Build an NVIDIA NeMo Fabric Adapter

Build against the published southbound contract. Keep the adapter thin: let NeMo Fabric own planning and consumer-facing behavior, and let the adapter own only target translation and lifecycle state.

Read the Contract

Read the current adapter contract before changing code. Start with the overview, choose an integration shape, then follow the numbered descriptor, configuration, execution, results, registration, and verification stages. Read the custom-agent page when the target loads application-defined agents or workflows. Read the optional native OpenAI streaming page only when the adapter claims that capability.

Use the committed adapter-contract JSON Schemas or the schemas installed with the matching NeMo Fabric release for exact wire shapes. Do not reconstruct a schema from examples or copy field lists into adapter code.

Establish the Boundary

Establish the adapter boundary before defining its descriptor:

1. Identify the adapter implementation and its stable adapter_id.
2. Choose a harness adapter, a shared framework adapter with registered

targets, or a dedicated custom-agent adapter.

1. Reuse one shared adapter across custom agents when the framework provides

stable loading and invocation semantics. Use a dedicated adapter when the agent itself is the only unambiguous execution boundary.

1. List the normalized fields the target can actually enforce.
2. Separate adapter-wide harness.settings, per-target workflow.settings,

and typed extensions.

1. Keep installation, environment preparation, Relay orchestration, caller

scheduling, and consumer result enrichment outside the adapter.

If the requested behavior cannot be expressed by the current contract, surface the gap. Do not silently consume an unsupported northbound field or hide it in an unrelated extension.

Define the Descriptor First

Create one self-contained *.fabric-adapter.json before implementing target translation:

• Set the current contractversion, a globally stable adapterid,

adapter_kind, and runner binding.

• Declare only normalized config.accepts fields the implementation enforces.
• When accepting instructions.system, declare the exact supported

config.systeminstructionmodes. New descriptors must not rely on the legacy omitted-value behavior, which means replace only.

• Declare mcp.auth.oauth2 or mcp.auth.service_account only when the adapter

implements the corresponding MCP authentication mode.

• Publish closed settingsschema, modelschema, tooldefinitionschema,

and extensionschemas where applicable. Use modelschema only for static model/provider compatibility and model settings; keep credential validity and provider availability in startup validation.

• Declare runtime requirements and telemetry outputs without secret values.
• Leave optional capability flags false unless the installed NeMo Fabric runtime

exposes and tests that adapter operation. Set capabilities.streaming only when the adapter implements native OpenAI Chat Completions streaming through invokeopenaistream. Relay-backed ATOF streaming is independent and does not require this capability.

If the adapter loads registered targets, list their types in targettypes. Create one *.fabric-target.json per target. The target record owns its adapterid, type-specific entry point, and workflow settings schema. It uses the same contract_version as the Adapter Descriptor.

Validate descriptor schemas without importing adapter code. Keep all schema references local to the descriptor document; do not rely on HTTP or file references.

Package Discovery Metadata

Install the descriptor in the standard shared-data location. For setuptools:

[tool.setuptools.data-files]
"share/nemo-fabric/adapters/acme" = ["acme.fabric-adapter.json"]
"share/nemo-fabric/targets/acme" = ["email.fabric-target.json"]

Depend on nemo-fabric-adapter-contract for typed standard-library dataclasses. Install its optional pydantic extra only for Pydantic interoperability. Add nemo-fabric-adapters-common only if the adapter chooses its lifecycle or Relay helpers. A bare adapter package should not depend on the NeMo Fabric runtime.

For a TypeScript adapter, depend on nemo-fabric-adapter-contract. Import descriptor, configuration, runtime-context, request, and result types from the package root, matching the Python package's single model namespace. TypeScript types do not validate data received from a process or network boundary; validate untrusted values against the JSON Schemas included with the package.

Map AgentConfig

Accept a validated AgentConfig and translate each declared field once at the adapter boundary:

• Resolve named model roles into target-native model clients or settings.
• Apply normalized instructions and runtime limits only when declared. Validate

instructions.system.mode at the adapter startup boundary as well as during planning; replace discards the harness default, while append preserves it and adds the configured content after it.

• Convert MCP servers, tool definitions, tool policy, and skills into native

target constructs.

• Resolve workflow entry points and construction settings during start in

the task environment.

• Read identity, environment, artifacts, and telemetry from RuntimeContext,

not from workflow settings.

Reject unsupported values with stable, safe error codes. Do not log complete configs, environment values, headers, credentials, or arbitrary user input.

Use typed extension models and publish their schemas at the exact descriptor extension point. Never treat extensions as an unchecked dictionary escape hatch.

Implement the Lifecycle

Implement exactly one start, zero or more ordered invoke operations, and one stop for each NeMo Fabric runtime.

• Construct and retain target state in start.
• Accept AgentRunRequest and RuntimeContext, then return one

AgentRunResult from invoke.

