Enhance provider override to support baseUrl-only mode

Builds on #406 to support simpler proxy use case:
- Override just baseUrl to route built-in provider through proxy
- All built-in models preserved, no need to redefine them
- Full replacement still works when models array is provided

packages/coding-agent/test/model-registry.test.ts

@@ -53,7 +53,116 @@ describe("ModelRegistry", () => {
 		return registry.getAll().filter((m) => m.provider === provider);
 	}
 
-	describe("provider override", () => {
+	/** Create a baseUrl-only override (no custom models) */
+	function overrideConfig(baseUrl: string, headers?: Record<string, string>) {
+		return { baseUrl, ...(headers && { headers }) };
+	}
+
+	/** Write raw providers config (for mixed override/replacement scenarios) */
+	function writeRawModelsJson(providers: Record<string, unknown>) {
+		writeFileSync(modelsJsonPath, JSON.stringify({ providers }));
+	}
+
+	describe("baseUrl override (no custom models)", () => {
+		test("overriding baseUrl keeps all built-in models", () => {
+			writeRawModelsJson({
+				anthropic: overrideConfig("https://my-proxy.example.com/v1"),
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			const anthropicModels = getModelsForProvider(registry, "anthropic");
+
+			// Should have multiple built-in models, not just one
+			expect(anthropicModels.length).toBeGreaterThan(1);
+			expect(anthropicModels.some((m) => m.id.includes("claude"))).toBe(true);
+		});
+
+		test("overriding baseUrl changes URL on all built-in models", () => {
+			writeRawModelsJson({
+				anthropic: overrideConfig("https://my-proxy.example.com/v1"),
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			const anthropicModels = getModelsForProvider(registry, "anthropic");
+
+			// All models should have the new baseUrl
+			for (const model of anthropicModels) {
+				expect(model.baseUrl).toBe("https://my-proxy.example.com/v1");
+			}
+		});
+
+		test("overriding headers merges with model headers", () => {
+			writeRawModelsJson({
+				anthropic: overrideConfig("https://my-proxy.example.com/v1", {
+					"X-Custom-Header": "custom-value",
+				}),
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			const anthropicModels = getModelsForProvider(registry, "anthropic");
+
+			for (const model of anthropicModels) {
+				expect(model.headers?.["X-Custom-Header"]).toBe("custom-value");
+			}
+		});
+
+		test("baseUrl-only override does not affect other providers", () => {
+			writeRawModelsJson({
+				anthropic: overrideConfig("https://my-proxy.example.com/v1"),
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			const googleModels = getModelsForProvider(registry, "google");
+
+			// Google models should still have their original baseUrl
+			expect(googleModels.length).toBeGreaterThan(0);
+			expect(googleModels[0].baseUrl).not.toBe("https://my-proxy.example.com/v1");
+		});
+
+		test("can mix baseUrl override and full replacement", () => {
+			writeRawModelsJson({
+				// baseUrl-only for anthropic
+				anthropic: overrideConfig("https://anthropic-proxy.example.com/v1"),
+				// Full replacement for google
+				google: providerConfig(
+					"https://google-proxy.example.com/v1",
+					[{ id: "gemini-custom" }],
+					"google-generative-ai",
+				),
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+
+			// Anthropic: multiple built-in models with new baseUrl
+			const anthropicModels = getModelsForProvider(registry, "anthropic");
+			expect(anthropicModels.length).toBeGreaterThan(1);
+			expect(anthropicModels[0].baseUrl).toBe("https://anthropic-proxy.example.com/v1");
+
+			// Google: single custom model
+			const googleModels = getModelsForProvider(registry, "google");
+			expect(googleModels).toHaveLength(1);
+			expect(googleModels[0].id).toBe("gemini-custom");
+		});
+
+		test("refresh() picks up baseUrl override changes", () => {
+			writeRawModelsJson({
+				anthropic: overrideConfig("https://first-proxy.example.com/v1"),
+			});
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+
+			expect(getModelsForProvider(registry, "anthropic")[0].baseUrl).toBe("https://first-proxy.example.com/v1");
+
+			// Update and refresh
+			writeRawModelsJson({
+				anthropic: overrideConfig("https://second-proxy.example.com/v1"),
+			});
+			registry.refresh();
+
+			expect(getModelsForProvider(registry, "anthropic")[0].baseUrl).toBe("https://second-proxy.example.com/v1");
+		});
+	});
+
+	describe("provider replacement (with custom models)", () => {
 		test("custom provider with same name as built-in replaces built-in models", () => {
 			writeModelsJson({
 				anthropic: providerConfig("https://my-proxy.example.com/v1", [{ id: "claude-custom" }]),
@@ -81,7 +190,11 @@ describe("ModelRegistry", () => {
 		test("multiple built-in providers can be overridden", () => {
 			writeModelsJson({
 				anthropic: providerConfig("https://anthropic-proxy.example.com/v1", [{ id: "claude-proxy" }]),
-				google: providerConfig("https://google-proxy.example.com/v1", [{ id: "gemini-proxy" }], "google-generative-ai"),
+				google: providerConfig(
+					"https://google-proxy.example.com/v1",
+					[{ id: "gemini-proxy" }],
+					"google-generative-ai",
+				),
 			});
 
 			const registry = new ModelRegistry(authStorage, modelsJsonPath);