jax-ml/jax · error · nb::value_error

Argument to get_c_api_topology was not a pjrt_c_api capsule.

Error message

Argument to get_c_api_topology was not a pjrt_c_api capsule.

What it means

get_c_api_topology requires a capsule whose name is exactly 'pjrt_c_api' so its data can be safely cast to PJRT_Api*; any other name is rejected before the topology query.

Source

Thrown at jaxlib/jax.cc:607

      nb::arg("transfer_server_factory").none() = std::nullopt,
      nb::arg("force_dcn_cross_host_transfers") = false,
      nb::arg("sort_devices_by_process_index") = true);
  // TODO(b/322357665): Delete this method after TPU plugin changes to use the
  // standard registration.
  m.def("get_default_c_api_topology",
        [](std::string platform_name, std::string topology_name,
           const absl::flat_hash_map<std::string, xla::PjRtValueType>& options)
            -> std::shared_ptr<xla::ifrt::Topology> {
          return std::make_shared<xla::ifrt::PjRtTopology>(xla::ValueOrThrow(
              xla::GetCApiTopology(platform_name, topology_name, options)));
        });
  m.def("get_c_api_topology",
        [](nb::capsule c_api, std::string topology_name,
           const absl::flat_hash_map<std::string, xla::PjRtValueType>& options)
            -> std::shared_ptr<xla::ifrt::Topology> {
          if (c_api.name() == nullptr ||
              std::string_view(c_api.name()) != "pjrt_c_api") {
            throw nb::value_error(
                "Argument to get_c_api_topology was not a pjrt_c_api capsule.");
          }
          if (c_api.data() == nullptr) {
            throw nb::value_error(
                "Argument to get_c_api_topology contained a null pointer.");
          }
          return std::make_shared<xla::ifrt::PjRtTopology>(xla::ValueOrThrow(
              xla::GetCApiTopology(static_cast<const PJRT_Api*>(c_api.data()),
                                   topology_name, options)));
        });
  m.def("get_topology_for_devices",
        [](const std::vector<nb_class_ptr<PyDevice>>& py_devices) {
          if (py_devices.empty()) {
            throw nb::value_error(
                "get_topology_for_devices requires >= 1 devices.");
          }
          auto client = py_devices[0]->client();
          absl::InlinedVector<xla::ifrt::Device*, 1> ifrt_devices;

View on GitHub (pinned to 1e1c6a8fc0)

Solutions

1. Obtain the capsule via jax.extend.backend.get_backend_c_api() for the target platform
2. Check capsule name == 'pjrt_c_api' before calling

Example fix

# before
jaxlib.get_c_api_topology(cap, 'topo')
# after
from jax.extend import backend as jeb
jaxlib.get_c_api_topology(jeb.get_backend_c_api('tpu'), 'topo')

from jax.extend import backend as jeb
cap = jeb.get_backend_c_api(platform)
assert cap is not None

When it happens

Trigger: jaxlib.get_c_api_topology(capsule, topology_name, options) with a capsule not produced by load_pjrt_plugin/get_backend_c_api, or whose name is null.

Common situations: Custom backend topology discovery passing a wrong or stale capsule object.

Related errors

• c_api argument to load_pjrt_plugin is not a pjrt_c_api capsu
• Argument to get_c_api_topology contained a null pointer.
• Argument to register_custom_call_partitioner was not a pjrt_
• get_topology_for_devices requires >= 1 devices.
• {full_name} must be a pytree prefix with bool leaves or a tu

AI-assisted analysis of jax-ml/jax@1e1c6a8fc0 (2026-08-27). Data as JSON: /api/errors/dd2ebcdcc9a75895. Report an issue: GitHub.