tools/file: output diff format (context=bare, new=+, changes=unified diff)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

go.mod

@@ -3,3 +3,5 @@ module ollie
 go 1.25.6
 
 require gopkg.in/yaml.v3 v3.0.1
+
+require github.com/aymanbagabas/go-udiff v0.4.1

go.sum

@@ -1,3 +1,5 @@
+github.com/aymanbagabas/go-udiff v0.4.1 h1:OEIrQ8maEeDBXQDoGCbbTTXYJMYRCRO1fnodZ12Gv5o=
+github.com/aymanbagabas/go-udiff v0.4.1/go.mod h1:0L9PGwj20lrtmEMeyw4WKJ/TMyDtvAoK9bf2u/mNo3w=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=

pkg/tools/file/diff.go

@@ -0,0 +1,25 @@
+package file
+
+import (
+	"strings"
+
+	"github.com/aymanbagabas/go-udiff"
+)
+
+// unifiedDiff produces a unified diff of oldContent → newContent for display.
+func unifiedDiff(path, oldContent, newContent string) string {
+	return udiff.Unified(path, path, oldContent, newContent)
+}
+
+// plusLines formats content as diff "added" lines, each prefixed with "+".
+// Used by file_write for new files.
+func plusLines(content string) string {
+	lines := strings.Split(strings.TrimRight(content, "\n"), "\n")
+	sb := strings.Builder{}
+	for _, l := range lines {
+		sb.WriteByte('+')
+		sb.WriteString(l)
+		sb.WriteByte('\n')
+	}
+	return sb.String()
+}

pkg/tools/file/edit.go

@@ -16,7 +16,7 @@ var ToolEdit = tools.ToolInfo{
 
 Usage:
 - You must use your ` + "`" + `Read` + "`" + ` tool at least once in the conversation before editing. This tool will error if you attempt an edit without reading the file.
-- When editing text from Read tool output, ensure you preserve the exact indentation (tabs/spaces) as it appears AFTER the line number prefix. The line number prefix format is: line number + "| ". Everything after that is the actual file content to match. Never include any part of the line number prefix in the old_string or new_string.
+- When editing text from Read tool output, ensure you preserve the exact indentation (tabs/spaces) as it appears AFTER the leading "+" prefix. Never include the leading "+" in old_string or new_string.
 - ALWAYS prefer editing existing files in the codebase. NEVER write new files unless explicitly required.
 - Only use emojis if the user explicitly requests it. Avoid adding emojis to files unless asked.
 - The edit will FAIL if ` + "`" + `old_string` + "`" + ` is not unique in the file. Either provide a larger string with more surrounding context to make it unique or use ` + "`" + `replace_all` + "`" + ` to change every instance of ` + "`" + `old_string` + "`" + `.
@@ -67,7 +67,7 @@ func (s *Server) dispatchEdit(_ context.Context, raw json.RawMessage) (string, e
 		return "", fmt.Errorf("writing file: %w", err)
 	}
 
-	return "File edited successfully", nil
+	return unifiedDiff(a.FilePath, original, newContent), nil
 }
 
 // robustReplace tries exact → whitespace-normalized → indentation-flexible → trimmed-boundary.

pkg/tools/file/read.go

@@ -22,7 +22,7 @@ Usage:
 - By default, it reads up to 2000 lines starting from the beginning of the file
 - You can optionally specify a line offset and limit (especially handy for long files), but it's recommended to read the whole file by not providing these parameters
 - Any lines longer than 2000 characters will be truncated
-- Results are returned with line numbers in the format: line number + "| " + content, starting at 1
+- Results are returned in diff format: existing content lines have no prefix
 - This tool can only read files, not directories. To read a directory, use an ls command via the Bash tool.
 - You can call multiple tools in a single response. It is always better to speculatively read multiple potentially useful files in parallel.
 - If you read a file that exists but has empty contents "(empty file)" will be returned.`,
@@ -82,7 +82,7 @@ func (s *Server) dispatchRead(_ context.Context, raw json.RawMessage) (string, e
 			if len(line) > maxLineLen {
 				line = line[:maxLineLen] + "..."
 			}
-			lines = append(lines, fmt.Sprintf("%05d| %s", lineNum, line))
+			lines = append(lines, line)
 		}
 		lineNum++
 		if lineNum > off+lim {
@@ -97,7 +97,7 @@ func (s *Server) dispatchRead(_ context.Context, raw json.RawMessage) (string, e
 	}
 
 	s.markRead(a.FilePath)
-	return strings.Join(lines, "\n"), nil
+	return strings.Join(lines, "\n") + "\n", nil
 }
 
 // openChecked validates and opens a regular file within size limits.

pkg/tools/file/write.go

@@ -47,10 +47,19 @@ func (s *Server) dispatchWrite(_ context.Context, raw json.RawMessage) (string,
 		return errText("existing file must be read first. Use the Read tool to examine the file contents."), nil
 	}
 
+	var oldContent string
+	if exists {
+		oldContent, _ = readFileChecked(a.FilePath)
+	}
+
 	if err := os.WriteFile(a.FilePath, []byte(a.Content), 0644); err != nil { //nolint:gosec
 		return "", fmt.Errorf("writing file: %w", err)
 	}
 
 	s.markRead(a.FilePath)
-	return "File written successfully", nil
+
+	if !exists {
+		return plusLines(a.Content), nil
+	}
+	return unifiedDiff(a.FilePath, oldContent, a.Content), nil
 }