diff --git a/lib/db/posts.test.ts b/lib/db/posts.test.ts
new file mode 100644
index 0000000..cb75a1b
--- /dev/null
+++ b/lib/db/posts.test.ts
@@ -0,0 +1,80 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const from = vi.fn();
+
+vi.mock("@/lib/supabase/server", () => ({
+  createSupabaseServerClient: () => ({ from }),
+}));
+
+describe("fetchRelatedPosts", () => {
+  beforeEach(() => {
+    vi.resetModules();
+    vi.clearAllMocks();
+  });
+
+  it("scopes the no-tag-match fallback to the requested agent", async () => {
+    const fallbackQuery = {
+      select: vi.fn(),
+      eq: vi.fn(),
+      neq: vi.fn(),
+      order: vi.fn(),
+      limit: vi.fn(),
+    };
+    fallbackQuery.select.mockReturnValue(fallbackQuery);
+    fallbackQuery.eq.mockReturnValue(fallbackQuery);
+    fallbackQuery.neq.mockReturnValue(fallbackQuery);
+    fallbackQuery.order.mockReturnValue(fallbackQuery);
+    fallbackQuery.limit.mockResolvedValue({
+      data: [
+        {
+          id: "post-2",
+          case_number: "APM-0002",
+          title: "Another Claude failure",
+          agents: { slug: "claude", name: "Claude" },
+          post_tags: [],
+        },
+      ],
+    });
+
+    from.mockImplementation((table: string) => {
+      if (table === "tags") {
+        return {
+          select: () => ({
+            in: vi.fn().mockResolvedValue({ data: [{ id: "tag-1" }] }),
+          }),
+        };
+      }
+
+      if (table === "post_tags") {
+        return {
+          select: () => ({
+            in: vi.fn().mockResolvedValue({ data: [] }),
+          }),
+        };
+      }
+
+      if (table === "posts") return fallbackQuery;
+      throw new Error(`Unexpected table ${table}`);
+    });
+
+    const { fetchRelatedPosts } = await import("./posts");
+    const posts = await fetchRelatedPosts(
+      "APM-0001",
+      "claude",
+      ["hallucination"],
+      2,
+    );
+
+    expect(posts).toHaveLength(1);
+    expect(fallbackQuery.select).toHaveBeenCalledWith(
+      `*, agents!inner(slug, name, company), post_tags(tags(slug, label))`,
+    );
+    expect(fallbackQuery.eq).toHaveBeenCalledWith("agents.slug", "claude");
+    expect(fallbackQuery.eq).toHaveBeenCalledWith("status", "approved");
+    expect(fallbackQuery.neq).toHaveBeenCalledWith("case_number", "APM-0001");
+    expect(fallbackQuery.order).toHaveBeenCalledWith("vote_score", {
+      ascending: false,
+    });
+    expect(fallbackQuery.limit).toHaveBeenCalledWith(2);
+  });
+});
diff --git a/lib/db/posts.ts b/lib/db/posts.ts
index 5d2c906..166bf42 100644
--- a/lib/db/posts.ts
+++ b/lib/db/posts.ts
@@ -260,7 +260,10 @@ export async function fetchRelatedPosts(
     // Fallback: same agent
     const { data } = await supabase
       .from("posts")
-      .select(`*, agents(slug, name, company), post_tags(tags(slug, label))`)
+      .select(
+        `*, agents!inner(slug, name, company), post_tags(tags(slug, label))`,
+      )
+      .eq("agents.slug", agentSlug)
       .eq("status", "approved")
       .neq("case_number", caseNumber)
       .order("vote_score", { ascending: false })