NVIDIA NeMo Fabric Codex Adapter

The nvidia.fabric.codex adapter uses the official Codex Python SDK behind Fabric's normalized invocation contract. It does not resolve or execute a separately installed codex command. The SDK package owns its pinned app-server runtime and typed JSON-RPC protocol.

Install

To install just the Codex adapter by itself:

pip install "nemo-fabric[codex]"

To install just the Codex adapter along with the NeMo Fabric Runtime:

pip install "nemo-fabric[codex, runtime]"

Authentication

Fabric reuses the authentication state that Codex stores under CODEX_HOME (default: ~/.codex). Fabric does not perform an interactive login, copy credentials, or mutate the user's Codex configuration.

Codex supports two OpenAI authentication modes:

• ChatGPT login: Sign in through Codex with a ChatGPT plan. Fabric can then run without OPENAI_API_KEY while that cached login remains valid.
• API key login: Provision the same Codex credential store with an OpenAI API key. This mode uses OpenAI Platform billing rather than ChatGPT plan credits.

For a nondefault credential store, set CODEX_HOME before both login and the Fabric invocation. Treat CODEX_HOME/auth.json as a secret when Codex uses file-based credential storage. Refer to the Codex authentication documentation for login, headless setup, and credential-storage options.

The adapter forwards OPENAI_API_KEY and a selected model's api_key_env to the SDK runtime. The current real-agent acceptance path validates an existing Codex login; it does not yet claim a raw environment variable as a complete login flow.

When models.default.provider is nvidia, the adapter defines a Codex model provider for the configured NVIDIA Responses endpoint. Fabric.run(...) owns that provider for one invocation, while Fabric.start_runtime(...) fixes it for the lifetime of the persistent runtime. The adapter reads the credential from api_key_env (default: NVIDIA_API_KEY) and isolates Codex state under the Fabric artifact root, so execution does not depend on or modify a user's Codex login. Set the endpoint in models.default.settings.base_url or NVIDIA_FRONTIER_BASE_URL; the adapter does not assume a default frontier endpoint.

The adapter depends on the Codex SDK, which installs and selects its matching app-server runtime. Fabric does not declare the runtime package directly or treat it as a user-installed command or adapter descriptor requirement.

A codex command on PATH is not selected implicitly. To override the SDK-selected runtime intentionally, set harness.settings.codex_bin to an app-server path that is absolute or relative to the explicit base_dir. Fabric passes the resolved path through CodexConfig.codex_bin; the SDK remains the execution driver.

Execution Model

Each Fabric runtime currently starts one local adapter host and retains one AsyncCodex client and one Codex thread. The Codex SDK starts and controls its pinned local codex app-server subprocess over JSON-RPC. Ordered Runtime.invoke(...) calls reuse that client and thread directly; the adapter closes the SDK client and app-server transport during Runtime.stop(). Codex owns the transcript; Fabric owns runtime-to-thread correlation, timeout, cancellation, and cleanup.

The result includes the SDK's typed terminal response, turn status, token usage, timing, and completed thread items. It does not expose CLI commands, return codes, stdout, or stderr.

Configuration

Use normalized FabricConfig fields for portable configuration:

• models selects the Codex model. The adapter supports the built-in openai provider and NVIDIA-hosted Responses-compatible models through the nvidia provider.
• environment.workspace sets the working directory.
• mcp maps stdio, HTTP, and streamable HTTP servers into the Codex thread's mcp_servers configuration. For stdio, Fabric parses url as a command plus arguments.
• skills.paths names skill directories that contain SKILL.md. The adapter registers each directory as a process-scoped Codex skill root so Codex can select matching skills through its normal discovery behavior.
• telemetry enables native OpenTelemetry or NeMo Relay observability.

The Codex adapter does not declare tools.blocked support. The current Codex runtime has per-MCP-server tool filters, but it does not provide one complete deny boundary for built-in, local, MCP, and hosted tools. Fabric therefore routes normalized blocked-tool policy as unsupported instead of applying a partial policy.

