Update to determinate by default (#208 )

* Update to determinate by default Implements https://github.com/DeterminateSystems/nix-src/issues/201 * Fixup the version verification script
Drop x86_64-darwin (#207 )
2025-11-10 09:48:50 -05:00 · 2025-11-09 22:55:22 -05:00 · 2025-11-07 13:02:42 -03:00 · 2025-11-07 15:52:50 +00:00 · 2025-10-24 22:24:28 -04:00 · 2025-09-12 14:50:59 +02:00
26 changed files with 59842 additions and 54536 deletions
@@ -7,9 +7,8 @@ EXPECTED_VERSION="${1}"

 INSTALLED_NIX_VERSION_OUTPUT=$(nix --version)
 INSTALLED_NIX_VERSION=$(echo "${INSTALLED_NIX_VERSION_OUTPUT}" | awk '{print $NF}')
-EXPECTED_OUTPUT="nix (Nix) ${EXPECTED_VERSION}"

-if [ "${INSTALLED_NIX_VERSION_OUTPUT}" != "${EXPECTED_OUTPUT}" ]; then
+if [ "${INSTALLED_NIX_VERSION}" != "${EXPECTED_VERSION}" ]; then
  echo "Nix version ${INSTALLED_NIX_VERSION} didn't match expected version ${EXPECTED_VERSION}"
  exit 1
 else
@@ -11,9 +11,9 @@ jobs:
    runs-on: ubuntu-22.04
    needs:
      - check-dist-up-to-date
-      - install-nix-linux
-      - install-nix-macos
+      - install-nix
      - install-with-non-default-source-inputs
+      - install-no-id-token
    # NOTE(cole-h): GitHub treats "skipped" as "OK" for the purposes of required checks on branch
    # protection, so we take advantage of this fact and fail if any of the dependent actions failed,
    # or "skip" (which is a success for GHA's purposes) if none of them did.
@@ -28,7 +28,7 @@ jobs:
    steps:
      - uses: actions/checkout@v4
      - uses: DeterminateSystems/nix-installer-action@main
-      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - uses: DeterminateSystems/flakehub-cache-action@main
      - name: pnpm install
        run: nix develop --command pnpm install
      - name: prettier format
@@ -44,14 +44,22 @@ jobs:
      - name: Ensure no staged changes
        run: git diff --exit-code

-  install-nix-linux:
-    name: Run test suite for Linux systems
+  install-nix:
+    name: "Test: ${{ matrix.runner }}${{ matrix.determinate && ' with determinate' || '' }}"
    strategy:
+      fail-fast: false
      matrix:
        runner:
          - ubuntu-latest
          - nscloud-ubuntu-22.04-amd64-4x16
          - namespace-profile-default-arm64
+          # - macos-12-large # determinate-nixd is broken on macos-12
+          - macos-13-large
+          - macos-14-large
+          - macos-14-xlarge # arm64
+        determinate:
+          - true
+          - false
    runs-on: ${{ matrix.runner }}
    permissions:
      contents: read
@@ -65,11 +73,12 @@ jobs:
          log-directives: nix_installer=trace
          backtrace: full
          _internal-strict-mode: true
+          determinate: ${{ matrix.determinate }}
      - name: echo $PATH
        run: echo $PATH

      - name: Render the devshell
-        if: success() || failure()
+        if: (success() || failure()) && matrix.runner != 'macos-13-large' && matrix.runner != 'macos-14-large'
        run: |
          nix develop --command date

@@ -82,91 +91,6 @@ jobs:
          nix store gc
          nix run nixpkgs#hello

-      - name: Test bash
-        run: nix-instantiate -E 'builtins.currentTime' --eval
-        if: success() || failure()
-        shell: bash --login {0}
-      - name: Test sh
-        run: nix-instantiate -E 'builtins.currentTime' --eval
-        if: success() || failure()
-        shell: sh -l {0}
-      - name: Install Nix again (noop)
-        uses: ./
-        with:
-          logger: pretty
-          log-directives: nix_installer=trace
-          backtrace: full
-          _internal-strict-mode: true
-      - name: Test `nix` with `$GITHUB_PATH`
-        if: success() || failure()
-        run: |
-          nix run nixpkgs#hello
-          nix profile install nixpkgs#hello
-          hello
-          nix store gc
-          nix run nixpkgs#hello
-      - name: Reinstall Nix
-        uses: ./
-        with:
-          logger: pretty
-          log-directives: nix_installer=trace
-          backtrace: full
-          reinstall: true
-          extra-conf: |
-            use-sqlite-wal = true
-          _internal-strict-mode: true
-      - name: Test `nix` with `$GITHUB_PATH`
-        if: success() || failure()
-        run: |
-          nix run nixpkgs#hello
-          nix profile install nixpkgs#hello
-          hello
-          nix store gc
-          nix run nixpkgs#hello
-      - name: Verify the generated nix.conf
-        run: |
-          cat -n /etc/nix/nix.conf
-          grep -E "^trusted-users = .*$USER" /etc/nix/nix.conf
-          grep -E "^use-sqlite-wal = true" /etc/nix/nix.conf
-      - name: Breakpoint if tests failed
-        if: failure()
-        uses: namespacelabs/breakpoint-action@v0
-        with:
-          duration: 5m
-          authorized-users: grahamc
-
-  install-nix-macos:
-    name: Run test suite for macOS systems
-    strategy:
-      matrix:
-        runner:
-          # x86_64-darwin
-          - macos-12
-          # aarch64-darwin
-          - macos-latest-xlarge
-    runs-on: ${{ matrix.runner }}
-    permissions:
-      contents: read
-      id-token: write
-    steps:
-      - uses: actions/checkout@v4
-      - name: Install Nix
-        uses: ./
-        with:
-          logger: pretty
-          log-directives: nix_installer=trace
-          backtrace: full
-          _internal-strict-mode: true
-      - name: echo $PATH
-        run: echo $PATH
-      - name: Test `nix` with `$GITHUB_PATH`
-        if: success() || failure()
-        run: |
-          nix run nixpkgs#hello
-          nix profile install nixpkgs#hello
-          hello
-          nix store gc
-          nix run nixpkgs#hello
      - name: Test bash
        run: nix-instantiate -E 'builtins.currentTime' --eval
        if: success() || failure()
@@ -176,9 +100,8 @@ jobs:
        if: success() || failure()
        shell: sh -l {0}
      - name: Test zsh
-        run: nix-instantiate -E 'builtins.currentTime' --eval
+        run: if (zsh --help > /dev/null); then zsh --login --interactive -c "nix-instantiate -E 'builtins.currentTime' --eval"; fi
        if: success() || failure()
-        shell: zsh --login --interactive {0}
      - name: Install Nix again (noop)
        uses: ./
        with:
@@ -186,6 +109,7 @@ jobs:
          log-directives: nix_installer=trace
          backtrace: full
          _internal-strict-mode: true
+          determinate: ${{ matrix.determinate }}
      - name: Test `nix` with `$GITHUB_PATH`
        if: success() || failure()
        run: |
@@ -204,6 +128,7 @@ jobs:
          extra-conf: |
            use-sqlite-wal = true
          _internal-strict-mode: true
+          determinate: ${{ matrix.determinate }}
      - name: Test `nix` with `$GITHUB_PATH`
        if: success() || failure()
        run: |
@@ -214,15 +139,10 @@ jobs:
          nix run nixpkgs#hello
      - name: Verify the generated nix.conf
        run: |
-          cat /etc/nix/nix.conf
-          grep -E "^trusted-users = .*$USER" /etc/nix/nix.conf
-          grep -E "^use-sqlite-wal = true" /etc/nix/nix.conf
-      - name: Breakpoint if tests failed
-        if: failure()
-        uses: namespacelabs/breakpoint-action@v0
-        with:
-          duration: 5m
-          authorized-users: grahamc
+          nix config show
+          cat -n /etc/nix/nix.conf
+          nix config show | grep -E "^trusted-users = .*$USER"
+          nix config show | grep -E "^use-sqlite-wal = true"

