diff --git a/src/shared/model-key.test.ts b/src/shared/model-key.test.ts
new file mode 100644
index 000000000000..f3d9511e0d13
--- /dev/null
+++ b/src/shared/model-key.test.ts
@@ -0,0 +1,28 @@
+// Model key tests cover canonical provider/model key construction.
+import { describe, expect, it } from "vitest";
+import { modelKey } from "./model-key.js";
+
+describe("modelKey", () => {
+  it("joins provider and model into a canonical key", () => {
+    expect(modelKey("openai", "gpt-5")).toBe("openai/gpt-5");
+    expect(modelKey("anthropic", "claude-opus-4-8")).toBe("anthropic/claude-opus-4-8");
+  });
+
+  it("returns the model alone when provider is empty", () => {
+    expect(modelKey("", "gpt-5")).toBe("gpt-5");
+    expect(modelKey("   ", "gpt-5")).toBe("gpt-5");
+  });
+
+  it("returns the provider alone when model is empty", () => {
+    expect(modelKey("openai", "")).toBe("openai");
+    expect(modelKey("openai", "   ")).toBe("openai");
+  });
+
+  it("avoids double prefix when model already contains provider prefix", () => {
+    expect(modelKey("openai", "openai/gpt-5")).toBe("openai/gpt-5");
+  });
+
+  it("trims whitespace from both arguments", () => {
+    expect(modelKey(" openai ", " gpt-5 ")).toBe("openai/gpt-5");
+  });
+});