ -270,6 +270,18 @@ export class DebugMCPServer {
         }, async (args: { fileFullPath: string; line: number; condition?: string }) =>
             this.runTool('add_breakpoint', () => debuggingHandler.handleAddBreakpoint(args)));
 
+        // Add logpoint tool
+        server.registerTool('add_logpoint', {
+            description: 'Add a logpoint: a breakpoint that logs a message instead of pausing execution. Ideal for tracing values across many iterations or hot paths without stopping, or where a hard pause would distort timing. Embed expressions in curly braces to interpolate runtime values, e.g. "user id={user.id}, count={items.length}". Provide an optional condition to only log when it evaluates to true.',
+            inputSchema: {
+                fileFullPath: z.string().describe('Full path to the file'),
+                line: z.number().int().describe('Line number (1-based) where the logpoint should be set'),
+                logMessage: z.string().describe('Message to log when the line is reached. Wrap expressions in {curly braces} to interpolate runtime values.'),
+                condition: z.string().optional().describe('Optional condition expression. When provided, the message is only logged if this expression evaluates to true.'),
+            },
+        }, async (args: { fileFullPath: string; line: number; logMessage: string; condition?: string }) =>
+            this.runTool('add_logpoint', () => debuggingHandler.handleAddLogpoint(args)));
+
         // Remove breakpoint tool
         server.registerTool('remove_breakpoint', {
             description: 'Remove a breakpoint that is no longer needed.',

src/debugMCPServer.ts

@@ -33,7 +33,7 @@ export interface IDebuggingExecutor {
     continue(): Promise<void>;
     pause(): Promise<void>;
     restart(): Promise<void>;
-    addBreakpoint(uri: vscode.Uri, line: number, condition?: string): Promise<void>;
+    addBreakpoint(uri: vscode.Uri, line: number, condition?: string, logMessage?: string): Promise<void>;
     removeBreakpoint(uri: vscode.Uri, line: number): Promise<void>;
     getCurrentDebugState(numNextLines: number): Promise<DebugState>;
     getVariables(frameId: number, scope?: 'local' | 'global' | 'all'): Promise<any>;
@@ -326,14 +326,17 @@ export class DebuggingExecutor implements IDebuggingExecutor {
     /**
      * Add a breakpoint at specified location. An optional condition makes it a
      * conditional breakpoint that only pauses execution when the expression
-     * evaluates to true.
+     * evaluates to true. An optional logMessage makes it a logpoint that logs
+     * the message (with {expressions} interpolated) instead of pausing.
      */
-    public async addBreakpoint(uri: vscode.Uri, line: number, condition?: string): Promise<void> {
+    public async addBreakpoint(uri: vscode.Uri, line: number, condition?: string, logMessage?: string): Promise<void> {
         try {
             const breakpoint = new vscode.SourceBreakpoint(
                 new vscode.Location(uri, new vscode.Position(line - 1, 0)),
                 true,
-                condition
+                condition,
+                undefined,
+                logMessage
             );
             vscode.debug.addBreakpoints([breakpoint]);
         } catch (error) {

src/debuggingExecutor.ts

@@ -19,6 +19,7 @@ export interface IDebuggingHandler {
     handlePause(): Promise<string>;
     handleRestart(): Promise<string>;
     handleAddBreakpoint(args: { fileFullPath: string; line: number; condition?: string }): Promise<string>;
+    handleAddLogpoint(args: { fileFullPath: string; line: number; logMessage: string; condition?: string }): Promise<string>;
     handleRemoveBreakpoint(args: { fileFullPath: string; line: number }): Promise<string>;
     handleClearAllBreakpoints(): Promise<string>;
     handleListBreakpoints(): Promise<string>;
@@ -325,6 +326,39 @@ export class DebuggingHandler implements IDebuggingHandler {
         }
     }
 
