audit-context-building by trailofbits

---
name: audit-context-building
description: Enables ultra-granular, line-by-line code analysis to build deep architectural context before vulnerability or bug finding.
---

# Deep Context Builder Skill (Ultra-Granular Pure Context Mode)

## 1. Purpose

This skill governs **how Claude thinks** during the context-building phase of an audit.

When active, Claude will:
- Perform **line-by-line / block-by-block** code analysis by default.
- Apply **First Principles**, **5 Whys**, and **5 Hows** at micro scale.
- Continuously link insights → functions → modules → entire system.
- Maintain a stable, explicit mental model that evolves with new evidence.
- Identify invariants, assumptions, flows, and reasoning hazards.

This skill defines a structured analysis format (see Example: Function Micro-Analysis below) and runs **before** the vulnerability-hunting phase.

---

## 2. When to Use This Skill

Use when:
- Deep comprehension is needed before bug or vulnerability discovery.
- You want bottom-up understanding instead of high-level guessing.
- Reducing hallucinations, contradictions, and context loss is critical.
- Preparing for security auditing, architecture review, or threat modeling.

Do **not** use for:
- Vulnerability findings
- Fix recommendations
- Exploit reasoning
- Severity/impact rating

---

## 3. How This Skill Behaves

When active, Claude will:
- Default to **ultra-granular analysis** of each block and line.
- Apply micro-level First Principles, 5 Whys, and 5 Hows.
- Build and refine a persistent global mental model.
- Update earlier assumptions when contradicted ("Earlier I thought X; now Y.").
- Periodically anchor summaries to maintain stable context.
- Avoid speculation; express uncertainty explicitly when needed.

Goal: **deep, accurate understanding**, not conclusions.

---

## Rationalizations (Do Not Skip)

| Rationalization | Why It's Wrong | Required Action |
|-----------------|----------------|-----------------|
| "I get the gist" | Gist-level understanding misses edge cases | Line-by-line analysis required |
| "This function is simple" | Simple functions compose into complex bugs | Apply 5 Whys anyway |
| "I'll remember this invariant" | You won't. Context degrades. | Write it down explicitly |
| "External call is probably fine" | External = adversarial until proven otherwise | Jump into code or model as hostile |
| "I can skip this helper" | Helpers contain assumptions that propagate | Trace the full call chain |
| "This is taking too long" | Rushed context = hallucinated vulnerabilities later | Slow is fast |

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## 4. Phase 1 — Initial Orientation (Bottom-Up Scan)

Before deep analysis, Claude performs a minimal mapping:

1. Identify major modules/files/contracts.
2. Note obvious public/external entrypoints.
3. Identify likely actors (users, owners, relayers, oracles, other contracts).
4. Identify important storage variables, dicts, state structs, or cells.
5. Build a preliminary structure without assuming behavior.

This establishes anchors for detailed analysis.

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## 5. Phase 2 — Ultra-Granular Function Analysis (Default Mode)

Every non-trivial function receives full micro analysis.

### 5.1 Per-Function Microstructure Checklist

For each function:

1. **Purpose**
   - Why the function exists and its role in the system.

2. **Inputs & Assumptions**
   - Parameters and implicit inputs (state, sender, env).
   - Preconditions and constraints.

3. **Outputs & Effects**
   - Return values.
   - State/storage writes.
   - Events/messages.
   - External interactions.

4. **Block-by-Block / Line-by-Line Analysis**
   For each logical block:
   - What it does.
   - Why it appears here (ordering logic).
   - What assumptions it relies on.
   - What invariants it establishes or maintains.
   - What later logic depends on it.

   Apply per-block:
   - **First Principles**
   - **5 Whys**
   - **5 Hows**

---

### 5.2 Cross-Function & External Flow Analysis
*(Full Integration of Jump-Into-External-Code Rule)*

When encountering calls, **continue the same micro-first analysis across boundaries.**

#### Internal Calls
- Jump into the callee immediately.
- Perform block-by-block analysis of relevant code.
- Track flow of data, assumptions, and invariants:
  caller → callee → return → caller.
- Note if callee logic behaves differently in this specific call context.

#### External Calls — Two Cases

**Case A — External Call to a Contract Whose Code Exists in the Codebase**
Treat as an internal call:
- Jump into the target contract/function.
- Continue block-by-block micro-analysis.
- Propagate invariants and assumptions seamlessly.
- Consider edge cases based on the *actual* code, not a black-box guess.

**Case B — External Call Without Available Code (True External / Black Box)**
Analyze as adversarial:
- Describe payload/value/gas or parameters sent.
- Identify assumptions about the target.
- Consider all outcomes:
  - revert
  - incorrect/strange return values
  - unexpected state changes
  - misbehavior
  - reentrancy (if applicable)

#### Continuity Rule
Treat the entire call chain as **one continuous execution flow**.
Never reset context.
All invariants, assumptions, and data dependencies must propagate across calls.

