diff --git a/packages/agent-auth/src/__tests__/plugin-collision.test.ts b/packages/agent-auth/src/__tests__/plugin-collision.test.ts
new file mode 100644
index 000000000..b85ea4a78
--- /dev/null
+++ b/packages/agent-auth/src/__tests__/plugin-collision.test.ts
@@ -0,0 +1,111 @@
+/**
+ * Regression test for better-auth/better-auth#9885.
+ *
+ * agent-auth registers a global `before` hook. It used to claim any request
+ * carrying a 3-segment Bearer JWT, then hard-reject the request with
+ * INVALID_JWT when the token's protected-header `typ` wasn't "host+jwt" or
+ * "agent+jwt". That collided with other plugins composed on the same
+ * betterAuth() instance (e.g. @better-auth/infra's dash() plugin), whose own
+ * JWTs were rejected before their owning routes could validate them.
+ *
+ * The hook now claims requests by token ownership (typ), so foreign Bearer
+ * JWTs pass through untouched to the plugin that owns them.
+ */
+import { createAuthEndpoint } from "@better-auth/core/api";
+import { getTestInstance } from "better-auth/test";
+import { exportJWK, generateKeyPair, importJWK, SignJWT } from "jose";
+import { beforeAll, describe, expect, it } from "vitest";
+import { agentAuth, agentAuthClientPlugin, BASE, createTestClient } from "./helpers";
+
+const TEST_CAPABILITIES = [{ name: "check_balance", description: "Check account balance" }];
+
+/**
+ * Minimal stand-in for @better-auth/infra's dash() plugin: it owns /dash/*
+ * routes and validates its own infra-issued Bearer JWT inside the endpoint.
+ */
+function dashPlugin() {
+  return {
+    id: "infra-dash",
+    endpoints: {
+      dashValidate: createAuthEndpoint(
+        "/dash/validate",
+        { method: "GET" },
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        async (ctx: any) => {
+          const auth = ctx.headers?.get("authorization");
+          const bearer = auth?.replace(/^Bearer\s+/i, "");
+          // The owning plugin performs its own validation here; for the test we
+          // just confirm the request reached us with its token intact.
+          return ctx.json({ ok: true, sawToken: !!bearer && bearer !== auth });
+        },
+      ),
+    },
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  } as any;
+}
+
+/** Sign a JWT with an arbitrary `typ` that does NOT belong to agent-auth. */
+async function signForeignJWT(typ: string): Promise<string> {
+  const { privateKey } = await generateKeyPair("EdDSA", { crv: "Ed25519", extractable: true });
+  const jwk = await exportJWK(privateKey);
+  const key = await importJWK(jwk, "EdDSA");
+  return new SignJWT({})
+    .setProtectedHeader({ alg: "EdDSA", typ })
+    .setSubject("infra-dashboard")
+    .setAudience(BASE)
+    .setIssuedAt()
+    .setExpirationTime("60s")
+    .setJti(globalThis.crypto.randomUUID())
+    .sign(key);
+}
+
+describe("agent-auth + other-plugin composition (#9885)", () => {
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  let client: ReturnType<typeof createTestClient>;
+
+  beforeAll(async () => {
+    const t = await getTestInstance(
+      {
+        plugins: [
+          agentAuth({
+            providerName: "collision-test",
+            capabilities: TEST_CAPABILITIES,
+            modes: ["delegated"],
+          }),
+          dashPlugin(),
+        ],
+      },
+      { clientOptions: { plugins: [agentAuthClientPlugin()] } },
+    );
+    client = createTestClient((req) => t.auth.handler(req));
+  });
+
+  it("passes a foreign Bearer JWT through to the route that owns it", async () => {
+    const dashToken = await signForeignJWT("infra+jwt");
+
+    const res = await client.api("/dash/validate", {
+      method: "GET",
+      headers: { authorization: `Bearer ${dashToken}` },
+    });
+
+    // Before the fix this was 401 INVALID_JWT from agent-auth's before-hook.
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as { ok: boolean; sawToken: boolean };
+    expect(body.ok).toBe(true);
+    // The foreign token reaches the owning plugin unchanged.
+    expect(body.sawToken).toBe(true);
+  });
+
+  it("does not reject a foreign token even on a path agent-auth would otherwise match", async () => {
+    // A token with an unrecognized typ must never produce agent-auth's
+    // INVALID_JWT — agent-auth simply doesn't own it.
+    const foreignToken = await signForeignJWT("JWT");
+
+    const res = await client.api("/dash/validate", {
+      method: "GET",
+      headers: { authorization: `Bearer ${foreignToken}` },
+    });
+
+    expect(res.status).toBe(200);
+  });
+});
diff --git a/packages/agent-auth/src/middleware.ts b/packages/agent-auth/src/middleware.ts
index b87be8ea1..80362c505 100644
--- a/packages/agent-auth/src/middleware.ts
+++ b/packages/agent-auth/src/middleware.ts
@@ -75,7 +75,15 @@ export function createAgentAuthBeforeHook(
       if (!auth) return false;
       const bearer = auth.replace(/^Bearer\s+/i, "");
       if (!bearer || bearer === auth) return false;
-      return bearer.split(".").length === 3;
+      if (bearer.split(".").length !== 3) return false;
+      // Only claim agent-auth's own tokens; let other plugins' JWTs pass through. (#9885)
+      let typ: string | undefined;
+      try {
+        typ = decodeProtectedHeader(bearer).typ;
+      } catch {
+        return false;
+      }
+      return typ === "host+jwt" || typ === "agent+jwt";
     },
     handler: createAuthMiddleware(async (ctx) => {
       const challenge = agentAuthChallenge(ctx.context.baseURL);