Context engineer

Context Engineer Skill

The Phase 0.5 skill — takes the output from data-scientist and builds the actual loading strategy, scripts, and transformations that fill the agent's context window with maximum signal.

Context engineering is not prompt engineering. Prompt engineering is about what you tell the agent to do. Context engineering is about what the agent sees when it starts working.

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Core Principle

Find the smallest possible set of high-signal tokens that maximizes the likelihood of desired outcomes.

Every token in the context window costs attention budget. Context is a finite resource with diminishing returns — more data does not mean better results. The right 5,000 tokens outperform the wrong 50,000.

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The Context Budget

The 50-60% Rule

Never fill more than 50-60% of the context window with input data. The remaining capacity is needed for:

Reserve	What It's For	Typical %
Agent reasoning	Chain-of-thought, analysis, planning	15-20%
Tool calls & results	Tool inputs, outputs, intermediate results	10-15%
Output generation	The agent's final response	10-15%
System prompt + tools	Instructions and tool definitions	5-10%

Budget Calculation

Available context = Model context window × 0.55
System prompt + tools ≈ measure actual tokens
Data budget = Available context - System prompt - Tools

Example (200K context model):
  Available: 200,000 × 0.55 = 110,000 tokens
  System prompt + tools: ~5,000 tokens
  Data budget: ~105,000 tokens
  ≈ 75,000 words ≈ 300 pages of text

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Operation: DESIGN — Creating a Context Strategy

Input Required

This skill requires the output from data-scientist — specifically:

• File inventory with sizes and token estimates
• Data profiles showing structure and content
• Signal vs noise classification per file
• Quality issues identified
• Context loading recommendations

Step 1 — Prioritize by Signal Value

Rank all data by how directly it helps solve the task:

Priority	What Goes Here	Loading Order
P0 — Critical	Data without which the task cannot be solved	Load first, always
P1 — Important	Data that significantly improves accuracy	Load second
P2 — Supporting	Data that provides helpful context	Load if budget allows
P3 — Reference	Data that might be needed for edge cases	Make available via tools, don't preload
P4 — Noise	Data that adds tokens without value	Never load

Step 2 — Choose a Loading Strategy

Strategy	When to Use	How It Works
Full load	Total data fits in 50% of context	Load everything relevant, no transformation needed
Filtered load	Data fits after removing noise	Strip noise lines/fields, load the rest
Summarized load	Data too large even after filtering	Summarize or aggregate, load summaries
Progressive load	Data much larger than context	Load critical data upfront, give agent tools to load more on demand
Hybrid	Mixed data types and sizes	Combine strategies per file

Step 3 — Design Transformations

For each file that needs transformation before loading:

## Transformation Plan

### File: app.log (12,847 lines → target: ~2,000 lines)
1. Filter to ERROR and WARN only (removes 11,401 DEBUG/INFO lines)
2. Deduplicate consecutive identical messages (keeps first + count)
3. Strip common prefix boilerplate from each line
4. Result: ~1,446 lines ≈ 4,300 tokens

### File: results.json (45 entries → target: 45 entries, compressed)
1. Remove null 'details' fields (8 entries)
2. Flatten nested metadata to single level
3. Remove internal IDs not relevant to task
4. Result: ~45 entries ≈ 2,100 tokens

### File: trace.pcap (5.1 MB binary → text summary)
1. Cannot load binary into context
2. Pre-extract SIP messages and GTPv2 summaries using tshark
3. Load extracted text summary only
4. Result: ~500 lines ≈ 1,500 tokens

Step 4 — Define the Context Layout

Structure how data appears in the context window. Order matters — information placed earlier gets stronger attention.

## Context Layout

[SYSTEM PROMPT]                          ← Agent instructions
[TOOL DEFINITIONS]                       ← Available capabilities

--- DATA SECTION (loaded by context-engineer) ---

## Task
[User's task description]

## Key Facts
[Critical entities: phone numbers, IPs, timeline, etc.]

## Primary Evidence
[P0 data — the most important files/sections]

## Supporting Evidence
[P1 data — important but secondary]

## Additional Context
[P2 data — if budget allows]

--- END DATA SECTION ---

[AGENT REASONING SPACE]                  ← Reserved for the agent

Layout rules:

• Place the task description and key facts FIRST — the agent needs to know what it's solving before seeing data
• Place most critical data immediately after the task
• Place supporting data later
• Long documents go at the TOP (before instructions) for long-context models

Document tag pattern — for long documents in context:

When loading large documents, wrap them in structured tags so the model can identify source and boundaries:

<document>
  <source>sip_log.txt (filtered to REGISTER/INVITE only)</source>
  <document_content>
    [file content here]
  </document_content>
</document>

<document>
  <source>probe_log.txt (deduplicated, 1,446 entries)</source>
  <document_content>
    [file content here]
  </document_content>
</document>

This helps the model cite specific sources and distinguish between multiple documents in context. Instruct the agent to quote relevant sections before reasoning — this improves grounding and reduces hallucination.

