The Right 300 Tokens Beat 100k Noisy Ones: The Architecture of Context Engineering

QCon London 2026

Session agentic coding

The Right 300 Tokens Beat 100k Noisy Ones: The Architecture of Context Engineering

Wednesday Mar 18 / 10:35AM GMT, Fleming (3rd Fl.) at The QEII Centre, London

Abstract

Your agent has 100k tokens of context. It still forgets what you told it two messages ago.

Prompt engineering taught us to craft the perfect instruction. Context engineering asks a different question: what does your model need to see and what should it never see at all? It's the shift from writing prompts to designing context.

In this talk, we'll dissect four antipatterns killing your AI agents and the architectural fixes that actually work:

  • The Stuffed Prompt : You crammed everything upfront and hoped for the best. But static context doesn't scale. We'll explore dynamic loading and context refinement : fetching what's needed when it's needed, and staying within your context window without losing signal. (And yes, we'll bust the myth that position doesn't matter—models do lose track of what's buried in the middle.)
  • The Wrong Tool for the Job : You picked one retrieval method and used it everywhere. But RAG isn't always the answer. Neither are tools. Neither is an exact match. We'll break down when embeddings help, when MCP gives you precision, and when a simple lookup beats both.
  • The Goldfish Agent : Your AI agent forgets everything between sessions. Or worse, remembers everything forever. We'll explore short-term and long-term memory, pruning and compaction strategies : what to persist, what to summarize, where to store it, and when to let go.
  • The Vibes Eval : You shipped because it "felt right." But you can't improve what you don't measure. We'll build eval strategies that prove your context choices work or expose the tokens you're wasting.

Your context window called. It wants its tokens back!

Bonus: We'll use a coding agent to explain these patterns so you'll learn how they work under the hood ; but everything also applies to AI agents in general.

Interview

Everyone's building AI agents. Most of them are broken in the same four ways, and the fix is better context architecture. We'll show, live on stage, how the same agent with the same model goes from unreliable to production-grade just by changing what it sees and when it sees it. Senior developers are the ones making these architectural decisions right now, and most of the available guidance skips the practitioner details. This doesn't.

Because the models are good enough. That's no longer the bottleneck. The teams that are struggling with AI agents are feeding the model the wrong context at the wrong time. Context engineering is becoming the real differentiator between agents that impress in a demo and agents that hold up on a Tuesday afternoon in production. And unlike waiting for the next model release, this is something teams can fix today.

The biggest one: treating the context window like a junk drawer. Teams dump everything they might need into the prompt (system instructions, RAG results, conversation history, tool outputs) and hope the model sorts it out. It doesn't. Then there's the retrieval problem: everyone defaults to embeddings and vector search even when a simple exact match would be faster and more accurate. And finally, memory. Most agents have no memory strategy at all. They either start fresh every session or accumulate everything forever with no sense of what's worth keeping. These aren't edge cases. They're the default for most teams building agents today.

Audit your context. Take your agent's prompt, count the tokens, and ask for each chunk: does the model actually need this right now, for this specific task? If the answer is no, pull it out and load it dynamically. That single exercise usually improves agent reliability more than any model upgrade.

Topics

agentic coding DevOps technical leadership AI Context Engineering AI Developer Tools
76% senior dev or higher
1:11 speaker ratio
60+ practitioners

QCon London 2026 is a three day conference for senior software engineers, architects and team leads. An international program committee of working engineers selects every session. Patterns and practices, not products and pitches.

Share

From the same track

Wednesday 18 March

10:35 Fleming (3rd Fl.) Session agentic coding The Right 300 Tokens Beat 100k Noisy Ones: The Architecture of Context Engineering Patrick Debois, Baruch Sadogursky 11:45 Fleming (3rd Fl.) Session AI/ML Beyond Benchmarks: How Evaluations Ensure Safety at Scale in LLM Applications Clara Matos Director of Applied AI @Sword Health, Focused on Building and Scaling Machine Learning Systems 13:35 Whittle (3rd Fl.) Session data platform engineering Building an AI Ready Global Scale Data Platform George Peter Hantzaras Engineering Director, Core Platforms @MongoDB, Open Source Ambassador, Published Author 14:45 Whittle (3rd Fl.) Session Your Agent Sandbox Doesn't Know My Authz Model: A Standard-Shaped Hole Paul Carleton Member of Technical Staff @Anthropic, Core Maintainer of MCP 15:55 Whittle (3rd Fl.) Session Explicit Semantics for AI Applications: Ontologies in Practice Jesús Barrasa Field CTO for AI @Neo4j