diff --git a/cmd/sin-code/chat_tools.go b/cmd/sin-code/chat_tools.go
index e0186d9a..8e8dd768 100644
--- a/cmd/sin-code/chat_tools.go
+++ b/cmd/sin-code/chat_tools.go
@@ -32,11 +32,13 @@ const (
 // and network calls. Production defaults point to the real implementations.
 var (
 	toolReadFn           = toolRead
-	toolWriteFn          = toolWrite
-	toolEditFn           = toolEdit
-	toolBashFn           = toolBash
-	toolSearchFn         = toolSearch
-	toolBootstrapSkillFn = toolBootstrapSkill
+	toolWriteFn           = toolWrite
+	toolEditFn            = toolEdit
+	toolApplyDiffFn       = toolApplyDiff
+	toolGenerateDiffFn    = toolGenerateDiff
+	toolBashFn            = toolBash
+	toolSearchFn          = toolSearch
+	toolBootstrapSkillFn  = toolBootstrapSkill
 	toolSearchWalkErrFn  = func(_ string, err error) error { return nil }
 	metaBootstrapSkillFn = meta.BootstrapSkill
 )
@@ -57,7 +59,11 @@ func builtinSpecs() []agentloopToolSpecAlias {
 			InputSchema: obj(map[string]any{"path": str("file path"), "content": str("full file content")}, "path", "content")},
 		{Name: "sin_edit", Description: "Replace the first exact occurrence of old with new in a file.",
 			InputSchema: obj(map[string]any{"path": str("file path"), "old": str("exact text to replace"), "new": str("replacement text")}, "path", "old", "new")},
-		{Name: "sin_bash", Description: "Run a shell command in the workspace (120s timeout).",
+		{Name: "sin_apply_diff", Description: "Apply a unified diff to a file. Validates each hunk before applying and reports applied/rejected hunks. (issue #365)",
+			InputSchema: obj(map[string]any{"path": str("file path"), "diff": str("unified diff string")}, "path", "diff")},
+		{Name: "sin_generate_diff", Description: "Generate a unified diff from old and new content. (issue #365)",
+			InputSchema: obj(map[string]any{"old_content": str("original content"), "new_content": str("updated content")}, "old_content", "new_content")},
+		{Name: "sin_bash", Description: "Run a shell command in the workspace (120s timeout).", 
 			InputSchema: obj(map[string]any{"command": str("shell command")}, "command")},
 		{Name: "sin_search", Description: "Search files for a substring; returns file:line matches.",
 			InputSchema: obj(map[string]any{"pattern": str("substring to search"), "dir": str("directory (default .)")}, "pattern")},
@@ -78,6 +84,10 @@ func builtinTool(ctx context.Context, workspace, name string, args map[string]an
 		return toolWriteFn(argStr(args, "path"), argStr(args, "content"))
 	case "sin_edit":
 		return toolEditFn(argStr(args, "path"), argStr(args, "old"), argStr(args, "new"))
+	case "sin_apply_diff":
+		return toolApplyDiffFn(argStr(args, "path"), argStr(args, "diff"))
+	case "sin_generate_diff":
+		return toolGenerateDiffFn(argStr(args, "old_content"), argStr(args, "new_content"))
 	case "sin_bash":
 		return toolBashFn(ctx, argStr(args, "command"))
 	case "sin_search":
@@ -192,6 +202,31 @@ func toolEdit(path, old, new string) (string, error) {
 	return result, nil
 }
 
+func toolApplyDiff(path, diff string) (string, error) {
+	if path == "" || diff == "" {
+		return "", fmt.Errorf("sin_apply_diff: path and diff required")
+	}
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return "", err
+	}
+	applier := agentloop.NewDiffApplier()
+	updated, err := applier.Apply(string(data), diff)
+	if err != nil {
+		return "", fmt.Errorf("sin_apply_diff: %w", err)
+	}
+	if err := os.WriteFile(path, []byte(updated), 0o644); err != nil {
+		return "", err
+	}
+	result := "applied diff to " + path
+	result += maybeGenerateTest(path)
+	return result, nil
+}
+
+func toolGenerateDiff(oldContent, newContent string) (string, error) {
+	return agentloop.GenerateDiff(oldContent, newContent), nil
+}
+
 // sandboxConfig controls OS-level isolation for sin_bash (issue #367).
 var sandboxConfig struct {
 	enabled   bool
diff --git a/cmd/sin-code/internal/agentloop/apply_diff.go b/cmd/sin-code/internal/agentloop/apply_diff.go
index 8b08311b..886bf3ad 100644
--- a/cmd/sin-code/internal/agentloop/apply_diff.go
+++ b/cmd/sin-code/internal/agentloop/apply_diff.go
@@ -325,3 +325,63 @@ func splitLines(text string) []string {
 	}
 	return lines
 }
+
+// GenerateDiff produces a unified diff string from old and new content.
+// It uses a simple line-based algorithm that groups consecutive changes into
+// one hunk. The output is valid for ApplyDiff and human-readable.
+func GenerateDiff(oldContent, newContent string) string {
+	oldLines := splitLines(oldContent)
+	newLines := splitLines(newContent)
+
+	// Find common prefix.
