diff --git a/apps/server/src/auth/http.ts b/apps/server/src/auth/http.ts
index 780aaabde25..c23298ba14d 100644
--- a/apps/server/src/auth/http.ts
+++ b/apps/server/src/auth/http.ts
@@ -27,8 +27,10 @@ import { parseAllowedOAuthScope } from "@t3tools/shared/oauthScope";
 import { causeErrorTag } from "@t3tools/shared/observability";
 import * as DateTime from "effect/DateTime";
 import * as Effect from "effect/Effect";
+import * as FileSystem from "effect/FileSystem";
 import { identity } from "effect/Function";
 import * as Layer from "effect/Layer";
+import * as Result from "effect/Result";
 import * as Cookies from "effect/unstable/http/Cookies";
 import * as HttpEffect from "effect/unstable/http/HttpEffect";
 import { HttpServerRequest, HttpServerResponse } from "effect/unstable/http";
@@ -37,7 +39,12 @@ import * as HttpApiBuilder from "effect/unstable/httpapi/HttpApiBuilder";
 import * as EnvironmentAuth from "./EnvironmentAuth.ts";
 import * as SessionStore from "./SessionStore.ts";
 import { traceAuthenticatedRelayRequest, traceRelayRequest } from "../cloud/traceRelayRequest.ts";
-import { deriveAuthClientMetadata } from "./utils.ts";
+import * as ServerConfig from "../config.ts";
+import {
+  decodeDevelopmentSessionCookieName,
+  deriveAuthClientMetadata,
+  planStaleDevelopmentSessionCookieSweep,
+} from "./utils.ts";
 import { verifyRequestDpopProof } from "./dpop.ts";
 
 const CREDENTIAL_RESPONSE_HEADERS = {
@@ -203,6 +210,8 @@ export const authHttpApiLayer = HttpApiBuilder.group(
   Effect.fnUntraced(function* (handlers) {
     const serverAuth = yield* EnvironmentAuth.EnvironmentAuth;
     const sessions = yield* SessionStore.SessionStore;
+    const serverConfig = yield* ServerConfig.ServerConfig;
+    const fileSystem = yield* FileSystem.FileSystem;
 
     return handlers
       .handle(
@@ -228,13 +237,52 @@ export const authHttpApiLayer = HttpApiBuilder.group(
               args.payload.credential,
               deriveAuthClientMetadata({ request }),
             );
+            const requestCookieNames = Object.keys(request.cookies);
+            const cookieNamesToExpire =
+              serverConfig.devUrl === undefined
+                ? []
+                : yield* Effect.gen(function* () {
+                    const stateDirs = new Set(
+                      requestCookieNames.flatMap((name) => {
+                        const decoded = decodeDevelopmentSessionCookieName(name);
+                        return decoded !== null && "stateDir" in decoded ? [decoded.stateDir] : [];
+                      }),
+                    );
+                    const stateDirExistence = new Map(
+                      yield* Effect.all(
+                        Array.from(stateDirs, (stateDir) =>
+                          fileSystem.exists(stateDir).pipe(
+                            Effect.orElseSucceed(() => true),
+                            Effect.map((exists) => [stateDir, exists] as const),
+                          ),
+                        ),
+                        { concurrency: "unbounded" },
+                      ),
+                    );
+                    return planStaleDevelopmentSessionCookieSweep({
+                      ownCookieName: sessions.cookieName,
+                      ownPort: serverConfig.port,
+                      requestCookieNames,
+                      stateDirExists: (stateDir) => stateDirExistence.get(stateDir) ?? true,
+                    });
+                  });
             const sessionCookies = yield* Effect.fromResult(
-              Cookies.set(Cookies.empty, sessions.cookieName, result.sessionToken, {
-                expires: DateTime.toDate(result.response.expiresAt),
-                httpOnly: true,
-                path: "/",
-                sameSite: "lax",
-              }),
+              cookieNamesToExpire.reduce(
+                (cookies, name) =>
+                  Result.flatMap(cookies, (current) =>
+                    Cookies.expireCookie(current, name, {
+                      httpOnly: true,
+                      path: "/",
+                      sameSite: "lax",
+                    }),
+                  ),
+                Cookies.set(Cookies.empty, sessions.cookieName, result.sessionToken, {
+                  expires: DateTime.toDate(result.response.expiresAt),
+                  httpOnly: true,
+                  path: "/",
+                  sameSite: "lax",
+                }),
+              ),
             ).pipe(Effect.catch(() => failEnvironmentInternal("browser_session_cookie_failed")));
 
             yield* HttpEffect.appendPreResponseHandler((_request, response) =>
diff --git a/apps/server/src/auth/utils.test.ts b/apps/server/src/auth/utils.test.ts
index edc58f71131..3ad8fd7d9cb 100644
--- a/apps/server/src/auth/utils.test.ts
+++ b/apps/server/src/auth/utils.test.ts
@@ -1,8 +1,11 @@
 import { describe, expect, it } from "vite-plus/test";
 
 import {
+  base64UrlEncode,
+  decodeDevelopmentSessionCookieName,
   deriveAuthClientMetadata,
   isRemoteReachableHost,
+  planStaleDevelopmentSessionCookieSweep,
   resolveSessionCookieName,
 } from "./utils.ts";
 
