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

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)
-	if err != nil {
-		return UserResult(fmt.Sprintf("File edited: %s\n[diff unavailable: %v]", path, err))
+	summary := fmt.Sprintf("File edited: %s", path)
+	if exceedsDiffPreviewLimits(before, after) {
+		return SilentResult(summary + "\n" + diffPreviewSkippedMessage)
 	}
 
-	content := fmt.Sprintf("File edited: %s\n```diff\n%s\n```", path, diff)
-	return UserResult(content)
+	diff, err := buildUnifiedDiff(path, before, after)
+	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 == "" {