diff --git a/packages/sandbox/daemon-go/internal/routes/fs.go b/packages/sandbox/daemon-go/internal/routes/fs.go
index 2a98f74127..65b3ee9650 100644
--- a/packages/sandbox/daemon-go/internal/routes/fs.go
+++ b/packages/sandbox/daemon-go/internal/routes/fs.go
@@ -22,6 +22,7 @@ import (
 
 const (
 	maxImageBytes    = 5 * 1024 * 1024
+	maxEditFileBytes = 10 * 1024 * 1024
 	maxTransferBytes = 500 * 1024 * 1024
 	transferDeadline = 5 * time.Minute
 )
@@ -419,6 +420,10 @@ func Edit(deps FsDeps) http.HandlerFunc {
 			httpx.Error(w, 400, "old_string and new_string must differ")
 			return
 		}
+		if stat, err := os.Stat(filePath); err == nil && stat.Size() > maxEditFileBytes {
+			httpx.Error(w, 400, fmt.Sprintf("File too large (%d bytes; cap is %d)", stat.Size(), maxEditFileBytes))
+			return
+		}
 		raw, err := os.ReadFile(filePath)
 		if err != nil {
 			httpx.Error(w, 400, fmt.Sprintf("File not found: %s", body.Path))
diff --git a/packages/sandbox/daemon-go/internal/routes/fs_edit_test.go b/packages/sandbox/daemon-go/internal/routes/fs_edit_test.go
new file mode 100644
index 0000000000..6ae25a4987
--- /dev/null
+++ b/packages/sandbox/daemon-go/internal/routes/fs_edit_test.go
@@ -0,0 +1,64 @@
+package routes
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func newEditRequest(t *testing.T, path, oldString, newString string) *http.Request {
+	t.Helper()
+	body, err := json.Marshal(map[string]any{
+		"path":       path,
+		"old_string": oldString,
+		"new_string": newString,
+	})
+	if err != nil {
+		t.Fatalf("marshal body: %v", err)
+	}
+	return httptest.NewRequest(http.MethodPost, "/_sandbox/edit", bytes.NewReader(body))
+}
+
+// TestEditRejectsOversizedFile guards the same untrusted-size boundary as the
+// read route: without a cap, editing a large file reads it entirely into
+// memory to compute the replacement, which can OOM the daemon.
+func TestEditRejectsOversizedFile(t *testing.T) {
+	repoDir := t.TempDir()
+	big := strings.Repeat("a", maxEditFileBytes+1)
+	filePath := filepath.Join(repoDir, "big.txt")
+	if err := os.WriteFile(filePath, []byte(big), 0o644); err != nil {
+		t.Fatalf("write fixture: %v", err)
+	}
+
+	handler := Edit(FsDeps{AppRoot: repoDir, RepoDir: repoDir})
+	rec := httptest.NewRecorder()
+	handler(rec, newEditRequest(t, "big.txt", "a", "b"))
+
+	if rec.Code != 400 {
+		t.Fatalf("status = %d, want 400; body = %s", rec.Code, rec.Body.String())
+	}
+	if !strings.Contains(rec.Body.String(), "too large") {
+		t.Fatalf("body = %s, want a too-large error", rec.Body.String())
+	}
+}
+
+func TestEditAllowsFileUnderCap(t *testing.T) {
+	repoDir := t.TempDir()
+	filePath := filepath.Join(repoDir, "small.txt")
+	if err := os.WriteFile(filePath, []byte("hello world"), 0o644); err != nil {
+		t.Fatalf("write fixture: %v", err)
+	}
+
+	handler := Edit(FsDeps{AppRoot: repoDir, RepoDir: repoDir})
+	rec := httptest.NewRecorder()
+	handler(rec, newEditRequest(t, "small.txt", "hello", "goodbye"))
+
+	if rec.Code != 200 {
+		t.Fatalf("status = %d, want 200; body = %s", rec.Code, rec.Body.String())
+	}
+}