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### 5.3 Complete Analysis Example

See [FUNCTION_MICRO_ANALYSIS_EXAMPLE.md](resources/FUNCTION_MICRO_ANALYSIS_EXAMPLE.md) for a complete walkthrough demonstrating:
- Full micro-analysis of a DEX swap function
- Application of First Principles, 5 Whys, and 5 Hows
- Block-by-block analysis with invariants and assumptions
- Cross-function dependency mapping
- Risk analysis for external interactions

This example demonstrates the level of depth and structure required for all analyzed functions.

---

### 5.4 Output Requirements

When performing ultra-granular analysis, Claude MUST structure output following the format defined in [OUTPUT_REQUIREMENTS.md](resources/OUTPUT_REQUIREMENTS.md).

Key requirements:
- **Purpose** (2-3 sentences minimum)
- **Inputs & Assumptions** (all parameters, preconditions, trust assumptions)
- **Outputs & Effects** (returns, state writes, external calls, events, postconditions)
- **Block-by-Block Analysis** (What, Why here, Assumptions, First Principles/5 Whys/5 Hows)
- **Cross-Function Dependencies** (internal calls, external calls with risk analysis, shared state)

Quality thresholds:
- Minimum 3 invariants per function
- Minimum 5 assumptions documented
- Minimum 3 risk considerations for external interactions
- At least 1 First Principles application
- At least 3 combined 5 Whys/5 Hows applications

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### 5.5 Completeness Checklist

Before concluding micro-analysis of a function, verify against the [COMPLETENESS_CHECKLIST.md](resources/COMPLETENESS_CHECKLIST.md):

- **Structural Completeness**: All required sections present (Purpose, Inputs, Outputs, Block-by-Block, Dependencies)
- **Content Depth**: Minimum thresholds met (invariants, assumptions, risk analysis, First Principles)
- **Continuity & Integration**: Cross-references, propagated assumptions, invariant couplings
- **Anti-Hallucination**: Line number citations, no vague statements, evidence-based claims

Analysis is complete when all checklist items are satisfied and no unresolved "unclear" items remain.

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## 6. Phase 3 — Global System Understanding

After sufficient micro-analysis:

1. **State & Invariant Reconstruction**
   - Map reads/writes of each state variable.
   - Derive multi-function and multi-module invariants.

2. **Workflow Reconstruction**
   - Identify end-to-end flows (deposit, withdraw, lifecycle, upgrades).
   - Track how state transforms across these flows.
   - Record assumptions that persist across steps.

3. **Trust Boundary Mapping**
   - Actor → entrypoint → behavior.
   - Identify untrusted input paths.
   - Privilege changes and implicit role expectations.

4. **Complexity & Fragility Clustering**
   - Functions with many assumptions.
   - High branching logic.
   - Multi-step dependencies.
   - Coupled state changes across modules.

These clusters help guide the vulnerability-hunting phase.

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## 7. Stability & Consistency Rules
*(Anti-Hallucination, Anti-Contradiction)*

Claude must:

- **Never reshape evidence to fit earlier assumptions.**
  When contradicted:
  - Update the model.
  - State the correction explicitly.

- **Periodically anchor key facts**
  Summarize core:
  - invariants
  - state relationships
  - actor roles
  - workflows

- **Avoid vague guesses**
  Use:
  - "Unclear; need to inspect X."
  instead of:
  - "It probably…"

- **Cross-reference constantly**
  Connect new insights to previous state, flows, and invariants to maintain global coherence.

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## 8. Subagent Usage

Claude may spawn subagents for:
- Dense or complex functions.
- Long data-flow or control-flow chains.
- Cryptographic / mathematical logic.
- Complex state machines.
- Multi-module workflow reconstruction.

Use the **`function-analyzer`** agent for per-function deep analysis.
It follows the full microstructure checklist, cross-function flow
rules, and quality thresholds defined in this skill, and enforces
the pure-context-building constraint.

Subagents must:
- Follow the same micro-first rules.
- Return summaries that Claude integrates into its global model.

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## 9. Relationship to Other Phases

This skill runs **before**:
- Vulnerability discovery
- Classification / triage
- Report writing
- Impact modeling
- Exploit reasoning

It exists solely to build:
- Deep understanding
- Stable context
- System-level clarity

---

## 10. Non-Goals

While active, Claude should NOT:
- Identify vulnerabilities
- Propose fixes
- Generate proofs-of-concept
- Model exploits
- Assign severity or impact

This is **pure context building** only.