  install-with-non-default-source-inputs:
    name: Install Nix using non-default source-${{ matrix.inputs.key }}
@@ -230,14 +150,10 @@ jobs:
    strategy:
      matrix:
        inputs:
-          # https://github.com/DeterminateSystems/nix-installer/blob/v0.18.0
+          # https://github.com/DeterminateSystems/nix-installer/blob/v3.11.3
          - key: url
-            value: https://github.com/DeterminateSystems/nix-installer/releases/download/v0.18.0/nix-installer-x86_64-linux
-            nix-version: "2.21.2"
-          # https://github.com/DeterminateSystems/nix-installer/tree/7011c077ec491da410fbc39f68676b0908b9ce7e
-          - key: revision
-            value: 7011c077ec491da410fbc39f68676b0908b9ce7e
-            nix-version: "2.19.2"
+            value: https://github.com/DeterminateSystems/nix-installer/releases/download/v3.11.3/nix-installer-x86_64-linux
+            nix-version: "2.31.2" # 3.11.3 is based on 2.31.2

    steps:
      - uses: actions/checkout@v4
@@ -248,3 +164,13 @@ jobs:
          _internal-strict-mode: true
      - name: Ensure that the expected Nix version ${{ matrix.inputs.nix-version }} is installed via alternative source-${{ matrix.inputs.key }}
        run: .github/verify-version.sh ${{ matrix.inputs.nix-version }}
+
+  install-no-id-token:
+    name: Install Nix without an ID token
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ./
+        with:
+          _internal-strict-mode: true
+          determinate: true
@@ -1,3 +1,5 @@
 # Submitting Pull Requests

+Run `pnpm install` to install necessary JS tools.
+
 This action is based off https://github.com/actions/javascript-action. As part of your contributing flow you **must** run `npm run all` before we can merge.
@@ -34,7 +34,7 @@ jobs:

 ### With FlakeHub

-To fetch private flakes from FlakeHub, update the `permissions` block and pass `flakehub: true`:
+To fetch private flakes from FlakeHub and Nix builds from FlakeHub Cache, update the `permissions` block and use [`determinate-nix-action`][determinate-nix-action]:

 ```yaml
 on:
@@ -51,14 +51,25 @@ jobs:
      contents: "read"
    steps:
      - uses: actions/checkout@v4
-      - uses: DeterminateSystems/nix-installer-action@main
-        with:
-          flakehub: true
+      - uses: DeterminateSystems/determinate-nix-action@v3
      - run: nix build .
 ```

 See [`.github/workflows/ci.yml`](.github/workflows/ci.yml) for a full example.

+### Pinning the version
+
+This GitHub Action uses the most recent version of the Determinate Nix Installer, even when the Action itself is pinned.
+If you wish to pin your CI workflows to a specific version, use the [`determinate-nix-action`][determinate-nix-action].
+That Action is updated and tagged for every Determinate release.
+
+The `DeterminateSystems/determinate-nix-action@v3.5.2` reference, for example, always installs Determinate Nix v3.5.2.
+
+Additionally, an extra tag on the major version is kept up to date with the current release.
+The `DeterminateSystems/determinate-nix-action@v3` reference, for example, installs the most recent release in the `v3.x.y` series.
+
+If you do tag to a specific version, please [use Dependabot to update your actions][dependabot-actions].
+
 ### Advanced Usage

