fix(tools) limit edit diff preview size for user and model

pkg/tools/result_test.go

@@ -44,9 +44,6 @@ func TestSilentResult(t *testing.T) {
 func TestDiffResult(t *testing.T) {
 	result := DiffResult("pkg/tools/fs/edit.go", []byte("hello world\n"), []byte("hello universe\n"))
 
-	if result.ForLLM != result.ForUser {
-		t.Fatalf("Expected ForLLM and ForUser to match, got %q vs %q", result.ForLLM, result.ForUser)
-	}
 	if result.Silent {
 		t.Error("Expected Silent to be false")
 	}
@@ -56,6 +53,12 @@ func TestDiffResult(t *testing.T) {
 	if result.Async {
 		t.Error("Expected Async to be false")
 	}
+	if result.ForLLM == result.ForUser {
+		t.Fatalf("Expected ForLLM to omit the full diff, got %q", result.ForLLM)
+	}
+	if len(result.ForLLM) >= len(result.ForUser) {
+		t.Fatalf("Expected ForLLM to stay smaller than ForUser, got %d vs %d", len(result.ForLLM), len(result.ForUser))
+	}
 
 	for _, want := range []string{
 		"File edited: pkg/tools/fs/edit.go",
@@ -80,6 +83,9 @@ func TestDiffResult_NormalizesAbsolutePathsAndHandlesNoOpChanges(t *testing.T) {
 	if !strings.Contains(result.ForUser, "(no content change)") {
 		t.Fatalf("Expected no-content-change marker, got %q", result.ForUser)
 	}
+	if !strings.Contains(result.ForLLM, "(no content change)") {
+		t.Fatalf("Expected compact no-op summary in ForLLM, got %q", result.ForLLM)
+	}
 }
 
 func TestAsyncResult(t *testing.T) {

pkg/tools/shared/diff_result.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"path/filepath"
 	"strings"
+	"unicode/utf8"
 
 	"github.com/pmezard/go-difflib/difflib"
 )
@@ -12,6 +13,11 @@ import (
 const (
 	noContentChangeDiffMessage = "(no content change)"
 	noNewlineAtEOFMarker       = `\ No newline at end of file`
+	diffPreviewSkippedMessage  = "[diff preview skipped: file too large for inline preview]"
+	diffPreviewTruncatedNote   = "[diff preview truncated; call read_file for the full edited contents]"
+	maxDiffInputBytes          = 64 * 1024
+	maxDiffInputLines          = 2000
+	maxUserDiffPreviewBytes    = 16 * 1024
 )
 
 // DiffResult creates a user-visible tool result containing a unified diff for
@@ -19,13 +25,36 @@ const (
 // the follow-up assistant response can reason about the resulting change set,
 // including EOF newline transitions.
 func DiffResult(path string, before, after []byte) *ToolResult {
-	diff, err := buildUnifiedDiff(path, before, after)
+	summary := fmt.Sprintf("File edited: %s", path)
-	if err != nil {
+	if exceedsDiffPreviewLimits(before, after) {
-		return UserResult(fmt.Sprintf("File edited: %s\n[diff unavailable: %v]", path, err))
+		return SilentResult(summary + "\n" + diffPreviewSkippedMessage)
 	}
 
-	content := fmt.Sprintf("File edited: %s\n```diff\n%s\n```", path, diff)
+	diff, err := buildUnifiedDiff(path, before, after)
-	return UserResult(content)
+	if err != nil {
+		return UserResult(fmt.Sprintf("%s\n[diff unavailable: %v]", summary, err))
+	}
+
+	userDiff, truncated := truncateDiffPreview(diff, maxUserDiffPreviewBytes)
+	userContent := fmt.Sprintf("%s\n```diff\n%s\n```", summary, userDiff)
+	if truncated {
+		userContent += "\n" + diffPreviewTruncatedNote
+	}
+
+	llmContent := summary
+	if diff == noContentChangeDiffMessage {
+		llmContent = summary + "\n" + noContentChangeDiffMessage
+	} else if truncated {
+		llmContent = summary + "\n" + diffPreviewTruncatedNote
+	}
+
+	return &ToolResult{
+		ForLLM:  llmContent,
+		ForUser: userContent,
+		Silent:  false,
+		IsError: false,
+		Async:   false,
+	}
 }
 
 func buildUnifiedDiff(path string, before, after []byte) (string, error) {
@@ -78,6 +107,52 @@ func lacksTrailingNewline(content []byte) bool {
 	return len(content) > 0 && !bytes.HasSuffix(content, []byte("\n"))
 }
 
