diff --git a/.github/workflows/openapi-version-check.yml b/.github/workflows/openapi-version-check.yml
new file mode 100644
index 0000000..592ffe3
--- /dev/null
+++ b/.github/workflows/openapi-version-check.yml
@@ -0,0 +1,34 @@
+name: OpenAPI Version Check
+
+on:
+  pull_request:
+    paths:
+      - public/openapi.json
+      - CHANGELOG.md
+      - scripts/check-openapi-version.mjs
+      - .github/workflows/openapi-version-check.yml
+
+permissions:
+  contents: read
+
+jobs:
+  openapi-version:
+    name: OpenAPI version bump
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          fetch-depth: 0
+
+      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+        with:
+          node-version: '22'
+
+      - name: Load base OpenAPI spec
+        run: |
+          git fetch --no-tags --depth=1 origin "${{ github.base_ref }}"
+          git show "origin/${{ github.base_ref }}:public/openapi.json" > /tmp/openapi.base.json || echo '{}' > /tmp/openapi.base.json
+
+      - name: Check OpenAPI version policy
+        run: node scripts/check-openapi-version.mjs /tmp/openapi.base.json public/openapi.json
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..e32cde0
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,17 @@
+# OpenAPI Spec Changelog
+
+History of `public/openapi.json` `info.version` bumps. See the
+[spec-versioning policy](CONTRIBUTING.md#openapi-spec-versioning) for when and how
+to bump. Every change to API paths or response schemas gets a one-line entry here;
+the [OpenAPI Version Check](.github/workflows/openapi-version-check.yml) CI job
+enforces that a bump has a matching entry.
+
+## 2.1.0 — 2026-05-21
+
+- Align `/account` response schema with the live flat shape (#230).
+
+## 2.0.0
+
+- Baseline versioned spec. (Note: `/sports/{sportId}` and `/sportsbooks/{bookId}`
+  were removed under this version in #218 without a bump — the gap that motivated
+  the versioning policy in #233.)
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index b9640cb..a640813 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -27,6 +27,39 @@ Before opening a PR:
 
 PRs that fix a single issue are easier to review than batched ones.
 
+## OpenAPI spec versioning
+
+`public/openapi.json` is the published API contract. Some consumers pin against it
+and run CI-graded contract testing, so spec changes must be programmatically
+detectable.
+
+**Bump `info.version` (SemVer) whenever you change `paths` or response schemas:**
+
+| Bump | When |
+|------|------|
+| MAJOR (`x.0.0`) | Backward-incompatible redesign; removed or renamed response field; breaking schema change |
+| MINOR (`2.x.0`) | New path or field; a shape fix that aligns the spec to the live response. **Removed paths bump the MINOR at minimum.** |
+| PATCH (`2.1.x`) | Description-only edits, examples, doc clarifications |
+
+**Enforcement.** [`.github/workflows/openapi-version-check.yml`](.github/workflows/openapi-version-check.yml)
+runs on every PR that touches `public/openapi.json`. It fails if paths/schemas
+changed without an `info.version` bump, and if a bump has no matching
+[`CHANGELOG.md`](CHANGELOG.md) entry. Run it locally before pushing:
+
+```bash
+git show "origin/main:public/openapi.json" > /tmp/openapi.base.json
+node scripts/check-openapi-version.mjs /tmp/openapi.base.json public/openapi.json
+```
+
+**Consumer signal.** Clients can poll the lightweight sidecar at
+[`https://docs.sharpapi.io/openapi-version.json`](https://docs.sharpapi.io/openapi-version.json)
+(`{ "version", "x-generated-at", "x-commit-sha" }`) to detect changes without
+downloading the full spec. The same provenance is also stamped into
+`info["x-generated-at"]` / `info["x-commit-sha"]` inside `openapi.json` at build time.
+
+**History.** Each bump gets a one-line entry in [`CHANGELOG.md`](CHANGELOG.md);
+deploys additionally cut a dated GitHub Release.
+
 ## Style
 