 - If KVM is available, the installer sets up KVM so that Nix can use it ,and exports the `DETERMINATE_NIX_KVM` environment variable set to 1.
@@ -85,10 +96,11 @@ Differing from the upstream [Nix](https://github.com/NixOS/nix) installer script
 | Parameter               | Description                                                                                                                                                                                                                                                                    | Type                                       | Default                                                        |
 | :---------------------- | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :----------------------------------------- | :------------------------------------------------------------- |
 | `backtrace`             | The setting for [`RUST_BACKTRACE`][backtrace]                                                                                                                                                                                                                                  | string                                     |                                                                |
+| `determinate`           | Whether to install [Determinate Nix](https://determinate.systems/enterprise) and log in to FlakeHub for private Flakes and binary caches.                                                                                                                                      | Boolean                                    | `true`                                                         |
 | `extra-args`            | Extra arguments to pass to the planner (prefer using structured `with:` arguments unless using a custom [planner]!)                                                                                                                                                            | string                                     |                                                                |
 | `extra-conf`            | Extra configuration lines for `/etc/nix/nix.conf` (includes `access-tokens` with `secrets.GITHUB_TOKEN` automatically if `github-token` is set)                                                                                                                                | string                                     |                                                                |
-| `flakehub`              | Log in to FlakeHub to pull private flakes using the GitHub Actions [JSON Web Token](https://jwt.io) (JWT), which is bound to the `api.flakehub.com` audience.                                                                                                                  | Boolean                                    | `false`                                                        |
-| `force-docker-shim`     | Force the use of Docker as a process supervisor. This setting is automatically enabled when necessary.                                                                                                                                                                         | Boolean                                    | `false`                                                        |
+| `flakehub`              | Deprecated. Implies `determinate`.                                                                                                                                                                                                                                             | Boolean                                    | `false`                                                        |
+| `force-no-systemd`      | Force using other methods than systemd to launch the daemon. This setting is automatically enabled when necessary.                                                                                                                                                             | Boolean                                    | `false`                                                        |
 | `github-token`          | A [GitHub token] for making authenticated requests (which have a higher rate-limit quota than unauthenticated requests)                                                                                                                                                        | string                                     | `${{ github.token }}`                                          |
 | `github-server-url`     | The URL for the GitHub server, to use with the `github-token` token. Defaults to the current GitHub server, supporting GitHub Enterprise Server automatically. Only change this value if the provided `github-token` is for a different GitHub server than the current server. | string                                     | `${{ github.server }}`                                         |
 | `init`                  | The init system to configure (requires `planner: linux-multi`)                                                                                                                                                                                                                 | enum (`none` or `systemd`)                 |                                                                |
@@ -110,7 +122,7 @@ Differing from the upstream [Nix](https://github.com/NixOS/nix) installer script
 | `source-pr`             | The pull request of `nix-installer` to use (conflicts with `source-tag`, `source-revision`, and `source-branch`)                                                                                                                                                               | integer                                    |                                                                |
 | `source-revision`       | The revision of `nix-installer` to use (conflicts with `source-tag`, `source-branch`, and `source-pr`)                                                                                                                                                                         | string                                     |                                                                |
 | `source-tag`            | The tag of `nix-installer` to use (conflicts with `source-revision`, `source-branch`, `source-pr`)                                                                                                                                                                             | string                                     |                                                                |
-| `source-url`            | A URL pointing to a `nix-installer.sh` script                                                                                                                                                                                                                                  | URL                                        | `https://install.determinate.systems/nix`                      |
+| `source-url`            | A URL pointing to the `nix-installer` binary                                                                                                                                                                                                                                   | URL                                        | n/a (calculated)                                               |
 | `nix-package-url`       | The Nix package URL                                                                                                                                                                                                                                                            | URL                                        |                                                                |
 | `planner`               | The installation [planner] to use                                                                                                                                                                                                                                              | enum (`linux` or `macos`)                  |                                                                |
 | `reinstall`             | Force a reinstall if an existing installation is detected (consider backing up `/nix/store`)                                                                                                                                                                                   | Boolean                                    | `false`                                                        |
@@ -122,6 +134,8 @@ Differing from the upstream [Nix](https://github.com/NixOS/nix) installer script

 [apfs]: https://en.wikipedia.org/wiki/Apple_File_System
 [backtrace]: https://doc.rust-lang.org/std/backtrace/index.html#environment-variables
+[dependabot-actions]: https://github.com/DeterminateSystems/determinate-nix-action?tab=readme-ov-file#-automate-updates-with-dependabot
+[determinate-nix-action]: https://github.com/DeterminateSystems/determinate-nix-action
 [github token]: https://docs.github.com/en/actions/security-guides/automatic-token-authentication
 [planner]: https://github.com/determinateSystems/nix-installer#usage
 [profile]: https://nixos.org/manual/nix/stable/package-management/profiles
@@ -7,6 +7,10 @@ inputs:
  backtrace:
    description: The setting for `RUST_BACKTRACE` (see https://doc.rust-lang.org/std/backtrace/index.html#environment-variables)
    required: false
+  determinate:
+    description: |
+      Whether to install [Determinate Nix](https://determinate.systems/enterprise) and log in to FlakeHub for private Flakes and binary caches.
+    default: true
  extra-args:
    description: Extra args to pass to the planner (prefer using structured `with:` arguments unless using a custom planner!)
    required: false
@@ -14,11 +18,11 @@ inputs:
    description: Extra configuration lines for `/etc/nix/nix.conf` (includes `access-tokens` with `secrets.GITHUB_TOKEN` automatically if `github-token` is set)
    required: false
  flakehub:
-    description: Automatically log in to your [FlakeHub](https://flakehub.com) account, for accessing private flakes.
+    description: Deprecated. Implies `determinate`.
    required: false
    default: false
-  force-docker-shim:
-    description: Force the use of Docker as a process supervisor. This setting is automatically enabled when necessary.
+  force-no-systemd:
+    description: Force using other methods than systemd to launch the daemon. This setting is automatically enabled when necessary.
    required: false
    default: false
  github-token:
@@ -30,6 +34,9 @@ inputs:
  init:
    description: "The init system to configure, requires `planner: linux-multi` (allowing the choice between `none` or `systemd`)"
    required: false
+  job-status:
+    description: The overall status of the job. Set automatically, for aggregate analysis of Nix stability.
+    default: ${{ job.status }}
  kvm:
    description: Automatically configure the GitHub Actions Runner for NixOS test supports, if the host supports it.
    required: false
@@ -1,19 +0,0 @@
-# Determinate Nix Installer: Docker Shim
-#
-# This empty image exists to lean on Docker as a process supervisor when
-# systemd isn't available. Specifically intended for self-hosted GitHub
-# Actions runners using Docker-in-Docker.
-#
-# See: https://github.com/DeterminateSystems/nix-installer-action 
-
-FROM scratch
-
-ENTRYPOINT [ "/nix/var/nix/profiles/default/bin/nix-daemon"]
-CMD []
-
-HEALTHCHECK \
-    --interval=5m \
-    --timeout=3s \
-    CMD ["/nix/var/nix/profiles/default/bin/nix", "store", "ping", "--store", "daemon"]
-
-COPY ./Dockerfile /README.md
@@ -1,52 +0,0 @@
-# Determinate Nix Installer Action: Docker Shim
-
-The image in this repository is a product of the contained Dockerfile.
-It is an otherwise empty image with a configuration layer.
-
-This image is to be used in GitHub Actions runners which don't have systemd available, like self-hosted ARC runners.
-
-The image would have no layers / content at all, however Docker has a bug and refuses to export those images.
-This isn't a technical limitation preventing us from creating and distributing that image, but an ease-of-use limitation.
-Since some of Docker's inspection tools break on an empty image, the image contains a single layer containing a README.
-
-To build:
-
-```shell
-docker build . --tag determinate-nix-shim:latest
-docker image save determinate-nix-shim:latest | gzip --best > amd64.tar
-```
-
-Then, extract the tarball:
-
-```
-mkdir extract
-cd extract
-tar -xf ../amd64.tar
-```
-
-It'll look like this, though the hashes will be different.
-
-```
-.
-├── 771204abb853cdde06bbbc680001a02642050a1db1a7b0a48cf5f20efa8bdc5d.json
-├── c4088111818e553e834adfc81bda8fe6da281afa9a40012eaa82796fb5476e98
-│   ├── VERSION
-│   ├── json
-│   └── layer.tar
-├── manifest.json
-└── repositories
-```
-
-Ignore `manifest.json`, and edit the other two JSON documents to replace `amd64` with `arm64`, both in a key named "architecture:
-
-```
-"architecture":"amd64"
-```
-
-Then re-create the tar, from within the `extract` directory:
-
-```
-tar --options gzip:compression-level=9 -zcf ../arm64.tar.gz .
-```
-
-Then `git add` the two .tar.gz's and you're done.
@@ -9,7 +9,7 @@