+	prefix := 0
+	for prefix < len(oldLines) && prefix < len(newLines) && oldLines[prefix] == newLines[prefix] {
+		prefix++
+	}
+
+	// Find common suffix after the prefix.
+	suffix := 0
+	for suffix < len(oldLines)-prefix && suffix < len(newLines)-prefix &&
+		oldLines[len(oldLines)-1-suffix] == newLines[len(newLines)-1-suffix] {
+		suffix++
+	}
+
+	oldEnd := len(oldLines) - suffix
+	newEnd := len(newLines) - suffix
+
+	ctxBefore := 3
+	if prefix < ctxBefore {
+		ctxBefore = prefix
+	}
+	ctxAfter := 3
+	if oldEnd+ctxAfter > len(oldLines) {
+		ctxAfter = len(oldLines) - oldEnd
+		if ctxAfter < 0 {
+			ctxAfter = 0
+		}
+	}
+
+	oldStart := prefix - ctxBefore + 1
+	newStart := prefix - ctxBefore + 1
+	oldLinesCount := ctxBefore + (oldEnd - prefix) + ctxAfter
+	newLinesCount := ctxBefore + (newEnd - prefix) + ctxAfter
+
+	var b strings.Builder
+	fmt.Fprintf(&b, "--- a/file\n+++ b/file\n")
+	fmt.Fprintf(&b, "@@ -%d,%d +%d,%d @@\n", oldStart, oldLinesCount, newStart, newLinesCount)
+
+	for i := prefix - ctxBefore; i < prefix; i++ {
+		fmt.Fprintf(&b, " %s\n", oldLines[i])
+	}
+	for i := prefix; i < oldEnd; i++ {
+		fmt.Fprintf(&b, "-%s\n", oldLines[i])
+	}
+	for i := prefix; i < newEnd; i++ {
+		fmt.Fprintf(&b, "+%s\n", newLines[i])
+	}
+	for i := oldEnd; i < oldEnd+ctxAfter && i < len(oldLines); i++ {
+		fmt.Fprintf(&b, " %s\n", oldLines[i])
+	}
+
+	return b.String()
+}
diff --git a/cmd/sin-code/internal/agentloop/apply_diff_test.go b/cmd/sin-code/internal/agentloop/apply_diff_test.go
index 67c1da5b..ebc20b6c 100644
--- a/cmd/sin-code/internal/agentloop/apply_diff_test.go
+++ b/cmd/sin-code/internal/agentloop/apply_diff_test.go
@@ -198,6 +198,41 @@ func TestApplyDiff_FileHeadersSkipped(t *testing.T) {
 	}
 }
 
+func TestGenerateDiff_RoundTrip(t *testing.T) {
+	oldContent := "alpha\nbeta\ngamma\n"
+	newContent := "alpha\nBETA\ngamma\n"
+	diff := GenerateDiff(oldContent, newContent)
+	hunks, err := ParseUnifiedDiff(diff)
+	if err != nil {
+		t.Fatalf("ParseUnifiedDiff: %v", err)
+	}
+	result, err := ApplyDiff(oldContent, hunks)
+	if err != nil {
+		t.Fatalf("ApplyDiff: %v", err)
+	}
+	if result != newContent {
+		t.Errorf("round-trip = %q, want %q", result, newContent)
+	}
+}
+
+func TestGenerateDiff_Insertion(t *testing.T) {
+	oldContent := "a\nc\n"
+	newContent := "a\nb\nc\n"
+	diff := GenerateDiff(oldContent, newContent)
+	if !strings.Contains(diff, "+b") {
+		t.Errorf("diff missing inserted line: %s", diff)
+	}
+}
+
+func TestGenerateDiff_Deletion(t *testing.T) {
+	oldContent := "a\nb\nc\n"
+	newContent := "a\nc\n"
+	diff := GenerateDiff(oldContent, newContent)
+	if !strings.Contains(diff, "-b") {
+		t.Errorf("diff missing removed line: %s", diff)
+	}
+}
+
 // isBinary returns true if the content contains NUL bytes, which is the
 // standard heuristic for binary file detection.
 func isBinary(content string) bool {
diff --git a/cmd/sin-code/internal/permission_defaults.go b/cmd/sin-code/internal/permission_defaults.go
index 525e2c37..97b2181d 100644
--- a/cmd/sin-code/internal/permission_defaults.go
+++ b/cmd/sin-code/internal/permission_defaults.go
@@ -14,6 +14,8 @@ func DefaultPermissionRules() []permission.Rule {
 		{Tool: "sin_read", Policy: "allow"},
 		{Tool: "sin_write", Policy: "allow"},
 		{Tool: "sin_edit", Policy: "allow"},
+		{Tool: "sin_apply_diff", Policy: "allow"},   // v3.23.0: unified diff editor (issue #365)
+		{Tool: "sin_generate_diff", Policy: "allow"}, // v3.23.0: diff generator (issue #365)
 		{Tool: "sin_test", Policy: "allow"},
 		{Tool: "sin_quality_gate", Policy: "allow"}, // v3.21.0: Test-First Verify-Loop (RFC-test-automation)
 		{Tool: "sin_mutation", Policy: "allow"},