diff --git a/backend/internal/pkg/apicompat/responses_to_chatcompletions_tool_name_test.go b/backend/internal/pkg/apicompat/responses_to_chatcompletions_tool_name_test.go
new file mode 100644
index 0000000000..c30ca97389
--- /dev/null
+++ b/backend/internal/pkg/apicompat/responses_to_chatcompletions_tool_name_test.go
@@ -0,0 +1,44 @@
+package apicompat
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestResponsesEventToChatChunks_ArgumentsDeltaOmitsEmptyName pins that
+// streamed arguments-only deltas never re-emit the tool name as an empty
+// string. OpenAI-compatible clients accumulate the name from the first
+// tool-call delta; a trailing "name":"" would overwrite the accumulated name
+// and break tool dispatch ("unknown tool").
+func TestResponsesEventToChatChunks_ArgumentsDeltaOmitsEmptyName(t *testing.T) {
+	state := NewResponsesEventToChatState()
+
+	added := ResponsesEventToChatChunks(&ResponsesStreamEvent{
+		Type:        "response.output_item.added",
+		OutputIndex: 0,
+		Item: &ResponsesOutput{
+			Type:   "function_call",
+			CallID: "call_a",
+			Name:   "exec",
+		},
+	}, state)
+	require.Len(t, added, 1)
+	first, err := json.Marshal(added[0])
+	require.NoError(t, err)
+	require.Contains(t, string(first), `"name":"exec"`)
+
+	for _, fragment := range []string{`{"cmd":"ls"}`, `{"cmd":"ls","flags":"-la"}`} {
+		deltas := ResponsesEventToChatChunks(&ResponsesStreamEvent{
+			Type:        "response.function_call_arguments.delta",
+			OutputIndex: 0,
+			Delta:       fragment,
+		}, state)
+		require.Len(t, deltas, 1)
+		raw, err := json.Marshal(deltas[0])
+		require.NoError(t, err)
+		require.NotContains(t, string(raw), `"name"`, "arguments delta must not carry a name field")
+		require.Contains(t, string(raw), `"arguments"`)
+	}
+}
diff --git a/backend/internal/pkg/apicompat/types.go b/backend/internal/pkg/apicompat/types.go
index bcc8cd481e..9f2e8388e7 100644
--- a/backend/internal/pkg/apicompat/types.go
+++ b/backend/internal/pkg/apicompat/types.go
@@ -714,7 +714,9 @@ type ChatToolCall struct {
 
 // ChatFunctionCall contains the function name and arguments.
 type ChatFunctionCall struct {
-	Name      string `json:"name"`
+	// Empty name is omitted so streamed arguments-only deltas never overwrite
+	// the tool name a client accumulated from the first delta.
+	Name      string `json:"name,omitempty"`
 	Arguments string `json:"arguments"`
 }