diff --git a/internal/client/proto.go b/internal/client/proto.go
index 62a43b5884e01ae8fcd3242c68e95d1f76251c42..d07e46dc84bf09dccffbd609784f92c7ae9a9c67 100644
--- a/internal/client/proto.go
+++ b/internal/client/proto.go
@@ -423,12 +423,28 @@ func (c *Client) SendMessage(ctx context.Context, id string, sessionID, runID, p
 		return fmt.Errorf("failed to send message to agent: %w", err)
 	}
 	defer rsp.Body.Close()
-	if rsp.StatusCode != http.StatusOK {
+	if rsp.StatusCode != http.StatusOK && rsp.StatusCode != http.StatusAccepted {
+		if msg := decodeErrorMessage(rsp.Body); msg != "" {
+			return fmt.Errorf("failed to send message to agent: status code %d: %s", rsp.StatusCode, msg)
+		}
 		return fmt.Errorf("failed to send message to agent: status code %d", rsp.StatusCode)
 	}
 	return nil
 }
 
+// decodeErrorMessage attempts to decode the response body as a
+// proto.Error and returns its message. It returns an empty string
+// when the body is empty or cannot be decoded into a proto.Error
+// with a non-empty message, letting callers fall back to a
+// status-only error.
+func decodeErrorMessage(body io.Reader) string {
+	var e proto.Error
+	if err := json.NewDecoder(body).Decode(&e); err != nil {
+		return ""
+	}
+	return e.Message
+}
+
 // GetAgentSessionInfo retrieves the agent session info for a workspace.
 func (c *Client) GetAgentSessionInfo(ctx context.Context, id string, sessionID string) (*proto.AgentSession, error) {
 	rsp, err := c.get(ctx, fmt.Sprintf("/workspaces/%s/agent/sessions/%s", id, sessionID), nil, nil)
diff --git a/internal/client/proto_test.go b/internal/client/proto_test.go
index b5739ccc91c16b2bb0fc3c3f6dc2281687bd8e65..c7abd3e03d4ae6f575079c7c938369d6cb7cc30b 100644
--- a/internal/client/proto_test.go
+++ b/internal/client/proto_test.go
@@ -88,6 +88,76 @@ func TestSubscribeEventsContextCancelClosesEvents(t *testing.T) {
 	}
 }
 
+func TestSendMessageAcceptsStatusAccepted(t *testing.T) {
+	t.Parallel()
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusAccepted)
+	}))
+	defer srv.Close()
+
+	c := captureClient(t, srv)
+	require.NoError(t, c.SendMessage(context.Background(), "ws1", "sess1", "", "hello"))
+}
+
+func TestSendMessageAcceptsStatusOK(t *testing.T) {
+	t.Parallel()
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer srv.Close()
+
+	c := captureClient(t, srv)
+	require.NoError(t, c.SendMessage(context.Background(), "ws1", "sess1", "", "hello"))
+}
+
+func TestSendMessageDecodesErrorBody(t *testing.T) {
+	t.Parallel()
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusBadRequest)
+		_ = json.NewEncoder(w).Encode(proto.Error{Message: "session id is required"})
+	}))
+	defer srv.Close()
+
+	c := captureClient(t, srv)
+	err := c.SendMessage(context.Background(), "ws1", "", "", "hello")
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "status code 400")
+	require.Contains(t, err.Error(), "session id is required")
+}
+
+func TestSendMessageFallsBackOnMalformedErrorBody(t *testing.T) {
+	t.Parallel()
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+		_, _ = w.Write([]byte("not json"))
+	}))
+	defer srv.Close()
+
+	c := captureClient(t, srv)
+	err := c.SendMessage(context.Background(), "ws1", "sess1", "", "hello")
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "status code 500")
+	require.NotContains(t, err.Error(), "not json")
+}
+
+func TestSendMessageFallsBackOnEmptyErrorBody(t *testing.T) {
+	t.Parallel()
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	defer srv.Close()
+
+	c := captureClient(t, srv)
+	err := c.SendMessage(context.Background(), "ws1", "sess1", "", "hello")
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "status code 500")
+}
+
 func marshalSSEPayload(t *testing.T) []byte {
 	t.Helper()