context-engineering（情境工程）

Context Engineering

The discipline of curating the smallest high-signal token set that achieves desired outcomes.

Overview

Context engineering goes beyond prompt engineering. While prompts focus on *what* you ask, context engineering focuses on *everything* the model sees—system instructions, tool definitions, documents, message history, and tool outputs.

Key Insight: Context windows are constrained not by raw token capacity but by attention mechanics. As context grows, models experience degradation.

Overview

• Designing agent system prompts
• Optimizing RAG retrieval pipelines
• Managing long-running conversations
• Building multi-agent architectures
• Reducing token costs while maintaining quality

The "Lost in the Middle" Phenomenon

Models pay unequal attention across the context window:

Attention
Strength   ████████░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░████████
           ↑                                                      ↑
        START              MIDDLE (weakest attention)           END

Practical Implications:

The Five Context Layers

1. System Prompts (Identity Layer)

Establishes agent identity at the right "altitude":

TOO HIGH (vague):        "You are a helpful assistant"
TOO LOW (brittle):       "Always respond with exactly 3 bullet points..."
OPTIMAL (principled):    "You are a senior engineer who values clarity,
                          tests assumptions, and explains trade-offs"

Best Practices:

• Define role and expertise level
• State core principles (not rigid rules)
• Include what NOT to do (boundaries)
• Position at START of context

2. Tool Definitions (Capability Layer)

Tools steer behavior through descriptions:

# ❌ BAD: Ambiguous - when would you use this?
@tool
def search(query: str) -> str:
    """Search for information."""
    pass

# ✅ GOOD: Clear trigger conditions
@tool
def search_documentation(query: str) -> str:
    """
    Search internal documentation for technical answers.

    USE WHEN:
    - User asks about internal APIs or services
    - Question requires company-specific knowledge
    - Public information is insufficient

    DO NOT USE WHEN:
    - Question is general programming knowledge
    - User explicitly wants external sources
    """
    pass

Rule: If a human cannot definitively say which tool to use, an agent cannot either.

3. Retrieved Documents (Knowledge Layer)

Just-in-time loading beats pre-loading:

# ❌ BAD: Pre-load everything
context = load_all_documentation()  # 50k tokens!

# ✅ GOOD: Progressive disclosure
def build_context(query: str) -> str:
    # Stage 1: Lightweight retrieval (500 tokens)
    summaries = search_summaries(query, top_k=5)

    # Stage 2: Selective deep loading (only if needed)
    if needs_detail(summaries):
        full_docs = load_full_documents(summaries[:2])
        return summaries + full_docs

    return summaries

4. Message History (Memory Layer)

Treat as scratchpad, not permanent storage:

# Implement sliding window with compression
MAX_MESSAGES = 20
COMPRESSION_TRIGGER = 0.7  # 70% of context budget

def manage_history(messages: list, budget: int) -> list:
    current_tokens = count_tokens(messages)

    if current_tokens > budget * COMPRESSION_TRIGGER:
        # Compress older messages, keep recent
        old = messages[:-5]
        recent = messages[-5:]

        summary = summarize(old)  # Anchored compression
        return [summary] + recent

    return messages

5. Tool Outputs (Observation Layer)

Critical Finding: Tool outputs can reach 83.9% of total context usage!

# ❌ BAD: Return raw output
def search_web(query: str) -> str:
    results = web_search(query)
    return json.dumps(results)  # Could be 10k+ tokens!

# ✅ GOOD: Structured, bounded output
def search_web(query: str) -> str:
    results = web_search(query)

    # Extract only what's needed
    extracted = [
        {
            "title": r["title"],
            "snippet": r["snippet"][:200],  # Truncate
            "url": r["url"]
        }
        for r in results[:5]  # Limit count
    ]

    return json.dumps(extracted)  # ~500 tokens max

The 95% Finding

Research shows what actually drives agent performance:

┌────────────────────────────────────────────────────────────────┐
│  TOKEN USAGE        ████████████████████████████████████  80%  │
│  TOOL CALLS         █████  10%                                 │
│  MODEL CHOICE       ██  5%                                     │
│  OTHER              ██  5%                                     │
└────────────────────────────────────────────────────────────────┘

Key Insight: Optimize context efficiency BEFORE switching models.

Context Budget Management

Token Budget Calculator

def calculate_budget(model: str, task_type: str) -> dict:
    """Calculate optimal token allocation."""