MCP as context source:

If the project uses Model Context Protocol (MCP) servers, context can be loaded dynamically from external sources (databases, APIs, file systems) at runtime. Treat MCP tool results the same as loaded files — they consume context budget and should be filtered/transformed before use.

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Operation: BUILD — Writing the Loading Scripts

Script Pattern: Context Loader

"""
Context loader for [agent name].
Loads and transforms data files into optimized context.

Generated by context-engineer based on data-analyst output.
"""

import os
from pathlib import Path


def load_context(data_dir: str, max_tokens: int = 105_000) -> str:
    """Load and transform data files into agent context.

    Args:
        data_dir: Path to the data folder
        max_tokens: Maximum token budget for data (default: 105K)

    Returns:
        Formatted context string ready for the agent
    """
    data_dir = Path(data_dir)
    sections = []
    token_count = 0

    # P0: Critical data (always load)
    p0_files = [
        ("sip_log.txt", load_sip_log),
        ("probe_log.txt", load_probe_log),
    ]
    for filename, loader in p0_files:
        filepath = data_dir / filename
        if filepath.exists():
            content = loader(filepath)
            tokens = estimate_tokens(content)
            if token_count + tokens <= max_tokens:
                sections.append(content)
                token_count += tokens

    # P1: Important data (load if budget allows)
    p1_files = [
        ("device_info.json", load_device_info),
        ("screenshot_metadata.txt", load_screenshots),
    ]
    for filename, loader in p1_files:
        filepath = data_dir / filename
        if filepath.exists():
            content = loader(filepath)
            tokens = estimate_tokens(content)
            if token_count + tokens <= max_tokens * 0.85:  # leave margin
                sections.append(content)
                token_count += tokens

    return format_context(sections, token_count, max_tokens)


def load_sip_log(filepath: Path) -> str:
    """Load SIP log, filtered to REGISTER and INVITE messages only."""
    lines = filepath.read_text().splitlines()
    relevant = [l for l in lines if any(k in l for k in
                ["REGISTER", "INVITE", "200 OK", "403", "404", "480"])]
    return f"## SIP Log ({len(relevant)} relevant messages)\n\n" + "\n".join(relevant)


def load_probe_log(filepath: Path) -> str:
    """Load probe log, deduplicated and stripped of repeated polling."""
    lines = filepath.read_text().splitlines()
    seen = set()
    filtered = []
    for line in lines:
        # Deduplicate consecutive identical commands
        key = line.split("|", 2)[-1].strip() if "|" in line else line
        if key not in seen:
            seen.add(key)
            filtered.append(line)
    return f"## Probe Log ({len(filtered)} entries)\n\n" + "\n".join(filtered)


def estimate_tokens(text: str) -> int:
    """Rough token estimate: ~4 characters per token."""
    return len(text) // 4


def format_context(sections: list, used: int, budget: int) -> str:
    """Format sections with budget summary."""
    header = f"## Context Budget: {used:,} / {budget:,} tokens ({used*100//budget}% used)\n\n"
    return header + "\n\n---\n\n".join(sections)

Script Pattern: Transformation Functions

Common transformations you'll need:

# Filter log by severity
def filter_by_severity(lines, min_level="WARN"):
    levels = {"DEBUG": 0, "INFO": 1, "WARN": 2, "ERROR": 3, "FATAL": 4}
    min_val = levels.get(min_level, 0)
    return [l for l in lines if any(
        lvl in l and levels.get(lvl, 0) >= min_val for lvl in levels
    )]

# Deduplicate consecutive repeated lines
def dedup_consecutive(lines):
    result = []
    prev = None
    count = 0
    for line in lines:
        if line == prev:
            count += 1
        else:
            if count > 0:
                result.append(f"  [repeated {count} more times]")
            result.append(line)
            prev = line
            count = 0
    return result

# Extract time window
def filter_time_range(lines, start, end, time_parser):
    return [l for l in lines if start <= time_parser(l) <= end]

# Strip fields from JSON
def strip_fields(data, remove_keys):
    if isinstance(data, dict):
        return {k: strip_fields(v, remove_keys)
                for k, v in data.items() if k not in remove_keys}
    if isinstance(data, list):
        return [strip_fields(item, remove_keys) for item in data]
    return data