 - **Tone**: terse and concrete. Show the request, the response, and the interesting details. Skip filler.
diff --git a/public/openapi-version.json b/public/openapi-version.json
new file mode 100644
index 0000000..c11f84f
--- /dev/null
+++ b/public/openapi-version.json
@@ -0,0 +1,5 @@
+{
+  "version": "2.1.0",
+  "x-generated-at": "2026-05-20T23:30:42-04:00",
+  "x-commit-sha": "13f97e3"
+}
diff --git a/scripts/check-openapi-version.mjs b/scripts/check-openapi-version.mjs
new file mode 100644
index 0000000..0d50896
--- /dev/null
+++ b/scripts/check-openapi-version.mjs
@@ -0,0 +1,98 @@
+#!/usr/bin/env node
+// Enforces the OpenAPI spec-versioning policy (CONTRIBUTING.md, issue #233):
+// fails CI when public/openapi.json's paths or response schemas change without
+// an info.version bump, and when a version bump lacks a CHANGELOG.md entry.
+// Compares info only by version — the build stamps info["x-generated-at"] /
+// info["x-commit-sha"], which must not count as a semantic change.
+//
+// Usage: node scripts/check-openapi-version.mjs <base-spec.json> <head-spec.json>
+
+import { readFileSync, existsSync } from 'node:fs'
+
+const [, , basePath, headPath] = process.argv
+if (!basePath || !headPath) {
+  console.error('usage: check-openapi-version.mjs <base-spec> <head-spec>')
+  process.exit(2)
+}
+
+function load (p) {
+  try {
+    const raw = readFileSync(p, 'utf8').trim()
+    return raw ? JSON.parse(raw) : null
+  } catch (err) {
+    console.error(`[openapi-version] cannot parse ${p}: ${err.message}`)
+    process.exit(2)
+  }
+}
+
+// Stable stringify: recursively sort object keys so key-order churn isn't
+// mistaken for a semantic change.
+function canonical (value) {
+  if (Array.isArray(value)) return value.map(canonical)
+  if (value && typeof value === 'object') {
+    return Object.keys(value).sort().reduce((acc, k) => {
+      acc[k] = canonical(value[k])
+      return acc
+    }, {})
+  }
+  return value
+}
+
+function semantic (spec) {
+  return JSON.stringify({
+    paths: canonical(spec?.paths ?? {}),
+    schemas: canonical(spec?.components?.schemas ?? {})
+  })
+}
+
+const base = load(basePath)
+const head = load(headPath)
+
+if (!head) {
+  console.error('[openapi-version] head spec missing or empty')
+  process.exit(2)
+}
+
+// New spec (no base on the target branch) — nothing to compare against.
+if (!base || Object.keys(base).length === 0) {
+  console.log('[openapi-version] no base spec to compare; skipping.')
+  process.exit(0)
+}
+
+const baseVersion = base?.info?.version
+const headVersion = head?.info?.version
+
+if (semantic(base) === semantic(head)) {
+  console.log('[openapi-version] no path/schema changes detected; OK.')
+  process.exit(0)
+}
+
+if (baseVersion === headVersion) {
+  console.error(
+    `[openapi-version] paths or schemas changed but info.version is still ${headVersion}.\n` +
+    '  Bump info.version per the SemVer policy in CONTRIBUTING.md:\n' +
+    '    MAJOR (x.0.0)  backward-incompatible redesign / removed-renamed field\n' +
+    '    MINOR (2.x.0)  new path or field, or a shape fix aligning the spec to the live response\n' +
+    '    PATCH (2.1.x)  description-only edits\n' +
+    '  Removed paths and renamed fields bump the MINOR at minimum.'
+  )
+  process.exit(1)
+}
+
+// Version bumped — require a CHANGELOG.md entry for the new version.
+const CHANGELOG = 'CHANGELOG.md'
+if (existsSync(CHANGELOG)) {
+  const log = readFileSync(CHANGELOG, 'utf8')
+  const escaped = headVersion.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')
+  const hasEntry = new RegExp(`^##\\s.*${escaped}(\\s|$)`, 'm').test(log)
+  if (!hasEntry) {
+    console.error(
+      `[openapi-version] info.version bumped ${baseVersion} -> ${headVersion} but ` +
+      `CHANGELOG.md has no "## ... ${headVersion}" entry. Add one describing the change.`
+    )
+    process.exit(1)
+  }
+}
+
+console.log(`[openapi-version] info.version ${baseVersion} -> ${headVersion}; OK.`)
+process.exit(0)
diff --git a/scripts/stamp-openapi.mjs b/scripts/stamp-openapi.mjs
index b8bf8f1..83578a2 100644
--- a/scripts/stamp-openapi.mjs
+++ b/scripts/stamp-openapi.mjs
@@ -10,11 +10,15 @@
 //
 // Both fields are advisory — they don't affect the OpenAPI semantics. Tools
 // that don't recognise the `x-` extensions ignore them.
+//
+// It also emits public/openapi-version.json — a tiny sidecar consumers can poll
+// to detect spec changes without downloading the full spec (issue #233).
 
 import { execSync } from 'node:child_process'
 import { readFileSync, writeFileSync } from 'node:fs'
 
 const SPEC_PATH = 'public/openapi.json'
+const VERSION_SIDECAR_PATH = 'public/openapi-version.json'
 
 function git (args) {
   try {
@@ -39,3 +43,11 @@ spec.info['x-commit-sha'] = commitSHA
 
 writeFileSync(SPEC_PATH, JSON.stringify(spec, null, 2) + '\n')
 console.log(`[stamp-openapi] ${SPEC_PATH} → x-generated-at=${commitISO} x-commit-sha=${commitSHA}`)
+
+const versionSidecar = {
+  version: spec.info.version,
+  'x-generated-at': commitISO,
+  'x-commit-sha': commitSHA
+}
+writeFileSync(VERSION_SIDECAR_PATH, JSON.stringify(versionSidecar, null, 2) + '\n')
+console.log(`[stamp-openapi] ${VERSION_SIDECAR_PATH} → version=${spec.info.version}`)