fix: skip binary files in content search

pkg/tools/builtin/filesystem/filesystem.go

@@ -1,6 +1,7 @@
 package filesystem
 
 import (
+	"bytes"
 	"context"
 	"encoding/base64"
 	"encoding/json"
@@ -9,6 +10,7 @@ import (
 	"io"
 	"io/fs"
 	"log/slog"
+	"net/http"
 	"os"
 	"path/filepath"
 	"regexp"
@@ -1181,6 +1183,11 @@ func (t *ToolSet) handleSearchFilesContent(_ context.Context, args SearchFilesCo
 			return nil
 		}
 
+		binary, err := t.isBinaryFile(path)
+		if err != nil || binary {
+			return nil
+		}
+
 		content, err := t.readFile(path)
 		if err != nil {
 			return nil
@@ -1265,6 +1272,53 @@ func (t *ToolSet) handleSearchFilesContent(_ context.Context, args SearchFilesCo
 	}, nil
 }
 
+func (t *ToolSet) readFileHeader(resolved string, size int) ([]byte, error) {
+	if size <= 0 {
+		return nil, nil
+	}
+
+	root, rel, err := t.rootedAccess(resolved)
+	if err != nil {
+		return nil, err
+	}
+
+	var file *os.File
+	if root != nil {
+		file, err = root.Open(rel)
+	} else {
+		file, err = os.Open(resolved)
+	}
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	buf := make([]byte, size)
+	n, err := io.ReadFull(file, buf)
+	if err != nil && !errors.Is(err, io.EOF) && !errors.Is(err, io.ErrUnexpectedEOF) {
+		return nil, err
+	}
+	return buf[:n], nil
+}
+
+func (t *ToolSet) isBinaryFile(resolved string) (bool, error) {
+	header, err := t.readFileHeader(resolved, maxBinarySniffBytes)
+	if err != nil {
+		return false, err
+	}
+	return isBinaryContent(header), nil
+}
+
+func isBinaryContent(header []byte) bool {
+	if len(header) == 0 {
+		return false
+	}
+	if bytes.IndexByte(header, 0) >= 0 {
+		return true
+	}
+	return !strings.HasPrefix(http.DetectContentType(header), "text/")
+}
+
 func (t *ToolSet) handleWriteFile(ctx context.Context, args WriteFileArgs) (*tools.ToolCallResult, error) {
 	resolvedPath, err := t.resolveAndCheckPath(args.Path)
 	if err != nil {

pkg/tools/builtin/filesystem/filesystem_test.go

@@ -567,6 +567,52 @@ func TestFilesystemTool_SearchFilesContent_SkipsSymlinkToLargeFile(t *testing.T)
 	assert.NotContains(t, result.Output, "target.bin")
 }
 
+// TestFilesystemTool_SearchFilesContent_SkipsBinaryFiles verifies that
+// small binary files are skipped even when they contain the query bytes.
+func TestFilesystemTool_SearchFilesContent_SkipsBinaryFiles(t *testing.T) {
+	t.Parallel()
+	tmpDir := t.TempDir()
+	tool := New(tmpDir)
+
+	require.NoError(t, os.WriteFile(filepath.Join(tmpDir, "text.txt"), []byte("findme\n"), 0o644))
+	require.NoError(t, os.WriteFile(filepath.Join(tmpDir, "program.bin"), []byte{0x7f, 'E', 'L', 'F', 0, 'f', 'i', 'n', 'd', 'm', 'e', '\n'}, 0o644))
+	require.NoError(t, os.WriteFile(filepath.Join(tmpDir, "doc.pdf"), []byte("%PDF-1.7\nfindme\n"), 0o644))
+
+	result, err := tool.handleSearchFilesContent(t.Context(), SearchFilesContentArgs{
+		Path:  ".",
+		Query: "findme",
+	})
+	require.NoError(t, err)
+	assert.Contains(t, result.Output, "text.txt")
+	assert.NotContains(t, result.Output, "program.bin")
+	assert.NotContains(t, result.Output, "doc.pdf")
+	assert.Equal(t, SearchFilesContentMeta{MatchCount: 1, FileCount: 1}, result.Meta)
+}
+
+func TestIsBinaryContent(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name   string
+		header []byte
+		want   bool
+	}{
+		{name: "empty", header: nil, want: false},
+		{name: "plain text", header: []byte("package main\nfunc main() {}\n"), want: false},
+		{name: "utf8 text", header: []byte("Résumé\n"), want: false},
+		{name: "elf", header: []byte{0x7f, 'E', 'L', 'F', 0, 1, 1, 0}, want: true},
+		{name: "png", header: []byte{0x89, 'P', 'N', 'G', '\r', '\n', 0x1a, '\n'}, want: true},
+		{name: "pdf", header: []byte("%PDF-1.7\nfindme\n"), want: true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			assert.Equal(t, tc.want, isBinaryContent(tc.header))
+		})
+	}
+}
+
 func TestFilesystemTool_PostEditCommands(t *testing.T) {
 	t.Parallel()
 	tmpDir := t.TempDir()

pkg/tools/builtin/filesystem/limit.go

@@ -9,6 +9,10 @@ const maxFiles = 100
 // Skipping them keeps a recursive search over a large tree bounded.
 const maxSearchFileSize = 10 << 20 // 10 MiB
 
+// maxBinarySniffBytes is how much of a file header search_files_content
+// reads to decide whether the file is binary before loading the rest.
+const maxBinarySniffBytes = 512
+
 // maxSearchOutputBytes caps the total size of the joined match output.
 // Without it a broad search (e.g. across the home directory) produces a
 // multi-megabyte result that is then copied into the in-memory message