diff --git a/ui/src/app/impersonate/route.test.ts b/ui/src/app/impersonate/route.test.ts
new file mode 100644
index 00000000..472c5e7d
--- /dev/null
+++ b/ui/src/app/impersonate/route.test.ts
@@ -0,0 +1,227 @@
+// @vitest-environment node
+import { NextRequest } from "next/server";
+import { describe, expect, it, vi } from "vitest";
+
+import { GET } from "./route";
+
+const PROJECT_ID = "proj-123";
+
+vi.mock("@/lib/auth/config", () => ({
+    getStackConfig: vi.fn(async () => ({
+        projectId: "proj-123",
+        publishableClientKey: "pck_test",
+    })),
+}));
+
+function makeRequest(
+    url: string,
+    { cookie, forwardedProto }: { cookie?: string; forwardedProto?: string } = {},
+) {
+    const headers = new Headers();
+    if (cookie) headers.set("cookie", cookie);
+    if (forwardedProto) headers.set("x-forwarded-proto", forwardedProto);
+    return new NextRequest(url, { headers });
+}
+
+function parseSetCookie(header: string) {
+    const [nameValue, ...attrParts] = header.split("; ");
+    const eq = nameValue.indexOf("=");
+    const attrs = attrParts.map((a) => a.toLowerCase());
+    return {
+        name: nameValue.slice(0, eq),
+        value: decodeURIComponent(nameValue.slice(eq + 1)),
+        partitioned: attrs.includes("partitioned"),
+        secure: attrs.includes("secure"),
+        maxAge: Number(
+            attrParts.find((a) => a.toLowerCase().startsWith("max-age="))?.slice(8),
+        ),
+        domain: attrParts
+            .find((a) => a.toLowerCase().startsWith("domain="))
+            ?.slice(7),
+    };
+}
+
+describe("GET /impersonate", () => {
+    it("returns 400 when refresh_token is missing", async () => {
+        const response = await GET(
+            makeRequest("https://app.dograh.com/impersonate"),
+        );
+        expect(response.status).toBe(400);
+    });
+
+    it("redirects to redirect_path on the same origin", async () => {
+        const response = await GET(
+            makeRequest(
+                "https://app.dograh.com/impersonate?refresh_token=rt-new&redirect_path=/workflow/42",
+            ),
+        );
+        expect(response.status).toBe(307);
+        expect(response.headers.get("location")).toBe(
+            "https://app.dograh.com/workflow/42",
+        );
+    });
+
+    it("falls back to /workflow/create for a cross-origin redirect_path", async () => {
+        const response = await GET(
+            makeRequest(
+                "https://app.dograh.com/impersonate?refresh_token=rt-new&redirect_path=https://evil.com/phish",
+            ),
+        );
+        expect(response.headers.get("location")).toBe(
+            "https://app.dograh.com/workflow/create",
+        );
+    });
+
+    it("falls back to /workflow/create for a malformed redirect_path", async () => {
+        const response = await GET(
+            makeRequest(
+                "https://app.dograh.com/impersonate?refresh_token=rt-new&redirect_path=https%3A%2F%2F",
+            ),
+        );
+        expect(response.status).toBe(307);
+        expect(response.headers.get("location")).toBe(
+            "https://app.dograh.com/workflow/create",
+        );
+    });
+
+    it("clears presented session cookies in both jars and all domain scopes on https", async () => {
+        const hostRefresh = `__Host-hexclave-refresh-${PROJECT_ID}--default`;
+        const response = await GET(
+            makeRequest(
+                "https://app.dograh.com/impersonate?refresh_token=rt-new",
+                {
+                    cookie: [
+                        `${hostRefresh}=old-session`,
+                        "hexclave-access=old-access",
+                        `stack-refresh-${PROJECT_ID}=old-legacy`,
+                        `hexclave-refresh-${PROJECT_ID}--custom-abc=old-custom`,
+                        "stack-oauth-inner-xyz=oauth-state",
+                        "hexclave-is-https=true",
+                        "theme=dark",
+                    ].join("; "),
+                },
+            ),
+        );
+        const cookies = response.headers.getSetCookie().map(parseSetCookie);
+
+        // Deletions exist for both jars for a regular-named cookie.
+        const accessDeletions = cookies.filter(
+            (c) => c.name === "hexclave-access" && c.maxAge === 0,
+        );
+        expect(accessDeletions.some((c) => c.partitioned)).toBe(true);
+        expect(accessDeletions.some((c) => !c.partitioned)).toBe(true);
+        // ...and for the host-only plus each parent-domain scope.
+        const accessDomains = new Set(accessDeletions.map((c) => c.domain));
+        expect(accessDomains).toEqual(
+            new Set([undefined, "app.dograh.com", "dograh.com"]),
+        );
+
+        // Enumerated --custom-* cookies are cleared too.
+        expect(
+            cookies.some(
+                (c) =>
+                    c.name === `hexclave-refresh-${PROJECT_ID}--custom-abc` &&