@@ -58,24 +61,34 @@ describe("deriveAuthClientMetadata", () => {
 });
 
 describe("session cookie isolation", () => {
-  it("isolates loopback web servers by port and server state", () => {
+  it("isolates loopback web servers by port and encoded server state", () => {
+    const firstStateDir = "/tmp/t3-agent-one";
+    const secondStateDir = "/tmp/t3-agent-two";
     const first = resolveSessionCookieName({
       mode: "web",
       port: 5775,
       host: "127.0.0.1",
-      instanceKey: "/tmp/t3-agent-one",
+      instanceKey: firstStateDir,
       development: true,
     });
     const second = resolveSessionCookieName({
       mode: "web",
       port: 5775,
       host: "127.0.0.1",
-      instanceKey: "/tmp/t3-agent-two",
+      instanceKey: secondStateDir,
       development: true,
     });
 
-    expect(first).toMatch(/^t3_session_5775_[a-f0-9]{12}$/);
-    expect(second).toMatch(/^t3_session_5775_[a-f0-9]{12}$/);
+    expect(first).toBe(`t3_session_5775_${base64UrlEncode(firstStateDir)}`);
+    expect(second).toBe(`t3_session_5775_${base64UrlEncode(secondStateDir)}`);
+    expect(decodeDevelopmentSessionCookieName(first)).toEqual({
+      port: 5775,
+      stateDir: firstStateDir,
+    });
+    expect(decodeDevelopmentSessionCookieName(second)).toEqual({
+      port: 5775,
+      stateDir: secondStateDir,
+    });
     expect(first).not.toBe(second);
   });
 
@@ -113,15 +126,16 @@ describe("session cookie isolation", () => {
   });
 
   it("isolates development servers even when they bind a wildcard host", () => {
+    const stateDir = "/tmp/t3-wildcard-dev";
     expect(
       resolveSessionCookieName({
         mode: "web",
         port: 5775,
         host: "0.0.0.0",
-        instanceKey: "/tmp/t3-wildcard-dev",
+        instanceKey: stateDir,
         development: true,
       }),
-    ).toMatch(/^t3_session_5775_[a-f0-9]{12}$/);
+    ).toBe(`t3_session_5775_${base64UrlEncode(stateDir)}`);
   });
 
   it("classifies loopback aliases separately from remotely reachable hosts", () => {
@@ -133,3 +147,50 @@ describe("session cookie isolation", () => {
     expect(isRemoteReachableHost("192.168.1.50")).toBe(true);
   });
 });
+
+describe("development session cookie decoding", () => {
+  it("classifies encoded state directories, legacy hashes, and unrelated names", () => {
+    const stateDir = "/tmp/t3-agent-state";
+
+    expect(
+      decodeDevelopmentSessionCookieName(`t3_session_5775_${base64UrlEncode(stateDir)}`),
+    ).toEqual({ port: 5775, stateDir });
+    expect(decodeDevelopmentSessionCookieName("t3_session_5775_0123456789ab")).toEqual({
+      port: 5775,
+      legacyHash: "0123456789ab",
+    });
+    expect(decodeDevelopmentSessionCookieName("t3_session")).toBeNull();
+    expect(decodeDevelopmentSessionCookieName("t3_session_5775")).toBeNull();
+    expect(
+      decodeDevelopmentSessionCookieName(`t3_session_5775_${base64UrlEncode("relative/state")}`),
+    ).toBeNull();
+    expect(decodeDevelopmentSessionCookieName("other_5775_0123456789ab")).toBeNull();
+  });
+});
+
+describe("stale development session cookie sweep", () => {
+  it("expires only dead encoded siblings and same-port legacy cookies", () => {
+    const ownCookieName = `t3_session_5775_${base64UrlEncode("/tmp/own")}`;
+    const liveSibling = `t3_session_5776_${base64UrlEncode("/tmp/live")}`;
+    const staleSibling = `t3_session_5777_${base64UrlEncode("/tmp/stale")}`;
+    const samePortLegacy = "t3_session_5775_0123456789ab";
+    const otherPortLegacy = "t3_session_5778_abcdef012345";
+
+    expect(
+      planStaleDevelopmentSessionCookieSweep({
+        ownCookieName,
+        ownPort: 5775,
+        requestCookieNames: [
+          ownCookieName,
+          liveSibling,
+          staleSibling,
+          samePortLegacy,
+          otherPortLegacy,
+          "t3_session",
+          "t3_session_3773",
+        ],
+        stateDirExists: (stateDir) => stateDir === "/tmp/live",
+      }),
+    ).toEqual([staleSibling, samePortLegacy]);
+  });
+});
diff --git a/apps/server/src/auth/utils.ts b/apps/server/src/auth/utils.ts
index 32a6799b01f..fd6da64f6b8 100644
--- a/apps/server/src/auth/utils.ts
+++ b/apps/server/src/auth/utils.ts
@@ -22,6 +22,9 @@ const SESSION_COOKIE_NAME = "t3_session";
  * - **Desktop**, which scans upward from 3773 for a free port and binds
  *   127.0.0.1, so a second instance lands on a different port and the same host.
  *
+ * Dev names encode the state directory so a server can recognize and expire
+ * cookies belonging to worktrees whose state directories no longer exist.
+ *
  * Hosted deployments keep the stable production name: their public port can
  * change between releases, and scoping it would log every user out.
  */
@@ -40,14 +43,68 @@ export function resolveSessionCookieName(input: {
     return SESSION_COOKIE_NAME;
   }
 
