Context Builder — Agentic Skill

Generate a single, structured markdown file from any codebase directory. The output is optimized for LLM consumption with relevance-based file ordering, AST-aware code signatures, automatic token budgeting, and smart defaults.

Installation

# Requires Rust toolchain. Builds from source with cryptographic verification via crates.io.
cargo install context-builder --features tree-sitter-all

Pre-built binaries with SHA256 checksums are also available for manual download from GitHub Releases.

Verify: context-builder --version (expected: 0.8.3)

Security & Path Scoping

IMPORTANT: This tool reads file contents from the specified directory. Agents MUST follow these rules:

• Only target explicit project directories — always pass the exact project root (e.g., /home/user/projects/myapp). Never point at home directories, system paths, or credential stores (~/.ssh, ~/.aws, /etc, ~, /)
• Use scoped filters — use -f to limit to known source extensions (e.g., -f rs,toml,md), reducing exposure surface
• Output to project-local paths — write output to the project's docs/ folder or /tmp/, never to shared or public locations
• Review before sharing — the output may contain API keys, secrets, or credentials embedded in source files; always review or use .gitignore patterns to exclude sensitive files

Built-in protections (always active, no configuration needed):

• Excludes .git/, node_modules/, and 19 other heavy/sensitive directories at any depth
• Respects .gitignore rules when a .git directory is present
• Binary files are auto-detected and skipped via UTF-8 sniffing
• Output file and cache directory are auto-excluded to prevent self-ingestion

When to Use

• Deep code review — Feed an entire codebase to an LLM for architecture analysis or bug hunting
• Onboarding — Generate a project snapshot for understanding unfamiliar codebases
• Diff-based updates — After code changes, generate only the diffs to update an LLM's understanding
• AST signatures — Extract function/class signatures for token-efficient structural understanding
• Cross-project research — Quickly package a dependency's source for analysis

Core Workflow

1. Quick Context (whole project)

context-builder -d /path/to/project -y -o context.md

• -y skips confirmation prompts (recommended for agent workflows when path is explicitly scoped)
• Output includes: header → file tree → files sorted by relevance (config → source → tests → docs)

2. Scoped Context (specific file types)

context-builder -d /path/to/project -f rs,toml -i docs,assets -y -o context.md

• -f rs,toml includes only Rust and TOML files
• -i docs,assets excludes directories by name

3. AST Signatures Mode (minimal tokens)

context-builder -d /path/to/project --signatures -f rs,ts,py -y -o signatures.md

• Replaces full file content with extracted function/class signatures (~4K vs ~15K tokens per file)
• Supports 8 languages: Rust, JavaScript (.js/.jsx), TypeScript (.ts/.tsx), Python, Go, Java, C, C++
• Requires --features tree-sitter-all at install time

4. Signatures with Structural Summary

context-builder -d /path/to/project --signatures --structure -y -o context.md

• --structure appends a count summary (e.g., "6 functions, 2 structs, 1 impl block")
• Combine with --visibility public to show only public API surface

5. Budget-Constrained Context

context-builder -d /path/to/project --max-tokens 100000 -y -o context.md

• Caps output to ~100K tokens (estimated)
• Files are included in relevance order until budget is exhausted
• Automatically warns if output exceeds 128K tokens

6. Token Count Preview

context-builder -d /path/to/project --token-count

• Prints estimated token count without generating output
• Use this first to decide if filtering or --signatures is needed

7. Incremental Diffs

First, ensure context-builder.toml exists with:

timestamped_output = true
auto_diff = true

Then run twice:

# First run: baseline snapshot
context-builder -d /path/to/project -y

# After code changes: generates diff annotations
context-builder -d /path/to/project -y

For minimal output (diffs only, no full file bodies):

context-builder -d /path/to/project -y --diff-only

Smart Defaults

These behaviors require no configuration:

Feature	Behavior
Auto-ignore	node_modules, dist, build, __pycache__, .venv, vendor, and 12 more heavy dirs are excluded at any depth
Self-exclusion	Output file, cache dir, and context-builder.toml are auto-excluded
.gitignore	Respected automatically when .git directory exists
Binary detection	Binary files are skipped via UTF-8 sniffing
File ordering	Config/docs first → source (entry points before helpers) → tests → build/CI → lockfiles