+                    c.maxAge === 0 &&
+                    c.partitioned,
+            ),
+        ).toBe(true);
+
+        // __Host- deletions never carry a Domain attribute but still cover both jars.
+        const hostDeletions = cookies.filter(
+            (c) => c.name === hostRefresh && c.maxAge === 0,
+        );
+        expect(hostDeletions.length).toBeGreaterThan(0);
+        expect(hostDeletions.every((c) => c.domain === undefined)).toBe(true);
+        expect(hostDeletions.some((c) => c.partitioned)).toBe(true);
+
+        // The legacy raw refresh cookie is only ever deleted, never re-set.
+        expect(
+            cookies
+                .filter((c) => c.name === `stack-refresh-${PROJECT_ID}`)
+                .every((c) => c.maxAge === 0),
+        ).toBe(true);
+
+        // Non-identity cookies are left alone: app cookies, in-flight OAuth
+        // state, and the SDK's is-https flag.
+        expect(cookies.some((c) => c.name === "theme")).toBe(false);
+        expect(cookies.some((c) => c.name === "stack-oauth-inner-xyz")).toBe(
+            false,
+        );
+        expect(cookies.some((c) => c.name === "hexclave-is-https")).toBe(false);
+
+        // Deletions cover only cookies the browser presented (plus the fresh
+        // set) — no blanket static list bloating the header block.
+        expect(cookies.some((c) => c.name === "stack-access")).toBe(false);
+    });
+
+    it("sets the fresh refresh cookie once with Partitioned, after the deletions", async () => {
+        const hostRefresh = `__Host-hexclave-refresh-${PROJECT_ID}--default`;
+        const response = await GET(
+            makeRequest(
+                "https://app.dograh.com/impersonate?refresh_token=rt-new",
+                { cookie: `${hostRefresh}=old-session` },
+            ),
+        );
+        const cookies = response.headers.getSetCookie().map(parseSetCookie);
+
+        // Exactly one fresh set: CHIPS browsers store it partitioned (where
+        // the SDK writes), non-CHIPS browsers ignore the attribute and store
+        // it in the regular jar (also where the SDK writes). A second copy in
+        // the other jar would become a stale shadow the SDK never updates.
+        const freshSets = cookies.filter(
+            (c) => c.name === hostRefresh && c.maxAge > 0,
+        );
+        expect(freshSets).toHaveLength(1);
+        expect(freshSets[0].partitioned).toBe(true);
+        expect(freshSets[0].secure).toBe(true);
+        expect(JSON.parse(freshSets[0].value).refresh_token).toBe("rt-new");
+
+        // The fresh set must come after every deletion of the same name,
+        // otherwise the browser would apply a deletion last.
+        const lastDeletionIdx = cookies.reduce(
+            (acc, c, i) =>
+                c.name === hostRefresh && c.maxAge === 0 ? i : acc,
+            -1,
+        );
+        const firstFreshIdx = cookies.findIndex(
+            (c) => c.name === hostRefresh && c.maxAge > 0,
+        );
+        expect(firstFreshIdx).toBeGreaterThan(lastDeletionIdx);
+    });
+
+    it("honors x-forwarded-proto case-insensitively when the request is http", async () => {
+        const response = await GET(
+            makeRequest("http://app.dograh.com/impersonate?refresh_token=rt", {
+                forwardedProto: "HTTPS",
+            }),
+        );
+        const cookies = response.headers.getSetCookie().map(parseSetCookie);
+        const freshSets = cookies.filter(
+            (c) =>
+                c.name === `__Host-hexclave-refresh-${PROJECT_ID}--default` &&
+                c.maxAge > 0,
+        );
+        expect(freshSets).toHaveLength(1);
+        expect(freshSets[0].partitioned).toBe(true);
+    });
+
+    it("uses no __Host- prefix and no partitioned attribute on plain http", async () => {
+        const response = await GET(
+            makeRequest("http://localhost:3010/impersonate?refresh_token=rt", {
+                cookie: `hexclave-refresh-${PROJECT_ID}--default=old`,
+            }),
+        );
+        const cookies = response.headers.getSetCookie().map(parseSetCookie);
+
+        expect(cookies.some((c) => c.partitioned)).toBe(false);
+        expect(cookies.some((c) => c.domain !== undefined)).toBe(false);
+
+        const freshSets = cookies.filter(
+            (c) =>
+                c.name === `hexclave-refresh-${PROJECT_ID}--default` &&
+                c.maxAge > 0,
+        );
+        expect(freshSets).toHaveLength(1);
+        expect(freshSets[0].secure).toBe(false);
+    });
+});
diff --git a/ui/src/app/impersonate/route.ts b/ui/src/app/impersonate/route.ts
index 53f2823e..9a4acc5f 100644
--- a/ui/src/app/impersonate/route.ts
+++ b/ui/src/app/impersonate/route.ts
@@ -3,13 +3,91 @@ import { NextRequest, NextResponse } from "next/server";
 import { getStackConfig } from "@/lib/auth/config";
 
