feat(llms): add transformRequestBody hook and improve prompt assembly (#480)

* feat(llms): add transformRequestBody hook and refine prompt handling

* docs(website): document transformRequestBody usage

* refactor(extension): keep function-valued config handling consistent in useAgent

* feat: simplify `transformRequestBody`

---------

Co-authored-by: Simon <10131203+gaomeng1900@users.noreply.github.com>

packages/llms/src/OpenAIClient.ts

@@ -45,6 +45,17 @@ export class OpenAIClient implements LLMClient {
 		}
 
 		modelPatch(requestBody)
+		let transformedBody: Record<string, unknown> | undefined
+		try {
+			transformedBody = this.config.transformRequestBody(requestBody)
+		} catch (error) {
+			throw new InvokeError(
+				InvokeErrorType.CONFIG_ERROR,
+				`transformRequestBody failed: ${(error as Error).message}`,
+				error
+			)
+		}
+		const finalRequestBody = transformedBody ?? requestBody
 
 		// 2. Call API
 		let response: Response
@@ -55,7 +66,7 @@ export class OpenAIClient implements LLMClient {
 					'Content-Type': 'application/json',
 					...(this.config.apiKey && { Authorization: `Bearer ${this.config.apiKey}` }),
 				},
-				body: JSON.stringify(requestBody),
+				body: JSON.stringify(finalRequestBody),
 				signal: abortSignal,
 			})
 		} catch (error: unknown) {
@@ -225,7 +236,7 @@ export class OpenAIClient implements LLMClient {
 				reasoningTokens: data.usage?.completion_tokens_details?.reasoning_tokens,
 			},
 			rawResponse: data,
-			rawRequest: requestBody,
+			rawRequest: finalRequestBody,
 		}
 	}
 }

packages/llms/src/errors.ts

@@ -14,6 +14,7 @@ export const InvokeErrorType = {
 	UNKNOWN: 'unknown',
 
 	// Non-retryable
+	CONFIG_ERROR: 'config_error', // Invalid local configuration or hook
 	AUTH_ERROR: 'auth_error', // Authentication failed
 	CONTEXT_LENGTH: 'context_length', // Prompt too long
 	CONTENT_FILTER: 'content_filter', // Content filtered

packages/llms/src/index.ts

@@ -21,6 +21,7 @@ export function parseLLMConfig(config: LLMConfig): Required<LLMConfig> {
 		apiKey: config.apiKey || '',
 		temperature: config.temperature ?? DEFAULT_TEMPERATURE,
 		maxRetries: config.maxRetries ?? LLM_MAX_RETRIES,
+		transformRequestBody: config.transformRequestBody ?? ((requestBody) => requestBody),
 		disableNamedToolChoice: config.disableNamedToolChoice ?? false,
 		customFetch: (config.customFetch ?? fetch).bind(globalThis), // fetch will be illegal unless bound
 	}

packages/llms/src/types.ts

@@ -95,6 +95,16 @@ export interface LLMConfig {
 	temperature?: number
 	maxRetries?: number
 
+	/**
+	 * Transform the final request body before sending it to the provider.
+	 * Use this to implement provider-specific request tweaks such as caching hints or custom flags.
+	 *
+	 * Return a new object, or mutate the input object and return undefined.
+	 */
+	transformRequestBody?: (
+		requestBody: Record<string, unknown>
+	) => Record<string, unknown> | undefined
+
 	/**
 	 * remove the tool_choice field from the request.
 	 * @note fix "Invalid tool_choice type: 'object'" for some LLMs.