Codex-specific controls belong in harness.settings:

• sandbox: read-only, workspace-write, or danger-full-access
• approval_mode: auto_review or deny_all
• base_instructions and developer_instructions
• personality, reasoning_effort, service_name, and service_tier
• output_schema for SDK-native structured output
• codex_bin for an explicit Codex app-server runtime override
• config_overrides as dotted Codex configuration keys applied when the SDK runtime starts, such as Codex-only MCP timeout or required-server options
• timeout_seconds, defaulting to 1800
• env for variables explicitly forwarded to the Codex runtime
• nemo_relay_command for the optional external Relay gateway executable

Set model selection through models and the working directory through environment.workspace.

For Fabric.start_runtime(...), the model provider, MCP configuration, skill roots, and config_overrides are fixed when the runtime starts and cannot vary between Runtime.invoke(...) calls. Start a new runtime to change them. Fabric.run(...) starts the same runtime, invokes it once, and stops it, so the same settings are scoped to that single invocation.

The adapter filters the inherited environment. It retains portable OS and Codex state variables, the selected model's api_key_env, and explicit settings.env values while clearing unrelated parent-process secrets.

Relay Observability

Enable Relay through Fabric's normalized telemetry configuration. For each Relay-enabled Fabric runtime, the adapter:

1. Resolves one external nemo-relay executable.
2. Generates runtime-scoped gateway and plugin configuration.
3. Starts and health-checks nemo-relay --config ... --bind ....
4. Redirects the built-in OpenAI provider with runtime-scoped openai_base_url and passes Relay hooks through the Codex SDK's config argument.
5. Reuses the SDK client and gateway across turns, interrupting a timed-out turn.
6. Closes the SDK runtime and stops the gateway during runtime shutdown.

The SDK remains the Codex execution driver. Relay is a supervised sidecar and hook forwarder; the adapter never invokes a nemo-relay codex wrapper. The result reports the gateway config, URL, log, and collected Relay artifacts.

Fabric deliberately keeps Codex on its reserved built-in openai provider. Defining Relay as a custom model-provider alias breaks the Python SDK's ChatGPT-authenticated request path. Redirecting only openai_base_url preserves the SDK's supported authentication and host metadata while allowing Relay to capture Responses traffic. Fabric does not spoof the Codex CLI identity or fall back to CLI execution. Relay routes and observes requests; it does not provide OpenAI credentials or change the selected Codex authentication mode.

Relay-enabled Codex runs require models.default.provider: openai. The custom nvidia provider is not supported with Relay because its configured NVIDIA Responses base URL bypasses the built-in openai_base_url redirect. Run the nvidia provider without Relay and supply its credential through api_key_env (default: NVIDIA_API_KEY) and its endpoint through models.default.settings.base_url or NVIDIA_FRONTIER_BASE_URL.

Relay-enabled runs require the external nemo-relay CLI in addition to the Python package dependencies. Fabric accepts CLI versions >=0.6.0,<0.7.0. Until the request-decoding fix is released, install the tested PR revision:

git clone https://github.com/NVIDIA/NeMo-Relay.git nemo-relay
git -C nemo-relay fetch origin pull/452/head
git -C nemo-relay checkout --detach 0b02e01ac10d7d678da28830feba0ebf6743a7c0
cargo install --locked --path nemo-relay/crates/cli

Removal of this temporary source installation is tracked in TODO.md.

The nemo-relay Python package does not install this executable. Refer to the NeMo Relay installation guide for other supported installation methods.

Relay owns HTTP content decoding at the gateway boundary; Fabric does not configure Codex request compression. The Relay 0.6.0-alpha.20260716 tag cannot recover semantic fields from zstd-compressed SDK requests. Until a later 0.6.x release contains the fix, use the pinned NeMo Relay PR #452 revision above.

For Phoenix, native Codex OpenTelemetry targets the OTLP collector at http://localhost:4318/v1/traces and provides low-level app-server spans. Relay OpenInference provides the semantic chain, LLM, and tool hierarchy with decoded prompt, response, and token attributes. Prefer Relay OpenInference for agent-turn inspection.