DiffCI

Website · Test impact analysis guide · Open evidence study

Find test-selection opportunities in your CI before changing what it runs. DiffCI analyzes a commit's changes and dependency graph, then reports which test files it would select, why it falls back to a full run, and whether it can propose a test command. check also runs paired full and selected commands when it can infer them. The observe command and Action remain observation-only.

Try DiffCI

From a Git repository checkout, with Node.js 22.5+ and Git installed, run:

npx "@diffci.com/diffci@latest" check

check explains affected tests and, when it can infer safe commands, runs both the full and selected test commands to measure the difference. It sends nothing to DiffCI. Test commands can create files in the checkout. For analysis without test execution, use npx "@diffci.com/diffci@latest" observe --no-send.

To add instructions for coding agents, run:

npx "@diffci.com/diffci@latest" init

To also pin DiffCI as a development dependency and update the detected npm, pnpm, Yarn, or Bun lockfile, pass --install. Add --workflow for a separate non-blocking observation job:

npx "@diffci.com/diffci@latest" init --install --workflow

--install also adds diffci:check and diffci:observe package scripts. It preserves same-named scripts that the project already owns. After installation, CI or contributors can run npm run diffci:check or npm run diffci:observe without knowing the package name or version.

On Windows PowerShell, quote the package name:

npx "@diffci.com/diffci@latest" check

The copyable adoption kit includes an AGENTS.md instruction and maintainer PR text. AI-readable documentation is on Context7 CLI and Context7 Core. Use the GitHub Marketplace Action for a separate, non-blocking observation job. Required project CI remains authoritative.

One paired run is preliminary evidence; repeat comparisons and account for cache effects before claiming CI savings. On a full-validation fallback, check runs the full command once and reports 0% reduction.

Upgrade from 0.1.3: tests excluded by a source-only tsconfig.json could be discovered without their dependency edges, producing an incomplete selection. This is fixed in 0.1.4. Revalidate affected observations before using them as opportunity evidence; see the historical validation and release qualification.

The local default compares HEAD with its first parent; both commits must be available. For a specific comparison, add --base <base-sha> --head <head-sha>. DiffCI prints the selection, fallback reasons, and the path to a JSON report outside your checkout. REFUSED or ERROR is not a successful analysis; check the reported status even when the command exits successfully. See the support matrix for setup requirements and supported workloads. check runs inferred full and selected commands in the checkout and sends nothing by default. The commands may write generated files. Use observe --no-send for analysis without execution. See docs/ai-agents.md for Claude Code, Codex, Cursor, GitHub Copilot, and similar tools.

Measured example: a controlled Cal.com replay showed 44.2% net reduction in a job-equivalent install + pretest + test workload, including analysis overhead. This is one sandbox comparison, not Cal.com's production savings or a prediction for your repository. Read the timings and method.

Selection counts alone do not establish runtime savings. check reports a measured percentage only when both commands pass and the checked-out commit and worktree remain identical across both arms. The savings artifact embeds the base/head SHAs, observation SHA-256, commands, timings, and checkout snapshots; observe does not execute tests.

For an advanced paired runtime check, you can still run observe first and then run verify-savings against the observation report. It compares your normal full command with DiffCI's proposed selected command and writes JSON plus Markdown evidence; see docs/npm-adoption.md.

Observe in GitHub Actions

Save this as .github/workflows/diffci.yml to add a dedicated, non-blocking observation job:

name: DiffCI observation
on: [push, pull_request]
permissions:
  contents: read
jobs:
  diffci:
    runs-on: ubuntu-latest
    continue-on-error: true
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0
      - uses: DiffCI/DiffCI.com@5458e133a549b24f7224b1364811ef487fa4e153

Then check the workflow locally with npx "@diffci.com/diffci@latest" verify-workflow. Keep the observer out of required checks and other jobs' needs lists. The Action adds a job summary and a diffci-observation artifact to the run; it does not alter which tests your other jobs execute. The example pins release v0.2.9 to its qualified feature commit SHA for reproducibility.

