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# Response Synthesis
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Response generation and synthesis strategies for combining retrieved information into coherent answers in LlamaIndex.TS.
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## Import
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```typescript
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import { ResponseSynthesizer } from "llamaindex";
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```
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## Overview
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Response synthesis combines retrieved information from multiple sources into coherent, comprehensive answers using various strategies optimized for different use cases.
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## Base Synthesizer Interface
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```typescript { .api }
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interface BaseSynthesizer {
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  synthesize(query: string, nodes: BaseNode[]): Promise<EngineResponse>;
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}
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```
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## Response Synthesizer
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```typescript { .api }
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class ResponseSynthesizer implements BaseSynthesizer {
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  constructor(options?: {
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    responseMode?: ResponseMode;
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    serviceContext?: ServiceContext;
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  });
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  synthesize(query: string, nodes: BaseNode[]): Promise<EngineResponse>;
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  responseMode: ResponseMode;
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}
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type ResponseMode = "refine" | "compact" | "tree_summarize" | "simple_summarize";
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```
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## Response Modes
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### Tree Summarize
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Best for comprehensive answers from multiple sources.
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```typescript
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const treeSynthesizer = new ResponseSynthesizer({
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  responseMode: "tree_summarize",
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});
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```
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### Refine Mode
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Iteratively refines answers with each chunk.
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```typescript
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const refineSynthesizer = new ResponseSynthesizer({
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  responseMode: "refine",
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});
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```
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### Compact Mode
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Combines chunks to maximize context usage.
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```typescript
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const compactSynthesizer = new ResponseSynthesizer({
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  responseMode: "compact",
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});
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```
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## Usage with Query Engines
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```typescript
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import { RetrieverQueryEngine, ResponseSynthesizer } from "llamaindex";
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const queryEngine = new RetrieverQueryEngine({
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  retriever: index.asRetriever(),
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  responseSynthesizer: new ResponseSynthesizer({
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    responseMode: "tree_summarize",
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  }),
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});
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```
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## Factory Function
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```typescript { .api }
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function createResponseSynthesizer(mode: ResponseMode): BaseSynthesizer;
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```
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## Best Practices
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```typescript
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// Choose mode based on use case
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const synthesizer = createResponseSynthesizer(
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  documents.length > 10 ? "tree_summarize" : "refine"
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);
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```