  outputs = { self, flake-schemas, nixpkgs }:
    let
-      supportedSystems = [ "x86_64-linux" "aarch64-darwin" "aarch64-linux" "x86_64-darwin" ];
+      supportedSystems = [ "x86_64-linux" "aarch64-darwin" "aarch64-linux" ];
      forEachSupportedSystem = f: nixpkgs.lib.genAttrs supportedSystems (system: f {
        pkgs = import nixpkgs { inherit system; };
      });
@@ -11,7 +11,8 @@
    "check-fmt": "prettier --check .",
    "lint": "eslint src/**/*.ts",
    "package": "ncc build",
-    "all": "pnpm run format && pnpm run lint && pnpm run build && pnpm run package"
+    "test": "vitest --watch false",
+    "all": "pnpm run test && pnpm run format && pnpm run lint && pnpm run build && pnpm run package"
  },
  "repository": {
    "type": "git",
@@ -25,25 +26,27 @@
  },
  "homepage": "https://github.com/DeterminateSystems/nix-installer-action#readme",
  "dependencies": {
-    "@actions/core": "^1.10.1",
+    "@actions/core": "^1.11.1",
    "@actions/exec": "^1.1.1",
-    "@actions/github": "^5.1.1",
+    "@actions/github": "^6.0.1",
    "detsys-ts": "github:DeterminateSystems/detsys-ts",
-    "string-argv": "^0.3.2"
+    "got": "^14.6.2",
+    "string-argv": "^0.3.2",
+    "vitest": "^3.2.4"
  },
  "devDependencies": {
    "@trivago/prettier-plugin-sort-imports": "^4.3.0",
-    "@types/node": "^20.12.13",
+    "@types/node": "^20.19.24",
    "@types/uuid": "^9.0.8",
-    "@typescript-eslint/eslint-plugin": "^7.11.0",
-    "@vercel/ncc": "^0.38.1",
-    "eslint": "^8.57.0",
-    "eslint-import-resolver-typescript": "^3.6.1",
+    "@typescript-eslint/eslint-plugin": "^7.18.0",
+    "@vercel/ncc": "^0.38.4",
+    "eslint": "^8.57.1",
+    "eslint-import-resolver-typescript": "^3.10.1",
    "eslint-plugin-github": "^4.10.2",
-    "eslint-plugin-import": "^2.29.1",
-    "eslint-plugin-prettier": "^5.1.3",
-    "prettier": "^3.2.5",
-    "tsup": "^8.0.2",
-    "typescript": "^5.4.5"
+    "eslint-plugin-import": "^2.32.0",
+    "eslint-plugin-prettier": "^5.5.4",
+    "prettier": "^3.6.2",
+    "tsup": "^8.5.0",
+    "typescript": "^5.9.3"
  }
 }
@@ -0,0 +1,66 @@
+import * as core from "@actions/core";
+
+import type { Fix, FixHashesOutputV1, Mismatch } from "./fixHashes.js";
+
+function prettyDerivation(derivation: string): string {
+  return derivation.replace(/\/nix\/store\/\w+-/, "").replace(/.drv$/, "");
+}
+
+function annotateSingle(
+  file: string,
+  line: number,
+  { derivation, replacement }: Mismatch,
+): void {
+  const pretty = prettyDerivation(derivation);
+  core.error(`To correct the hash mismatch for ${pretty}, use ${replacement}`, {
+    file,
+    startLine: line,
+  });
+}
+
+function annotateMultiple(
+  file: string,
+  { line, found, mismatches }: Fix,
+): void {
+  const matches = mismatches
+    .map(({ derivation, replacement }) => {
+      const pretty = prettyDerivation(derivation);
+      return `* For the derivation ${pretty}, use ${replacement}`;
+    })
+    .join("\n");
+
+  core.error(
+    `There are multiple replacements for the expression ${found}:\n${matches}`,
+    {
+      file,
+      startLine: line,
+    },
+  );
+}
+
+function annotate(file: string, fix: Fix): void {
+  if (fix.mismatches.length === 1) {
+    annotateSingle(file, fix.line, fix.mismatches[0]);
+  } else {
+    annotateMultiple(file, fix);
+  }
+}
+
+/**
+ * Annotates fixed-output derivation hash mismatches using GitHub Actions'
+ *
+ * @param output The output of `determinate-nixd fix hashes --json`
+ * @returns The number of annotations reported to the user
+ */
+export function annotateMismatches(output: FixHashesOutputV1): number {
+  let count = 0;
+
+  for (const { file, fixes } of output.files) {
+    for (const fix of fixes) {
+      annotate(file, fix);
+      count++;
+    }
+  }
+
+  return count;
+}
@@ -0,0 +1,65 @@
+import { parseEvents, getRecentEvents } from "./events.js";
+import { expect, test } from "vitest";
+
+// Handy test for locally making sure you can fetch recent events:
+// eslint-disable-next-line no-constant-condition
+if (false) {
+  test("Parsing existing events", async () => {
+    expect(await getRecentEvents(new Date(Date.now() - 1000000))).toStrictEqual(
+      [{}],
+    );
+  });
+}
+
+test("Parsing existing events", () => {
+  const { events } = parseEvents([
+    {
+      v: "1",
+      c: "BuiltPathResponseEventV1",
+      drv: "/nix/store/m96zgji4fhi70s2zs6pq5pric6ch7p4h-stdenv-darwin.drv",
+      outputs: ["/nix/store/dalhfz3l75w4b4q06sxzqgb2wfydvkbv-stdenv-darwin"],
+      timing: null,
+    },
+    {
+      v: "1",
+      c: "BuiltPathResponseEventV1",
+      drv: "/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv",
+      outputs: ["/nix/store/qwlgz5da3pfb53gqpgdmazaj9jczrnly-dep-1"],
+      timing: {
+        startTime: "2025-04-11T14:38:02Z",
+        stopTime: "2025-04-11T14:38:05Z",
+        durationSeconds: 3,
+      },
+    },
+    {
+      v: "1",
+      c: "BuildFailureResponseEventV1",
+      drv: "/nix/store/ykvbksjqrza2zpj6nkbycrdfwgfdpr8g-hash-mismatch-md5-base16.drv",
+      timing: {
+        startTime: "2025-04-11T14:36:44Z",
+        stopTime: "2025-04-11T14:36:44Z",
+        durationSeconds: 0,
+      },
+    },
+  ]);
+  expect(events).toStrictEqual([
+    {
+      v: "1",
+      c: "BuiltPathResponseEventV1",
+      drv: "/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv",
+      timing: {
+        durationSeconds: 3,
+        startTime: new Date("2025-04-11T14:38:02Z"),
+      },
+    },
+    {
+      v: "1",
+      c: "BuildFailureResponseEventV1",
+      drv: "/nix/store/ykvbksjqrza2zpj6nkbycrdfwgfdpr8g-hash-mismatch-md5-base16.drv",
+      timing: {
+        durationSeconds: 0,
+        startTime: new Date("2025-04-11T14:36:44Z"),
+      },
+    },
+  ]);
+});
@@ -0,0 +1,89 @@
+import got from "got";
+
+export interface DEvent {
+  v: string;
