# diffctx — smart diff context for LLM code review
[](https://github.com/nikolay-e/diffctx/actions/workflows/ci.yml)
[](https://pypi.org/project/diffctx/)
[](https://crates.io/crates/diffctx)
[](https://www.npmjs.com/package/diffctx)
[](https://pypi.org/project/diffctx/)
**diffctx selects the minimum code an LLM needs to review a git diff.**
Instead of pasting whole files, it walks the dependency graph outward from the
changed lines and stops once more context stops paying for itself.
> Coming from [`treemapper`](https://pypi.org/project/treemapper/)? That name is
> deprecated. Every command, flag, and API call works unchanged:
> `treemapper` → `diffctx`, `treemapper-mcp` → `diffctx-mcp`.
## Why not just use `tree` or repomix?
| **Primary use case** | directory listing | full repo export | automated PR review | **diff context for code review** |
| Smart diff context | ✗ | ✗ | ✓ | ✓ |
| Works with any LLM | ✓ | ✓ | Claude only | ✓ |
| Free / local / offline | ✓ | ✓ | $15–25/review | ✓ |
| GitHub required | ✗ | ✗ | ✓ | ✗ |
| Multiple output formats | ✗ | limited | — | YAML/JSON/MD/txt |
| Python API | ✗ | ✗ | ✗ | ✓ |
| MCP server | ✗ | ✗ | ✗ | ✓ |
Fuller positioning — measured results, and when a whole-repo packer or a
persistent code-graph server fits better: [COMPARISON.md](COMPARISON.md).
## Install
```bash
uvx diffctx . --diff HEAD~1 # zero-install, run once via uv
pipx install diffctx # recommended: isolated CLI, no venv needed
pip install diffctx # or: into an active environment
pipx install 'diffctx[mcp]' # + MCP server for AI assistants
```
Without Python:
```bash
cargo install diffctx # native CLI from crates.io
npx diffctx . --diff HEAD~1 # npm wrapper over the native binary
docker run --rm -v "$PWD:/repo" ghcr.io/nikolay-e/diffctx . --diff HEAD~1
```
On Windows, via Scoop (this repository is the bucket):
```powershell
scoop bucket add diffctx https://github.com/nikolay-e/diffctx
scoop install diffctx/diffctx
```
The image runs as a non-root user (uid 10001) and writes to stdout — the
native binary has no `-o` flag, so redirect to capture: `... --diff HEAD~1 >
context.yaml`.
Prebuilt binaries for linux (x86_64/aarch64), macOS (arm64) and Windows (x64)
are attached to every [release](https://github.com/nikolay-e/diffctx/releases/latest).
The native binary covers diff mode with YAML/JSON output; tree mode, Markdown
output, the `graph` subcommand and the MCP server live in the Python package.
`cargo add diffctx` embeds the pipeline in a Rust project — the library is
imported as `_diffctx` (`use _diffctx::pipeline::build_diff_context`), since
the crate doubles as the Python extension module
([docs.rs](https://docs.rs/diffctx)).
## Quick start
```bash
diffctx . --diff HEAD~1 # smart context for last commit → paste into Claude/ChatGPT
diffctx . -f md -c # full codebase export → clipboard in Markdown
```
*`diffctx . --diff HEAD~1` selects only the fragments an LLM needs to review the
last commit, instead of dumping every changed file in full.*
## Diff context mode
Finds the minimal set of fragments needed to understand a change — imports,
callers, type definitions, config dependencies — across 50+ file types. It
builds a code graph (imports, co-changes, type refs), propagates relevance
outward from the changed lines, and stops when relevance drops below `--tau` or
the `--budget` token cap is hit.
