diff --git a/RUNNING.md b/RUNNING.md
index 82de3e832..4c4b55238 100644
--- a/RUNNING.md
+++ b/RUNNING.md
@@ -47,8 +47,14 @@ working instance of X" — read them before inventing a boot path.
   scenario source as `test.ts`.
 - `cd e2e && bun run serve` builds the viewer and serves the scenario ×
   target matrix over HTTP, bound to all interfaces (reachable over the
-  tailnet). Individual runs are at `#/<target>/<slug>` hash routes — when
-  handing results to the user, link those directly, not the bare matrix.
+  tailnet). It prefers port 8901 but walks forward to the next free port if
+  that's taken (so concurrent worktrees, or a leaked previous viewer, never
+  wedge each other) — read the printed `e2e viewer → …` URL for the actual
+  port. `PORT=…` pins a port explicitly and fails loudly if it's busy. The
+  built SPA is port- and mount-agnostic (relative assets + hash routing), so
+  whatever port it lands on just works. Individual runs are at
+  `#/<target>/<slug>` hash routes — when handing results to the user, link
+  those directly, not the bare matrix.
 - `bun e2e/scripts/pr-media.ts e2e/runs/<target>/<slug>` converts a run's
   recording to a gif, uploads it to the `e2e-media` branch, and prints
   PR-ready markdown.
diff --git a/e2e/scripts/serve.ts b/e2e/scripts/serve.ts
index 46b78f37f..a1f53eb97 100644
--- a/e2e/scripts/serve.ts
+++ b/e2e/scripts/serve.ts
@@ -1,15 +1,24 @@
 // Static server for runs/ — the review URL. Supports range requests so the
 // session videos seek/stream, gzips text assets, and marks vite's hashed
 // /assets/ as immutable so Monaco/React chunks download once, ever.
-// `bun e2e/scripts/serve.ts` → http://host:8901
+// `bun e2e/scripts/serve.ts` → prints the bound URL (default port 8901, but
+// it walks forward to the next free port if that's taken, so two worktrees —
+// or a leaked previous viewer — never wedge each other). `PORT=…` pins a port
+// explicitly and fails loudly if it's busy (the strictPort rule from
+// src/ports.ts). The SPA itself is port- and mount-agnostic (relative assets +
+// hash routing), so any port the server lands on just works in the browser.
 import { createReadStream, existsSync, statSync } from "node:fs";
 import { createServer } from "node:http";
+import type { AddressInfo } from "node:net";
 import { extname, join, normalize } from "node:path";
 import { fileURLToPath } from "node:url";
 import { createGzip } from "node:zlib";
 
 const ROOT = fileURLToPath(new URL("../runs/", import.meta.url));
-const PORT = Number(process.env.PORT ?? 8901);
+// Explicit PORT pins (and fails visibly if busy); otherwise 8901 is just a
+// starting preference we walk forward from.
+const PINNED = process.env.PORT !== undefined;
+const PREFERRED = Number(process.env.PORT ?? 8901);
 
 const MIME: Record<string, string> = {
   ".html": "text/html; charset=utf-8",
@@ -27,7 +36,7 @@ const MIME: Record<string, string> = {
 
 const COMPRESSIBLE = new Set([".html", ".js", ".css", ".map", ".svg", ".json", ".ts"]);
 
-createServer((req, res) => {
+const server = createServer((req, res) => {
   const url = new URL(req.url ?? "/", "http://x");
   let path = normalize(decodeURIComponent(url.pathname)).replace(/^([/\\])+/, "");
   if (path === "" || path === ".") path = "index.html";
@@ -78,4 +87,30 @@ createServer((req, res) => {
   }
   res.writeHead(200, { "content-type": type, "content-length": size, "accept-ranges": "bytes" });
   createReadStream(file).pipe(res);
-}).listen(PORT, () => console.log(`e2e viewer → http://localhost:${PORT}/`));
+});
+
+// Host omitted → bind every interface (reachable over the tailnet). On a busy
+// port: a pinned PORT is a hard error (predictable, matches --strictPort); an
+// unpinned default walks forward to the next free port instead of crashing.
+const MAX_WALK = 50;
+const listen = (port: number, attempt = 0): void => {
+  server.once("error", (err: NodeJS.ErrnoException) => {
+    if (err.code !== "EADDRINUSE") throw err;
+    if (PINNED) {
+      console.error(`e2e viewer: PORT=${port} is in use — free it or pick another port.`);
+      process.exit(1);
+    }
+    if (attempt >= MAX_WALK) {
+      console.error(`e2e viewer: no free port found in ${PREFERRED}..${PREFERRED + MAX_WALK}.`);
+      process.exit(1);
+    }
+    console.warn(`e2e viewer: port ${port} in use, trying ${port + 1}…`);
+    listen(port + 1, attempt + 1);
+  });
+  server.listen(port, () => {
+    const actual = (server.address() as AddressInfo).port;
+    console.log(`e2e viewer → http://localhost:${actual}/`);
+  });
+};
+
+listen(PREFERRED);