stack/apps/web/src/lib/api/chat.ts

/**
 * Chat API client
 * Handles LLM chat interactions via /api/llm/chat
 */

import { apiPost, fetchCsrfToken, getCsrfToken } from "./client";
import { API_BASE_URL } from "../config";

export interface ChatMessage {
  role: "system" | "user" | "assistant";
  content: string;
}

export interface ChatRequest {
  model: string;
  messages: ChatMessage[];
  stream?: boolean;
  temperature?: number;
  maxTokens?: number;
  systemPrompt?: string;
}

export interface ChatResponse {
  model: string;
  message: {
    role: "assistant";
    content: string;
  };
  done: boolean;
  totalDuration?: number;
  promptEvalCount?: number;
  evalCount?: number;
}

/**
 * Parsed SSE data chunk from the LLM stream
 */
interface SseChunk {
  error?: string;
  message?: {
    role: string;
    content: string;
  };
  model?: string;
  done?: boolean;
}

/**
 * Send a chat message to the LLM
 */
export async function sendChatMessage(request: ChatRequest): Promise<ChatResponse> {
  return apiPost<ChatResponse>("/api/llm/chat", request);
}

/**
 * Get or refresh the CSRF token for streaming requests.
 */
async function ensureCsrfTokenForStream(): Promise<string> {
  const existing = getCsrfToken();
  if (existing) {
    return existing;
  }
  return fetchCsrfToken();
}

/**
 * Stream a chat message from the LLM using SSE over fetch.
 *
 * The backend accepts stream: true in the request body and responds with
 * Server-Sent Events:
 *   data: {"message":{"content":"token"},...}\n\n  for each token
 *   data: [DONE]\n\n  when the stream is complete
 *   data: {"error":"message"}\n\n  on error
 *
 * @param request - Chat request (stream field will be forced to true)
 * @param onChunk - Called with each token string as it arrives
 * @param onComplete - Called when the stream finishes successfully
 * @param onError - Called if the stream encounters an error
 * @param signal - Optional AbortSignal for cancellation
 */
export function streamChatMessage(
  request: ChatRequest,
  onChunk: (chunk: string) => void,
  onComplete: () => void,
  onError: (error: Error) => void,
  signal?: AbortSignal
): void {
  void (async (): Promise<void> => {
    try {
      const csrfToken = await ensureCsrfTokenForStream();

      const response = await fetch(`${API_BASE_URL}/api/llm/chat`, {
        method: "POST",
        headers: {
          "Content-Type": "application/json",
          "X-CSRF-Token": csrfToken,
        },
        credentials: "include",
        body: JSON.stringify({ ...request, stream: true }),
        signal: signal ?? null,
      });

      if (!response.ok) {
        const errorText = await response.text().catch(() => response.statusText);
        throw new Error(`Stream request failed: ${errorText}`);
      }

      if (!response.body) {
        throw new Error("Response body is not readable");
      }

      const reader = response.body.getReader();
      const decoder = new TextDecoder("utf-8");
      let buffer = "";

      let readerDone = false;
      while (!readerDone) {
        const { done, value } = await reader.read();
        readerDone = done;
        if (done) {
          break;
        }

        buffer += decoder.decode(value, { stream: true });

        // SSE messages are separated by double newlines
        const parts = buffer.split("\n\n");
        // Keep the last (potentially incomplete) part
        buffer = parts.pop() ?? "";

        for (const part of parts) {
          const trimmed = part.trim();
          if (!trimmed) continue;

          for (const line of trimmed.split("\n")) {
            if (!line.startsWith("data: ")) continue;

            const data = line.slice("data: ".length).trim();

            if (data === "[DONE]") {
              onComplete();
              return;
            }

            try {
              const parsed = JSON.parse(data) as SseChunk;

              if (parsed.error) {
                throw new Error(parsed.error);
              }

              if (parsed.message?.content) {
                onChunk(parsed.message.content);
              }
            } catch (parseErr) {
              if (parseErr instanceof SyntaxError) {
                continue;
              }
              throw parseErr;
            }
          }
        }
      }

      // Natural end of stream without [DONE]
      onComplete();
    } catch (err: unknown) {
      if (err instanceof DOMException && err.name === "AbortError") {
        return;
      }
      onError(err instanceof Error ? err : new Error(String(err)));
    }
  })();
}