+  c: string;
+  drv: string;
+  timing: {
+    startTime: Date;
+    durationSeconds: number;
+  };
+}
+
+export interface ParsedEventsResult {
+  readonly events: DEvent[];
+  readonly hasMismatches: boolean;
+}
+
+export function parseEvents(data: unknown): ParsedEventsResult {
+  let hasMismatches = false;
+
+  if (!Array.isArray(data)) {
+    return { events: [], hasMismatches };
+  }
+
+  const events = data.flatMap((event) => {
+    // If this was a hash mismatch event, note it and move on
+    if (event.v === "1" && event.c === "HashMismatchResponseEventV1") {
+      hasMismatches = true;
+      return [];
+    }
+
+    // Otherwise, determine if it's an event we're interested in
+    if (
+      event.v === "1" &&
+      (event.c === "BuildFailureResponseEventV1" ||
+        event.c === "BuiltPathResponseEventV1") &&
+      Object.hasOwn(event, "drv") &&
+      typeof event.drv === "string" &&
+      Object.hasOwn(event, "timing") &&
+      typeof event.timing === "object" &&
+      event.timing !== null
+    ) {
+      const timing = event.timing as { [key: string]: unknown };
+
+      if (
+        Object.hasOwn(timing, "startTime") &&
+        typeof timing.startTime === "string" &&
+        Object.hasOwn(timing, "durationSeconds") &&
+        typeof timing.durationSeconds === "number"
+      ) {
+        const date = Date.parse(timing.startTime);
+        if (!Number.isNaN(date)) {
+          return [
+            {
+              v: event.v,
+              c: event.c,
+              drv: event.drv,
+              timing: {
+                startTime: new Date(date),
+                durationSeconds: timing.durationSeconds,
+              },
+            },
+          ];
+        }
+      }
+    }
+
+    return [];
+  });
+
+  return { events, hasMismatches };
+}
+
+export async function getRecentEvents(
+  since: Date,
+): Promise<ParsedEventsResult> {
+  const queryParam = encodeURIComponent(since.toISOString());
+
+  const resp = await got
+    .get(
+      `http://unix:/nix/var/determinate/determinate-nixd.socket:/events/recent?since=${queryParam}`,
+      {
+        enableUnixSockets: true,
+      },
+    )
+    .json();
+
+  return parseEvents(resp);
+}
@@ -0,0 +1,275 @@
+import { expect, test } from "vitest";
+import {
+  FailureSummary,
+  getBuildFailures,
+  summarizeFailures,
+} from "./failuresummary.js";
+
+/* eslint-disable @typescript-eslint/no-non-null-assertion */
+
+test("Select for failure events", () => {
+  const events = [
+    {
+      v: "1",
+      c: "BuildFailureResponseEventV1",
+      drv: `/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv`,
+      timing: {
+        startTime: new Date(1 * 1000),
+        durationSeconds: 1,
+      },
+    },
+    {
+      v: "1",
+      c: "BuiltPathResponseEventV1",
+      drv: `/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-2.drv`,
+      timing: {
+        startTime: new Date(2 * 1000),
+        durationSeconds: 2,
+      },
+    },
+    {
+      v: "1",
+      c: "BuildFailureResponseEventV1",
+      drv: `/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv`,
+      timing: {
+        startTime: new Date(3 * 1000),
+        durationSeconds: 3,
+      },
+    },
+  ];
+
+  expect(getBuildFailures(events)).toStrictEqual([
+    {
+      v: "1",
+      c: "BuildFailureResponseEventV1",
+      drv: `/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv`,
+      timing: {
+        startTime: new Date(1 * 1000),
+        durationSeconds: 1,
+      },
+    },
+    {
+      v: "1",
+      c: "BuildFailureResponseEventV1",
+      drv: `/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv`,
+      timing: {
+        startTime: new Date(3 * 1000),
+        durationSeconds: 3,
+      },
+    },
+  ]);
+});
+
+test("Summarize Failures", async () => {
+  const events = [
+    {
+      v: "1",
+      c: "BuildFailureResponseEventV1",
+      drv: `/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv`,
+      timing: {
+        startTime: new Date(1 * 1000),
+        durationSeconds: 1,
+      },
+    },
+    {
+      v: "1",
+      c: "BuiltPathResponseEventV1",
+      drv: `/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-2.drv`,
+      timing: {
+        startTime: new Date(2 * 1000),
+        durationSeconds: 2,
+      },
+    },
+    {
+      v: "1",
+      c: "BuildFailureResponseEventV1",
+      drv: `/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv`,
+      timing: {
+        startTime: new Date(3 * 1000),
+        durationSeconds: 3,
+      },
+    },
+  ];
+
+  const logMaker = async (drv: string): Promise<string | undefined> => {
+    if (drv.includes("dep-1")) {
+      return `${drv}\n`.repeat(9).trimEnd();
+    } else {
+      return `${drv}\n`.repeat(25).trimEnd();
+    }
+  };
+
+  const summary: FailureSummary = (await summarizeFailures(events, logMaker))!;
+
+  expect(summary.markdownLines.join("\n"))
+    .toStrictEqual(`### Build error review :boom:
+> [!NOTE]
+> 2 builds failed
+<details><summary>Failure log: <code>/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-<strong>dep-1</strong>.drv</code></summary>
+
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+
+</details>
+
+<details><summary>Failure log: <code>/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-<strong>dep-3</strong>.drv</code></summary>
+
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+
+</details>
+`);
+
+  expect(summary.logLines.join("\n"))
+    .toStrictEqual(`\u001b[38;2;255;0;0mBuild logs from 2 failures
+The following build logs are also available in the Markdown summary:
+::group::Failed build: /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+::endgroup::
+::group::Failed build: /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+::endgroup::`);
+});
+
+test("Omit some logs if there are too many", async () => {
