# diffctx — smart diff context for LLM code review

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**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.

> Formerly published as `treemapper` — every command, flag, and API call works unchanged.

## How it compares

Whole-repo packers (repomix and friends) seed on the repository and export
everything; persistent code-graph servers answer structural queries against a
maintained index. diffctx is **diff-seeded**: the input is a change, the
output is the fragments needed to understand it, packed under a hard token
budget — local, deterministic, no index, no model calls. Measured results and
when the other two families fit 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
```

Every [release](https://github.com/nikolay-e/diffctx/releases/latest) carries
prebuilt binaries; the current target set is Linux (x86_64/aarch64), macOS
(arm64/x86_64) and Windows (x64/arm64), and a release older than a target's
first build lacks it (v1.16.0 predates the macOS x86_64 and Windows arm64
archives). On Windows on ARM, `diffctx[mcp]` builds `cryptography` from source,
which needs OpenSSL: that project publishes no wheel for the platform.
Free-threaded CPython (`3.14t`) cannot load the `abi3` wheel, so each release
after 1.16.0 also ships a `cp314t` wheel per platform (the extension runs with
the GIL off); on 1.16.0 itself `pip` and `uv` fall back to compiling the sdist.
The native binary and Docker image cover diff mode with YAML/JSON output and
write to stdout (redirect to capture); tree mode, Markdown output, the `graph`
subcommand and the MCP server live in the Python package.

## 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 demo](https://raw.githubusercontent.com/nikolay-e/diffctx/main/docs/demo/demo.gif)

*`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.

`--diff` takes a git range (`HEAD~1..HEAD`, `main..feature`) or a **duration
window ending now** — `24h`, `8d`, `90min`, `1h30m`, `2w` (units `s`, `m`/`min`,
`h`, `d`, `w`, composable). A window diffs the working tree against the last
commit before it, so it covers the commits made inside the window *plus* the
uncommitted and untracked work on top — `diffctx . --diff 24h` is "everything I
touched today". A ref that happens to look like a duration (a branch `24h`)
keeps its git meaning.

| Flag        | Default | Description                                                              |
|-------------|---------|--------------------------------------------------------------------------|
| `--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); `rrf` = reciprocal-rank fusion of `ego` and `bm25` (widest recall, no scale calibration between the two signals); `pit` = the same fusion on score percentiles |
| `--budget`  | auto    | Cap in o200k_base tokens on the whole rendered artifact (see [Token counting](docs/product/token-budget.md)): every changed file gets one witness first, relevance fills the rest, and the renderer drops context from the tail until the document fits. A budget smaller than the summary yields the summary alone. `N` = fixed cap, `-1` disables it, `0` is a strict-zero floor (no fragments; use `--full` for changed files only) |
| `--alpha`   | 0.60    | PPR continuation probability: higher = relevance travels further from the change, lower = tighter around it (`--scoring ppr` only) |
| `--tau`     | 0.05    | 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 the run stops cooperatively and emits a partial artifact whose `coverage` block names the limit (exit 0). 124 is the watchdog behind it, 30 s later, for a phase that could not stop |
| `--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 |
| `--mode` | `pack` | `locate` emits the same ranked selection as compact `diffctx.locate.v1` JSON — path, lines, score, provenance reasons, a blast-radius `summary` and per-item impact `group` (`test`/`type`/`config`), NO source bodies. Adds a `coverage` block naming what the run could not see (`unparsed_files`, `zero_edge_files`, `ppr_truncated`, `next_up`, a heuristic `confidence`) and an `overflow` ranking of what the budget left behind — omitted entirely when there is nothing to disclose. `impact` answers, under 2k tokens, what the change reaches outside its diff: callers in files the diff does not touch, whether a test guards each, cross-commit overlap, and the public API and schema changes (text by default, `diffctx.impact.v1` JSON with `-f json`). The MCP tool takes both as `mode` |

Every JSON/YAML artifact opens with `schema: diffctx.context.v1` and validates
against [`schemas/diffctx.context.v1.json`](schemas/diffctx.context.v1.json),
generated from the engine's own type; it closes with a `provenance` block
(engine version, input object ids, the effective configuration and its hash,
the selection parameters, the tokenizer) and, when a limit stopped the run
short, a `coverage` block naming it.

### `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

<!-- BEGIN 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 24h                      # everything changed in the last 24 hours
diffctx . --diff 8d                       # same over 8 days (also 90s, 10min, 1h30m, 2w)
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
diffctx . --diff HEAD~1 --mode locate     # ranked navigation JSON, no source
```
<!-- END USAGE -->

Every run reports token count and size on stderr — `12,847 tokens
(o200k_base), 52.3 KB`. Counts are exact only for the GPT-4o family; Claude,
Gemini and others tokenize differently, so treat `--budget` as an upper bound
and leave headroom ([details](docs/product/token-budget.md)). Unreadable files
become placeholders like `<binary file: N bytes>`.

