diff --git a/.changeset/fix-bearer-cookie-parse-mutate-serialize.md b/.changeset/fix-bearer-cookie-parse-mutate-serialize.md
new file mode 100644
index 0000000000..a9967418d5
--- /dev/null
+++ b/.changeset/fix-bearer-cookie-parse-mutate-serialize.md
@@ -0,0 +1,9 @@
+---
+"better-auth": patch
+---
+
+The bearer plugin now produces a single entry per cookie name when merging
+its session token into the request `Cookie` header. Previously the merged
+header could carry two entries for the same name if the request already
+had a stale session cookie, which would surface to downstream code that
+picks the first occurrence.
diff --git a/packages/better-auth/src/cookies/cookie-utils.ts b/packages/better-auth/src/cookies/cookie-utils.ts
index d404842cc7..5d6f6a0e7b 100644
--- a/packages/better-auth/src/cookies/cookie-utils.ts
+++ b/packages/better-auth/src/cookies/cookie-utils.ts
@@ -173,6 +173,34 @@ export function toCookieOptions(
 	};
 }
 
+/**
+ * Add or replace a cookie in the request `Cookie` header.
+ *
+ * Cookie pairs are joined with `; `, but `headers.append("cookie", ...)`
+ * joins with `, ` in some runtimes (e.g. Deno, Cloudflare Workers) and
+ * breaks downstream cookie parsing. This builds the header value via
+ * parse-mutate-serialize.
+ */
+export function setRequestCookie(
+	headers: Headers,
+	name: string,
+	value: string,
+): void {
+	const cookieMap = new Map<string, string>();
+	for (const pair of (headers.get("cookie") || "").split(";")) {
+		const trimmed = pair.trim();
+		const eq = trimmed.indexOf("=");
+		if (eq > 0) cookieMap.set(trimmed.slice(0, eq), trimmed.slice(eq + 1));
+	}
+
+	cookieMap.set(name, value);
+
+	headers.set(
+		"cookie",
+		Array.from(cookieMap, ([k, v]) => `${k}=${v}`).join("; "),
+	);
+}
+
 export function setCookieToHeader(headers: Headers) {
 	return (context: { response: Response }) => {
 		const setCookieHeader = context.response.headers.get("set-cookie");
diff --git a/packages/better-auth/src/cookies/cookies.test.ts b/packages/better-auth/src/cookies/cookies.test.ts
index 2eed1335e7..5e65273b17 100644
--- a/packages/better-auth/src/cookies/cookies.test.ts
+++ b/packages/better-auth/src/cookies/cookies.test.ts
@@ -12,6 +12,7 @@ import {
 	HOST_COOKIE_PREFIX,
 	parseSetCookieHeader,
 	SECURE_COOKIE_PREFIX,
+	setRequestCookie,
 	stripSecureCookiePrefix,
 	toCookieOptions,
 } from "./cookie-utils";
@@ -372,6 +373,44 @@ describe("cookie-utils stripSecureCookiePrefix", () => {
 	});
 });
 
+/**
+ * @see https://github.com/better-auth/better-call/issues/54
+ * @see https://github.com/better-auth/better-auth/pull/8089
+ */
+describe("cookie-utils setRequestCookie", () => {
+	it("writes a cookie when the header is empty", () => {
+		const headers = new Headers();
+		setRequestCookie(headers, "better-auth.session_token", "abc");
+		expect(headers.get("cookie")).toBe("better-auth.session_token=abc");
+	});
+
+	it("preserves existing cookies and joins with `; ` per RFC 6265", () => {
+		const headers = new Headers({ cookie: "preference=dark; locale=en" });
+		setRequestCookie(headers, "better-auth.session_token", "abc");
+		expect(headers.get("cookie")).toBe(
+			"preference=dark; locale=en; better-auth.session_token=abc",
+		);
+	});
+
+	it("replaces an existing cookie of the same name rather than duplicating it", () => {
+		const headers = new Headers({
+			cookie: "better-auth.session_token=stale; locale=en",
+		});
+		setRequestCookie(headers, "better-auth.session_token", "fresh");
+		expect(headers.get("cookie")).toBe(
+			"better-auth.session_token=fresh; locale=en",
+		);
+	});
+
+	it("ignores malformed pairs in the existing header", () => {
+		const headers = new Headers({ cookie: "valid=1; ; =orphan; locale=en" });
+		setRequestCookie(headers, "better-auth.session_token", "abc");
+		expect(headers.get("cookie")).toBe(
+			"valid=1; locale=en; better-auth.session_token=abc",
+		);
+	});
+});
+
 describe("getSessionCookie", async () => {
 	it("should return the correct session cookie", async () => {
 		const { signInWithTestUser } = await getTestInstance();
diff --git a/packages/better-auth/src/plugins/bearer/index.ts b/packages/better-auth/src/plugins/bearer/index.ts
index 18a69ab5ea..d345301245 100644
--- a/packages/better-auth/src/plugins/bearer/index.ts
+++ b/packages/better-auth/src/plugins/bearer/index.ts
@@ -3,6 +3,7 @@ import { createAuthMiddleware } from "@better-auth/core/api";
 import { createHMAC } from "@better-auth/utils/hmac";
 import { serializeSignedCookie } from "better-call";
 import { parseSetCookieHeader } from "../../cookies";
+import { setRequestCookie } from "../../cookies/cookie-utils";
 import { PACKAGE_VERSION } from "../../version";
 
 declare module "@better-auth/core" {
@@ -105,14 +106,10 @@ export const bearer = (options?: BearerOptions | undefined) => {
 						const headers = new Headers({
 							...Object.fromEntries(existingHeaders?.entries()),
 						});
-						// Use headers.set() with "; " separator per RFC 6265.
-						// headers.append("cookie") joins with ", " in some runtimes
-						// (e.g. Deno, Cloudflare Workers), which breaks cookie parsing.
-						const existingCookie = headers.get("cookie");
-						const newCookie = `${c.context.authCookies.sessionToken.name}=${signedToken}`;
-						headers.set(
-							"cookie",
-							existingCookie ? `${existingCookie}; ${newCookie}` : newCookie,
+						setRequestCookie(
+							headers,
+							c.context.authCookies.sessionToken.name,
+							signedToken,
 						);
 						return {
 							context: {