Context Engineering

Context engineering is the discipline of systematically selecting, structuring, and delivering the smallest possible set of high-signal tokens that make a task plausibly solvable for an LLM — reliably, safely, and cost-effectively.

This skill provides actionable patterns, checklists, and templates for the full context pipeline — from first principles through production readiness.

When to Use

• Designing context pipelines for AI agents (what enters the LLM context window, in what order)
• Reviewing context quality and token efficiency of existing systems
• Building RAG systems (retrieval pipeline, chunking, packing, grounding)
• Implementing memory (session, working, long-term, organizational, tool-output)
• Designing agent harness (init/continuation prompts, state management, token budgets, caching)
• Multi-agent orchestration (context boundaries, payload schemas, fan-out merge)
• Production readiness assessment for AI systems (8-checklist gate)
• Privacy and compliance for AI features (multi-tenant isolation, consent, data residency)

When NOT to Use

• Prompt technique selection and template patterns → use prompt-engineering skill
• Security audit of individual prompts → use prompt-engineering skill → security-checklist.md
• Non-AI software engineering tasks → use software-engineer role + stack-specific role
• Infrastructure and deployment → use devops-engineer role

Context Engineering vs Prompt Engineering

These are complementary disciplines:

• Prompt engineering = designing the instruction text (technique selection, template patterns, formatting, few-shot)
• Context engineering = designing the full context PIPELINE (what enters the window, in what order, how it's managed across turns, how it degrades gracefully)

Context engineering is the system; prompt engineering is one layer within it.

Key Concepts

First Principles

1. Attention is finite — curate, don't stuff. "More context" is not monotonically better
2. Position effects — critical info at beginning and end ("lost-in-the-middle"). Models use tokens near the start and end more effectively than those buried in the middle
3. High-signal tokens > high-volume tokens — compact, discriminative context beats verbose dumps
4. Separation of concerns — policy, knowledge, examples, output contract in distinct layers. Enables independent swap, A/B testing, failure analysis
5. Context is a pipeline — produced by a system (retrieval, ranking, summarization, memory, policy filters, schema enforcement), not concatenated strings

The Context Stack (8-Layer Architecture)

Ordered layers, top = highest priority. Higher layers override lower on conflict.

Layer	What Goes Here	Design Notes
1. System policy & safety	Non-negotiable constraints, content policies, refusal rules	First in window — highest attention. Cacheable
2. Developer instructions	Role definitions, hard rules, reasoning protocols, conventions	Maps to: Codex package rules, role overlays, and AGENTS.md guidance
3. Tool contracts	Schemas, descriptions, permissions, failure modes, retry policies	Tool descriptions ARE prompts — optimize them
4. Runtime state	Current project context, AGENTS.md, active file, task state, structured state blob	Maps to: Codex AGENTS.md files
5. Knowledge context (RAG)	Retrieved documents, search results, file contents	Must be wrapped as untrusted data with grounding envelopes
6. Memory	Session, working, long-term, organizational, tool-output	Must be scoped, filtered, and conflict-resolved
7. Examples (few-shot)	Minimal, high-leverage exemplars aligned to output contract	Dynamic selection preferred; skip if schema suffices
8. Output contract	JSON Schema, format instructions, constraints, error handling	End of context — recency bias helps compliance

Rule: keep policy/instructions (Layers 1-2) separate from knowledge (Layer 5). Mixing them is a root cause of prompt injection and "obedience confusion."

→ Full reference: context-stack-model.md

Cacheable Prefix Design

Layers 1-3 rarely change → group all static content at the beginning of the prompt to enable KV cache reuse. Append dynamic content (Layer 4-8) at the end. Never interleave static and dynamic blocks.

Track cache_prefix_ratio = cached_tokens / total_input_tokens (target > 0.3).

Token Budget Allocation

Treat context window as a finite resource with explicit budget per layer:

Allocation	Suggested %	Notes
System + rules + tools (L1-3)	15-25%	Stable, cacheable prefix
Runtime state (L4)	5-10%	Structured state blob
Knowledge / RAG (L5)	30-40%	Dynamic, highest signal variance
Memory (L6)	10-15%	Filtered, relevance-ranked
Examples (L7)	0-10%	Only when needed
Output contract (L8)	5-10%	Schema + error handling

Overflow strategy: truncate lowest-priority layers first (L7 → L6 → L5).

Resource Files

File	Contents	Guide §§
context-stack-model.md	8-layer architecture, layer design patterns, position effects, mapping to Codex assets	§3
memory-engineering.md	Memory taxonomy, CRUD lifecycle, conflict handling, compression strategies	§6
agent-harness-patterns.md	Init/continuation, state blobs, token budgets, caching, failure modes, streaming	§7
rag-engineering.md	Pipeline blueprint (7 stages), chunking, packing, degradation signals	§5
multi-agent-patterns.md	Context boundaries, payload schemas, return contracts, fan-out, contamination	§7.6, §14
production-checklists.md	All 8 production readiness checklists	§11
reference-templates.md	Grounding envelope, untrusted wrapper, structured output, tool error envelope	§12
privacy-compliance-ai.md	Multi-tenant isolation, data controls, consent, regional compliance	§9

Integration

• Follows rules: prompt-engineer role (prompt-level techniques, security, eval-first quality)
• Used by workflows: ai-skills skill (context engineering validation), feature-dev skill (AI features), plan skill (AI work packages), code-review skill (context quality review)
• Companion skill: prompt-engineering skill (technique selection, template patterns, eval, security checklist)
• Collaborates with roles: solution-architect role (AI system design), product-manager role (agent contracts), sre-engineer role (context observability), software-engineer role (implementation)
• Primary source: guides/context_engineering_guide.md