initial version

api/config.py

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+from typing import Literal
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+
+class Settings(BaseSettings):
+    model_config = SettingsConfigDict(env_file=".env", env_file_encoding="utf-8")
+
+    ai_mode: Literal["online", "offline"] = "offline"
+
+    # Online (Claude) settings
+    anthropic_api_key: str = ""
+    claude_model: str = "claude-sonnet-4-6"
+
+    # Offline (Ollama) settings
+    ollama_host: str = "http://localhost:11434"
+    ollama_model: str = "llama3.2:3b"
+
+    cors_origins: list[str] = ["http://localhost:5173", "http://localhost:3000"]
+
+
+settings = Settings()

api/routers/chat.py

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+from fastapi import APIRouter, HTTPException
+from pydantic import BaseModel
+
+from api.services.ai import chat
+
+router = APIRouter()
+
+
+class Message(BaseModel):
+    role: str
+    content: str
+
+
+class ChatRequest(BaseModel):
+    messages: list[Message]
+    context: dict | None = None
+
+
+class ChatResponse(BaseModel):
+    reply: str
+
+
+@router.post("/chat", response_model=ChatResponse)
+async def chat_endpoint(request: ChatRequest):
+    try:
+        reply = await chat(
+            messages=[m.model_dump() for m in request.messages],
+            context=request.context,
+        )
+        return ChatResponse(reply=reply)
+    except Exception as e:
+        raise HTTPException(status_code=500, detail=str(e))

api/services/ai.py

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+from api.config import settings
+
+
+SYSTEM_PROMPT = """You are a friendly, beginner-aware music theory assistant for WhatTheFlat — a live jam helper app.
+You help musicians understand what's happening harmonically during a jam session.
+Keep answers short, warm, and jargon-free. If you use a music term, briefly explain it.
+Never say someone played a "wrong" note — always frame it as "try these instead".
+When given context about the current key and chord, use it in your response."""
+
+
+async def chat(messages: list[dict], context: dict | None = None) -> str:
+    system = SYSTEM_PROMPT
+    if context:
+        parts = []
+        if context.get("key"):
+            parts.append(f"Current key: {context['key']}")
+        if context.get("chord"):
+            parts.append(f"Current chord: {context['chord']}")
+        if parts:
+            system += "\n\nLive session context:\n" + "\n".join(parts)
+
+    if settings.ai_mode == "online":
+        return await _chat_claude(messages, system)
+    else:
+        return await _chat_ollama(messages, system)
+
+
+async def _chat_claude(messages: list[dict], system: str) -> str:
+    import anthropic
+
+    client = anthropic.AsyncAnthropic(api_key=settings.anthropic_api_key)
+    response = await client.messages.create(
+        model=settings.claude_model,
+        max_tokens=512,
+        system=system,
+        messages=messages,
+    )
+    return response.content[0].text
+
+
+async def _chat_ollama(messages: list[dict], system: str) -> str:
+    import ollama
+
+    client = ollama.AsyncClient(host=settings.ollama_host)
+    full_messages = [{"role": "system", "content": system}] + messages
+    response = await client.chat(
+        model=settings.ollama_model,
+        messages=full_messages,
+    )
+    return response.message.content