fix: use root LLM config as fallback for graph store instead of hardcoded OpenAI default (#4466)

Co-authored-by: utkarsh240799 <utkarsh240799@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mem0-ts/src/oss/tests/config-manager.test.ts

@@ -349,6 +349,78 @@ describe("ConfigManager", () => {
   });
 });
 
+// ─────────────────────────────────────────────────────────────────────────
+// Graph store LLM config propagation (issue #3425)
+// ─────────────────────────────────────────────────────────────────────────
+describe("mergeConfig - graph store LLM config (issue #3425)", () => {
+  const baseEmbedder = {
+    provider: "openai",
+    config: { apiKey: "test-key" },
+  };
+  const baseVectorStore = {
+    provider: "memory",
+    config: { collectionName: "test" },
+  };
+  const graphStoreNeo4j = {
+    provider: "neo4j",
+    config: {
+      url: "neo4j://localhost:7687",
+      username: "neo4j",
+      password: "password",
+    },
+  };
+
+  it("should NOT have a default graphStore.llm — root llm should be the fallback", () => {
+    const config = ConfigManager.mergeConfig({
+      embedder: baseEmbedder,
+      vectorStore: baseVectorStore,
+      llm: {
+        provider: "anthropic",
+        config: { model: "claude-sonnet-4-20250514" },
+      },
+      graphStore: graphStoreNeo4j,
+    });
+
+    // graphStore should NOT have its own llm after merge
+    expect(config.graphStore?.llm).toBeUndefined();
+    // root llm should be anthropic
+    expect(config.llm.provider).toBe("anthropic");
+    expect(config.llm.config.model).toBe("claude-sonnet-4-20250514");
+  });
+
+  it("should preserve explicit graphStore.llm when user provides it", () => {
+    const config = ConfigManager.mergeConfig({
+      embedder: baseEmbedder,
+      vectorStore: baseVectorStore,
+      llm: {
+        provider: "anthropic",
+        config: { model: "claude-sonnet-4-20250514" },
+      },
+      graphStore: {
+        ...graphStoreNeo4j,
+        llm: { provider: "openai", config: { model: "gpt-4o" } },
+      },
+    });
+
+    // graphStore should have its own llm
+    expect(config.graphStore?.llm?.provider).toBe("openai");
+    expect(config.graphStore?.llm?.config).toEqual({ model: "gpt-4o" });
+    // root llm should still be anthropic
+    expect(config.llm.provider).toBe("anthropic");
+  });
+
+  it("should not have graphStore.llm when user does not provide one", () => {
+    const config = ConfigManager.mergeConfig({
+      embedder: baseEmbedder,
+      vectorStore: baseVectorStore,
+      llm: { provider: "openai", config: { model: "gpt-4o" } },
+    });
+
+    // Default graphStore should not have llm
+    expect(config.graphStore?.llm).toBeUndefined();
+  });
+});
+
 // ─────────────────────────────────────────────────────────────────────────
 // Memory class – LM Studio end-to-end flow (mocked factories)
 // ─────────────────────────────────────────────────────────────────────────