Context Engineering

Context engineering is the practice of curating the knowledge an LLM needs to work effectively in a codebase. Good context makes the difference between an LLM that follows project conventions and one that generates plausible but inconsistent code.

Birgitta Boeckeler's key insight: the code design itself is a huge part of the context. Well-structured code communicates intent; the harness documentation makes that intent explicit and enforceable.

The Context Section of HARNESS.md

The Context section serves three audiences simultaneously:

1. New team members read it to understand the project's stack and conventions
2. LLM agents read it as structured input when reviewing code or generating enforcement
3. The plugin reads it to understand what tools might be appropriate for enforcement

Stack Declaration

Declare the factual baseline: languages, build system, test framework, container strategy. Keep this section factual — no aspirations, just what is true today. The harness-discoverer agent populates this during /harness-init.

Conventions

Conventions are the core of context engineering. Each convention must be concrete enough that a reviewer could objectively say whether code follows it. The test: could two independent reviewers agree without discussing it?

For detailed examples of well-written vs poorly-written conventions, consult references/convention-patterns.md.

Writing Effective Conventions

The Enforceability Test

Before adding a convention, ask:

1. Can I describe what code that follows this convention looks like?
2. Can I describe what code that violates it looks like?
3. Could a tool or agent check this without ambiguity?

If the answer to any question is no, the convention needs rewriting.

From Aspiration to Enforcement

Start with what the team values. Ask: "What would I see in code that follows this?" Write the observable properties as the convention.

Aspiration	Observable convention
"Write clean code"	Not enforceable — decompose further
"Keep functions short"	"Functions must not exceed 40 lines"
"Use meaningful names"	"Variables must be 3+ characters except i, j, k, err"
"Handle errors properly"	"Every returned error must be wrapped with context"

Convention Categories

Organise conventions by what they govern:

• Naming — casing, length, domain vocabulary
• File structure — one type per file, co-located tests, package boundaries
• Error handling — wrapping, recovery, logging
• Documentation — what must have doc comments, what the first sentence must contain
• Dependencies — injection patterns, banned packages, version policies
• Testing — coverage thresholds, test naming, fixture patterns

What Not to Put in Conventions

• Implementation instructions ("use dependency injection") — describe the observable outcome instead
• Tool configuration — that belongs in the tool's config file
• Aspirational quality statements — decompose into observable properties
• Temporary rules — use constraint scope and GC rules instead

Keeping Context Fresh

Context rots. The harness-gc agent checks for documentation freshness as a garbage collection rule. Symptoms of stale context:

• Conventions reference functions, files, or patterns that no longer exist
• Stack declaration lists a framework version that has been upgraded
• Naming conventions describe a style the codebase has drifted from

The /harness-audit command detects some of these automatically by cross-referencing HARNESS.md against the project's actual state.

Additional Resources

Reference Files

• references/convention-patterns.md — Detailed examples of enforceable vs unenforceable conventions, the enforceability spectrum, and techniques for converting aspirations to observable rules