The CLI sends no report with --no-send. The Action uploads a GitHub artifact by default; sending to DiffCI's hosted service requires an explicitly configured endpoint and token. Installation details · Seven-day pilot

Use DiffCI

Surface	Use it for	Current distribution
@diffci.com/diffci	Try observe locally, run an opt-in runtime pilot, or install the GitHub Action from this repository	Published npm CLI and Action

The CLI bundles a pinned revision of the Core engine from GitHub. Users install only @diffci.com/diffci; the check command above uses it directly. Core performs Git analysis, dependency graphs, impact, path baseline, and selected-command planning. The report format and non-interfering GitHub Action remain in this repository. See docs/package-relationship.md for the source relationship. For evaluation results and their limits, start with docs/adoption-evidence.md.

Agent Adoption

Add DiffCI instructions to a repository:

npx "@diffci.com/diffci@latest" init

Then ask your coding agent to run:

npx "@diffci.com/diffci@latest" check

Agent-specific docs: Codex · Claude Code · Cursor · GitHub Copilot · Grok.

Live discovery files: llms.txt · AI agents.

Adoption materials: outreach copy · metrics · targets.

For native agent integrations, DiffCI provides a stateless, read-only HTTPS MCP endpoint for validation guidance:

https://diffci.com/mcp

Use the stdio MCP server when tools need to inspect the local checkout or run tests:

npx -p "@diffci.com/diffci@latest" diffci-mcp

See the MCP server page for Codex, Claude Code, Cursor, and VS Code setup, or docs/mcp.md for repository documentation.

Project background

Public Core and private Cloud

DiffCI's public-good analysis engine is released separately from its commercial hosted product. The project lives in the DiffCI GitHub organization. This repository was transferred to DiffCI/DiffCI.com on 2026-09-16.

Component	Scope	Licensing
DiffCI Core	Standalone dependency/change analysis, CI graph inference, safety/fallback, advisory test selection, synthetic benchmarking and local compute measurement	AGPL-3.0-only; public
DiffCI Cloud	Hosted infrastructure, billing, enterprise dashboard, organization management, proprietary data/services, and managed acceleration	Proprietary
Optional enterprise code	Separately scoped, visible and auditable enterprise capabilities	Source available, with commercial production rights controlled by DiffCI

This mixed repository is public. The reviewed Core extraction was published separately, with fresh Git history. This repository retains its existing engine snapshot while package integration is migrated separately. No blanket AGPL license applies to this repository. Public Core is advisory-only; energy/carbon/cost are modeled estimates, not verified environmental savings. The exact extraction is recorded in the Core release audit.

The funded Core should run independently of DiffCI Cloud. AGPL permits commercial use and competing hosting; it adds source-sharing obligations for covered modifications, including qualifying remote network use. Grant eligibility depends on each grant's agreement and funded deliverables.

See the licensing boundaries, organization and migration plan, and prepared organization profile.

This repository moved out of the DentalPresence.in monorepo (previously diffci/ there) into its own repo on 2026-08-21, once the project outgrew being a subfolder. DentalPresence.in remains DiffCI's original dogfooding target - some code (the planner DentalPresence-specific PATH baseline/task registry, a few fixture tests) still reflects that origin - but the research and shadow-validation pipelines are generic and have been exercised against dozens of real third-party repositories.

Current state

Language expansion: initial Vue SFC, Go package-level, and conventional Maven reactor analysis is implemented through repository adapters. See the support matrix and setup requirements for exact scope, fallback behavior, and validation boundaries.

