feat(coding-agent): support per-model overrides in models.json

Add modelOverrides field to provider config that allows customizing
individual built-in models without replacing the entire provider.

Example:
  {
    "providers": {
      "openrouter": {
        "modelOverrides": {
          "anthropic/claude-sonnet-4": {
            "compat": { "openRouterRouting": { "only": ["amazon-bedrock"] } }
          }
        }
      }
    }
  }

Overrides are deep-merged with built-in model definitions. Supports:
- name, reasoning, input, contextWindow, maxTokens
- Partial cost overrides (e.g. only change input cost)
- headers (merged with existing)
- compat settings (merged with existing)

Works alongside baseUrl overrides on the same provider.

closes #1062

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

@@ -248,6 +248,240 @@ describe("ModelRegistry", () => {
 		});
 	});
 
+	describe("modelOverrides (per-model customization)", () => {
+		test("model override applies to a single built-in model", () => {
+			writeRawModelsJson({
+				openrouter: {
+					modelOverrides: {
+						"anthropic/claude-sonnet-4": {
+							name: "Custom Sonnet Name",
+						},
+					},
+				},
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			const models = getModelsForProvider(registry, "openrouter");
+
+			const sonnet = models.find((m) => m.id === "anthropic/claude-sonnet-4");
+			expect(sonnet?.name).toBe("Custom Sonnet Name");
+
+			// Other models should be unchanged
+			const opus = models.find((m) => m.id === "anthropic/claude-opus-4");
+			expect(opus?.name).not.toBe("Custom Sonnet Name");
+		});
+
+		test("model override with compat.openRouterRouting", () => {
+			writeRawModelsJson({
+				openrouter: {
+					modelOverrides: {
+						"anthropic/claude-sonnet-4": {
+							compat: {
+								openRouterRouting: { only: ["amazon-bedrock"] },
+							},
+						},
+					},
+				},
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			const models = getModelsForProvider(registry, "openrouter");
+
+			const sonnet = models.find((m) => m.id === "anthropic/claude-sonnet-4");
+			expect((sonnet?.compat as any)?.openRouterRouting).toEqual({ only: ["amazon-bedrock"] });
+		});
+
+		test("model override deep merges compat settings", () => {
+			writeRawModelsJson({
+				openrouter: {
+					modelOverrides: {
+						"anthropic/claude-sonnet-4": {
+							compat: {
+								openRouterRouting: { order: ["anthropic", "together"] },
+							},
+						},
+					},
+				},
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			const models = getModelsForProvider(registry, "openrouter");
+			const sonnet = models.find((m) => m.id === "anthropic/claude-sonnet-4");
+
+			// Should have both the new routing AND preserve other compat settings
+			expect((sonnet?.compat as any)?.openRouterRouting).toEqual({ order: ["anthropic", "together"] });
+		});
+
+		test("multiple model overrides on same provider", () => {
+			writeRawModelsJson({
+				openrouter: {
+					modelOverrides: {
+						"anthropic/claude-sonnet-4": {
+							compat: { openRouterRouting: { only: ["amazon-bedrock"] } },
+						},
+						"anthropic/claude-opus-4": {
+							compat: { openRouterRouting: { only: ["anthropic"] } },
+						},
+					},
+				},
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			const models = getModelsForProvider(registry, "openrouter");
+
+			const sonnet = models.find((m) => m.id === "anthropic/claude-sonnet-4");
+			const opus = models.find((m) => m.id === "anthropic/claude-opus-4");
+
+			expect((sonnet?.compat as any)?.openRouterRouting).toEqual({ only: ["amazon-bedrock"] });
+			expect((opus?.compat as any)?.openRouterRouting).toEqual({ only: ["anthropic"] });
+		});
+
+		test("model override combined with baseUrl override", () => {
+			writeRawModelsJson({
+				openrouter: {
+					baseUrl: "https://my-proxy.example.com/v1",
+					modelOverrides: {
+						"anthropic/claude-sonnet-4": {
+							name: "Proxied Sonnet",
+						},
+					},
+				},