| `--scoring` | `ego` | `ego` = bounded expansion around changed nodes (fast, predictable radius); `ppr` = Personalized PageRank (global, smoother decay, slower); `bm25` = lexical retrieval against the diff hunks (baseline for sparse graphs) |
| `--budget` | auto | Hard cap in o200k_base tokens (see [Token counting](docs/product/token-budget.md)): `N` enforces a fixed cap, `-1` disables it, `0` is a strict-zero floor (empty selection; use `--full` for changed files only) |
| `--alpha` | 0.60 | PPR damping; higher = context clusters tighter around changes (`--scoring ppr` only) |
| `--tau` | 0.12 | Relevance threshold for full fragment content; lower-scoring fragments are stubbed or dropped (lower = more context) |
| `--full` | false | Only the changed files, every fragment, no related-code context |
| `--timeout` | 300 | Wall-clock deadline in seconds; on expiry diffctx exits 124 instead of hanging |
| `--with-raw-diff` | false | Also embed git's raw unified diff ahead of the selected fragments — additive (selection unchanged), not charged to `--budget`, lock/ignored/secret-like sections omitted. Python CLI only |
Calibration of `--alpha`, `--tau`, and the edge-weight priors:
[`docs/engineering/parameter-strategy.md`](docs/engineering/parameter-strategy.md).
Theory:
[diffctx: Budgeted Typed-Graph Retrieval for Diff-Aware Code Context
Selection (Zenodo, 2026)](https://doi.org/10.5281/zenodo.18824579).
### `graph` subcommand
Explore the underlying dependency graph directly, without a diff:
```bash
diffctx graph . # Mermaid graph of directory deps (default)
diffctx graph . --summary # cycles, hotspots, coupling metrics
diffctx graph . --level fragment -f json # fragment-level graph as JSON
diffctx graph . --level file -f graphml -o g.xml # file-level graph as GraphML
```
## Usage
```bash
# full codebase export:
diffctx . # Markdown to stdout + token count
diffctx . -f md -c # Markdown → clipboard
diffctx . -f json -o tree.json # JSON → file
diffctx . --no-content # structure only, no file contents
diffctx . --max-depth 3 # limit depth
diffctx . -i custom.ignore # custom ignore patterns
# diff context mode (requires git repo):
diffctx . --diff # uncommitted changes (working tree vs HEAD)
diffctx . --diff HEAD~1 # context for last commit
diffctx . --diff main..feature # context for feature branch
diffctx . --diff HEAD~1 --budget 30000 # limit to ~30k tokens
diffctx . --diff HEAD~1 -c # diff context to clipboard
diffctx . --diff HEAD~1 --with-raw-diff # raw patch + selected context
```
Every run reports token count and size on stderr — `12,847 tokens
(o200k_base), 52.3 KB` (tiktoken, the GPT-4o tokenizer; `~`-prefixed above
1 MB). `-c/--copy` copies output via `pbcopy` (macOS), `clip` (Windows), or
`wl-copy`/`xclip`/`xsel` (Linux). Unreadable files become placeholders like
`<binary file: N bytes>`, `<file too large: N bytes>`, or
`<unreadable content: not utf-8>`.
Counts come from tiktoken's `o200k_base` encoder and are exact only for the
GPT-4o family; Claude, Gemini and others tokenize differently, so treat
`--budget` as an upper bound in o200k tokens and leave headroom. Details:
[`docs/product/token-budget.md`](docs/product/token-budget.md).