+func exceedsDiffPreviewLimits(before, after []byte) bool {
+	return len(before) > maxDiffInputBytes ||
+		len(after) > maxDiffInputBytes ||
+		countDiffLines(before) > maxDiffInputLines ||
+		countDiffLines(after) > maxDiffInputLines
+}
+
+func countDiffLines(content []byte) int {
+	if len(content) == 0 {
+		return 0
+	}
+
+	lines := bytes.Count(content, []byte{'\n'})
+	if !bytes.HasSuffix(content, []byte("\n")) {
+		lines++
+	}
+	return lines
+}
+
+func truncateDiffPreview(diff string, maxBytes int) (string, bool) {
+	if maxBytes <= 0 || len(diff) <= maxBytes {
+		return diff, false
+	}
+
+	truncated := diff[:maxBytes]
+	for len(truncated) > 0 && !utf8.ValidString(truncated) {
+		truncated = truncated[:len(truncated)-1]
+	}
+
+	lastNewline := strings.LastIndexByte(truncated, '\n')
+	if lastNewline > 0 {
+		truncated = truncated[:lastNewline]
+	}
+
+	truncated = strings.TrimRight(truncated, "\n")
+	if truncated == "" {
+		truncated = diff[:maxBytes]
+		for len(truncated) > 0 && !utf8.ValidString(truncated) {
+			truncated = truncated[:len(truncated)-1]
+		}
+		truncated = strings.TrimRight(truncated, "\n")
+	}
+
+	return truncated, true
+}
+
 func diffDisplayPath(path string) string {
 	displayPath := strings.TrimLeft(filepath.ToSlash(path), "/")
 	if displayPath == "" {

pkg/tools/shared/diff_result_test.go

@@ -1,6 +1,7 @@
 package toolshared
 
 import (
+	"bytes"
 	"strings"
 	"testing"
 )
@@ -11,15 +12,21 @@ func TestDiffResult_UserVisibleUnifiedDiff(t *testing.T) {
 	if result == nil {
 		t.Fatal("DiffResult() returned nil")
 	}
-	if result.ForLLM != result.ForUser {
-		t.Fatalf("expected ForLLM and ForUser to match, got %q vs %q", result.ForLLM, result.ForUser)
-	}
 	if result.Silent {
 		t.Fatal("expected DiffResult to be user-visible")
 	}
 	if result.IsError {
 		t.Fatal("expected DiffResult to be successful")
 	}
+	if result.ForLLM == result.ForUser {
+		t.Fatal("expected compact model context instead of duplicating the full diff")
+	}
+	if len(result.ForLLM) >= len(result.ForUser) {
+		t.Fatalf("expected ForLLM to stay smaller than ForUser, got %d vs %d", len(result.ForLLM), len(result.ForUser))
+	}
+	if result.ForLLM != "File edited: /tmp/example.txt" {
+		t.Fatalf("expected compact summary in ForLLM, got %q", result.ForLLM)
+	}
 
 	for _, want := range []string{
 		"File edited: /tmp/example.txt",
@@ -100,6 +107,43 @@ func TestBuildUnifiedDiff_UsesNormalizedDisplayPaths(t *testing.T) {
 	}
 }
 
+func TestDiffResult_SkipsPreviewForLargeFiles(t *testing.T) {
+	before := bytes.Repeat([]byte("a"), maxDiffInputBytes+1)
+	after := bytes.Repeat([]byte("b"), maxDiffInputBytes+1)
+
+	result := DiffResult("big.txt", before, after)
+
+	if !result.Silent {
+		t.Fatal("expected large diff previews to be skipped silently")
+	}
+	if result.ForUser != "" {
+		t.Fatalf("expected no user-facing preview when skipped, got %q", result.ForUser)
+	}
+	if !strings.Contains(result.ForLLM, diffPreviewSkippedMessage) {
+		t.Fatalf("expected skipped-preview note, got %q", result.ForLLM)
+	}
+}
+
+func TestDiffResult_TruncatesLargeUserPreview(t *testing.T) {
+	after := []byte(strings.Repeat("abcd", maxUserDiffPreviewBytes/4) + "\n")
+
+	result := DiffResult("preview.txt", []byte("before\n"), after)
+
+	if result.Silent {
+		t.Fatal("expected preview to remain user-visible below the input caps")
+	}
+	if !strings.Contains(result.ForUser, diffPreviewTruncatedNote) {
+		t.Fatalf("expected truncated preview note, got %q", result.ForUser)
+	}
+	if !strings.Contains(result.ForLLM, diffPreviewTruncatedNote) {
+		t.Fatalf("expected model summary to mention truncation, got %q", result.ForLLM)
+	}
+	if len(result.ForLLM) >= len(result.ForUser) {
+		t.Fatalf("expected ForLLM to remain smaller than ForUser, "+
+			"got %d vs %d", len(result.ForLLM), len(result.ForUser))
+	}
+}
+
 func TestDiffDisplayPath(t *testing.T) {
 	tests := []struct {
 		name string