Three completed research stages plus an in-progress prospective-validation stage, in order:

• Stage 0 - a 2,000-delta historical benchmark across 20 real repositories, run through a real Cloudflare orchestrator. Verdict: GO WITH CONDITIONS.
• Stage 1A - forensic root-cause investigation of every repository/delta where Stage 0's confidence model degraded to UNSAFE, and of every historical "unsafe miss" candidate. Identified the top 3 highest-leverage fixes.
• Stage 1B - implemented those 3 fixes (reachability-aware confidence narrowing, an improved historical safety-measurement methodology, a tsconfig-scope + package.json-diffing fix), validated them live against real repositories (coverage improved, zero contradicted safety cases), and ran a real wall-clock FULL/PATH/DiffCI runtime pilot.
• Stage 2 (current) - prospective shadow validation on real, currently-arriving CI events, not more historical benchmarking. A live pipeline (Cloudflare Sandbox Containers + Worker, D1 + R2) observes real repositories, predicts before their outcome is known, and later reconciles against the real CI result. Current verdict: EXTEND SHADOW VALIDATION - the pipeline is real and defect-free, and since 2026-08-21 it runs autonomously: a Cron Trigger polls enrolled repositories every 10 minutes (src/research/cloudflare/shadow-cron.ts), and the registered DiffCI Shadow GitHub App (read-only; see docs/github-app-registration.md) delivers push/workflow events to /v1/shadow/webhook for instant predictions and exactly-on-time reconciliation - this repository shadow-observes itself through that App. See docs/research/2026-08-21-stage2-final-report.md for the full picture; what's honestly still missing is real observation volume, working GitHub Actions on our own repositories (account billing), and real design-partner repositories.

Every dated report behind these stages lives in docs/research/ - start with 2026-08-21-stage2-architecture.md for the fullest current picture of what's built vs not, or the Stage 0/1A/1B reports for the historical-validation story.

For the next product milestone, see docs/alpha-readiness.md. It tracks the private-alpha bar: install DiffCI, keep CI unchanged, collect real shadow observations, and render a trustworthy potential-savings report.

Architecture

DiffCI is now framed as an open-core product:

DiffCI
|
├── Open-source core
|   ├── DiffCI engine
|   ├── CLI / npm package
|   ├── Local analysis
|   └── Basic GitHub Action
|       |
|       └── Tidelift package support
|
└── Commercial DiffCI
    ├── Hosted service / DiffCI Cloud
    ├── Organization dashboard
    ├── Historical analytics
    ├── Advanced CI/CD optimization
    ├── Enterprise policies
    ├── Managed runners
    ├── Team features
    └── Support / enterprise services

The open-source core is the trust and adoption surface. It runs locally or in the host repository's own CI, writes a report, and changes nothing about CI execution. Commercial DiffCI adds hosted history, organization views, policy, managed operations, runners, and support. Tidelift belongs to the supported open-source package path, not the hosted product feature boundary. See docs/open-core-packaging.md and docs/tidelift-package-support.md.

The source tree follows that split:

• src/git/, src/repo/, src/planner/, and src/client/ are the installable OSS observer path.
• action.yml wraps the observer as a basic non-blocking GitHub Action.
• src/research/ and src/shadow/ run validation, GitHub App shadow observation, and reconciliation.
• src/product/, src/auth/, src/billing/, src/ingest/, src/ledger/, src/runner/, and src/usage/ are the commercial/control-plane layer.
• docs/oss-boundary.md records what is allowed into the npm package.

Commands

# Type-check and run the full test suite
npm run check

# Generate an example delta / impact / plan for the current repo's latest commit
npm run diffci
npm run impact

# Run a real Stage 0-style historical benchmark locally
npm run research:stage0

# Deploy the Cloudflare research/shadow Worker (D1 + R2 + Sandbox Containers)
npm run research:sandbox:deploy

Install Surfaces

DiffCI is intended to be installable as infrastructure, not only as a hosted shadow experiment:

- uses: DiffCI/DiffCI.com@5458e133a549b24f7224b1364811ef487fa4e153

npx "@diffci.com/diffci@latest" observe
npx "@diffci.com/diffci@latest" check
npx "@diffci.com/diffci@latest" init
npx "@diffci.com/diffci@latest" verify-workflow

The GitHub Action and npm CLI establish the OSS/package distribution path. The hosted GitHub App and DiffCI Cloud build on that trust boundary for teams that want shared reports and history. See docs/distribution.md for the package and Action positioning, docs/open-core-packaging.md for the commercial split, and docs/npm-adoption.md for copy-paste pilot material.