+  const events = [
+    {
+      v: "1",
+      c: "BuildFailureResponseEventV1",
+      drv: `/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv`,
+      timing: {
+        startTime: new Date(1 * 1000),
+        durationSeconds: 1,
+      },
+    },
+    {
+      v: "1",
+      c: "BuiltPathResponseEventV1",
+      drv: `/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-2.drv`,
+      timing: {
+        startTime: new Date(2 * 1000),
+        durationSeconds: 2,
+      },
+    },
+    {
+      v: "1",
+      c: "BuildFailureResponseEventV1",
+      drv: `/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv`,
+      timing: {
+        startTime: new Date(3 * 1000),
+        durationSeconds: 3,
+      },
+    },
+  ];
+
+  const logMaker = async (drv: string): Promise<string | undefined> => {
+    return `${drv}\n`.repeat(5).trimEnd();
+  };
+
+  const summary: FailureSummary = (await summarizeFailures(
+    events,
+    logMaker,
+    500,
+  ))!;
+
+  expect(summary.markdownLines.join("\n"))
+    .toStrictEqual(`### Build error review :boom:
+> [!NOTE]
+> 2 builds failed
+<details><summary>Failure log: <code>/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-<strong>dep-1</strong>.drv</code></summary>
+
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+
+</details>
+
+
+> [!NOTE]
+> The following failure has been omitted due to GitHub Actions' summary length limitations.
+> The full logs are available in the post-run phase of the Nix Installer Action.
+> * \`/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv\``);
+
+  expect(summary.logLines.join("\n"))
+    .toStrictEqual(`\u001b[38;2;255;0;0mBuild logs from 2 failures
+The following build logs are also available in the Markdown summary:
+::group::Failed build: /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv
+::endgroup::
+The following build logs are NOT available in the Markdown summary:
+::group::Failed build: /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+    /nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-3.drv
+::endgroup::`);
+});
@@ -0,0 +1,124 @@
+import { getExecOutput } from "@actions/exec";
+import { DEvent } from "./events.js";
+import { stripVTControlCharacters } from "node:util";
+
+// CI summaries have a max length of "1024k" which I assume to be 1048576 bytes.
+// Generously, the mermaid doc is about 50,000 bytes.
+// Rounding it all down a bit further for wiggle room, that leaves lots of log space.
+const defaultMaxSummaryLength = 995_000;
+
+export function getBuildFailures(events: DEvent[]): DEvent[] {
+  return events.filter((event: DEvent): Boolean => {
+    return event.c === "BuildFailureResponseEventV1";
+  });
+}
+
+export interface FailureSummary {
+  logLines: string[];
+  markdownLines: string[];
+}
+
+export async function summarizeFailures(
+  events: DEvent[],
+  getLog: (drv: string) => Promise<string | undefined> = getLogFromNix,
+  maxLength: number = defaultMaxSummaryLength,
+): Promise<FailureSummary | undefined> {
+  const failures = getBuildFailures(events);
+
+  if (failures.length === 0) {
+    return undefined;
+  }
+
+  const logLines = [];
+  const markdownLines = [];
+
+  logLines.push(
+    `\u001b[38;2;255;0;0mBuild logs from ${failures.length} failure${failures.length === 1 ? "" : "s"}`,
+  );
+  logLines.push(
+    `The following build logs are also available in the Markdown summary:`,
+  );
+  markdownLines.push(`### Build error review :boom:`);
+  markdownLines.push("> [!NOTE]");
+  markdownLines.push(
+    `> ${failures.length} build${failures.length === 1 ? "" : "s"} failed`,
+  );
+
+  const markdownLogChunks: {
+    drv: string;
+    txtLines: string[];
+    mdLines: string[];
+  }[] = [];
+  for (const event of failures) {
+    const markdownLogChunk = [];
+    const txtLogChunk = [];
+    txtLogChunk.push(`::group::Failed build: ${event.drv}`);
+
+    const log =
+      (await getLog(event.drv)) ??
+      "(failure reading the log for this derivation.)";
+    const indented = log.split("\n").map((line) => `    ${line}`);
+
+    markdownLogChunk.push(
+      `<details><summary>Failure log: <code>${event.drv.replace(/^(\/nix[^-]*-)(.*)(\.drv)$/, "$1<strong>$2</strong>$3")}</code></summary>`,
+    );
+    markdownLogChunk.push("");
+
+    for (const line of indented) {
+      txtLogChunk.push(line);
+      markdownLogChunk.push(stripVTControlCharacters(line));
+    }
+    markdownLogChunk.push("");
+    markdownLogChunk.push("</details>");
+    markdownLogChunk.push("");
+
+    markdownLogChunks.push({
+      drv: event.drv,
+      mdLines: markdownLogChunk,
+      txtLines: txtLogChunk,
+    });
+    txtLogChunk.push(`::endgroup::`);
+  }
+
+  const skippedChunks = [];
+
+  // Add markdown log chunks until we exceed the max length
+  let markdownLength = markdownLines.join("\n").length;
+  for (const chunk of markdownLogChunks) {
+    const chunkLength = chunk.mdLines.join("\n").length;
+    if (markdownLength + chunkLength > maxLength) {
+      skippedChunks.push(chunk);
+    } else {
+      logLines.push(...chunk.txtLines);
+      markdownLines.push(...chunk.mdLines);
+      markdownLength += chunkLength;
+    }
+  }
+
+  if (skippedChunks.length > 0) {
+    markdownLines.push(
+      "",
+      "> [!NOTE]",
+      `> The following ${skippedChunks.length === 1 ? "failure has" : "failures have"} been omitted due to GitHub Actions' summary length limitations.`,
+      "> The full logs are available in the post-run phase of the Nix Installer Action.",
+    );
+
+    logLines.push(
+      "The following build logs are NOT available in the Markdown summary:",
+    );
+    for (const chunk of skippedChunks) {
+      markdownLines.push(`> * \`${chunk.drv}\``);
+      logLines.push(...chunk.txtLines);
+    }
+  }
+
+  return { logLines, markdownLines };