# Truncate to token budget
def truncate_to_budget(text, max_tokens):
    estimated = len(text) // 4
    if estimated <= max_tokens:
        return text
    char_limit = max_tokens * 4
    truncated = text[:char_limit]
    return truncated + f"\n\n[TRUNCATED: {estimated - max_tokens} tokens omitted]"

---

Operation: OPTIMIZE — Improving Context Quality

Technique 1 — Compaction

When context approaches the limit during long agent runs, compress the conversation:

• Preserve: decisions made, unresolved issues, key findings
• Discard: raw tool outputs already processed, repeated messages
• Summarize: multi-turn reasoning into conclusions

Technique 2 — Structured Note-Taking

For long-running tasks, the agent should write notes to external memory:

# Agent maintains a NOTES.md file
notes = {
    "key_findings": ["PGW rejected WLAN bearer", "Error code 72"],
    "decisions_made": ["Focus on GTPv2 frames 551-614"],
    "remaining_tasks": ["Check HSS subscriber profile"],
    "important_values": {"a_party": "+306971279001", "error_code": 72},
}

This lets the agent offload details and reclaim context space.

Technique 3 — Progressive Disclosure via Tools

Instead of preloading everything, give the agent tools to load data on demand:

@tool
def read_log_section(filename: str, start_line: int, end_line: int) -> str:
    """Read a specific section of a log file.
    Use this when you need to examine a particular time range
    or section in detail. Prefer this over loading entire files."""
    lines = Path(filename).read_text().splitlines()
    section = lines[start_line:end_line]
    return "\n".join(section)

This is the just-in-time strategy — lightweight identifiers upfront, detailed data loaded at runtime.

Technique 4 — Sub-Agent Context Isolation

For complex analysis requiring multiple data sources:

Main agent context: task + summary + coordination (small)
    └─ Sub-agent 1: loads and analyzes log files (full exploration)
       └─ Returns: 1,000-token summary of findings
    └─ Sub-agent 2: loads and analyzes PCAP data (full exploration)
       └─ Returns: 1,000-token summary of findings
Main agent: synthesizes sub-agent summaries into final answer

Each sub-agent gets a clean context window for deep analysis. The main agent only sees condensed summaries.

Choosing the Right Technique

Task Type	Best Technique	Why
Extensive back-and-forth (debugging, Q&A)	Compaction	Conversation grows linearly, compress older turns
Iterative development with milestones (build, test, fix)	Note-taking	Track progress across steps, recall decisions
Complex research / parallel exploration	Sub-agents	Each source gets full context, main agent synthesizes
Large dataset, unpredictable queries	Progressive disclosure	Load metadata upfront, fetch details on demand
Small dataset, single task	Full load	Everything fits — don't over-engineer

Start with the simplest technique that works. Full load → filtered load → progressive disclosure → sub-agents. Escalate only when simpler approaches hit limits.

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Output: Context Engineering Specification

The final deliverable is a spec that the builder agent uses:

## Context Engineering Spec

### Model & Budget
- Model: [name] with [X]K context
- Data budget: [Y]K tokens (55% of context)
- System prompt + tools: ~[Z]K tokens

### Loading Strategy: [full / filtered / summarized / progressive / hybrid]

### Context Layout (in order)
1. Task description + key facts (500 tokens)
2. [P0 file] filtered to [criteria] (~X tokens)
3. [P0 file] with [transformation] (~X tokens)
4. [P1 file] if budget allows (~X tokens)
5. Tools for on-demand loading of P2/P3 data

### Transformations Required
- [file]: [transformation] → saves [X] tokens
- [file]: [transformation] → saves [X] tokens

### Loading Script
- Location: src/context_loader.py
- Entry point: load_context(data_dir, max_tokens)

### Optimization Techniques Enabled
- [ ] Compaction (for long-running tasks)
- [ ] Note-taking (for multi-step tasks)
- [ ] Progressive disclosure (for large datasets)
- [ ] Sub-agent isolation (for multi-source analysis)

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Core Rules

1. Context is finite — treat every token as a cost against the attention budget
2. 50-60% maximum — never fill more than 60% with input data
3. Signal over volume — 5K relevant tokens beats 50K of everything
4. Order matters — critical data goes first, noise never loads
5. Measure, don't guess — estimate tokens for every file and transformation
6. Build scripts, not manual processes — context loading must be repeatable
7. Design for the agent, not for humans — what helps the model reason, not what looks organized to you

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What NOT To Do

• Dump all files into context and hope the agent figures it out
• Exceed 60% of context window with input data
• Load data without transforming or filtering first
• Ignore token costs ("it's a big context window, it'll be fine")
• Load binary files directly into text context
• Assume more data means better results (context rot is real)
• Build context strategies without first running data-scientist
• Hardcode file paths instead of building reusable loading scripts