diff --git a/.changeset/fair-sessions-retry.md b/.changeset/fair-sessions-retry.md
new file mode 100644
index 0000000000..34107f118c
--- /dev/null
+++ b/.changeset/fair-sessions-retry.md
@@ -0,0 +1,5 @@
+---
+"better-auth": patch
+---
+
+Fix `customSession` treating session lookup failures as signed-out users. Server-side `getSession` now throws an API error, and HTTP clients receive an error response.
diff --git a/packages/better-auth/src/plugins/custom-session/custom-session.test.ts b/packages/better-auth/src/plugins/custom-session/custom-session.test.ts
index ddebb268c1..7cd001c2b0 100644
--- a/packages/better-auth/src/plugins/custom-session/custom-session.test.ts
+++ b/packages/better-auth/src/plugins/custom-session/custom-session.test.ts
@@ -1,4 +1,4 @@
-import { describe, expect, expectTypeOf, it } from "vitest";
+import { describe, expect, expectTypeOf, it, vi } from "vitest";
 import { createAuthClient } from "../../client";
 import { parseSetCookieHeader } from "../../cookies";
 import { getTestInstance } from "../../test-utils/test-instance";
@@ -65,6 +65,29 @@ describe("Custom Session Plugin Tests", async () => {
 		expect(session?.newData).toEqual({ message: "Hello, World!" });
 	});
 
+	/**
+	 * @see https://github.com/better-auth/better-auth/issues/10566
+	 */
+	it("propagates session lookup failures to server and HTTP clients", async () => {
+		const { auth, client, signInWithTestUser } = await getTestInstance({
+			plugins: [customSession(async (session) => session)],
+		});
+		const { headers } = await signInWithTestUser();
+		const context = await auth.$context;
+		const lookup = vi
+			.spyOn(context.internalAdapter, "findSession")
+			.mockRejectedValue(new Error("database unavailable"));
+		try {
+			await expect(auth.api.getSession({ headers })).rejects.toMatchObject({
+				status: "INTERNAL_SERVER_ERROR",
+			});
+			const result = await client.getSession({ fetchOptions: { headers } });
+			expect(result.error?.status).toBe(500);
+		} finally {
+			lookup.mockRestore();
+		}
+	});
+
 	it("should return set cookie headers as separate entries", async () => {
 		const { headers } = await signInWithTestUser();
 		await client.getSession({
diff --git a/packages/better-auth/src/plugins/custom-session/index.ts b/packages/better-auth/src/plugins/custom-session/index.ts
index ab0c556a4c..3e8426d9a3 100644
--- a/packages/better-auth/src/plugins/custom-session/index.ts
+++ b/packages/better-auth/src/plugins/custom-session/index.ts
@@ -104,8 +104,6 @@ export const customSession = <
 						asResponse: false,
 						headers: ctx.headers,
 						returnHeaders: true,
-					}).catch((e) => {
-						return null;
 					});
 					if (!session?.response) {
 						return ctx.json(null);