diff --git a/pkg/agents/toolbox/file_replace_lines.go b/pkg/agents/toolbox/file_replace_lines.go
deleted file mode 100644
index 0319beb..0000000
--- a/pkg/agents/toolbox/file_replace_lines.go
+++ /dev/null
@@ -1,133 +0,0 @@
-package toolbox
-
-import (
-	"fmt"
-	"os"
-	"strings"
-
-	toolutil "git.mlow.ca/mlow/lmcli/pkg/agents/toolbox/util"
-	"git.mlow.ca/mlow/lmcli/pkg/api"
-)
-
-const FILE_REPLACE_LINES_DESCRIPTION = `Replace or remove a range of lines within a file, must specify path.
-
-Useful for re-writing snippets/blocks of code or entire functions.
-
-Plan your edits carefully and ensure any new content matches the flow and indentation of surrounding text.`
-
-var FileReplaceLinesTool = api.ToolSpec{
-	Name:        "file_replace_lines",
-	Description: FILE_REPLACE_LINES_DESCRIPTION,
-	Parameters: []api.ToolParameter{
-		{
-			Name:        "path",
-			Type:        "string",
-			Description: "Path of the file to be modified, relative to the current working directory.",
-			Required:    true,
-		},
-		{
-			Name:        "start_line",
-			Type:        "integer",
-			Description: `Line number which specifies the start of the replacement range (inclusive).`,
-			Required:    true,
-		},
-		{
-			Name:        "end_line",
-			Type:        "integer",
-			Description: `Line number which specifies the end of the replacement range (inclusive). If unset, range extends to end of file.`,
-		},
-		{
-			Name:        "content",
-			Type:        "string",
-			Description: `Content to replace specified range. Omit to remove the specified range.`,
-		},
-	},
-	Impl: func(tool *api.ToolSpec, args map[string]interface{}) (string, error) {
-		tmp, ok := args["path"]
-		if !ok {
-			return "", fmt.Errorf("path parameter to write_file was not included.")
-		}
-		path, ok := tmp.(string)
-		if !ok {
-			return "", fmt.Errorf("Invalid path in function arguments: %v", tmp)
-		}
-		var start_line int
-		tmp, ok = args["start_line"]
-		if ok {
-			tmp, ok := tmp.(float64)
-			if !ok {
-				return "", fmt.Errorf("Invalid start_line in function arguments: %v", tmp)
-			}
-			start_line = int(tmp)
-		}
-		var end_line int
-		tmp, ok = args["end_line"]
-		if ok {
-			tmp, ok := tmp.(float64)
-			if !ok {
-				return "", fmt.Errorf("Invalid end_line in function arguments: %v", tmp)
-			}
-			end_line = int(tmp)
-		}
-		var content string
-		tmp, ok = args["content"]
-		if ok {
-			content, ok = tmp.(string)
-			if !ok {
-				return "", fmt.Errorf("Invalid content in function arguments: %v", tmp)
-			}
-		}
-
-		result := fileReplaceLines(path, start_line, end_line, content)
-		ret, err := result.ToJson()
-		if err != nil {
-			return "", fmt.Errorf("Could not serialize result: %v", err)
-		}
-		return ret, nil
-	},
-}
-
-func fileReplaceLines(path string, startLine int, endLine int, content string) api.CallResult {
-	ok, reason := toolutil.IsPathWithinCWD(path)
-	if !ok {
-		return api.CallResult{Message: reason}
-	}
-
-	// Read the existing file's content
-	data, err := os.ReadFile(path)
-	if err != nil {
-		if !os.IsNotExist(err) {
-			return api.CallResult{Message: fmt.Sprintf("Could not read path: %s", err.Error())}
-		}
-		_, err = os.Create(path)
-		if err != nil {
-			return api.CallResult{Message: fmt.Sprintf("Could not create new file: %s", err.Error())}
-		}
-		data = []byte{}
-	}
-
-	if startLine < 1 {
-		return api.CallResult{Message: "start_line cannot be less than 1"}
-	}
-
-	lines := strings.Split(string(data), "\n")
-	contentLines := strings.Split(strings.Trim(content, "\n"), "\n")
-
-	if endLine == 0 || endLine > len(lines) {
-		endLine = len(lines)
-	}
-
-	before := lines[:startLine-1]
-	after := lines[endLine:]
-
-	lines = append(before, append(contentLines, after...)...)