+}
+
+async function getLogFromNix(drv: string): Promise<string | undefined> {
+  const output = await getExecOutput("nix", ["log", drv], {
+    silent: true,
+  });
+
+  return output.stdout;
+}
@@ -0,0 +1,38 @@
+import { getExecOutput } from "@actions/exec";
+
+export interface Mismatch {
+  readonly derivation: string;
+  readonly replacement: string;
+}
+
+export interface Fix {
+  readonly line: number;
+  readonly found: string;
+  readonly mismatches: readonly Mismatch[];
+}
+
+export interface FileFix {
+  readonly file: string;
+  readonly fixes: readonly Fix[];
+}
+
+export interface FixHashesOutputV1 {
+  readonly version: "v1";
+  readonly files: readonly FileFix[];
+}
+
+export async function getFixHashes(since: string): Promise<FixHashesOutputV1> {
+  const output = await getExecOutput(
+    "determinate-nixd",
+    ["fix", "hashes", "--json", "--since", since],
+    { silent: true },
+  );
+
+  if (output.exitCode !== 0) {
+    throw new Error(
+      `determinate-nixd fix hashes returned non-zero exit code ${output.exitCode} with the following error output:\n${output.stderr}`,
+    );
+  }
+
+  return JSON.parse(output.stdout);
+}
@@ -0,0 +1,207 @@
+import { mermaidify, makeMermaidReport } from "./mermaid.js";
+import { DEvent, parseEvents } from "./events.js";
+import { expect, test } from "vitest";
+
+/* eslint-disable @typescript-eslint/no-non-null-assertion */
+
+function generateEvents(count: number): DEvent[] {
+  const events: DEvent[] = [];
+
+  for (let i = 0; i < count; i++) {
+    events.push({
+      v: "1",
+      c: "BuiltPathResponseEventV1",
+      drv: `/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-${i}.drv`,
+      timing: {
+        startTime: new Date(i * 1000),
+        durationSeconds: i,
+      },
+    });
+  }
+
+  return events;
+}
+
+test("Empty event list returns no report", () => {
+  const report = makeMermaidReport([]);
+
+  expect(report).toBeUndefined();
+});
+
+test("Create a very large report doc and make sure it is small enough", () => {
+  const report = makeMermaidReport(generateEvents(2500))!;
+
+  // Assert the `.drv` suffix was pruned (1 reference = the NOTE at the end)
+  expect(report.match(/\.drv/g)!.length).equals(1);
+
+  // Assert the `/nix/store` prefix was pruned (1 reference = the NOTE at the end)
+  expect(report.match(/\/nix\/store\//g)!.length).equals(1);
+
+  // Assert that some events were pruned
+  expect(report.match(/dep-/g)!.length).lessThan(2500);
+  expect(report.match(/dep-/g)!.length).greaterThan(1500);
+
+  expect(report).toContain("suffix, and builds that took less than ");
+
+  expect(report.length).lessThan(50200);
+  expect(report.length).greaterThan(49000);
+});
+
+test("Create a medium large report doc and make sure it is small enough", () => {
+  const eventCount = 675;
+  const report = makeMermaidReport(generateEvents(eventCount))!;
+
+  // Assert the `.drv` suffix was pruned (1 reference = the NOTE at the end)
+  expect(report.match(/\.drv/g)!.length).equals(1);
+
+  // Assert the `/nix/store` prefix was pruned (1 reference = the NOTE at the end)
+  expect(report.match(/\/nix\/store\//g)!.length).equals(1);
+
+  // Assert that no lines were pruned
+  expect(report.match(/dep-/g)!.length).toStrictEqual(eventCount);
+
+  expect(report).toContain(
+    "suffixes have been removed to make the graph small enough to render",
+  );
+
+  expect(report.length).lessThan(50200);
+  expect(report.length).greaterThan(18000);
+});
+
+test("Create a small report doc and make sure it isn't pruned", () => {
+  const report = makeMermaidReport(generateEvents(100))!;
+
+  // Assert 100 events have the `.drv` suffix, ie: were not pruned
+  expect(report.match(/\.drv/g)!.length).equals(100);
+
+  // Assert 100 events have the `.drv` suffix, ie: were not pruned
+  expect(report.match(/\/nix\/store\//g)!.length).equals(100);
+
+  expect(report.length).lessThan(50000);
+});
+
+test("Generate a really big report and shrink it", () => {
+  const events = generateEvents(1000);
+
+  const originalLength = mermaidify(events, -1)!.length;
+  const limitedLengthZero = mermaidify(events, 0)!.length;
+  const limitedLengthOne = mermaidify(events, 1)!.length;
+  const limitedLengthTwo = mermaidify(events, 2)!.length;
+
+  expect(originalLength).greaterThan(limitedLengthZero);
+  expect(limitedLengthZero).greaterThan(limitedLengthOne);
+  expect(limitedLengthOne).greaterThan(limitedLengthTwo);
+});
+
+test("Generate a rough report of various length", () => {
+  const { events } = parseEvents([
+    {
+      v: "1",
+      c: "BuiltPathResponseEventV1",
+      drv: "/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-0.drv",
+      outputs: ["/nix/store/qwlgz5da3pfb53gqpgdmazaj9jczrnly-dep-0"],
+      timing: {
+        startTime: "2025-04-11T14:38:02Z",
+        stopTime: "2025-04-11T14:38:05Z",
+        durationSeconds: 0,
+      },
+    },
+    {
+      v: "1",
+      c: "BuiltPathResponseEventV1",
+      drv: "/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv",
+      outputs: ["/nix/store/qwlgz5da3pfb53gqpgdmazaj9jczrnly-dep-1"],
+      timing: {
+        startTime: "2025-04-11T14:38:02Z",
+        stopTime: "2025-04-11T14:38:05Z",
+        durationSeconds: 1,
+      },
+    },
+    {
+      v: "1",
+      c: "BuiltPathResponseEventV1",
+      drv: "/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-2.drv",
+      outputs: ["/nix/store/qwlgz5da3pfb53gqpgdmazaj9jczrnly-dep-2"],
+      timing: {
+        startTime: "2025-04-11T14:38:02Z",
+        stopTime: "2025-04-11T14:38:05Z",
+        durationSeconds: 2,
+      },
+    },
+    {
+      v: "1",
+      c: "BuildFailureResponseEventV1",
+      drv: "/nix/store/ykvbksjqrza2zpj6nkbycrdfwgfdpr8g-hash-mismatch-md5-base16.drv",
+      timing: {
+        startTime: "2025-04-11T14:38:05Z",