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			const models = getModelsForProvider(registry, "openrouter");
+			const sonnet = models.find((m) => m.id === "anthropic/claude-sonnet-4");
+
+			// Both overrides should apply
+			expect(sonnet?.baseUrl).toBe("https://my-proxy.example.com/v1");
+			expect(sonnet?.name).toBe("Proxied Sonnet");
+
+			// Other models should have the baseUrl but not the name override
+			const opus = models.find((m) => m.id === "anthropic/claude-opus-4");
+			expect(opus?.baseUrl).toBe("https://my-proxy.example.com/v1");
+			expect(opus?.name).not.toBe("Proxied Sonnet");
+		});
+
+		test("model override for non-existent model ID is ignored", () => {
+			writeRawModelsJson({
+				openrouter: {
+					modelOverrides: {
+						"nonexistent/model-id": {
+							name: "This should not appear",
+						},
+					},
+				},
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			const models = getModelsForProvider(registry, "openrouter");
+
+			// Should not create a new model
+			expect(models.find((m) => m.id === "nonexistent/model-id")).toBeUndefined();
+			// Should not crash or show error
+			expect(registry.getError()).toBeUndefined();
+		});
+
+		test("model override can change cost fields partially", () => {
+			writeRawModelsJson({
+				openrouter: {
+					modelOverrides: {
+						"anthropic/claude-sonnet-4": {
+							cost: { input: 99 },
+						},
+					},
+				},
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			const models = getModelsForProvider(registry, "openrouter");
+			const sonnet = models.find((m) => m.id === "anthropic/claude-sonnet-4");
+
+			// Input cost should be overridden
+			expect(sonnet?.cost.input).toBe(99);
+			// Other cost fields should be preserved from built-in
+			expect(sonnet?.cost.output).toBeGreaterThan(0);
+		});
+
+		test("model override can add headers", () => {
+			writeRawModelsJson({
+				openrouter: {
+					modelOverrides: {
+						"anthropic/claude-sonnet-4": {
+							headers: { "X-Custom-Model-Header": "value" },
+						},
+					},
+				},
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			const models = getModelsForProvider(registry, "openrouter");
+			const sonnet = models.find((m) => m.id === "anthropic/claude-sonnet-4");
+
+			expect(sonnet?.headers?.["X-Custom-Model-Header"]).toBe("value");
+		});
+
+		test("refresh() picks up model override changes", () => {
+			writeRawModelsJson({
+				openrouter: {
+					modelOverrides: {
+						"anthropic/claude-sonnet-4": {
+							name: "First Name",
+						},
+					},
+				},
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			expect(
+				getModelsForProvider(registry, "openrouter").find((m) => m.id === "anthropic/claude-sonnet-4")?.name,
+			).toBe("First Name");
+
+			// Update and refresh
+			writeRawModelsJson({
+				openrouter: {
+					modelOverrides: {
+						"anthropic/claude-sonnet-4": {
+							name: "Second Name",
+						},
+					},
+				},
+			});
+			registry.refresh();
+
+			expect(
+				getModelsForProvider(registry, "openrouter").find((m) => m.id === "anthropic/claude-sonnet-4")?.name,
+			).toBe("Second Name");
+		});
+
+		test("removing model override restores built-in values", () => {
+			writeRawModelsJson({
+				openrouter: {
+					modelOverrides: {
+						"anthropic/claude-sonnet-4": {
+							name: "Custom Name",
+						},
+					},
+				},
+			});
+
+			const registry = new ModelRegistry(authStorage, modelsJsonPath);
+			const customName = getModelsForProvider(registry, "openrouter").find(
+				(m) => m.id === "anthropic/claude-sonnet-4",
+			)?.name;
+			expect(customName).toBe("Custom Name");
+
+			// Remove override and refresh
+			writeRawModelsJson({});
+			registry.refresh();
+
+			const restoredName = getModelsForProvider(registry, "openrouter").find(
+				(m) => m.id === "anthropic/claude-sonnet-4",
+			)?.name;
+			expect(restoredName).not.toBe("Custom Name");
+		});
+	});
+
 	describe("API key resolution", () => {
 		/** Create provider config with custom apiKey */
 		function providerWithApiKey(apiKey: string) {