[SDK] Codex provider for .harness()

[santoshkumarradha]

Overview

Implement the Codex provider. Python uses CLI subprocess (codex exec --json). TypeScript uses native SDK (@openai/codex-sdk).

Branch: feat/harness-v2
Design doc: docs/design/harness-v2-design.md (Sections 4, 10.2)

Scope

Python (sdk/python/agentfield/harness/providers/)

• codex.py — CodexProvider implementing HarnessProvider:
  • CLI subprocess: codex exec --json [--full-auto] [-C cwd] prompt
  • Parse JSONL event stream from stdout
  • Extract final result text, messages, metrics
  • Support configurable binary path via codex_bin
• _cli.py — Shared async subprocess utilities:
  • async run_cli(cmd, env, cwd, timeout) → stdout, stderr, returncode
  • JSONL line parser
• Register in _factory.py for provider name "codex"
• Tests with mocked subprocess

TypeScript (sdk/typescript/src/harness/providers/)

• codex.ts — CodexProvider implementing HarnessProvider:
  • Uses @openai/codex-sdk (native TS SDK, optional peer dep)
  • Maps HarnessOptions → Codex SDK options
  • Collects results
• cli.ts — Shared subprocess utilities (for other CLI providers)
• Register in factory.ts
• Tests with mocked SDK

Key Details

• Codex CLI outputs JSONL events to stdout
• --full-auto maps to permission_mode: "auto"
• -C <path> sets working directory
• --json enables JSONL output mode
• Binary path configurable via HarnessConfig.codex_bin

Acceptance Criteria

• CLI command constructed correctly for all option combinations
• JSONL parsing extracts messages and final result
• Handles binary not found (FileNotFoundError → clear error)
• Handles non-zero exit codes
• Shared CLI utilities reusable by Gemini/OpenCode providers
• All tests pass

Dependencies

• Depends on: [SDK] Core types & provider interface for .harness() #199 (types), [SDK] HarnessRunner with retry & schema orchestration for .harness() #201 (runner)
• Blocks: [SDK] Wire .harness() into Agent class #204

[github-actions]

Thanks for picking this up, @santoshkumarradha! 🎉

Here's what to expect:

• We'll check in after 7 days if we haven't heard an update
• After 14 days, we'll send a second reminder
• After 21 days without activity, we'll unassign to let others contribute

Tips for success:

• Drop a comment when you start working on it
• Share any blockers - we're here to help!
• Link your PR with Fixes #203 in the description

Happy coding! 🚀