diff --git a/.github/workflows/compatibility-fixtures.yml b/.github/workflows/compatibility-fixtures.yml
new file mode 100644
index 0000000..9055cb9
--- /dev/null
+++ b/.github/workflows/compatibility-fixtures.yml
@@ -0,0 +1,29 @@
+name: Compatibility fixtures
+
+on:
+  push:
+    branches:
+      - main
+  schedule:
+    - cron: "23 7 * * *"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  compare:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1
+        with:
+          repository: cardmagic/solid-objects-ruby
+          ref: main
+          path: solid-objects-ruby
+          sparse-checkout: compatibility
+      - uses: actions/setup-node@249970729cb0ef3589644e2896645e5dc5ba9c38
+        with:
+          node-version: "24.15.0"
+      - run: node scripts/check-compatibility-fixtures.mjs compatibility solid-objects-ruby/compatibility
diff --git a/CHANGELOG.md b/CHANGELOG.md
index f69e33c..9b52921 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,19 @@
 # Changelog
 
+## Unreleased
+
+- Add `scripts/check-compatibility-fixtures.mjs` and a workflow with the name
+  `Compatibility fixtures`. The script compares the `*.json` files in two
+  directories by name and by bytes. It fails with one line for each file that
+  is missing on one side or different. The workflow runs the script on this
+  repository's `compatibility/` directory and on the `compatibility/` directory
+  of solid-objects-ruby at `main`. It runs on each push to `main`, one time
+  each day, and on demand. Before this change, no job compared the shared
+  fixtures.
+- `docs/parity.md` now cites Ruby `solid_objects` 0.17.3. That Ruby release
+  adds guides, tests, and one development dependency, and does not change the
+  runtime, so the ledger has no new row.
+
 ## 0.17.6 - 2026-10-09
 
 - Add three problem guides in `docs/guides/`. Each one reproduces a failure,
diff --git a/docs/parity.md b/docs/parity.md
index 05a6749..26e2080 100644
--- a/docs/parity.md
+++ b/docs/parity.md
@@ -4,7 +4,7 @@ This ledger tracks capability parity with the Ruby `solid_objects` gem.
 Parity preserves a capability and its correctness or security boundary. It does
 not copy a Rails API into Node.
 
-Reference: Ruby `solid_objects` 0.17.2. The JavaScript package began at the Ruby
+Reference: Ruby `solid_objects` 0.17.3. The JavaScript package began at the Ruby
 design's `0.12` capability generation; that version number did not imply earlier
 JavaScript releases.
 
@@ -284,6 +284,14 @@ and Rails to Node) ran in solid-objects-ruby#49; the one disagreement it
 found (the optional `arguments` default) is fixed and pinned by the shared
 fixture.
 
+The `Compatibility fixtures` workflow compares each `compatibility/*.json` file
+with the same file in solid-objects-ruby at `main`, byte for byte. It runs on
+each push to `main`, one time each day, and on demand. It does not run on a
+pull request, because the other repository holds the old fixture until the two
+pull requests of a paired change merge. Run the same check on a local clone of
+the Ruby repository with
+`node scripts/check-compatibility-fixtures.mjs compatibility ../solid-objects-ruby/compatibility`.
+
 ## JavaScript-only: Cloudflare hosting
 