-  // Cookies are scoped by host, not port. Loopback development servers need an
-  // instance-specific name or parallel agents overwrite each other's session,
-  // and a server that later reuses the port receives a token signed elsewhere.
-  const instanceHash = NodeCrypto.createHash("sha256")
-    .update(input.instanceKey)
-    .digest("hex")
-    .slice(0, 12);
-  return `${SESSION_COOKIE_NAME}_${input.port}_${instanceHash}`;
+  return `${SESSION_COOKIE_NAME}_${input.port}_${base64UrlEncode(input.instanceKey)}`;
+}
+
+export type DevelopmentSessionCookieName =
+  | { readonly port: number; readonly stateDir: string }
+  | { readonly port: number; readonly legacyHash: string };
+
+export function decodeDevelopmentSessionCookieName(
+  name: string,
+): DevelopmentSessionCookieName | null {
+  const match = /^t3_session_(\d+)_([A-Za-z0-9_-]+)$/.exec(name);
+  const port = Number(match?.[1]);
+  const suffix = match?.[2];
+  if (!Number.isSafeInteger(port) || suffix === undefined) {
+    return null;
+  }
+
+  if (/^[0-9a-f]{12}$/.test(suffix)) {
+    return { port, legacyHash: suffix };
+  }
+
+  const decoded = Encoding.decodeBase64UrlString(suffix);
+  if (Result.isFailure(decoded) || !isAbsolutePathLike(decoded.success)) {
+    return null;
+  }
+  return { port, stateDir: decoded.success };
+}
+
+// POSIX root, Windows drive, or UNC. Format only: the sweep decides existence.
+function isAbsolutePathLike(value: string): boolean {
+  return value.startsWith("/") || /^[A-Za-z]:[\\/]/.test(value) || value.startsWith("\\\\");
+}
+
+/**
+ * Plans cookie expiry from already-observed directory existence. Encoding the
+ * path creates a localhost-only path-existence oracle for pages that craft
+ * cookie names and observe expiry. This is accepted for development servers,
+ * where such a page already runs code on the same machine.
+ */
+export function planStaleDevelopmentSessionCookieSweep(input: {
+  readonly ownCookieName: string;
+  readonly ownPort: number;
+  readonly requestCookieNames: Iterable<string>;
+  readonly stateDirExists: (stateDir: string) => boolean;
+}): ReadonlyArray<string> {
+  const namesToExpire: Array<string> = [];
+  for (const name of input.requestCookieNames) {
+    if (name === input.ownCookieName) {
+      continue;
+    }
+    const decoded = decodeDevelopmentSessionCookieName(name);
+    if (decoded === null) {
+      continue;
+    }
+    if (
+      ("legacyHash" in decoded && decoded.port === input.ownPort) ||
+      ("stateDir" in decoded && !input.stateDirExists(decoded.stateDir))
+    ) {
+      namesToExpire.push(name);
+    }
+  }
+  return namesToExpire;
 }
 
