feat: generate LLM summary in __end__ node via ReAct loop

Instead of hardcoding 'completed: moderator returned END', the engine now
calls a summarizer (ReAct loop with cas_get tool) to produce a meaningful
summary of the workflow outcome before writing the __end__ node.

- New summarizer.ts following supervisor.ts pattern
- Uses extract scene LLM provider (falls back to raw completion.summary on failure)
- Tracks step contentHashes for summarizer context
- Schema: z.object({ summary: z.string() })

Refs #187, #188

小橘 <xiaoju@shazhou.work>

packages/workflow-execute/src/engine/engine.ts

@@ -26,6 +26,7 @@ import { END, START } from "@uncaged/workflow-runtime";
 import { err, type LogFn, ok, type Result } from "@uncaged/workflow-util";
 
 import { createExtract } from "../extract/index.js";
+import { createSummarizer, type SummarizeFn } from "./summarizer.js";
 import { runSupervisor } from "./supervisor.js";
 import {
   appendThreadHistoryEntry,
@@ -53,6 +54,7 @@ async function resolveEngineRegistryRuntime(
   Result<
     {
       extract: ReturnType<typeof createExtract>;
+      summarize: SummarizeFn;
       workflowConfig: WorkflowConfig;
     },
     string
@@ -76,7 +78,11 @@ async function resolveEngineRegistryRuntime(
     apiKey: ex.apiKey,
     model: ex.model,
   };
-  return ok({ extract: createExtract(llmProvider, { cas }), workflowConfig: cfg });
+  return ok({
+    extract: createExtract(llmProvider, { cas }),
+    summarize: createSummarizer(llmProvider, cas),
+    workflowConfig: cfg,
+  });
 }
 
 async function appendStateForStep(params: {
@@ -250,6 +256,7 @@ async function driveWorkflowGenerator(params: {
   bundleDir: string;
   startHash: string;
   chain: ChainState;
+  summarize: SummarizeFn;
 }): Promise<WorkflowResult> {
   const {
     fn,
@@ -262,6 +269,7 @@ async function driveWorkflowGenerator(params: {
     cas,
     bundleDir,
     startHash,
+    summarize,
   } = params;
   let chain: ChainState = params.chain;
   const gen = fn(thread, runtime);
@@ -270,6 +278,10 @@ async function driveWorkflowGenerator(params: {
     role: s.role,
     summary: JSON.stringify(s.meta),
   }));
+  const summarizerSteps: { role: string; contentHash: string }[] = thread.steps.map((s) => ({
+    role: s.role,
+    contentHash: s.contentHash,
+  }));
 
   while (true) {
     if (executeOptions.signal.aborted) {
@@ -288,13 +300,20 @@ async function driveWorkflowGenerator(params: {
 
     if (iterResult.done) {
       logger("F3HN8QKP", `thread ${threadId} generator finished`);
+      const rawCompletion = iterResult.value;
+      const llmSummary = await summarize({
+        prompt: thread.start.content,
+        recentSteps: summarizerSteps,
+        fallback: rawCompletion.summary,
+        logger,
+      });
       return await finalizeThread({
         cas,
         bundleDir,
         threadId,
         startHash,
         chain,
-        completion: iterResult.value,
+        completion: { ...rawCompletion, summary: llmSummary },
       });
     }
 
@@ -320,6 +339,7 @@ async function driveWorkflowGenerator(params: {
       role: step.role,
       summary: JSON.stringify(step.meta),
     });
+    summarizerSteps.push({ role: step.role, contentHash: step.contentHash });
 
     await Promise.race([
       executeOptions.awaitAfterEachYield(),
@@ -499,5 +519,6 @@ export async function executeThread(
     bundleDir,
     startHash,
     chain,
+    summarize: registryRuntime.value.summarize,
   });
 }

