app/agent/geminiBackend.ts

 // Google AI Studio (Gemini API) as a remote alternative to local

 // Ollama -- mainly to isolate how much of this project's slowness is

 // local-model compute vs. the surrounding system, since a production-

 // grade remote serving stack removes the local-hardware variable

 // entirely. Verified request/response shape directly against Google's

 // own API reference (contents[]/parts[], systemInstruction,

 // functionCall/functionResponse) rather than assumed.

 import type { ChatBackend, ChatMessage } from "./chatBackend.js";

 import type { ToolDef } from "./tools.js";

 function apiKey(): string {

  const key = process.env.GEMINI_API_KEY ?? process.env.GOOGLE_AI_API_KEY;

  if (!key) throw new Error('Gemini backend needs an API key: set GEMINI_API_KEY (or GOOGLE_AI_API_KEY) in your environment or app/.env. Get a free-tier key at https://aistudio.google.com/apikey -- never paste the key itself into chat.');

  return key;

 }

 function toGeminiTools(tools: ToolDef[]) {

  return [{ functionDeclarations: tools.map((t) => ({ name: t.name, description: t.description, parameters: t.parameters })) }];

 }

 interface GeminiPart {

  text?: string;

  functionCall?: { name: string; args: Record<string, unknown>; thoughtSignature?: string };

  functionResponse?: { name: string; response: Record<string, unknown> };

 }

 // Gemini has no "system" role and no "tool" role: the system message

 // becomes systemInstruction (pulled out separately, never part of

 // contents[]), and a tool result becomes a "user"-role functionResponse

 // part -- its own real convention, not a guess, per the verified docs.

 function toGeminiContents(messages: ChatMessage[]): { systemInstruction: { parts: GeminiPart[] } | undefined; contents: { role: string; parts: GeminiPart[] }[] } {

  let systemInstruction: { parts: GeminiPart[] } | undefined;

  const contents: { role: string; parts: GeminiPart[] }[] = [];

  for (const m of messages) {

  if (m.role === "system") {

  systemInstruction = { parts: [{ text: m.content }] };

  continue;

  }

  if (m.role === "tool") {

  let response: Record<string, unknown>;

  try {

  response = JSON.parse(m.content);

  } catch {

  response = { result: m.content };

  }

  contents.push({ role: "user", parts: [{ functionResponse: { name: m.name ?? "unknown", response } }] });

  continue;

  }

  const parts: GeminiPart[] = [];

  if (m.content) parts.push({ text: m.content });

  for (const call of m.tool_calls ?? []) {

  // Prefer replaying the exact functionCall part Gemini gave us

  // (thoughtSignature and all) over reconstructing one -- a

  // reconstructed call missing thoughtSignature is a real,

  // confirmed 400 from the API, not just a quality nit.

  const raw = call.raw as GeminiPart["functionCall"] | undefined;

  parts.push({ functionCall: raw ?? { name: call.function.name, args: call.function.arguments } });

  }

  contents.push({ role: m.role === "assistant" ? "model" : "user", parts });

  }

  return { systemInstruction, contents };

 }

 function fromGeminiResponse(data: { candidates?: { content?: { parts?: GeminiPart[] } }[] }): ChatMessage {

  const parts = data.candidates?.[0]?.content?.parts ?? [];

  let content = "";

  const toolCalls: NonNullable<ChatMessage["tool_calls"]> = [];

  for (const p of parts) {

  if (p.text) content += p.text;

  if (p.functionCall) toolCalls.push({ function: { name: p.functionCall.name, arguments: p.functionCall.args }, raw: p.functionCall });

  }

  return { role: "assistant", content, tool_calls: toolCalls.length ? toolCalls : undefined };

 }

 export function createGeminiBackend(model: string): ChatBackend {

  return {

  name: `gemini:${model}`,

  async chat(messages, tools, onPartial) {

  const { systemInstruction, contents } = toGeminiContents(messages);

  const res = await fetch(`https://generativelanguage.googleapis.com/v1beta/models/${model}:generateContent`, {

  method: "POST",

  body: JSON.stringify({ contents, systemInstruction, ...(tools.length ? { tools: toGeminiTools(tools) } : {}) }),

  });

  if (!res.ok) throw new Error(`gemini chat failed: ${res.status} ${await res.text()}`);

  const data = await res.json();

  const message = fromGeminiResponse(data);

  // No streaming here yet (this call is non-streaming) -- still

  // report the final content once, so callers that only care about

  // "what's the latest text" (a TUI status line) don't need a

  // separate non-streaming code path.

  if (message.content) onPartial?.(message.content);

  return message;

  },

  };

 }