Fix #18: add_breakpoint takes a line number instead of lineContent

Replace the substring-based `lineContent` parameter with a 1-based `line`
number, making add_breakpoint consistent with remove_breakpoint,
list_breakpoints, and the documented tool schema.

The handler now validates the line (integer >= 1 and within the file's
line count) and sets a single breakpoint at that line, failing clearly
instead of substring-matching (which could set multiple/unexpected
breakpoints). Updated the MCP tool schema, routing handler/test, README,
SKILL.md, and the C# troubleshooting doc.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 6ea24b92-40ad-4539-aa13-61aa6ff8751f

Author: ozzafar

README.md

@@ -60,7 +60,7 @@ DebugMCP is an MCP server that gives AI coding agents full control over the VS C
 | **continue_execution** | Continue until next breakpoint | None |
 | **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>`lineContent` (required)<br>`condition` (optional) |
+| **add_breakpoint** | Add a breakpoint at a specific line (optionally conditional) | `fileFullPath` (required)<br>`line` (required, 1-based)<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 |
@@ -371,7 +371,7 @@ Yes. DebugMCP supports `.cs` files and `.csproj` project files for C#/.NET debug
 - **Symptom**: Breakpoints are set but execution doesn't pause
 - **Solution**:
   - Ensure the correct file is being debugged
-  - Check that the breakpoint line content matches exactly
+  - Check that the breakpoint line number is correct
   - Verify the relevant language debugger extension is installed
 
 #### Configuration Not Auto-Detected

skills/debug-live/SKILL.md

@@ -46,9 +46,8 @@ If you can step through the code in a few tool calls, do that instead of specula
 
 ## Core workflow
 
-1. **Set a starting breakpoint.** Use `add_breakpoint` with the file path and the exact
-   line content you want to pause on (line content matching is more robust than line
-   numbers — it survives small edits). Place it at the earliest point that's still
+1. **Set a starting breakpoint.** Use `add_breakpoint` with the file path and the 1-based
+   line number you want to pause on. Place it at the earliest point that's still
    relevant to the suspected issue.
 2. **Optionally add strategic breakpoints.** Decision points, error-handling branches,
    data boundaries (where input enters, where output is produced).
@@ -168,8 +167,8 @@ Before ending the debug session, confirm you can answer:
 
 - **Start broad, then narrow.** Begin at the entry point of the suspect function. As
   you isolate the issue, add tighter breakpoints around the problematic region.
-- **Match line content, not numbers.** `add_breakpoint` takes the exact line text so
-  breakpoints survive small edits and refactors.
+- **Use line numbers.** `add_breakpoint` takes a 1-based `line`; re-check the line after
+  edits since numbers shift when code changes.
 - **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
@@ -182,7 +181,7 @@ Before ending the debug session, confirm you can answer:
 
 ### Investigating a bug in `calculate.py`
 ```text
-add_breakpoint  fileFullPath=/repo/src/calculate.py  lineContent="result = parse(raw)"
+add_breakpoint  fileFullPath=/repo/src/calculate.py  line=42
 start_debugging fileFullPath=/repo/src/calculate.py  workingDirectory=/repo
 # session pauses on the breakpoint
 get_variables_values scope=local
@@ -194,7 +193,7 @@ clear_all_breakpoints
 
 ### Debugging a single xUnit test in C#
 ```text
-add_breakpoint  fileFullPath=C:\Repo\Calculator.Tests\CalculatorTests.cs  lineContent="Assert.Equal(5, _calc.Add(2, 3));"
+add_breakpoint  fileFullPath=C:\Repo\Calculator.Tests\CalculatorTests.cs  line=18
 start_debugging fileFullPath=C:\Repo\Calculator.Tests\CalculatorTests.cs  workingDirectory=C:\Repo  testName=Add_ReturnsSum
 # pauses inside the test
 step_into

skills/debug-live/references/troubleshooting/csharp.md

@@ -33,7 +33,7 @@ DebugMCP provides enhanced support for debugging C# applications using the C# De
 3. Set appropriate working directories
 
 ### Breakpoint Management
-1. Use line content matching for reliable breakpoint placement
+1. Use the 1-based line number for reliable breakpoint placement
 2. Clear breakpoints when stopping debug sessions
 3. Verify breakpoints are hit in the expected files
 

src/debugMCPServer.ts

@@ -271,10 +271,10 @@ export class DebugMCPServer {
             description: 'Set a breakpoint to pause execution at a critical line of code. Essential for debugging: pause before potential errors, examine state at decision points, or verify code paths. Breakpoints let you inspect variables and control flow at exact moments. Provide an optional condition to create a conditional breakpoint that only pauses when the expression evaluates to true (e.g. "i == 5" or "user.id === null").',
             inputSchema: {
                 fileFullPath: z.string().describe('Full path to the file'),
-                lineContent: z.string().describe('Line content'),
+                line: z.number().int().describe('Line number (1-based) where the breakpoint should be set'),
                 condition: z.string().optional().describe('Optional condition expression. When provided, execution only pauses if this expression evaluates to true at the breakpoint location.'),
             },
-        }, async (args: { fileFullPath: string; lineContent: string; condition?: string }) =>
+        }, async (args: { fileFullPath: string; line: number; condition?: string }) =>
             this.runTool('add_breakpoint', () => debuggingHandler.handleAddBreakpoint(args)));
 
