diff --git a/src/typeschema/register.ts b/src/typeschema/register.ts
index 141dfcca..cf45d674 100644
--- a/src/typeschema/register.ts
+++ b/src/typeschema/register.ts
@@ -93,6 +93,7 @@ const mkPackageAwareResolver = async (
     if (acc[pkgId]) return acc[pkgId];
 
     const index = mkEmptyPkgIndex(pkg);
+    acc[pkgId] = index;
     for (const resource of await manager.search({ package: pkg })) {
         const rawUrl = resource.url;
         if (!rawUrl) continue;
@@ -115,7 +116,6 @@ const mkPackageAwareResolver = async (
         resolutionOptions.sort((a, b) => a.deep - b.deep);
     }
 
-    acc[pkgId] = index;
     return index;
 };
 
diff --git a/test/unit/typeschema/register.test.ts b/test/unit/typeschema/register.test.ts
index 22108515..fc76adb0 100644
--- a/test/unit/typeschema/register.test.ts
+++ b/test/unit/typeschema/register.test.ts
@@ -1,7 +1,12 @@
 import { describe, expect, it } from "bun:test";
 import type { FHIRSchema } from "@atomic-ehr/fhirschema";
 import { type CanonicalUrl, enrichFHIRSchema, type Name } from "@root/typeschema/types";
-import { mergeFsElementProps, registerFromPackageMetas, resolveFsElementGenealogy } from "@typeschema/register";
+import {
+    mergeFsElementProps,
+    registerFromManager,
+    registerFromPackageMetas,
+    resolveFsElementGenealogy,
+} from "@typeschema/register";
 
 type PFS = Partial<FHIRSchema>;
 
@@ -150,3 +155,32 @@ describe("Register tests", async () => {
         });
     });
 });
+
+describe("cyclic package dependencies", () => {
+    it("registerFromManager terminates on a dependency cycle", async () => {
+        // a ↔ b mutually depend (like hl7.terminology.r5 ↔ hl7.fhir.uv.extensions.r5).
+        // Without memoizing each package before recursing into its deps, the walk recurses
+        // forever; each unique package should be visited exactly once. The search-call cap
+        // makes a regression fail fast (red) instead of slowly exhausting memory.
+        const deps: Record<string, string[]> = { a: ["b"], b: ["a"] };
+        let searchCalls = 0;
+        const manager = {
+            packages: async () => Object.keys(deps).map((name) => ({ name, version: "1.0.0" })),
+            packageJson: async (name: string) => ({
+                name,
+                version: "1.0.0",
+                dependencies: Object.fromEntries((deps[name] ?? []).map((d) => [d, "1.0.0"])),
+            }),
+            search: async () => {
+                if (++searchCalls > 10) throw new Error("dependency walk did not terminate on cycle");
+                return [];
+            },
+        } as unknown as Parameters<typeof registerFromManager>[0];
+
+        const reg = await registerFromManager(manager, { focusedPackages: [{ name: "a", version: "1.0.0" }] });
+
+        expect(searchCalls).toBe(2); // each package scanned exactly once
+        expect(reg.resolver["a#1.0.0"]).toBeDefined();
+        expect(reg.resolver["b#1.0.0"]).toBeDefined();
+    });
+});