CLI Reference (Agent-Relevant Flags)

Flag	Purpose	Agent Guidance
-d <PATH>	Input directory	Always use absolute paths for reliability
-o <FILE>	Output path	Write to project docs/ or /tmp/
-f <EXT>	Filter by extension	Comma-separated: -f rs,toml,md
-i <NAME>	Ignore dirs/files	Comma-separated: -i tests,docs,assets
--max-tokens <N>	Token budget cap	Use 100000 for most models, 200000 for Gemini
--token-count	Dry-run token estimate	Run first to check if filtering is needed
-y	Skip all prompts	Use only with explicit, scoped project paths
--preview	Show file tree only	Quick exploration without generating output
--diff-only	Output only diffs	Minimizes tokens for incremental updates
--signatures	AST signature extraction	Requires tree-sitter-all feature at install
--structure	Structural summary	Pair with --signatures for compact output
--visibility <V>	Filter by visibility	all (default), public (public API only)
--truncate <MODE>	Truncation strategy	smart (AST-aware) or simple
--init	Create config file	Auto-detects project file types
--clear-cache	Reset diff cache	Use if diff output seems stale

Recipes

Recipe: Deep Think Code Review

Generate a scoped context file, then prompt an LLM for deep analysis:

# Step 1: Generate focused context
context-builder -d /path/to/project -f rs,toml --max-tokens 120000 -y -o docs/deep_think_context.md

# Step 2: Feed to LLM with a review prompt
# Attach docs/deep_think_context.md and ask for:
# - Architecture review
# - Bug hunting
# - Performance analysis

Recipe: API Surface Review (signatures only)

# Extract only public signatures — typically 80-90% fewer tokens than full source
context-builder -d /path/to/project --signatures --visibility public -f rs -y -o docs/api_surface.md

Recipe: Compare Two Versions

# Generate context for both versions
context-builder -d ./v1 -f py -y -o /tmp/v1_context.md
context-builder -d ./v2 -f py -y -o /tmp/v2_context.md

# Feed both to an LLM for comparative analysis

Recipe: Monorepo Slice

# Focus on a specific package within a monorepo
context-builder -d /path/to/monorepo/packages/core -f ts,tsx -i __tests__,__mocks__ -y -o core_context.md

Recipe: Quick Size Check Before Deciding Strategy

# Check if the project fits in context
context-builder -d /path/to/project --token-count

# If > 128K tokens, try signatures mode first:
context-builder -d /path/to/project --signatures --token-count

# Or scope it down:
context-builder -d /path/to/project -f rs,toml --max-tokens 100000 --token-count

Configuration File (Optional)

Create context-builder.toml in the project root for persistent settings:

output = "docs/context.md"
output_folder = "docs"
filter = ["rs", "toml"]
ignore = ["target", "benches"]
timestamped_output = true
auto_diff = true
max_tokens = 120000
signatures = true
structure = true
visibility = "public"

Initialize one automatically with context-builder --init.

Output Format

The generated markdown follows this structure:

# Directory Structure Report
[metadata: project name, filters, content hash]

## File Tree
[visual tree of included files]

## Files
### File: src/main.rs
[code block with file contents, syntax-highlighted by extension]

### File: src/lib.rs
...

Files appear in relevance order (not alphabetical), prioritizing config and entry points so LLMs build understanding faster.

When --signatures is active, file contents are replaced with extracted signatures:

### File: src/lib.rs
```rust
pub fn run_with_args(args: Args, config: Config, prompter: &dyn Prompter) -> Result<()>
pub fn generate_markdown_with_diff(...) -> Result<String>
```

Error Handling

• If context-builder is not installed, install with cargo install context-builder --features tree-sitter-all
• If --signatures shows no output for a file, the language may not be supported or the feature was not enabled at install
• If output exceeds token limits, add --max-tokens or narrow with -f / -i, or use --signatures
• If the project has no .git directory, auto-ignores still protect against dependency flooding
• Use --clear-cache if diff output seems stale or incorrect