What is context engineering and how does it differ from prompt engineering?

// Context engineering: assemble what the model sees for ONE agent step from
// several sources, under a token budget. Token counts are given per item
// (in a real system they come from the provider tokenizer).
const WINDOW = 5000, RESERVE_OUTPUT = 1200;
const BUDGET = WINDOW - RESERVE_OUTPUT;

const system = { id: "rules", tok: 600 };
const tools = [
  ["search_orders", "orders"], ["refund_order", "orders"], ["get_invoice", "billing"], ["update_card", "billing"],
  ["create_ticket", "support"], ["send_email", "support"], ["run_sql", "admin"], ["deploy_service", "admin"],
  ["read_logs", "admin"], ["translate", "misc"], ["weather", "misc"], ["calendar", "misc"],
].map(([id, tag]) => ({ id, tag, tok: 180 }));
const memory = [
  { id: "prefers email over phone", tok: 20, score: 0.81 },
  { id: "plan: Pro, since 2024", tok: 15, score: 0.77 },
  { id: "asked about a pet name in March", tok: 25, score: 0.12 },
];
const chunks = [
  { id: "refund-policy#2", tok: 400, score: 0.91, text: "Refunds within 30 days..." },
  { id: "refund-policy#2-copy", tok: 400, score: 0.9, text: "Refunds within 30 days..." },
  { id: "double-charge-faq#1", tok: 350, score: 0.84, text: "If you were charged twice..." },
  { id: "card-update-guide#4", tok: 380, score: 0.58, text: "To update a card..." },
  { id: "pricing-2023#1", tok: 420, score: 0.55, text: "Old 2023 prices...", stale: true },
  { id: "careers-page#1", tok: 300, score: 0.21, text: "We are hiring..." },
];
const history = Array.from({ length: 12 }, (_, i) => ({ id: `turn ${i + 1}`, tok: 160 }));
const toolResult = { id: "search_orders result", tok: 2600, keepTok: 220 }; // raw JSON vs the 3 fields needed
const question = { id: "user question", tok: 40 };
const TASK_TAGS = new Set(["orders", "billing", "support"]);

const sum = (xs) => xs.reduce((a, x) => a + x.tok, 0);
const naive = system.tok + sum(tools) + sum(memory) + sum(chunks) + sum(history) + toolResult.tok + question.tok;
console.log(`naive: everything in = ${naive} tokens, budget ${BUDGET} (window ${WINDOW} - ${RESERVE_OUTPUT} for output) -> ${naive > BUDGET ? "does not fit" : "fits"}`);

const dropped = [];
const sections = [];
// 1. stable prefix first (cache friendly): rules + only the tools this task needs
const keptTools = tools.filter((t) => TASK_TAGS.has(t.tag));
tools.filter((t) => !TASK_TAGS.has(t.tag)).forEach((t) => dropped.push(`${t.id} (tool not needed for this task)`));
sections.push(["rules", 1, 1, system.tok], ["tools", tools.length, keptTools.length, sum(keptTools)]);
// 2. select: memory above a relevance threshold
const keptMem = memory.filter((m) => m.score >= 0.5);
memory.filter((m) => m.score < 0.5).forEach((m) => dropped.push(`memory "${m.id}" (score ${m.score})`));
sections.push(["memory", memory.length, keptMem.length, sum(keptMem)]);
// 3. compress: old turns become a summary, the last 4 stay verbatim; tool output trimmed to needed fields
const recent = history.slice(-4);
const summaryTok = Math.round(sum(history.slice(0, -4)) * 0.1);
sections.push(["history", history.length, `${recent.length}+summary`, sum(recent) + summaryTok]);
sections.push(["tool result", 1, "trimmed", toolResult.keepTok]);
sections.push(["question", 1, 1, question.tok]);
// 4. select: retrieved chunks by score, deduplicated, while budget remains
let used = sections.reduce((a, s) => a + s[3], 0);
const seen = new Set(), keptChunks = [];
for (const c of [...chunks].sort((a, b) => b.score - a.score)) {
  if (seen.has(c.text)) { dropped.push(`${c.id} (duplicate text)`); continue; }
  if (c.score < 0.5) { dropped.push(`${c.id} (score ${c.score} below 0.5)`); continue; }
  if (c.stale) { dropped.push(`${c.id} (superseded source)`); continue; }
  if (used + c.tok > BUDGET) { dropped.push(`${c.id} (no budget left)`); continue; }
  seen.add(c.text); keptChunks.push(c); used += c.tok;
}
sections.splice(3, 0, ["retrieved", chunks.length, keptChunks.length, sum(keptChunks)]);

console.log("\nsection      candidates  kept        tokens");
for (const [name, cand, kept, t] of sections) console.log(`${name.padEnd(12)} ${String(cand).padStart(10)}  ${String(kept).padEnd(10)} ${String(t).padStart(6)}`);
console.log(`${"total".padEnd(12)} ${"".padStart(10)}  ${"".padEnd(10)} ${String(used).padStart(6)} of ${BUDGET} (${((100 * used) / BUDGET).toFixed(1)}%)`);
console.log(`saved vs naive: ${naive - used} tokens (${((100 * (naive - used)) / naive).toFixed(1)}%)`);

console.log("\nfinal order (stable first, question last):");
console.log("  " + ["rules", "tools", "memory", "retrieved: " + keptChunks.map((c) => c.id).join(", "), "history summary", "last 4 turns", "tool result (trimmed)", "user question"].join("\n  "));
console.log("\ndropped:");
dropped.forEach((d) => console.log("  - " + d));