## 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 = no fragments; -1 = uncapped; N = cap on the whole artifact
    alpha=0.6,
    tau=0.05,
    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

[![MCP Registry](https://img.shields.io/badge/MCP_Registry-io.github.nikolay--e%2Fdiffctx-blue)](https://registry.modelcontextprotocol.io/v0/servers?search=io.github.nikolay-e/diffctx)
[![diffctx MCP server](https://glama.ai/mcp/servers/nikolay-e/diffctx/badges/score.svg)](https://glama.ai/mcp/servers/nikolay-e/diffctx)

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. It is published in the official MCP registry
as `io.github.nikolay-e/diffctx`. One-line setup (zero-install via
[uv](https://docs.astral.sh/uv/)):

```bash
# Claude Code plugin: the server plus /diffctx:diffctx and /diffctx:impact
claude plugin marketplace add nikolay-e/diffctx
claude plugin install diffctx@diffctx
# Claude Code, server only
claude mcp add diffctx -- uvx --from 'diffctx[mcp]' diffctx-mcp
# Codex CLI
codex mcp add diffctx -- uvx --from 'diffctx[mcp]' diffctx-mcp
# Gemini CLI
gemini mcp add diffctx uvx -- --from 'diffctx[mcp]' diffctx-mcp
# VS Code
code --add-mcp '{"name":"diffctx","command":"uvx","args":["--from","diffctx[mcp]","diffctx-mcp"]}'
```

With `pip install 'diffctx[mcp]'` already done, replace the
`uvx --from 'diffctx[mcp]' diffctx-mcp` tail with plain `diffctx-mcp`.

The server exposes one tool, `diffctx_context`, that assistants call when
reviewing PRs, explaining changes, or investigating broken tests. It ranks the
code that explains a diff, then reads only the fragments the assistant picked —
two calls that pay for the selection instead of a whole pack. The wider
`get_tree_map` and `get_file_context` tools are opt-in via
`DIFFCTX_MCP_LEGACY_TOOLS=1`. Filesystem confinement via
`DIFFCTX_ALLOWED_PATHS`: see [SECURITY.md](SECURITY.md).

Every stdio client takes the same server shape; only the config file differs:

| Client | Config file | Key |
|---|---|---|
| Claude Code (project) | `.mcp.json` | `mcpServers` |
| Claude Desktop | `claude_desktop_config.json` | `mcpServers` |
| Cursor | `~/.cursor/mcp.json` | `mcpServers` |
| Windsurf | `~/.codeium/windsurf/mcp_config.json` | `mcpServers` |
| Continue | `~/.continue/config.json` | `experimental.modelContextProtocolServers` (transport object) |
| Zed | `~/.config/zed/settings.json` | `context_servers` (`command.path`) |

```json
{
  "mcpServers": {
    "diffctx": {
      "command": "uvx",
      "args": ["--from", "diffctx[mcp]", "diffctx-mcp"]
    }
  }
}
```

With `pip install 'diffctx[mcp]'` already done, `"command": "diffctx-mcp"` with
no args works everywhere instead. Use the `diffctx-mcp` entry point, not the
`diffctx mcp` subcommand: the latter only exists from 1.12.3 onward and would
map a directory named `mcp` on older releases.

## Ignore patterns

Respects `.gitignore` and `.diffctx/ignore` automatically — hierarchically at
every directory level, with full gitignore semantics (negation `!important.log`,
anchored `/root_only.txt`), and in tree mode the output file is always
auto-ignored (`--diff` reports a saved `tree.md` like any other change). Three
controls are tree mode only and are refused with `--diff` and `graph`: `.diffctx/whitelist`
(`-w`) as an include-only filter, `-i` for an extra ignore file, and
`--no-default-ignores` / `--no-ignores` to drop the built-in patterns or every
ignore rule.

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 in the OS cache directory (e.g.
`~/Library/Caches/diffctx/token-cache`) — a pure speedup, safe to delete.
`DIFFCTX_TOKEN_CACHE_DIR` relocates it; `DIFFCTX_TOKEN_CACHE_MAX_BYTES` caps
its size (default 512 MB, `0` disables eviction).

## Exit codes

| Code | Meaning |
|------|---------|
| `0`  | Success — output contains content |
| `1`  | Runtime error (bad path, permission denied, etc.) |
| `2`  | Usage error (invalid flags/arguments, an empty `--diff ""`, a malformed duration such as `1.5h`, an out-of-range `--alpha`/`--tau`/`--budget`/`--timeout`, or a tree-mode flag — `-i`/`-w`/`--no-default-ignores`/`--no-ignores` — given with `--diff` or `graph`) |
| `3`  | Environment error (`--diff` outside a git repo, in a bare one, or on a missing path, git not installed or refusing to start — its `fatal:` line is printed, no commits yet, a revision missing from a shallow clone) |
| `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` ran 30 s past the `--timeout` deadline without stopping cooperatively (the deadline itself yields a partial artifact and exit 0) |
| `130`| Interrupted (Ctrl-C) |
| `141`| Broken pipe (e.g. piping into `head`) |

## License

Apache 2.0

<!-- mcp-name: io.github.nikolay-e/diffctx -->
<!-- Ownership marker read from the PyPI description by the MCP registry. -->
<!-- Must survive edits verbatim: one space after the colon, case-sensitive. -->

---

- [Documentation site](https://diffctx.com/) — the pipeline
  end to end: diff → fragments → graph → relevance → selection
- [Command-line reference](docs/product/cli.md) — every flag with its default
  and meaning, rendered from `diffctx --help`
- [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
- [Benchmarks](BENCHMARKS.md) — every published number with what it was
  measured on, and how to reproduce it
- [FAQ](docs/product/faq.md) — heuristic or oracle, whose tokens, monorepos,
  the raw diff, secrets
- [Paper](https://doi.org/10.5281/zenodo.18824579) — budgeted typed-graph
  retrieval for diff-aware context selection (Zenodo, 2026)
- [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