 The experimental `solid-objects/cloudflare` backend hosts each actor identity
diff --git a/scripts/check-compatibility-fixtures.mjs b/scripts/check-compatibility-fixtures.mjs
new file mode 100644
index 0000000..18da5a5
--- /dev/null
+++ b/scripts/check-compatibility-fixtures.mjs
@@ -0,0 +1,53 @@
+import fs from "node:fs"
+import path from "node:path"
+import process from "node:process"
+
+const directories = process.argv.slice(2)
+
+if (directories.length !== 2) {
+  process.stderr.write(
+    "usage: node scripts/check-compatibility-fixtures.mjs <directory> <directory>\n",
+  )
+  process.exit(1)
+}
+
+const absentDirectories = directories.filter(
+  (directory) => fs.statSync(directory, { throwIfNoEntry: false })?.isDirectory() !== true,
+)
+
+if (absentDirectories.length > 0) {
+  process.stderr.write(
+    `${absentDirectories.map((directory) => `${directory} is not a directory`).join("\n")}\n`,
+  )
+  process.exit(1)
+}
+
+const fixtureNames = [
+  ...new Set(
+    directories.flatMap((directory) =>
+      fs.readdirSync(directory).filter((name) => name.endsWith(".json")),
+    ),
+  ),
+].sort()
+const differences = fixtureNames.flatMap(findDifferences)
+
+if (differences.length > 0) {
+  process.stderr.write(`${differences.join("\n")}\n`)
+  process.exitCode = 1
+}
+
+function findDifferences(name) {
+  const directoriesWithoutFile = directories.filter(
+    (directory) => !fs.existsSync(path.join(directory, name)),
+  )
+  if (directoriesWithoutFile.length > 0) {
+    return directoriesWithoutFile.map((directory) => `${name} is missing from ${directory}`)
+  }
+
+  const [first, second] = directories.map((directory) =>
+    fs.readFileSync(path.join(directory, name)),
+  )
+  if (first.equals(second)) return []
+
+  return [`${name} is different in ${directories.join(" and ")}`]
+}
diff --git a/test/check-compatibility-fixtures.test.ts b/test/check-compatibility-fixtures.test.ts
new file mode 100644
index 0000000..71687ce
--- /dev/null
+++ b/test/check-compatibility-fixtures.test.ts
@@ -0,0 +1,101 @@
+import { spawnSync } from "node:child_process"
+import fs from "node:fs"
+import os from "node:os"
+import path from "node:path"
+import { afterEach, describe, expect, it } from "vitest"
+
+const scriptPath = path.resolve("scripts/check-compatibility-fixtures.mjs")
+
+type Fixtures = Record<string, string>
+
+const fixtures: Fixtures = {
+  "json-values.json": '{"cases":[]}\n',
+  "transmit-envelopes.json": '{"version":1}\n',
+}
+
+let workspace: string | undefined
+
+afterEach(() => {
+  if (workspace !== undefined) fs.rmSync(workspace, { recursive: true, force: true })
+  workspace = undefined
+})
+
+describe("compatibility fixture check", () => {
+  it("passes for equal sets", () => {
+    const result = compare({ javascript: fixtures, ruby: fixtures })
+
+    expect(result.stderr).toBe("")
+    expect(result.status).toBe(0)
+  })
+
+  it("fails for a changed byte", () => {
+    const result = compare({
+      javascript: fixtures,
+      ruby: { ...fixtures, "json-values.json": '{"cases":[]}' },
+    })
+
+    expect(result.status).not.toBe(0)
+    expect(result.stderr).toContain("json-values.json is different")
+    expect(result.stderr).not.toContain("transmit-envelopes.json")
+  })
+
+  it("fails for a file present on one side only", () => {
+    const result = compare({
+      javascript: { ...fixtures, "sync-timeout.json": "{}\n" },
+      ruby: { ...fixtures, "telemetry-events.json": "{}\n" },
+    })
+
+    expect(result.status).not.toBe(0)
+    expect(result.stderr).toContain(`sync-timeout.json is missing from ${directoryFor("ruby")}`)
+    expect(result.stderr).toContain(
+      `telemetry-events.json is missing from ${directoryFor("javascript")}`,
+    )
+  })
+
+  it("fails for a missing directory", () => {
+    writeFixtures({ name: "javascript", files: fixtures })
+    const result = runScript([directoryFor("javascript"), directoryFor("ruby")])
+
+    expect(result.status).not.toBe(0)
+    expect(result.stderr).toContain(`${directoryFor("ruby")} is not a directory`)
+  })
+
+  it("ignores a file that is not JSON", () => {
+    const result = compare({ javascript: { ...fixtures, "README.md": "notes\n" }, ruby: fixtures })
+
+    expect(result.stderr).toBe("")
+    expect(result.status).toBe(0)
+  })
+
+  it("fails when a directory is not given", () => {
+    const result = runScript([])
+
+    expect(result.status).not.toBe(0)
+    expect(result.stderr).toContain("usage:")
+  })
+})
+
+function compare({ javascript, ruby }: { javascript: Fixtures; ruby: Fixtures }) {
+  writeFixtures({ name: "javascript", files: javascript })
+  writeFixtures({ name: "ruby", files: ruby })
+
+  return runScript([directoryFor("javascript"), directoryFor("ruby")])
+}
+
+function runScript(directories: string[]) {
+  return spawnSync(process.execPath, [scriptPath, ...directories], { encoding: "utf8" })
+}
+
+function writeFixtures({ name, files }: { name: string; files: Fixtures }) {
+  const directory = directoryFor(name)
+  fs.mkdirSync(directory)
+  for (const [fileName, content] of Object.entries(files)) {
+    fs.writeFileSync(path.join(directory, fileName), content)
+  }
+}
+
+function directoryFor(name: string) {
+  workspace ??= fs.mkdtempSync(path.join(os.tmpdir(), "compatibility-fixtures-"))
+
+  return path.join(workspace, name)
+}