+    /**
+     * Add a logpoint: a breakpoint that logs a message (with {expressions}
+     * interpolated by the debug adapter) instead of pausing execution. An
+     * optional condition only logs when the expression is true.
+     */
+    public async handleAddLogpoint(args: { fileFullPath: string; line: number; logMessage: string; condition?: string }): Promise<string> {
+        const { fileFullPath, line, logMessage, condition } = args;
+
+        try {
+            if (!Number.isInteger(line) || line < 1) {
+                throw new Error(`Invalid line number: ${line}. Provide a 1-based line number.`);
+            }
+            if (!logMessage) {
+                throw new Error('A non-empty logMessage is required for a logpoint.');
+            }
+
+            // Validate the line exists so we fail clearly instead of setting an
+            // unbound logpoint past the end of the file.
+            const document = await vscode.workspace.openTextDocument(vscode.Uri.file(fileFullPath));
+            if (line > document.lineCount) {
+                throw new Error(`Line ${line} is out of range: ${fileFullPath} has ${document.lineCount} lines.`);
+            }
+
+            const uri = vscode.Uri.file(fileFullPath);
+            await this.executor.addBreakpoint(uri, line, condition, logMessage);
+
+            const conditionInfo = condition ? ` (condition: ${condition})` : '';
+            return `Logpoint added at ${fileFullPath}:${line}${conditionInfo}`;
+        } catch (error) {
+            throw new Error(`Error adding logpoint: ${error}`);
+        }
+    }
+
     /**
      * Remove a breakpoint from specified location
      */
@@ -372,7 +406,9 @@ export class DebuggingHandler implements IDebuggingHandler {
                     const fileName = bp.location.uri.fsPath.split(/[/\\]/).pop();
                     const line = bp.location.range.start.line + 1;
                     const conditionInfo = bp.condition ? ` (condition: ${bp.condition})` : '';
-                    breakpointList += `${index + 1}. ${fileName}:${line}${conditionInfo}\n`;
+                    const kind = bp.logMessage ? `Logpoint` : `Breakpoint`;
+                    const logInfo = bp.logMessage ? ` (log: ${bp.logMessage})` : '';
+                    breakpointList += `${index + 1}. ${kind} ${fileName}:${line}${conditionInfo}${logInfo}\n`;
                 } else if (bp instanceof vscode.FunctionBreakpoint) {
                     breakpointList += `${index + 1}. Function: ${bp.functionName}\n`;
                 }

src/debuggingHandler.ts

@@ -204,6 +204,10 @@ export class RoutingDebuggingHandler implements IDebuggingHandler {
 		return this.forward('handleAddBreakpoint', args, args.fileFullPath);
 	}
 
+	public handleAddLogpoint(args: { fileFullPath: string; line: number; logMessage: string; condition?: string }): Promise<string> {
+		return this.forward('handleAddLogpoint', args, args.fileFullPath);
+	}
+
 	public handleRemoveBreakpoint(args: { fileFullPath: string; line: number }): Promise<string> {
 		return this.forward('handleRemoveBreakpoint', args, args.fileFullPath);
 	}

src/routingDebuggingHandler.ts

@@ -28,6 +28,7 @@ class RecordingHandler implements IDebuggingHandler {
 	handlePause() { return this.record('pause', {}); }
 	handleRestart() { return this.record('restart', {}); }
 	handleAddBreakpoint(args: any) { return this.record('addBp', args); }
+	handleAddLogpoint(args: any) { return this.record('addLp', args); }
 	handleRemoveBreakpoint(args: any) { return this.record('removeBp', args); }
 	handleClearAllBreakpoints() { return this.record('clearBp', {}); }
 	handleListBreakpoints() { return this.record('listBp', {}); }
@@ -125,6 +126,25 @@ suite('Multi-window routing', () => {
 		assert.strictEqual(handlerB.calls.length, 0);
 	});
 
+	test('add_logpoint routes by fileFullPath before any start_debugging', async () => {
+		const repoA = path.join(dir, 'repoA');
+		const repoB = path.join(dir, 'repoB');
+		const handlerA = new RecordingHandler('A');
+		const handlerB = new RecordingHandler('B');
+		await startWindow('a.json', [repoA], handlerA);
+		await startWindow('b.json', [repoB], handlerB);
+
+		const routing = new RoutingDebuggingHandler(new WorkspaceRegistry(process.pid, dir));
+		const result = await routing.handleAddLogpoint({
+			fileFullPath: path.join(repoA, 'src', 'y.py'),
+			line: 1,
+			logMessage: 'i={i}'
+		});
+
+		assert.strictEqual(result, 'A:addLp');
+		assert.strictEqual(handlerB.calls.length, 0);
+	});
+
 	test('throws a helpful error when no window owns the path', async () => {
 		const repoA = path.join(dir, 'repoA');
 		const repoB = path.join(dir, 'repoB');