Revert "cleanup: remove ModelId"

This reverts commit 7466a88.

Author: NikolaiSviridov

src/ModelId.ts

@@ -0,0 +1,48 @@
+import type {ReasoningEffort, ServiceTier} from "./app-server";
+import type {Model} from "./app-server/v2";
+
+/**
+ * ACP Model ID, combining the base model ID, reasoning effort level, and optional service tier.
+ * @example
+ * const id = ModelId.fromString("gpt-5.2[high]");
+ * const fastId = ModelId.fromString("gpt-5.2[high]@fast");
+ */
+export class ModelId {
+    private constructor(
+        public readonly model: string,
+        public readonly effort: string,
+        public readonly serviceTier: ServiceTier | null = null
+    ) {}
+
+    static fromComponents(model: Model, effort: ReasoningEffort, serviceTier: ServiceTier | null = null): ModelId {
+        return new ModelId(model.id, effort, serviceTier);
+    }
+
+    static create(modelId: string, effort: ReasoningEffort, serviceTier: ServiceTier | null = null): ModelId {
+        return new ModelId(modelId, effort, serviceTier);
+    }
+
+    static fromString(modelId: string): ModelId {
+        const bracketMatch = modelId.match(/^(?<model>[^\[]+)\[(?<effort>[^\]]+)](?:@(?<serviceTier>.+))?$/);
+        const model = bracketMatch?.groups?.["model"];
+        const effort = bracketMatch?.groups?.["effort"];
+        const serviceTier = bracketMatch?.groups?.["serviceTier"] ?? null;
+
+        if (!model || !effort) {
+            throw new Error(`Unsupported format of modelId: ${modelId}. Expected: modelId[effort] or modelId[effort]@fast.`);
+        }
+
+        // The generated app-server ServiceTier type also includes "flex", but ACP model IDs
+        // only expose Fast variants for now because model/list advertises Fast support.
+        if (serviceTier !== null && serviceTier !== "fast") {
+            throw new Error(`Unsupported service tier ${serviceTier} for modelId: ${modelId}.`);
+        }
+
+        return new ModelId(model, effort, serviceTier);
+    }
+
+    toString(): string {
+        const suffix = this.serviceTier === null ? "" : `@${this.serviceTier}`;
+        return `${this.model}[${this.effort}]${suffix}`;
+    }
+}

src/__tests__/ModelId.test.ts

@@ -0,0 +1,23 @@
+import {describe, expect, it} from "vitest";
+import {ModelId} from "../ModelId";
+
+describe("ModelId", () => {
+    it("formats and parses normal model IDs", () => {
+        const modelId = ModelId.create("gpt-5.2", "medium");
+
+        expect(modelId.toString()).toBe("gpt-5.2[medium]");
+        expect(ModelId.fromString("gpt-5.2[medium]")).toEqual(modelId);
+    });
+
+    it("formats and parses fast model IDs", () => {
+        const modelId = ModelId.create("gpt-5.2", "medium", "fast");
+
+        expect(modelId.toString()).toBe("gpt-5.2[medium]@fast");
+        expect(ModelId.fromString("gpt-5.2[medium]@fast")).toEqual(modelId);
+    });
+
+    it("rejects unknown service tiers", () => {
+        expect(() => ModelId.fromString("gpt-5.2[medium]@flex"))
+            .toThrow("Unsupported service tier flex");
+    });
+});