         // Remove breakpoint tool

src/debuggingHandler.ts

@@ -18,7 +18,7 @@ export interface IDebuggingHandler {
     handleContinue(): Promise<string>;
     handlePause(): Promise<string>;
     handleRestart(): Promise<string>;
-    handleAddBreakpoint(args: { fileFullPath: string; lineContent: string; condition?: string }): Promise<string>;
+    handleAddBreakpoint(args: { fileFullPath: string; line: number; condition?: string }): Promise<string>;
     handleRemoveBreakpoint(args: { fileFullPath: string; line: number }): Promise<string>;
     handleClearAllBreakpoints(): Promise<string>;
     handleListBreakpoints(): Promise<string>;
@@ -300,40 +300,26 @@ export class DebuggingHandler implements IDebuggingHandler {
      * Add a breakpoint at specified location. An optional condition makes it a
      * conditional breakpoint that only pauses when the expression is true.
      */
-    public async handleAddBreakpoint(args: { fileFullPath: string; lineContent: string; condition?: string }): Promise<string> {
-        const { fileFullPath, lineContent, condition } = args;
-        
+    public async handleAddBreakpoint(args: { fileFullPath: string; line: number; condition?: string }): Promise<string> {
+        const { fileFullPath, line, condition } = args;
+
         try {
-            // Find the line number containing the line content
-            const document = await vscode.workspace.openTextDocument(vscode.Uri.file(fileFullPath));
-            const text = document.getText();
-            const lines = text.split(/\r?\n/);
-            const matchingLineNumbers: number[] = [];
-            
-            for (let i = 0; i < lines.length; i++) {
-                if (lines[i].includes(lineContent)) {
-                    matchingLineNumbers.push(i + 1); // Convert to 1-based line numbers
-                }
+            if (!Number.isInteger(line) || line < 1) {
+                throw new Error(`Invalid line number: ${line}. Provide a 1-based line number.`);
             }
-            
-            if (matchingLineNumbers.length === 0) {
-                throw new Error(`Could not find any lines containing: ${lineContent}`);
+
+            // Validate the line exists so we fail clearly instead of setting an
+            // unbound breakpoint 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);
-            
-            // Add breakpoints to all matching lines
-            for (const lineNumber of matchingLineNumbers) {
-                await this.executor.addBreakpoint(uri, lineNumber, condition);
-            }
-            
+            await this.executor.addBreakpoint(uri, line, condition);
+
             const conditionInfo = condition ? ` (condition: ${condition})` : '';
-            if (matchingLineNumbers.length === 1) {
-                return `Breakpoint added at ${fileFullPath}:${matchingLineNumbers[0]}${conditionInfo}`;
-            } else {
-                const linesList = matchingLineNumbers.join(', ');
-                return `Breakpoints added at ${matchingLineNumbers.length} locations in ${fileFullPath}: lines ${linesList}${conditionInfo}`;
-            }
+            return `Breakpoint added at ${fileFullPath}:${line}${conditionInfo}`;
         } catch (error) {
             throw new Error(`Error adding breakpoint: ${error}`);
         }

src/routingDebuggingHandler.ts

@@ -198,7 +198,7 @@ export class RoutingDebuggingHandler implements IDebuggingHandler {
 		return this.forward('handleRestart', {});
 	}
 
-	public handleAddBreakpoint(args: { fileFullPath: string; lineContent: string; condition?: string }): Promise<string> {
+	public handleAddBreakpoint(args: { fileFullPath: string; line: number; condition?: string }): Promise<string> {
 		return this.forward('handleAddBreakpoint', args, args.fileFullPath);
 	}
 

src/test/routing.test.ts

@@ -115,7 +115,7 @@ suite('Multi-window routing', () => {
 		const routing = new RoutingDebuggingHandler(new WorkspaceRegistry(process.pid, dir));
 		const result = await routing.handleAddBreakpoint({
 			fileFullPath: path.join(repoA, 'src', 'y.py'),
-			lineContent: 'return 1'
+			line: 1
 		});
 
 		assert.strictEqual(result, 'A:addBp');