• Make stop safe after partial startup and failed invocation.
• Isolate mutable state between independent runtimes.
• If the descriptor declares capabilities.streaming, implement

async invokeopenaistream(request, context, emit). Execute the target exactly once, await emit(chunk) only for the openai.chat_completions.chunk/v1 profile, and return one AgentRunResult. Each chunk requires non-empty id and model, a nonnegative integer created, the exact chat.completion.chunk discriminator, and structurally valid choices. An invocation that emits no chunks is valid.

• Do not add an adapter streaming method for Relay-backed

Runtime.invoke_stream(); execute ordinary invoke and use the provided telemetry context.

For native OpenAI streaming, the SDK owns the authenticated loopback HTTP transport with chunked NDJSON framing. The common host validates the transport, removes its credentials from the adapter payload, and supplies the emit callback. Do not persist or log stream credentials, write chunks to stdout, add SSE framing, or forward other target-native event profiles.

For a Python adapter that opts into the common host:

from nemo_fabric_adapter_contract.models import AgentConfig
from nemo_fabric_adapter_contract.models import AgentRunRequest
from nemo_fabric_adapter_contract.models import AgentRunResult
from nemo_fabric_adapter_contract.models import AgentRunStatus
from nemo_fabric_adapter_contract.models import RuntimeContext
from nemo_fabric_adapters.common import lifecycle


class TargetRuntime:
    async def start(self, payload):
        config: AgentConfig = payload["config"]
        ...

    async def invoke(
        self,
        request: AgentRunRequest,
        context: RuntimeContext,
    ) -> AgentRunResult:
        native = await self.target.run(request.input)
        return AgentRunResult(
            status=AgentRunStatus.SUCCEEDED,
            output={"response": native.text},
        )

    async def invoke_openai_stream(self, request, context, emit):
        async for chunk in self.target.stream(request.input):
            await emit(chunk)
        return AgentRunResult(
            status=AgentRunStatus.SUCCEEDED,
            output={"response": self.target.final_text},
        )

    async def stop(self):
        ...


def main() -> None:
    lifecycle.serve(TargetRuntime, config_loader=AgentConfig.from_mapping)

The common host decodes the internal lifecycle envelope before calling the adapter and encodes its terminal result afterward. Adapter code does not parse the transport envelope or infer failure from fields inside output. Return AgentRunStatus.FAILED with an AgentRunError when the target completes with a failed outcome. Raise an exception when the adapter cannot produce a normalized terminal result.

For in-process Relay SDK telemetry where the adapter owns the invocation-level Agent scope, wrap that scope with nemofabricadapters.common.utils.relayrequestcontext(context.requestid). The helper uses a UUID request ID as Relay's propagated root and always returns nemofabricrequestid metadata, including for non-UUID request IDs. Do not apply this pattern to an external Relay gateway or an upstream integration that creates an isolated scope context unless its boundary accepts a per-turn propagation context.

Handle Custom Agents

For a shared framework adapter, select the registered target with FabricConfig.workflow.targetid. Use the selected Adapter Target Descriptor's spec.entrypoint.kind for adapter-scoped resolution semantics and ref for the factory identity. Validate workflow.settings against that target's spec.settingsschema.

• Define only entry-point kinds that the shared adapter resolves

unambiguously. The v1alpha2 contract does not define a global kind catalog.

• Map a declared factory intent to the corresponding target-native factory.

The current NeMo Agent Toolkit reference maps fabric.agent.react to its ReAct workflow factory.

• Supply factories an adapter-defined build context containing already

resolved native values; do not require custom agents to parse FabricConfig or AgentConfig.

• Use a dedicated adapter without an artificial workflow entry point when the

selected adapter already identifies one application-owned agent.

Compare the NeMo Agent Toolkit shared adapter with the dedicated LangGraph example before choosing the custom-agent boundary.

Validate Before Handoff

Complete these checks before handing off an adapter:

1. Install the built wheel in an isolated adapter environment.
2. Confirm discovery below share/nemo-fabric and inspect the resolved adapter

and target descriptors in Fabric().plan(...).

1. Exercise one accepted normalized config and rejection for unsupported

fields and each declared schema.

1. Run doctor(...) with both missing and satisfied requirements.
2. Test start, success, target failure, malformed output, repeated invocation,

stop, partial-start cleanup, EOF cleanup, and two-runtime isolation.

1. If native OpenAI streaming is claimed, test empty and multi-chunk streams,

malformed and oversized records, invalid chunks, sequence and identity mismatches, a missing end record, early consumer close without cancellation, a separate terminal result, one active turn, and exactly one target invocation.

1. Test Relay correlation separately if telemetry support is claimed.
2. Report the adapter package version, contract version, required-profile

result, and every optional capability as supported or unsupported.

Do not claim automated NeMo Fabric conformance until the published conformance suite exists and the exact adapter release passes it.