 /**
- * Helper route that receives a Stack refresh token via query parameters, stores
- * it as the regular Stack SDK cookie *for the current sub-domain only* and finally
- * redirects the user to the requested path.
+ * Helper route that receives a Stack refresh token via query parameters, wipes
+ * every Stack SDK session cookie the browser presented, stores the impersonated
+ * session as a fresh cookie and finally redirects to the requested path.
+ *
+ * On HTTPS Chrome the Stack SDK writes its cookies into the CHIPS-partitioned
+ * jar (`Secure; SameSite=None; Partitioned`), which coexists with the regular
+ * jar under the same cookie name. A Set-Cookie without the `Partitioned`
+ * attribute can neither delete nor overwrite the partitioned copy, and the
+ * SDK's own cookie parsing is first-occurrence-wins — so a leftover partitioned
+ * cookie from a previous session silently keeps winning over anything this
+ * route sets in the regular jar, resurfacing the old user. Deletions below are
+ * therefore emitted for BOTH jars (and for possible parent-domain scopes),
+ * which is also why headers are appended manually: `response.cookies.set`
+ * dedupes Set-Cookie by name.
+ *
+ * The fresh cookie is deliberately written only ONCE, with the `Partitioned`
+ * attribute: CHIPS browsers store it in the partitioned jar and non-CHIPS
+ * browsers ignore the attribute and store it in the regular jar — in both
+ * cases exactly the jar the SDK's own writes will later overwrite. Writing
+ * both jars instead would plant a copy the SDK never updates, recreating the
+ * stale-session bug this route exists to fix.
  *
  * Example usage (client side):
  *   /impersonate?refresh_token=<REFRESH>&redirect_path=/workflow/123
  */
+
+// Stack SDK cookies that hold session identity: hexclave/stack access cookies
+// and every refresh-cookie variant (bare legacy, project-scoped legacy,
+// --default, --custom-<domain>, __Host- prefixed). Deliberately excludes
+// non-identity SDK cookies (is-https flags, in-flight OAuth state) so an
+// impersonation redirect can't abort an unrelated concurrent sign-in.
+const SESSION_COOKIE_RE = /^(?:__Host-)?(?:stack|hexclave)-(?:access|refresh)(?:-|$)/;
+
+/**
+ * Domains a cookie could have been scoped to from this host, e.g.
+ * "app.dograh.com" -> ["app.dograh.com", "dograh.com"]. Returns [] for
+ * localhost / IP hosts. Stops before the last label, which over-generates for
+ * multi-label public suffixes (app.example.co.uk also yields co.uk) — the
+ * browser just rejects those deletions, so the cost is a wasted header.
+ */
+function parentDomains(hostname: string): string[] {
+    if (!hostname.includes(".") || /^[\d.]+$/.test(hostname)) {
+        return [];
+    }
+    const parts = hostname.split(".");
+    const domains: string[] = [];
+    for (let i = 0; i + 2 <= parts.length; i++) {
+        domains.push(parts.slice(i).join("."));
+    }
+    return domains;
+}
+
+interface SetCookieAttrs {
+    maxAge: number;
+    secure?: boolean;
+    domain?: string;
+    partitioned?: boolean;
+}
+
+// No HttpOnly: the Stack SDK reads these cookies from document.cookie.
+function serializeSetCookie(
+    name: string,
+    value: string,
+    attrs: SetCookieAttrs,
+): string {
+    const parts = [
+        `${name}=${encodeURIComponent(value)}`,
+        "Path=/",
+        `Max-Age=${attrs.maxAge}`,
+    ];
+    if (attrs.domain) {
+        parts.push(`Domain=${attrs.domain}`);
+    }
+    if (attrs.partitioned) {
+        // CHIPS requires Secure and SameSite=None.
+        parts.push("Secure", "SameSite=None", "Partitioned");
+    } else {
+        if (attrs.secure) {
+            parts.push("Secure");
+        }
+        parts.push("SameSite=Lax");
+    }
+    return parts.join("; ");
+}
+
 export async function GET(request: NextRequest) {
     const { searchParams } = new URL(request.url);
 
