diff --git a/README.md b/README.md
index 81408bc..23c2c9d 100644
--- a/README.md
+++ b/README.md
@@ -25,6 +25,8 @@ Open [http://localhost:3000](http://localhost:3000).
 | `/`           | Marketing landing page                                           |
 | `/open`       | URL scheme receiver — accepts `?owner`, `?repo`, `?issue` params and triggers `worktree://open?...` |
 
+The `/open` route is the public URL API consumed by the GitHub Action, CLI, and integrations. See [docs/api-versioning.md](docs/api-versioning.md) for the versioning policy and parameter reference.
+
 ## License
 
 MIT
diff --git a/docs/api-versioning.md b/docs/api-versioning.md
new file mode 100644
index 0000000..1a0a687
--- /dev/null
+++ b/docs/api-versioning.md
@@ -0,0 +1,47 @@
+# API Versioning Policy
+
+## Overview
+
+The primary "API surface" of this site is the `/open` URL scheme bridge. External consumers (GitHub Action comments, CI scripts, integrations) construct `/open?...` URLs. Breaking changes to accepted parameters must not invalidate old links.
+
+## Current version: v1 (unversioned)
+
+All existing `/open` URLs are treated as **v1**. The `v` query parameter is optional and defaults to `1`.
+
+### v1 parameter sets
+
+**GitHub issue**
+```
+/open?owner=<owner>&repo=<repo>&issue=<number>
+```
+
+**GitLab issue**
+```
+/open?owner=<owner>&repo=<repo>&gitlab_issue=<number>
+```
+
+**Jira issue**
+```
+/open?owner=<owner>&repo=<repo>&jira_issue_key=<key>&jira_host=<host>
+```
+
+## Versioning convention
+
+| Version | Path             | Status  |
+| ------- | ---------------- | ------- |
+| v1      | `/open`          | Current |
+| v2+     | `/open?v=2&...`  | Future  |
+
+### Rules
+
+1. **Never remove or rename params from an existing version.** Old links must keep working.
+2. **Introduce a new version for breaking changes.** Add `v=2` (or higher) when dropping, renaming, or reinterpreting existing params.
+3. **New optional params are non-breaking.** They may be added to an existing version without a version bump.
+4. **The `v` param must be a positive integer string.** Unknown values return an error state with a clear message.
+
+## Adding a new version
+
+1. Extend the `resolveParams` function in `src/app/open/page.tsx` with a branch for the new version.
+2. Add the new param set to this document under a new `## v<N> parameter sets` section.
+3. Keep the old version branch intact — do not delete it.
+4. Update the version table above.
diff --git a/src/app/open/page.tsx b/src/app/open/page.tsx
index 9dd3d01..4011a14 100644
--- a/src/app/open/page.tsx
+++ b/src/app/open/page.tsx
@@ -3,16 +3,18 @@
 import { useEffect, useRef, useState } from "react";
 import { type IssueParams, buildWorktreeUrl } from "./issue-card";
 import { NoParamsView } from "./no-params-view";
+import { UnknownVersionView } from "./unknown-version-view";
 import { OpeningView } from "./opening-view";
 import { InstallView } from "./install-view";
 import { OpenNav, OpenFooter } from "./open-layout";
 
 type Phase =
-  | "loading"    // reading URL params
-  | "opening"    // scheme triggered, awaiting user confirmation
-  | "success"    // user confirmed it opened
-  | "install"    // user says it didn't open → show install guide
-  | "no-params"; // no valid params in URL
+  | "loading"         // reading URL params
+  | "opening"         // scheme triggered, awaiting user confirmation
+  | "success"         // user confirmed it opened
+  | "install"         // user says it didn't open → show install guide
+  | "no-params"       // no valid params in URL
+  | "unknown-version"; // unrecognized v= param
 
 function get(sp: URLSearchParams, key: string): string {
   const val = sp.get(key);
@@ -42,13 +44,23 @@ function resolveParams(sp: URLSearchParams): IssueParams | null {
   return { kind: "github", owner, repo, issue };
 }
 
+const SUPPORTED_VERSIONS: string[] = ["1"];
+
 export default function OpenPage() {
   const [phase, setPhase] = useState<Phase>("loading");
   const [params, setParams] = useState<IssueParams | null>(null);
+  const [urlVersion, setUrlVersion] = useState<string>("1");
   const schemeTriggered = useRef(false);
 
   useEffect(() => {
     const sp = new URLSearchParams(window.location.search);
+    const raw = sp.get("v");
+    const v = raw !== null ? raw : "1";
+    setUrlVersion(v);
+    if (!SUPPORTED_VERSIONS.includes(v)) {
+      setPhase("unknown-version");
+      return;
+    }
     const p = resolveParams(sp);
     if (!p) { setPhase("no-params"); return; }
     setParams(p);
@@ -71,6 +83,7 @@ export default function OpenPage() {
       <main className="open-main">
         <div className="open-content">
           {phase === "no-params" && <NoParamsView />}
+        {phase === "unknown-version" && <UnknownVersionView version={urlVersion} />}
           {phase === "loading" && (
             <div className="loading-center">
               <div className="spinner-lg anim-spin" />
diff --git a/src/app/open/unknown-version-view.tsx b/src/app/open/unknown-version-view.tsx
new file mode 100644
index 0000000..92d2e17
--- /dev/null
+++ b/src/app/open/unknown-version-view.tsx
@@ -0,0 +1,23 @@
+import Link from "next/link";
+import { AlertTriangle } from "lucide-react";
+
+export function UnknownVersionView({ version }: { version: string }) {
+  return (
+    <div className="no-params-center anim-fade-up">
+      <div className="no-params-icon">
+        <AlertTriangle size={20} color="#9090a8" strokeWidth={1.5} />
+      </div>
+      <h1 className="no-params-title">Unsupported link version</h1>
+      <p className="no-params-body">
+        This link uses{" "}
+        <code className="inline-code">v={version}</code>, which is not
+        supported by this version of the site. The link may have been
+        generated by a newer version of the Worktree runner. Please update
+        your runner to open this workspace.
+      </p>
+      <Link href="/" className="btn-ghost">
+        ← Back to home
+      </Link>
+    </div>
+  );
+}