packages/workflow-execute/src/engine/summarizer.ts

@@ -0,0 +1,112 @@
+import type { CasStore } from "@uncaged/workflow-cas";
+import { createLlmFn, createThreadReactor } from "@uncaged/workflow-reactor";
+import type { LlmProvider } from "@uncaged/workflow-runtime";
+import type { LogFn } from "@uncaged/workflow-util";
+import * as z from "zod/v4";
+
+import { extractFunctionToolFromZodSchema } from "../extract/index.js";
+
+const SUMMARIZER_MAX_REACT_ROUNDS = 4;
+const SUMMARIZER_RECENT_STEP_LIMIT = 20;
+
+const summarySchema = z.object({ summary: z.string() }).meta({
+  title: "workflow_summary",
+  description: "A concise summary of the completed workflow's results and outcome.",
+});
+
+type SummarizerThreadContext = {
+  cas: CasStore;
+};
+
+const CAS_GET_TOOL_DEFINITION = {
+  type: "function" as const,
+  function: {
+    name: "cas_get",
+    description:
+      "Read a Merkle DAG node from content-addressed storage by its hash. Returns YAML-formatted node with type, payload, and refs.",
+    parameters: {
+      type: "object",
+      properties: {
+        hash: { type: "string", description: "The CAS hash to retrieve" },
+      },
+      required: ["hash"],
+    },
+  },
+};
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null && !Array.isArray(value);
+}
+
+function buildSummarizerInput(args: {
+  prompt: string;
+  recentSteps: readonly { role: string; contentHash: string }[];
+}): string {
+  const recent = args.recentSteps.slice(-SUMMARIZER_RECENT_STEP_LIMIT);
+  const stepsBlock = recent
+    .map((s, i) => `${i + 1}. [${s.role}] contentHash: ${s.contentHash}`)
+    .join("\n");
+  return `Original task:\n${args.prompt}\n\nCompleted steps (oldest first):\n${stepsBlock === "" ? "(none)" : stepsBlock}\n\nUse cas_get to read step content if needed. Summarize the workflow outcome concisely.`;
+}
+
+export type SummarizeFn = (args: {
+  prompt: string;
+  recentSteps: readonly { role: string; contentHash: string }[];
+  fallback: string;
+  logger: LogFn;
+}) => Promise<string>;
+
+export function createSummarizer(provider: LlmProvider, cas: CasStore): SummarizeFn {
+  const reactor = createThreadReactor<SummarizerThreadContext>({
+    llm: createLlmFn(provider),
+    maxRounds: SUMMARIZER_MAX_REACT_ROUNDS,
+    staticTools: [CAS_GET_TOOL_DEFINITION],
+    structuredToolFromSchema: (schema) => {
+      const t = extractFunctionToolFromZodSchema(schema);
+      return {
+        name: t.name,
+        tool: {
+          type: "function" as const,
+          function: {
+            name: t.name,
+            description: t.description,
+            parameters: t.parameters,
+          },
+        },
+      };
+    },
+    systemPromptForStructuredTool: (structuredToolName) =>
+      `You summarize completed workflow threads. You have access to cas_get to read step content by hash. After reviewing the steps, call the ${structuredToolName} tool with a concise summary of the workflow outcome and results. Or reply with only a JSON object such as {"summary":"..."}.`,
+    toolHandler: async (call, thread) => {
+      if (call.function.name !== "cas_get") {
+        return `Unknown tool: ${call.function.name}`;
+      }
+      let hash: string;
+      try {
+        const ta = JSON.parse(call.function.arguments) as unknown;
+        if (!isRecord(ta) || typeof ta.hash !== "string") {
+          return 'cas_get requires {"hash": "<cas-hash>"}.';
+        }
+        hash = ta.hash;
+      } catch {
+        return "cas_get arguments were not valid JSON.";
+      }
+      const blob = await thread.cas.get(hash);
+      return blob === null ? "null" : blob;
+    },
+  });
+
+  return async (args) => {
+    const result = await reactor({
+      thread: { cas },
+      input: buildSummarizerInput(args),
+      schema: summarySchema,
+    });
+    if (!result.ok) {
+      args.logger("P2WX7KNR", `summarizer failed: ${result.error}`);
+      return args.fallback;
+    }
+    args.logger("Q5MT3VBF", "summarizer produced workflow summary");
+    return result.value.summary;
+  };
+}