## Python API
```python
from pathlib import Path
from diffctx import build_diff_context, map_directory, to_json, to_markdown, to_text, to_yaml
ctx = build_diff_context(
Path("."),
"HEAD~1..HEAD",
budget_tokens=None, # None = auto; 0 = strict-zero floor (empty); -1 = uncapped; N = hard cap
alpha=0.6,
tau=0.12,
full=False,
scoring_mode="ego",
timeout=300,
with_raw_diff=False, # True also embeds the raw unified diff (not charged to budget)
)
print(to_markdown(ctx))
tree = map_directory(
".",
max_depth=None,
no_content=False,
max_file_bytes=None,
ignore_file=None,
no_default_ignores=False,
whitelist_file=None,
)
print(to_yaml(tree))
```
## MCP server
diffctx includes an [MCP](https://modelcontextprotocol.io) server that lets AI
assistants (Claude Code, Cursor, Windsurf, etc.) call diff context analysis
automatically during code review. Install with `pip install 'diffctx[mcp]'`,
then register it — for Claude Code:
```bash
claude mcp add diffctx -- diffctx-mcp
```
The server exposes three tools — `get_diff_context`, `get_tree_map`, and
`get_file_context` — that assistants call when reviewing PRs, explaining
changes, or investigating broken tests. Tool reference and configs for
Cursor, Continue, Windsurf, and Zed:
[`src/diffctx/mcp/README.md`](src/diffctx/mcp/README.md).
## Ignore patterns
Respects `.gitignore` and `.diffctx/ignore` automatically — hierarchically at
every directory level, with full gitignore semantics (negation `!important.log`,
anchored `/root_only.txt`). `.diffctx/whitelist` acts as an include-only filter,
and the output file is always auto-ignored. `--no-default-ignores` disables the
built-in patterns; `--no-ignores` disables all ignore rules (tree mode only).
An excluded path never appears in the output in any role: in diff mode it is
dropped both from `changed_files` and from the candidate universe, so it cannot
come back as a related-context fragment either (including under `--full`). The
same guarantee covers secret-like paths (`id_rsa`, `*.pem`, `*.key`, ...),
which are filtered even without an ignore entry.
## Token cache
Diff mode caches per-blob tokenization under
`~/Library/Caches/diffctx/token-cache` (macOS),
`$XDG_CACHE_HOME/diffctx/token-cache` (Linux) or
`%LOCALAPPDATA%\diffctx\token-cache` (Windows). It is a pure speedup: deleting
it only costs one cold run.
| `DIFFCTX_TOKEN_CACHE_DIR` | Relocate the cache |
| `DIFFCTX_TOKEN_CACHE_MAX_BYTES` | Size cap, default `536870912` (512 MB); `0` disables eviction |
Eviction is amortized: each run trims one of the cache's 256 shards back under
its share of the cap, oldest entries first.
## Exit codes
| `0` | Success — output contains content |
| `1` | Runtime error (bad path, permission denied, etc.) |
| `2` | Usage error (invalid flags/arguments) |
| `3` | Environment error (`--diff` outside a git repo, git not installed, no commits yet) |
| `4` | `--diff` produced no semantic context (clean tree, binary-only, everything filtered); output is still emitted. Deletion/rename/lockfile-only diffs list `deleted_files`/`renamed_files`/`lockfile_changes` and exit `0` |
| `124`| `--diff` exceeded the `--timeout` wall-clock deadline |
| `130`| Interrupted (Ctrl-C) |
| `141`| Broken pipe (e.g. piping into `head`) |
## Repository layout
Rust product code lives in `crates/diffctx-native/`; the Python CLI and MCP
package live in `src/diffctx/`. Evaluation code, immutable datasets, and paper
artifacts are intentionally separated under `eval/`, `datasets/`, and `paper/`.
See [repository ownership boundaries](docs/architecture/repository-layout.md)
for the lifecycle and entry point of each area.
## License
Apache 2.0
---
- [Documentation site](https://nikolay-e.github.io/diffctx/) — the pipeline
end to end: diff → fragments → graph → relevance → selection
- [GitHub Action](docs/product/github-action.md) — diff context as a CI step
for LLM review
- [Token counting](docs/product/token-budget.md) — which encoder, and what
`--budget` means for non-GPT models
- [Comparison](COMPARISON.md) — measured results, and when a whole-repo packer
or a persistent code-graph server fits better
- [Changelog](CHANGELOG.md)
- [Security policy](SECURITY.md) — threat model and vulnerability reporting
- [Parameter strategy](docs/engineering/parameter-strategy.md) — how `--alpha`,
`--tau`, and edge weights are calibrated