-	newContent := strings.Join(lines, "\n")
-
-	// Join the lines and write back to the file
-	err = os.WriteFile(path, []byte(newContent), 0644)
-	if err != nil {
-		return api.CallResult{Message: fmt.Sprintf("Could not write to path: %s", err.Error())}
-	}
-
-	return api.CallResult{Result: newContent}
-}
diff --git a/pkg/agents/toolbox/modify_file.go b/pkg/agents/toolbox/modify_file.go
new file mode 100644
index 0000000..53657c0
--- /dev/null
+++ b/pkg/agents/toolbox/modify_file.go
@@ -0,0 +1,178 @@
+package toolbox
+
+import (
+	"fmt"
+	"os"
+	"strings"
+
+	"git.mlow.ca/mlow/lmcli/pkg/agents/toolbox/util"
+	toolutil "git.mlow.ca/mlow/lmcli/pkg/agents/toolbox/util"
+	"git.mlow.ca/mlow/lmcli/pkg/api"
+)
+
+var MODIFY_FILE_DESCRIPTION = []string{
+    "Modify a file. If the file does not exist, it will be created.",
+    "",
+    "Content can be either inserted, replaced, or removed through a combination of the start, stop, and content parameters.",
+    "Use the start and stop line numbers to limit the range of modification to the file.",
+    "If both `start` and `stop` are left unset (or set to 0), the entire file's contents will be updated.",
+    "If `start` is set to n and `stop` to n+1, content will be inserted at line n (the content that was at line n will be shifted below the newly inserted content).",
+    "If only `start` is set, content from the given line and onwards will be updated.",
+    "If only `stop` is set, content up to but not including the given line will be updated.",
+    "",
+    "Examples:",
+    "1. Append to a file:",
+    "   {\"path\": \"example.txt\", \"start\": <last_line_number + 1>, \"content\": \"New content to append\"}",
+    "",
+    "2. Insert at a specific line:",
+    "   {\"path\": \"example.txt\", \"start\": 5, \"stop\": 5, \"content\": \"New line inserted above the previous line 5\"}",
+    "",
+    "3. Replace a range of lines:",
+    "   {\"path\": \"example.txt\", \"start\": 3, \"stop\": 7, \"content\": \"New content replacing lines 3-7\"}",
+    "",
+    "4. Remove a range of lines:",
+    "   {\"path\": \"example.txt\", \"start\": 2, \"stop\": 5}",
+    "",
+    "5. Replace entire file contents:",
+    "   {\"path\": \"example.txt\", \"content\": \"New file contents\"}",
+    "",
+    "6. Update from a specific line to the end of the file:",
+    "   {\"path\": \"example.txt\", \"start\": 10, \"content\": \"New content from line 10 onwards\"}",
+    "",
+    "7. Update from the beginning of the file to a specific line:",
+    "   {\"path\": \"example.txt\", \"stop\": 6, \"content\": \"New content for first 5 lines\"}",
+    "",
+    "Note: Always use specific line numbers based on the current file content. Avoid using arbitrarily large numbers for start or stop.",
+}
+
+var ModifyFile = api.ToolSpec{
+	Name:        "modify_file",
+	Description: strings.Join(MODIFY_FILE_DESCRIPTION, "\n"),
+	Parameters: []api.ToolParameter{
+		{
+			Name:        "path",
+			Type:        "string",
+			Description: "Path of the file to be modified, relative to the current working directory.",
+			Required:    true,
+		},
+		{
+			Name:        "start",
+			Type:        "integer",
+			Description: `Start line of the range to modify (inclusive). If omitted, the beginning of the file is implied.`,
+		},
+		{
+			Name:        "stop",
+			Type:        "integer",
+			Description: `End line of the range to modify (inclusive). If omitted, the end of the file is implied.`,
+		},
+		{
+			Name:        "content",
+			Type:        "string",
+			Description: "Content to insert/replace at the range defined by `start` and `stop`. If omitted, the range is removed.",
+		},
+	},
+	Impl: func(tool *api.ToolSpec, args map[string]interface{}) (string, error) {
+		tmp, ok := args["path"]
+		if !ok {
+			return "", fmt.Errorf("path parameter to modify_file was not included.")
+		}
+		path, ok := tmp.(string)
+		if !ok {
+			return "", fmt.Errorf("Invalid path in function arguments: %v", tmp)
+		}
+		var start int
+		tmp, ok = args["start"]
+		if ok {
+			tmp, ok := tmp.(float64)
+			if !ok {
+				return "", fmt.Errorf("Invalid start in function arguments: %v", tmp)
+			}
+			start = int(tmp)
+		}
+		var stop int
+		tmp, ok = args["stop"]
+		if ok {
+			tmp, ok := tmp.(float64)
+			if !ok {
+				return "", fmt.Errorf("Invalid stop in function arguments: %v", tmp)
+			}
+			stop = int(tmp)
+		}
+		var content string
+		tmp, ok = args["content"]
+		if ok {
+			content, ok = tmp.(string)
+			if !ok {
+				return "", fmt.Errorf("Invalid content in function arguments: %v", tmp)
+			}
+		}
+
+		result := fileModifyContents(path, start, stop, content)
+		ret, err := result.ToJson()
+		if err != nil {
+			return "", fmt.Errorf("Could not serialize result: %v", err)
+		}
+		return ret, nil
+	},
+}
+
+
+func fileModifyContents(path string, startLine int, stopLine int, content string) api.CallResult {
+    ok, reason := toolutil.IsPathWithinCWD(path)
+    if !ok {
+        return api.CallResult{Message: reason}
+    }
+
+    // Read the existing file's content
+    data, err := os.ReadFile(path)
+    if err != nil {
+        if !os.IsNotExist(err) {
+            return api.CallResult{Message: fmt.Sprintf("Could not read path: %s", err.Error())}
+        }
+        _, err = os.Create(path)
+        if err != nil {
+            return api.CallResult{Message: fmt.Sprintf("Could not create new file: %s", err.Error())}
+        }
+        data = []byte{}
+    }
+
+    lines := strings.Split(string(data), "\n")
+    contentLines := strings.Split(strings.TrimSuffix(content, "\n"), "\n")
+
+    // If both start and stop are unset, update the entire file
+    if startLine == 0 && stopLine == 0 {
+        lines = contentLines
+    } else {
+        if startLine < 1 {
+            startLine = 1
+        }
+        if stopLine == 0 || stopLine > len(lines) {
+            stopLine = len(lines)
+        }
+
+        before := lines[:startLine-1]
+        after := lines[stopLine:]
+
+        // Handle insertion case
+        if startLine == stopLine {
+            lines = append(before, append(contentLines, lines[startLine-1:]...)...)