 export function isRemoteReachableHost(host: string | undefined): boolean {
diff --git a/apps/server/src/server.test.ts b/apps/server/src/server.test.ts
index f339ff84fae..b1a351d092a 100644
--- a/apps/server/src/server.test.ts
+++ b/apps/server/src/server.test.ts
@@ -60,6 +60,7 @@ import * as Schema from "effect/Schema";
 import * as Stream from "effect/Stream";
 import * as TestClock from "effect/testing/TestClock";
 import { ChildProcessSpawner } from "effect/unstable/process";
+import * as Cookies from "effect/unstable/http/Cookies";
 import {
   FetchHttpClient,
   HttpBody,
@@ -141,6 +142,7 @@ import * as ReviewService from "./review/ReviewService.ts";
 import * as SourceControlRepositoryService from "./sourceControl/SourceControlRepositoryService.ts";
 import * as ServerSecretStore from "./auth/ServerSecretStore.ts";
 import * as EnvironmentAuth from "./auth/EnvironmentAuth.ts";
+import { resolveSessionCookieName } from "./auth/utils.ts";
 import * as CloudManagedEndpointRuntime from "./cloud/ManagedEndpointRuntime.ts";
 import * as CloudCliTokenManager from "./cloud/CliTokenManager.ts";
 import * as ProcessDiagnostics from "./diagnostics/ProcessDiagnostics.ts";
@@ -1594,6 +1596,80 @@ it.layer(NodeServices.layer)("server router seam", (it) => {
     }).pipe(Effect.provide(NodeHttpServer.layerTest)),
   );
 
+  it.effect("sweeps stale development session cookies when pairing", () =>
+    Effect.gen(function* () {
+      const fileSystem = yield* FileSystem.FileSystem;
+      const liveStateDir = yield* fileSystem.makeTempDirectoryScoped({
+        prefix: "t3-live-dev-cookie-",
+      });
+      const staleStateDir = yield* fileSystem.makeTempDirectory({
+        prefix: "t3-stale-dev-cookie-",
+      });
+      yield* fileSystem.remove(staleStateDir, { recursive: true });
+
+      const config = yield* buildAppUnderTest({
+        config: {
+          mode: "web",
+          port: 5775,
+          devUrl: new URL("http://127.0.0.1:5173"),
+        },
+      });
+      const ownCookieName = resolveSessionCookieName({
+        mode: config.mode,
+        port: config.port,
+        host: config.host,
+        instanceKey: config.stateDir,
+        development: true,
+      });
+      const staleCookieName = resolveSessionCookieName({
+        mode: "web",
+        port: 5776,
+        host: "127.0.0.1",
+        instanceKey: staleStateDir,
+        development: true,
+      });
+      const liveCookieName = resolveSessionCookieName({
+        mode: "web",
+        port: 5777,
+        host: "127.0.0.1",
+        instanceKey: liveStateDir,
+        development: true,
+      });
+      const legacyCookieName = `t3_session_${config.port}_0123456789ab`;
+
+      const { response } = yield* bootstrapBrowserSession(defaultDesktopBootstrapToken, {
+        headers: {
+          cookie: [
+            `${staleCookieName}=stale-token`,
+            `${liveCookieName}=live-token`,
+            `${legacyCookieName}=legacy-token`,
+          ].join("; "),
+        },
+      });
+      const setCookieHeaders = Cookies.toSetCookieHeaders(response.cookies);
+      const ownSetCookie = setCookieHeaders.find((header) =>
+        header.startsWith(`${ownCookieName}=`),
+      );
+      const staleSetCookie = setCookieHeaders.find((header) =>
+        header.startsWith(`${staleCookieName}=`),
+      );
+      const legacySetCookie = setCookieHeaders.find((header) =>
+        header.startsWith(`${legacyCookieName}=`),
+      );
+
+      assert.equal(response.status, 200);
+      assert.isDefined(ownSetCookie);
+      assert.notInclude(ownSetCookie ?? "", "Max-Age=0");
+      for (const expiredCookie of [staleSetCookie, legacySetCookie]) {
+        assert.isDefined(expiredCookie);
+        assert.include(expiredCookie ?? "", "Max-Age=0");
+        assert.include(expiredCookie ?? "", "Path=/");
+        assert.include(expiredCookie ?? "", "HttpOnly");
+      }
+      assert.notInclude(setCookieHeaders.join("\n"), liveCookieName);
+    }).pipe(Effect.provide(NodeHttpServer.layerTest)),
+  );
+
   it.effect("exchanges a bootstrap grant for a scoped bearer access token", () =>
     Effect.gen(function* () {
       yield* buildAppUnderTest();
diff --git a/docs/internals/environment-auth.md b/docs/internals/environment-auth.md
index 5f4f5b6e960..430cdd14392 100644
--- a/docs/internals/environment-auth.md
+++ b/docs/internals/environment-auth.md
@@ -37,6 +37,11 @@ browser session cookie. The cookie is an HTTP transport adapter for the same
 scoped session model; the response never exposes the session secret to browser
 JavaScript.
 
+Loopback development servers use a per-instance cookie name containing the
+server port and a base64url-encoded state directory, so parallel worktrees do
+not overwrite each other's sessions. Pairing also expires cookies whose encoded
+state directories no longer exist, while preserving live sibling worktrees.
+
 ### Bearer Access Token
 
 Non-browser clients use `POST /oauth/token` with an