@@ -27,85 +105,80 @@ export async function GET(request: NextRequest) {
         return new Response("Stack auth is not configured", { status: 400 });
     }
 
-    const requestedRedirectUrl = new URL(redirectPath, request.url);
     const fallbackRedirectUrl = new URL("/workflow/create", request.url);
-    const redirectUrl =
-        requestedRedirectUrl.origin === request.nextUrl.origin
-            ? requestedRedirectUrl.toString()
-            : fallbackRedirectUrl.toString();
+    let redirectUrl = fallbackRedirectUrl.toString();
+    try {
+        const requestedRedirectUrl = new URL(redirectPath, request.url);
+        if (requestedRedirectUrl.origin === request.nextUrl.origin) {
+            redirectUrl = requestedRedirectUrl.toString();
+        }
+    } catch {
+        // Malformed redirect_path (e.g. "https://") — keep the fallback.
+    }
 
     const response = NextResponse.redirect(redirectUrl);
 
+    const forwardedProto = request.headers
+        .get("x-forwarded-proto")
+        ?.split(",")[0]
+        ?.trim()
+        .toLowerCase();
     const isSecure =
-        request.nextUrl.protocol === "https:" ||
-        request.headers.get("x-forwarded-proto") === "https";
-    const refreshCookieName = `${isSecure ? "__Host-" : ""}hexclave-refresh-${stackConfig.projectId}--default`;
-    const accessCookieName = "hexclave-access";
-    const refreshMaxAge = 60 * 60 * 24 * 365;
+        request.nextUrl.protocol === "https:" || forwardedProto === "https";
 