+        } else {
+            // If content is omitted, remove the specified range
+            if content == "" {
+                lines = append(before, after...)
+            } else {
+                lines = append(before, append(contentLines, after...)...)
+            }
+        }
+    }
+
+    newContent := strings.Join(lines, "\n")
+
+    // Write back to the file
+    err = os.WriteFile(path, []byte(newContent), 0644)
+    if err != nil {
+        return api.CallResult{Message: fmt.Sprintf("Could not write to path: %s", err.Error())}
+    }
+
+    return api.CallResult{Result: util.AddLineNumbers(newContent)}
+}
diff --git a/pkg/agents/toolbox/read_file.go b/pkg/agents/toolbox/read_file.go
index 7427d0c..f41c8c2 100644
--- a/pkg/agents/toolbox/read_file.go
+++ b/pkg/agents/toolbox/read_file.go
@@ -3,7 +3,6 @@ package toolbox
 import (
 	"fmt"
 	"os"
-	"strings"
 
 	toolutil "git.mlow.ca/mlow/lmcli/pkg/agents/toolbox/util"
 	"git.mlow.ca/mlow/lmcli/pkg/api"
@@ -60,14 +59,7 @@ func readFile(path string) api.CallResult {
 	if err != nil {
 		return api.CallResult{Message: fmt.Sprintf("Could not read path: %s", err.Error())}
 	}
-
-	lines := strings.Split(string(data), "\n")
-	content := strings.Builder{}
-	for i, line := range lines {
-		content.WriteString(fmt.Sprintf("%d\t%s\n", i+1, line))
-	}
-
 	return api.CallResult{
-		Result: content.String(),
+		Result: toolutil.AddLineNumbers(string(data)),
 	}
 }
diff --git a/pkg/agents/toolbox/util/util.go b/pkg/agents/toolbox/util/util.go
index eb8a8b7..94ef811 100644
--- a/pkg/agents/toolbox/util/util.go
+++ b/pkg/agents/toolbox/util/util.go
@@ -65,3 +65,14 @@ func IsPathWithinCWD(path string) (bool, string) {
 	}
 	return true, ""
 }
+
+// AddLineNumbers takes a string of content and returns a new string with line
+// numbers prefixed
+func AddLineNumbers(content string) string {
+	lines := strings.Split(strings.TrimSuffix(content, "\n"), "\n")
+	result := strings.Builder{}
+	for i, line := range lines {
+		result.WriteString(fmt.Sprintf("%d\t%s\n", i+1, line))
+	}
+	return result.String()
+}
diff --git a/pkg/agents/tools.go b/pkg/agents/tools.go
index aba83ed..5fada49 100644
--- a/pkg/agents/tools.go
+++ b/pkg/agents/tools.go
@@ -8,12 +8,11 @@ import (
 )
 
 var AvailableTools map[string]api.ToolSpec = map[string]api.ToolSpec{
-	"dir_tree":           toolbox.DirTreeTool,
-	"read_dir":           toolbox.ReadDirTool,
-	"read_file":          toolbox.ReadFileTool,
-	"write_file":         toolbox.WriteFileTool,
-	"file_insert_lines":  toolbox.FileInsertLinesTool,
-	"file_replace_lines": toolbox.FileReplaceLinesTool,
+	"dir_tree":    toolbox.DirTreeTool,
+	"read_dir":    toolbox.ReadDirTool,
+	"read_file":   toolbox.ReadFileTool,
+	"modify_file": toolbox.ModifyFile,
+	"write_file":  toolbox.WriteFileTool,
 }
 
 func ExecuteToolCalls(calls []api.ToolCall, available []api.ToolSpec) ([]api.ToolResult, error) {