    MAX_CONTEXT = {
        "gpt-4o": 128_000,
        "claude-3": 200_000,
        "llama-3": 128_000,
    }

    # Reserve 20% for response generation
    available = MAX_CONTEXT[model] * 0.8

    # Allocation by task type
    ALLOCATIONS = {
        "chat": {
            "system": 0.05,      # 5%
            "tools": 0.05,       # 5%
            "history": 0.60,    # 60%
            "retrieval": 0.20,  # 20%
            "current": 0.10,    # 10%
        },
        "agent": {
            "system": 0.10,     # 10%
            "tools": 0.15,      # 15%
            "history": 0.30,    # 30%
            "retrieval": 0.25,  # 25%
            "observations": 0.20, # 20%
        },
    }

    alloc = ALLOCATIONS[task_type]
    return {k: int(v * available) for k, v in alloc.items()}

Compression Triggers

COMPRESSION_CONFIG = {
    "trigger_threshold": 0.70,    # Start compressing at 70%
    "target_threshold": 0.50,     # Compress down to 50%
    "preserve_recent": 5,         # Always keep last 5 messages
    "preserve_system": True,      # Never compress system prompt
}

Attention-Aware Positioning

Template Structure

[START - HIGH ATTENTION]
## System Identity
You are a {role} specialized in {domain}.

## Critical Constraints
- NEVER {dangerous_action}
- ALWAYS {required_behavior}

[MIDDLE - LOWER ATTENTION]
## Background Context
{retrieved_documents}
{older_conversation_history}

[END - HIGH ATTENTION]
## Current Task
{recent_messages}
{user_query}

## Response Guidelines
{output_format_instructions}

Priority Positioning Rules

1. Identity & Constraints → START (immutable)
2. Critical instructions → START or END
3. Retrieved documents → MIDDLE (expandable)
4. Conversation history → MIDDLE (compressible)
5. Current query → END (always visible)
6. Output format → END (guides generation)

Metrics: Tokens-Per-Task

Optimize for total task completion, not individual requests:

@dataclass
class TaskMetrics:
    task_id: str
    total_tokens: int = 0
    request_count: int = 0
    retrieval_tokens: int = 0
    generation_tokens: int = 0

    @property
    def tokens_per_request(self) -> float:
        return self.total_tokens / max(self.request_count, 1)

    @property
    def efficiency_ratio(self) -> float:
        """Lower is better - generation vs total context."""
        return self.generation_tokens / max(self.total_tokens, 1)

Anti-pattern: Aggressive compression that loses critical details forces expensive re-fetching, consuming MORE tokens overall.

Common Pitfalls

Integration with OrchestKit

Agent System Prompts

Apply attention-aware positioning to agent definitions:

# Agent: backend-system-architect

[HIGH ATTENTION - START]
## Identity
Senior backend architect with 15+ years experience.

## Constraints
- NEVER suggest unvalidated security patterns
- ALWAYS consider multi-tenant isolation

[LOWER ATTENTION - MIDDLE]
## Domain Knowledge
{dynamically_loaded_patterns}

[HIGH ATTENTION - END]
## Current Task
{user_request}

Skill Loading

Progressive skill disclosure:

# Stage 1: Load skill metadata only (~100 tokens)
skill_index = load_skill_summaries()

# Stage 2: Load relevant skill on demand (~500 tokens)
if task_matches("database"):
    full_skill = load_skill("pgvector-search")

CC 2.1.7: MCP Auto-Discovery and Deferral

MCP Search Mode

CC 2.1.7 introduces intelligent MCP tool discovery. When context usage exceeds 10% of the effective window, MCPs are automatically deferred to reduce token overhead.

Context < 10%:  MCP tools immediately available
Context > 10%:  MCP tools discovered via MCPSearch (deferred loading)

Savings: ~7200 tokens per session average

How Auto-Deferral Works

The context budget monitor tracks usage against the effective window:

1. Below 10%: MCP tool definitions loaded in context (~1200 tokens)
2. Above 10%: MCP tools deferred, available via MCPSearch on-demand
3. State file: /tmp/claude-mcp-defer-state-{session}.json

Best Practices for MCP with Auto-Deferral

1. Use MCPs early - Before context fills up
2. Batch MCP calls - Multiple queries in one turn
3. Cache MCP results - Store retrieved docs in context
4. Monitor statusline - Watch for mcp.deferred: true

Checking MCP Deferral State

cat /tmp/claude-mcp-defer-state-${CLAUDE_SESSION_ID}.json

Related Skills

• context-compression - Compression strategies and anchored summarization
• multi-agent-orchestration - Context isolation across agents
• rag-retrieval - Optimizing retrieved document context
• prompt-caching - Reducing redundant context transmission

Version: 1.0.0 (January 2026) Based on: Context Engineering research, BrowseComp evaluation findings Key Metric: 80% of agent performance variance explained by token usage

Capability Details

attention-mechanics

Keywords: context window, attention, lost in the middle, token budget Solves:

• Understand lost-in-the-middle effect (high attention at START/END)
• Position critical info strategically
• Optimize tokens-per-task not tokens-per-request

context-layers

Keywords: context anatomy, context structure, five layers Solves:

• Understand 5 context layers (system, tools, docs, history, outputs)
• Implement just-in-time document loading
• Manage tool output truncation

budget-allocation

Keywords: token budget, context budget, allocation Solves:

• Allocate tokens across context layers
• Implement compression triggers at 70% utilization
• Target 50% utilization after compression