-    // Store the refresh token using the cookie name/shape Stack's current
-    // nextjs-cookie token store reads. The old route only set the legacy
-    // refresh cookie, which let a stale access cookie keep the browser on the
-    // previous app-domain session.
-    response.cookies.set(
-        refreshCookieName,
-        JSON.stringify({
-            refresh_token: refreshToken,
-            updated_at_millis: Date.now(),
-        }),
-        {
-            path: "/",
-            maxAge: refreshMaxAge,
-            secure: isSecure,
-            httpOnly: false, // Must be accessible from the browser for Stack SDK
-            sameSite: "lax",
-        },
-    );
-
-    const staleCookieNames = new Set([
-        accessCookieName,
-        `stack-refresh-${stackConfig.projectId}`,
-        "stack-refresh",
-        `stack-refresh-${stackConfig.projectId}--default`,
-        `__Host-stack-refresh-${stackConfig.projectId}--default`,
-        `hexclave-refresh-${stackConfig.projectId}--default`,
-        `__Host-hexclave-refresh-${stackConfig.projectId}--default`,
-        "stack-access",
-    ]);
-    staleCookieNames.delete(refreshCookieName);
+    // Every scope a stale SDK cookie may live in: host-only plus each parent
+    // domain, each in the regular jar and (on https) its partitioned twin. The
+    // request's Cookie header is the complete list of names to clear: the SDK
+    // only sets Lax or None+Partitioned cookies, both of which the browser
+    // attaches to this top-level navigation.
+    const domains: (string | undefined)[] = [
+        undefined,
+        ...parentDomains(request.nextUrl.hostname),
+    ];
+    const jars = isSecure ? [false, true] : [false];
 
+    const setCookieHeaders: string[] = [];
     for (const cookie of request.cookies.getAll()) {
-        const name = cookie.name;
-        if (name !== refreshCookieName && (
-            name.startsWith(`hexclave-refresh-${stackConfig.projectId}--custom-`) ||
-            name.startsWith(`stack-refresh-${stackConfig.projectId}--custom-`) ||
-            name.startsWith(`__Host-hexclave-refresh-${stackConfig.projectId}--`) ||
-            name.startsWith(`__Host-stack-refresh-${stackConfig.projectId}--`)
-        )) {
-            staleCookieNames.add(name);
+        if (!SESSION_COOKIE_RE.test(cookie.name)) {
+            continue;
+        }
+        const isHostPrefixed = cookie.name.startsWith("__Host-");
+        for (const partitioned of jars) {
+            for (const domain of domains) {
+                if (isHostPrefixed && domain) {
+                    continue; // __Host- cookies never have a Domain attribute
+                }
+                setCookieHeaders.push(
+                    serializeSetCookie(cookie.name, "", {
+                        maxAge: 0,
+                        secure: isHostPrefixed || isSecure,
+                        domain,
+                        partitioned,
+                    }),
+                );
+            }
         }
     }
 
-    for (const name of staleCookieNames) {
-        response.cookies.set(name, "", {
-            path: "/",
-            maxAge: 0,
-            secure: name.startsWith("__Host-") || isSecure,
-            httpOnly: false,
-            sameSite: "lax",
-        });
-    }
-
-    // Keep writing the legacy project refresh cookie for compatibility with
-    // older Stack SDK builds, but the structured Hexclave cookies above are the
-    // source of truth for the current app.
-    response.cookies.set(`stack-refresh-${stackConfig.projectId}`, refreshToken, {
-        path: "/",
-        maxAge: refreshMaxAge,
-        secure: isSecure,
-        httpOnly: false, // Must be accessible from the browser for Stack SDK
-        sameSite: "lax",
+    // Fresh impersonated session, written AFTER the deletions so it survives
+    // them, in the name/shape Stack's nextjs-cookie token store reads. Single
+    // write with Partitioned (see the header comment for why).
+    const refreshCookieName = `${isSecure ? "__Host-" : ""}hexclave-refresh-${stackConfig.projectId}--default`;
+    const refreshCookieValue = JSON.stringify({
+        refresh_token: refreshToken,
+        updated_at_millis: Date.now(),
     });
+    setCookieHeaders.push(
+        serializeSetCookie(refreshCookieName, refreshCookieValue, {
+            maxAge: 60 * 60 * 24 * 365,
+            secure: isSecure,
+            partitioned: isSecure,
+        }),
+    );
+
+    for (const header of setCookieHeaders) {
+        response.headers.append("set-cookie", header);
+    }
 
     return response;
 }