N Agentic Harnesses For Codex

Use this skill as a router for designing, building, and evaluating agentic harnesses.

Read only the files you need. Do not load the entire reference set unless the request genuinely spans multiple subsystems.

Default posture:

• Bias toward lean, solo-maintainable architecture.
• Start with a single-agent design unless clear constraints justify more.
• Require an evaluation plan even for greenfield builds.
• Prefer explicit system boundaries, permission policy, and workflow state over prompt cleverness.
• Translate ideas into implementation phases, success criteria, and failure tests.

Step 1: Classify The Request

Choose one mode before reading reference files.

design

Use when the user is creating a new harness, planning a major rebuild, or asking for architecture, MVP, or implementation sequencing.

Default reads:

• references/01-principles-and-solo-dev-defaults.md
• references/02-harness-shapes-and-architecture.md
• references/08-design-and-build-playbook.md

Add subsystem files only as needed.

evaluation

Use when the user already has a harness and wants gaps, risks, missing primitives, UX upgrades, or architectural cleanup.

Default reads:

• references/01-principles-and-solo-dev-defaults.md
• references/09-evaluation-and-improvement-playbook.md

Add subsystem files only for the parts under review.

design + evaluation

Use when the user wants a target architecture and a way to verify it, compare it with an existing system, or define acceptance criteria before building.

Default reads:

• references/01-principles-and-solo-dev-defaults.md
• references/02-harness-shapes-and-architecture.md
• references/08-design-and-build-playbook.md
• references/09-evaluation-and-improvement-playbook.md

Step 2: Classify The Product Shape

Determine the closest product shape before going deeper:

• code agent
• chat assistant
• workflow orchestrator
• internal copilot
• embedded AI product feature
• hybrid system

If the request is ambiguous, pick the closest shape and state the assumption.

Step 3: Read The Smallest Useful Reference Set

Read these only when the request needs them:

• references/01-principles-and-solo-dev-defaults.md Use first for almost every request. It defines the default decision posture.
• references/02-harness-shapes-and-architecture.md Read when choosing system shape, boundaries, lifecycle, transports, or deployment structure.
• references/03-tools-execution-and-permissions.md Read when the request involves tool registries, tool calling, approval gates, sandboxes, or trust tiers.
• references/04-state-sessions-and-durability.md Read when the request involves sessions, resumability, retries, idempotency, approval waits, or long-running work.
• references/05-context-memory-and-evaluation.md Read when the request involves context windows, retrieval, memory, provenance, evals, replay tests, or regression detection.
• references/06-agents-and-extensibility.md Read when the request involves multi-agent design, plugins, hooks, skills, or extension surfaces.
• references/07-ux-observability-and-operations.md Read when the request involves streaming UX, health checks, logs, analytics, budgets, or supportability.
• references/08-design-and-build-playbook.md Read when the user needs a build-ready plan from idea to implementation.
• references/09-evaluation-and-improvement-playbook.md Read when the user needs findings, missing primitives, upgrade priorities, or acceptance tests.
• references/10-example-requests-and-output-patterns.md Read when you need prompt examples or response structure examples.
• references/11-codex-translation-notes.md Read only when adapting this Anthropic-style skill into a Codex-oriented version or when mapping concepts between the two environments.

Do not rely on reference-to-reference chains. This file is the index.

Operating Rules

• Convert vague ambitions into concrete harness primitives.
• Push back on unnecessary complexity.
• Treat workflow state, permissions, context assembly, and evaluation as first-class architecture, not cleanup tasks.
• Separate universal harness primitives from product-specific manifestation.
• For evaluation requests, present findings first and improvement sequence second.
• For design requests, include how the design will be tested before calling it done.

Output Contract

For design

Return:

• recommended harness shape
• core primitives and subsystem boundaries
• MVP boundary
• phased implementation plan
• verification and acceptance criteria

For evaluation

Return:

• findings ordered by severity or leverage
• missing or weak primitives
• user experience and operational gaps
• prioritized upgrade path
• tests or checks that confirm the fixes

For design + evaluation

Return:

• target architecture
• comparison against current or likely failure modes
• implementation phases
• acceptance criteria
• evaluation plan covering regressions, safety, and UX

Final Check Before Responding

• Did you keep the design lean enough for a solo developer unless the request clearly demanded more?
• Did you avoid recommending multi-agent coordination by default?
• Did you include evaluation, not just construction?
• Did you give the user an operational path forward instead of abstract theory?