editor-mcp

Instructions

Implementation Reference

• src/text_editor/server.py:564-732 (handler)

  The primary handler function implementing the 'overwrite' tool logic. Verifies selection integrity via ID check, processes new content lines (adding newlines as needed), generates a diff preview with context, performs syntax validation for Python (using Black) and JavaScript/JSX (using Babel), stores pending changes and diff for confirmation, and returns preview or syntax error details.
  async def overwrite( new_lines: dict, ) -> Dict[str, Any]: """ Overwrite the selected lines with new text. Amount of new lines can differ from the original selection Args: new_lines (dict): Example: {"lines":["line one", "second line"]} Returns: dict: Diff preview showing the proposed changes, and any syntax errors for JS or Python """ new_lines = new_lines.get("lines") if self.current_file_path is None: return {"error": "No file path is set. Use set_file first."} if ( self.selected_start is None or self.selected_end is None or self.selected_id is None ): return {"error": "No selection has been made. Use select tool first."} try: with open(self.current_file_path, "r", encoding="utf-8") as file: lines = file.readlines() except Exception as e: return {"error": f"Error reading file: {str(e)}"} start = self.selected_start end = self.selected_end id = self.selected_id current_content = "".join(lines[start - 1 : end]) computed_id = calculate_id(current_content, start, end) if computed_id != id: return { "error": "id verification failed. The content may have been modified since you last read it." } processed_new_lines = [] for line in new_lines: if not line.endswith("\n"): processed_new_lines.append(line + "\n") else: processed_new_lines.append(line) if ( processed_new_lines and end < len(lines) and not processed_new_lines[-1].endswith("\n") ): processed_new_lines[-1] += "\n" before = lines[: start - 1] after = lines[end:] modified_lines = before + processed_new_lines + after diff_result = generate_diff_preview(lines, modified_lines, start, end) error = None if self.current_file_path.endswith(".py"): full_content = "".join(modified_lines) try: black.format_file_contents( full_content, fast=True, mode=black.Mode(), ) except black.InvalidInput as e: error = { "error": f"Python syntax error: {str(e)}", "diff_lines": diff_result, "auto_cancel": self.fail_on_python_syntax_error, } except Exception as e: if not isinstance(e, NothingChanged): error = { "error": f"Black check raised {type(e)}: {str(e)}", "diff_lines": diff_result, } elif self.enable_js_syntax_check and self.current_file_path.endswith( (".jsx", ".js") ): with tempfile.NamedTemporaryFile( mode="w", suffix=".jsx", delete=False ) as temp: temp_path = temp.name temp.writelines(modified_lines) try: presets = ( ["@babel/preset-react"] if self.current_file_path.endswith(".jsx") else ["@babel/preset-env"] ) cmd = [ "npx", "babel", "--presets", ",".join(presets), "--no-babelrc", temp_path, "--out-file", "/dev/null", # Output to nowhere, we just want to check syntax ] # Execute Babel to transform (which validates syntax) process = subprocess.run(cmd, capture_output=True, text=True) if process.returncode != 0: error_output = process.stderr filtered_lines = [] for line in error_output.split("\n"): if "node_modules/@babel" not in line: filtered_lines.append(line) filtered_error = "\n".join(filtered_lines).strip() if not filtered_error: filtered_error = "JavaScript syntax error detected" error = { "error": f"JavaScript syntax error: {filtered_error}", "diff_lines": diff_result, "auto_cancel": self.fail_on_js_syntax_error, } except Exception as e: os.unlink(temp_path) error = { "error": f"Error checking JavaScript syntax: {str(e)}", "diff_lines": diff_result, } finally: if os.path.exists(temp_path): os.unlink(temp_path) self.pending_modified_lines = modified_lines self.pending_diff = diff_result result = { "status": "preview", "message": "Changes ready to apply. Use confirm() to apply or cancel() to discard.", "diff_lines": diff_result["diff_lines"], "start": start, "end": end, } if error: result.update(error) if error.get("auto_cancel", False): self.pending_modified_lines = None self.pending_diff = None result["status"] = "auto_cancelled" result["message"] = ( "Changes automatically cancelled due to syntax error. The lines are still selected." ) else: result["message"] += ( " It looks like there is a syntax error, but you can choose to fix it in the subsequent edits." ) return result
• src/text_editor/server.py:32-56 (helper)

  Helper function to compute a unique identifier for selected text content, combining optional line range prefix with truncated SHA-256 hash. Used in 'overwrite' for verifying that the selected content has not changed since selection.
  def calculate_id(text: str, start: int = None, end: int = None) -> str: """ Calculate a unique ID for content verification based on the text content. The ID is formed by combining a line prefix (if line numbers are provided) with a truncated SHA-256 id of the content. This allows quick verification that content hasn't changed between operations. Args: text (str): Content to generate ID for start (Optional[int]): Starting line number for the content end (Optional[int]): Ending line number for the content Returns: str: ID string in format: [LinePrefix]-[Truncatedid] Example: "L10-15-a7" for content spanning lines 10-15 Example: "L5-b3" for content on line 5 only """ prefix = "" if start and end: prefix = f"L{start}-{end}-" if start == end: prefix = f"L{start}-" return f"{prefix}{hashlib.sha256(text.encode()).hexdigest()[:2]}"
• src/text_editor/server.py:58-102 (helper)

  Helper function to generate a context-aware diff preview of proposed changes in 'overwrite' tool, showing removed (+/- prefixed), added lines, and surrounding context lines.
  def generate_diff_preview( original_lines: list, modified_lines: list, start: int, end: int ) -> dict: """ Generate a diff preview comparing original and modified content. Args: original_lines (list): List of original file lines modified_lines (list): List of modified file lines start (int): Start line number of the edit (1-based) end (int): End line number of the edit (1-based) Returns: dict: A dictionary with keys prefixed with + or - to indicate additions/deletions Format: [("-1", "removed line"), ("+1", "added line")] """ diffs = [] # Add some context lines before the change context_start = max(0, start - 1 - 3) # 3 lines of context before for i in range(context_start, start - 1): diffs.append((i + 1, original_lines[i].rstrip())) # Show removed lines for i in range(start - 1, end): diffs.append((f"-{i+1}", original_lines[i].rstrip())) # Show added lines new_content = "".join( modified_lines[ start - 1 : start - 1 + len(modified_lines) - len(original_lines) + (end - (start - 1)) ] ) new_lines = new_content.splitlines() for i, line in enumerate(new_lines): diffs.append((f"+{start+i}", line)) context_end = min(len(original_lines), end + 3) # 3 lines of context after for i in range(end, context_end): diffs.append((i + 1, original_lines[i].rstrip())) return { "diff_lines": diffs, }
• src/text_editor/server.py:564-564 (registration)

  The FastMCP decorator registering the overwrite handler function as a tool.
  async def overwrite(