+        stopTime: "2025-04-11T14:38:09Z",
+        durationSeconds: 4,
+      },
+    },
+  ]);
+
+  expect(mermaidify(events, -1)).toStrictEqual(`\`\`\`mermaid
+gantt
+    dateFormat X
+    axisFormat %Mm%Ss
+/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-0.drv (0s):d, 0, 0s
+/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-1.drv (1s):d, 0, 1s
+/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-2.drv (2s):d, 0, 2s
+/nix/store/ykvbksjqrza2zpj6nkbycrdfwgfdpr8g-hash-mismatch-md5-base16.drv (4s):crit, 3, 4s
+\`\`\``);
+
+  expect(mermaidify(events, 0)).toStrictEqual(`\`\`\`mermaid
+gantt
+    dateFormat X
+    axisFormat %Mm%Ss
+dep-0 (0s):d, 0, 0s
+dep-1 (1s):d, 0, 1s
+dep-2 (2s):d, 0, 2s
+hash-mismatch-md5-base16 (4s):crit, 3, 4s
+\`\`\``);
+
+  expect(mermaidify(events, 1)).toStrictEqual(`\`\`\`mermaid
+gantt
+    dateFormat X
+    axisFormat %Mm%Ss
+dep-1 (1s):d, 0, 1s
+dep-2 (2s):d, 0, 2s
+hash-mismatch-md5-base16 (4s):crit, 3, 4s
+\`\`\``);
+});
+
+test("Generate a really big report and shrink it", () => {
+  const events = generateEvents(1000);
+
+  const originalLength = mermaidify(events, -1)!.length;
+  const limitedLengthZero = mermaidify(events, 0)!.length;
+  const limitedLengthOne = mermaidify(events, 1)!.length;
+  const limitedLengthTwo = mermaidify(events, 2)!.length;
+
+  expect(originalLength).greaterThan(limitedLengthZero);
+  expect(limitedLengthZero).greaterThan(limitedLengthOne);
+  expect(limitedLengthOne).greaterThan(limitedLengthTwo);
+});
+
+test("Really long builds get multi-unit timestamps", () => {
+  const { events } = parseEvents([
+    {
+      v: "1",
+      c: "BuiltPathResponseEventV1",
+      drv: "/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-0.drv",
+      outputs: ["/nix/store/qwlgz5da3pfb53gqpgdmazaj9jczrnly-dep-0"],
+      timing: {
+        startTime: "2025-04-11T14:38:02Z",
+        stopTime: "2026-05-14T13:32:01Z",
+        durationSeconds: 34383239,
+      },
+    },
+  ]);
+
+  expect(mermaidify(events, -1)).toStrictEqual(`\`\`\`mermaid
+gantt
+    dateFormat X
+    axisFormat %Mm%Ss
+/nix/store/rz9hrpay90sjrid5hx3x8v606ji679xa-dep-0.drv (573053m59s):d, 0, 34383239s
+\`\`\``);
+});
@@ -0,0 +1,96 @@
+import { DEvent } from "./events.js";
+import { truncateDerivation } from "./util.js";
+
+export function makeMermaidReport(events: DEvent[]): string | undefined {
+  // # 50k is the max: https://github.com/mermaid-js/mermaid/blob/c269dc822c528e1afbde34e18a1cad03d972d4fe/src/defaultConfig.js#L55
+  const maxLength = 49900;
+  let mermaid = "";
+  let pruneLevel = -2;
+
+  do {
+    pruneLevel += 1;
+    mermaid = mermaidify(events, pruneLevel) ?? "";
+  } while (mermaid.length > maxLength);
+
+  if (!mermaid) {
+    return undefined;
+  }
+
+  const lines = [
+    "<details open><summary><strong>Build timeline</strong> :hourglass_flowing_sand:</summary>",
+    "", // load bearing whitespace, deleting it breaks the details expander / markdown
+    mermaid,
+    "", // load bearing whitespace, deleting it breaks the details expander / markdown
+  ];
+
+  if (pruneLevel === 0) {
+    lines.push("> [!NOTE]");
+    lines.push(
+      "> `/nix/store/[hash]` and the `.drv` suffixes have been removed to make the graph small enough to render.",
+    );
+  } else if (pruneLevel > 0) {
+    lines.push("> [!NOTE]");
+    lines.push(
+      `> \`/nix/store/[hash]\`, the \`.drv\` suffix, and builds that took less than ${formatDuration(pruneLevel)} have been removed to make the graph small enough to render.`,
+    );
+  }
+
+  lines.push(""); // load bearing whitespace, deleting it breaks the details expander / markdown
+  lines.push("</details>");
+
+  return lines.join("\n");
+}
+
+export function mermaidify(
+  allEvents: DEvent[],
+  pruneLevel: number,
+): string | undefined {
+  const events = allEvents
+    .filter(
+      (event) =>
+        event.c === "BuiltPathResponseEventV1" ||
+        event.c === "BuildFailureResponseEventV1",
+    )
+    .sort(
+      (a, b) => a.timing.startTime.getTime() - b.timing.startTime.getTime(),
+    );
+
+  const firstEvent = events.at(0);
+  if (firstEvent === undefined) {
+    return undefined;
+  }
+
+  const zeroMoment = firstEvent.timing.startTime.getTime();
+
+  const lines = [
+    "```mermaid",
+    "gantt",
+    "    dateFormat X",
+    "    axisFormat %Mm%Ss",
+  ];
+
+  for (const event of events) {
+    const duration = event.timing.durationSeconds;
+    if (duration < pruneLevel) {
+      continue;
+    }
+
+    const label = pruneLevel >= 0 ? truncateDerivation(event.drv) : event.drv;
+    const tag = event.c === "BuildFailureResponseEventV1" ? "crit" : "d";
+    const relativeStartTime =
+      (event.timing.startTime.getTime() - zeroMoment) / 1000;
+
+    lines.push(
+      `${label} (${formatDuration(duration)}):${tag}, ${relativeStartTime}, ${duration}s`,
+    );
+  }
+  lines.push("```");
+
+  return lines.join("\n");
+}
+
+function formatDuration(duration: number): string {
+  const durSeconds = duration % 60;
+  const durMinutes = (duration - durSeconds) / 60;
+  return `${durMinutes > 0 ? `${durMinutes}m` : ""}${durSeconds}s`;
+}
@@ -0,0 +1,3 @@
+export function truncateDerivation(drv: string): string {
+  return drv.replace(/^\/nix\/store\/[a-z0-9]+-/, "").replace(/\.drv$/, "");
+}
@@ -1,6 +1,6 @@
 {
  "compilerOptions": {
-    "target": "ES2020" /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', 'ES2018', 'ES2019' or 'ESNEXT'. */,
+    "target": "ES2022" /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', 'ES2018', 'ES2019' or 'ESNEXT'. */,
    "module": "Node16",
    "moduleResolution": "NodeNext",
    "outDir": "./dist",
@@ -11,5 +11,5 @@
    "resolveJsonModule": true,
    "declaration": true
  },
-  "exclude": ["node_modules", "**/*.test.ts", "dist"]
+  "exclude": ["node_modules", "dist"]
 }