Build extensible agent harness with interactive chat loop

src/agent.ts

@@ -0,0 +1,89 @@
+import type { AgentMessage, ModelClient } from "./types.js";
+import { ToolRegistry } from "./tools/registry.js";
+import type { EventBus } from "./events.js";
+
+export type AgentOptions = {
+  cwd: string;
+  maxTurns?: number;
+  events?: EventBus;
+};
+
+export class Agent {
+  private readonly history: AgentMessage[] = [];
+
+  constructor(
+    private readonly model: ModelClient,
+    private readonly tools: ToolRegistry,
+    private readonly options: AgentOptions
+  ) {}
+
+  async runTurn(prompt: string): Promise<string> {
+    const events = this.options.events;
+    const maxTurns = this.options.maxTurns ?? 20;
+
+    this.history.push({ role: "user", content: prompt });
+    await events?.emit("agent.started", { maxTurns });
+
+    for (let turn = 0; turn < maxTurns; turn += 1) {
+      await events?.emit("agent.turn.started", { turn: turn + 1 });
+      const result = await this.model.respond(this.history, this.tools.list());
+      this.history.push(...(result.output as AgentMessage[]));
+
+      if (result.toolCalls.length === 0) {
+        await events?.emit("agent.completed", { turn: turn + 1 });
+        return result.outputText;
+      }
+
+      for (const call of result.toolCalls) {
+        await events?.emit("tool.call.requested", { tool: call.name });
+        const tool = this.tools.get(call.name);
+
+        if (!tool) {
+          await events?.emit("tool.call.unknown", { tool: call.name });
+          this.history.push({
+            type: "function_call_output",
+            call_id: call.id,
+            output: JSON.stringify({ error: `Unknown tool: ${call.name}` })
+          });
+          continue;
+        }
+
+        try {
+          const parsed = JSON.parse(call.arguments);
+          await events?.emit("tool.call.started", { tool: call.name, input: parsed });
+          const output = await tool.execute(parsed, { cwd: this.options.cwd });
+          await events?.emit("tool.call.completed", { tool: call.name });
+
+          this.history.push({
+            type: "function_call_output",
+            call_id: call.id,
+            output: JSON.stringify(output)
+          });
+        } catch (error) {
+          await events?.emit("tool.call.failed", {
+            tool: call.name,
+            error: error instanceof Error ? error.message : "Tool execution failed"
+          });
+          this.history.push({
+            type: "function_call_output",
+            call_id: call.id,
+            output: JSON.stringify({
+              error: error instanceof Error ? error.message : "Tool execution failed"
+            })
+          });
+        }
+      }
+    }
+
+    await events?.emit("agent.failed", { reason: "max_turns_exceeded", maxTurns });
+    throw new Error(`Agent exceeded max turns (${maxTurns}).`);
+  }
+
+  getHistory(): AgentMessage[] {
+    return [...this.history];
+  }
+
+  reset(): void {
+    this.history.length = 0;
+  }
+}