feat: add chatWithSearch method to BaseAgent

Enables any agent to use web search tools (search_web, fetch_url)
via a tool-calling loop (max 5 rounds). Used by auditor for
eraResearch fact verification.

packages/core/src/agents/base.ts

@@ -1,5 +1,6 @@
-import type { LLMClient, LLMMessage, LLMResponse } from "../llm/provider.js";
-import { chatCompletion } from "../llm/provider.js";
+import type { LLMClient, LLMMessage, LLMResponse, AgentMessage, ToolDefinition } from "../llm/provider.js";
+import { chatCompletion, chatWithTools } from "../llm/provider.js";
+import { searchWeb, fetchUrl } from "../utils/web-search.js";
 
 export interface AgentContext {
   readonly client: LLMClient;
@@ -8,6 +9,31 @@ export interface AgentContext {
   readonly bookId?: string;
 }
 
+const SEARCH_TOOLS: ReadonlyArray<ToolDefinition> = [
+  {
+    name: "search_web",
+    description: "搜索互联网验证事实。用于核实年代、人物、事件、地理等真实信息。",
+    parameters: {
+      type: "object",
+      properties: {
+        query: { type: "string", description: "搜索关键词" },
+      },
+      required: ["query"],
+    },
+  },
+  {
+    name: "fetch_url",
+    description: "抓取指定URL的文本内容，用于读取搜索结果的详情页。",
+    parameters: {
+      type: "object",
+      properties: {
+        url: { type: "string", description: "要抓取的URL" },
+      },
+      required: ["url"],
+    },
+  },
+];
+
 export abstract class BaseAgent {
   protected readonly ctx: AgentContext;
 
@@ -22,5 +48,61 @@ export abstract class BaseAgent {
     return chatCompletion(this.ctx.client, this.ctx.model, messages, options);
   }
 
+  /**
+   * Chat with web search tools enabled.
+   * The agent can call search_web and fetch_url during the conversation.
+   * Runs a tool-calling loop (max 5 rounds) until the model stops calling tools.
+   */
+  protected async chatWithSearch(
+    messages: ReadonlyArray<LLMMessage>,
+    options?: { readonly temperature?: number; readonly maxTokens?: number },
+  ): Promise<LLMResponse> {
+    const agentMessages: AgentMessage[] = messages.map(m => ({
+      role: m.role,
+      content: m.content,
+    }));
+
+    const maxRounds = 5;
+    for (let round = 0; round < maxRounds; round++) {
+      const result = await chatWithTools(
+        this.ctx.client, this.ctx.model, agentMessages, SEARCH_TOOLS, options,
+      );
+
+      agentMessages.push({
+        role: "assistant" as const,
+        content: result.content || null,
+        ...(result.toolCalls.length > 0 ? { toolCalls: result.toolCalls } : {}),
+      });
+
+      if (result.toolCalls.length === 0) {
+        return { content: result.content, usage: { promptTokens: 0, completionTokens: 0, totalTokens: 0 } };
+      }
+
+      for (const toolCall of result.toolCalls) {
+        const args = JSON.parse(toolCall.arguments) as Record<string, unknown>;
+        let toolResult: string;
+        try {
+          if (toolCall.name === "search_web") {
+            const searchResult = await searchWeb(args.query as string);
+            toolResult = JSON.stringify(searchResult);
+          } else if (toolCall.name === "fetch_url") {
+            const text = await fetchUrl(args.url as string, 6000);
+            toolResult = JSON.stringify({ url: args.url, content: text });
+          } else {
+            toolResult = JSON.stringify({ error: `Unknown tool: ${toolCall.name}` });
+          }
+        } catch (e) {
+          toolResult = JSON.stringify({ error: String(e) });
+        }
+        agentMessages.push({ role: "tool" as const, toolCallId: toolCall.id, content: toolResult });
+      }
+    }
+
+    // If we exhausted rounds, return the last assistant content
+    const lastAssistant = [...agentMessages].reverse().find(m => m.role === "assistant");
+    const content = (lastAssistant && "content" in lastAssistant && lastAssistant.content) || "";
+    return { content, usage: { promptTokens: 0, completionTokens: 0, totalTokens: 0 } };
+  }
+
   abstract get name(): string;
 }