@@ -53,7 +53,10 @@ import {
 } from '../utils/dev-tls-contract.js';
 // The auth base-URL precedence chain, borrowed from the command that owns it
 // (#16734). ⛔ `dev` declares no chain of its own — see printMcpConnectHint.
-import { resolveAuthBaseUrl } from './serve.js';
+// The development auto-shift and its notice, borrowed from the command that owns
+// them (#26270): this process runs the same walk before it compiles, so the
+// config reads the port the `serve` child will bind.
+import { resolveAuthBaseUrl, getAvailablePort, formatDevPortShiftNotice } from './serve.js';
 import type { ResolvedProjectDatabaseUrl } from '@objectstack/runtime';
 
 /**
@@ -516,7 +519,7 @@ export default class Dev extends Command {
         printKV('Config', configPath, '📂');
       }
 
-      const port = flags.port ?? readEnvWithDeprecation('OS_PORT', 'PORT', { silent: true });
+      let port = flags.port ?? readEnvWithDeprecation('OS_PORT', 'PORT', { silent: true });
 
       // ── dev's own door on the ONE port contract (#12673) ──────────────
       // Everything about the port — the range, the reader, the refusal prose —
@@ -572,6 +575,43 @@ export default class Dev extends Command {
         if (flags.port !== undefined) Object.assign(process.env, portFlagEnv(parseRequestedPort(port) ?? undefined));
       }
 
+      const requestedPort = port ?? '3000';
+
+      // ── The port the server will bind is the port the config reads (#26270) ──
+      // The `serve` child shifts off a busy port, but it did so after the
+      // compile below and its own config load had both read the requested
+      // port. Measured on the showcase with 41310 held: `os dev -p 41310`
+      // served on 41311 while `/api/v1/meta/connector/showcase_status_api`
+      // named 41310 and the self-ping flow answered `400 FLOW_FAILED`.
+      //
+      // So this process runs the child's walk (`getAvailablePort`, from the
+      // same requested port) ahead of the first config evaluation. On a shift
+      // it says so in the child's words, exports the port on the one channel a
+      // config reads (`portFlagEnv`, for the compile, the watch recompiles and
+      // the child), and forwards it as the child's `--port`, so the child binds
+      // it without walking again and every restart keeps it.
+      //
+      // ⚠️ The probe releases the port before the child binds it. A port taken
+      // in that window makes the child shift once more: it says so, and exports
+      // its own port for the config it loads (`serve.ts`), but the artifact
+      // compiled here keeps this port until the next recompile. An exhausted
+      // walk is left to the child, which repeats it and prints why.
+      const askedFor = parseRequestedPort(requestedPort);
+      if (askedFor !== null) {
+        let bound = askedFor;
+        try {
+          bound = await getAvailablePort(askedFor);
+        } catch {
+          // The child's own walk reports this (`formatExhaustedPortSearchNotice`).
+        }
+        if (bound !== askedFor) {
+          // stderr, for the #7915 reason the port refusal above gives.
+          process.stderr.write(`${formatDevPortShiftNotice(askedFor, bound)}\n`);
+          Object.assign(process.env, portFlagEnv(bound));
+          port = String(bound);
+        }
+      }
+
       // Compile only when the boot's artifact is one this command builds — a
       // pinned artifact means "use this, don't rebuild" — and then by default
       // (#26130, `devCompilesAtBoot`).
@@ -822,7 +862,6 @@ export default class Dev extends Command {
       printKV('Database', redactConnectionUrl(effectiveDb), '🗄️');
 
       const binPath = process.argv[1];
-      const requestedPort = port ?? '3000';
 
       // ── Serve child under restart supervision (#5148) ───────────────
       // The watcher below rebuilds dist/objectstack.json, but a running
 
@@ -467,6 +467,26 @@ export function formatExhaustedPortSearchNotice(requestedPort: number, cause: un
   );
 }
 
+/**
+ * The notice for a development auto-shift that succeeded (#12543): the port
+ * asked for, that it could not be had, and the port taken instead.
+ *
+ * One function because two processes print it (#26270): `os serve` when its
+ * own walk shifts, and the `os dev` parent, which runs the same walk before it
+ * compiles so the config reads the port the server will bind. Whichever shifts
+ * says so in these words; the other never shifts on that boot.
+ */
+export function formatDevPortShiftNotice(requestedPort: number, port: number): string {
+  return (
+    '\n'
+    + chalk.yellow(`  ⚠ Port ${requestedPort} is in use — serving on ${port} instead.\n`)
+    + chalk.dim(`     Development auto-shift: ${requestedPort} was not free, so this server took\n`)
+    + chalk.dim(`     the next one that was (${port}). Anything still pointed at ${requestedPort} — a\n`)
+    + chalk.dim('     proxy, an OAuth callback URL, another terminal, a test harness — is\n')
+    + chalk.dim(`     talking to whatever holds ${requestedPort}, not to this server.`)
+  );
+}
+
 /**
  * The port the HTTP server ACTUALLY bound, read off the transport that bound it
  * (#13062).
@@ -2340,14 +2360,16 @@ export default class Serve extends Command {
         // line cannot be swallowed, and it survives a boot that dies later. A
         // drifted port is a plausible CAUSE of such a death, so the notice has
         // to outlive it.
-        printDiagnostic(
-          '\n'
-          + chalk.yellow(`  ⚠ Port ${requestedPort} is in use — serving on ${port} instead.\n`)
-          + chalk.dim(`     Development auto-shift: ${requestedPort} was not free, so this server took\n`)
-          + chalk.dim(`     the next one that was (${port}). Anything still pointed at ${requestedPort} — a\n`)
-          + chalk.dim('     proxy, an OAuth callback URL, another terminal, a test harness — is\n')
-          + chalk.dim(`     talking to whatever holds ${requestedPort}, not to this server.`),
-        );
+        printDiagnostic(formatDevPortShiftNotice(requestedPort, port));
+        // ── …and the shifted port is the port the config reads (#26270) ──
+        // The config is loaded further down in this process, and it may read
+        // `OS_PORT`/`PORT` at module evaluation (the showcase's self URL does),
+        // as may the dev compile a direct `os serve --dev` runs. Without this
+        // they read the port that was busy: measured on the showcase,
+        // `/api/v1/meta/connector/showcase_status_api` named the held port and
+        // the self-ping flow answered `400 FLOW_FAILED`. Exported on the same
+        // two names a port flag is (`portFlagEnv`), so nothing restates them.
+        Object.assign(process.env, portFlagEnv(port));
       }
     } else if (!(await isPortAvailable(requestedPort))) {
       // One write, for the reason spelled out at the "Nothing to serve" exit
 
@@ -425,6 +425,10 @@ export function portTextReadNotice(
  *    watch recompiles and the `serve` child all run in (#26243).
  *  - `os serve` merges it into its own `process.env` before it loads the
  *    config (#26243).
+ *  - A development auto-shift exports the port it shifted to the same way,
+ *    in the `os dev` parent before its compile and in `os serve` before its
+ *    config load, so the config reads the port bound rather than the busy
+ *    one it asked for (#26270).
  *
  * Measured before #26243, on `examples/app-showcase` with no `OS_PORT`/`PORT`:
  * `os dev -p 41302` and `os serve --port 41303` both served