clicktrack/lib/analysis/providers/anthropic.ts

import 'server-only';

import Anthropic from "@anthropic-ai/sdk";
import type { CTPDocument } from "@/lib/ctp/schema";
import type { AnalysisInput, AnalysisProvider } from "@/lib/analysis/providers";

// Extract the non-streaming Message type from the SDK without relying on internal paths
type AnthropicMessage = Extract<
  Awaited<ReturnType<Anthropic["messages"]["create"]>>,
  { content: unknown[] }
>;

const client = new Anthropic();

// ─── JSON Schema for structured output ───────────────────────────────────────

export const CTP_SCHEMA = {
  type: "object",
  additionalProperties: false,
  required: ["version", "metadata", "count_in", "sections"],
  properties: {
    version: { type: "string", enum: ["1.0"] },
    metadata: {
      type: "object",
      additionalProperties: false,
      required: [
        "title", "artist", "mbid", "duration_seconds",
        "contributed_by", "verified", "created_at",
      ],
      properties: {
        title:            { type: "string" },
        artist:           { type: "string" },
        mbid:             { type: ["string", "null"] },
        duration_seconds: { type: "number" },
        contributed_by:   { type: "string" },
        verified:         { type: "boolean" },
        created_at:       { type: "string" },
      },
    },
    count_in: {
      type: "object",
      additionalProperties: false,
      required: ["enabled", "bars", "use_first_section_tempo"],
      properties: {
        enabled:                 { type: "boolean" },
        bars:                    { type: "integer", minimum: 1, maximum: 8 },
        use_first_section_tempo: { type: "boolean" },
      },
    },
    sections: {
      type: "array",
      minItems: 1,
      items: {
        type: "object",
        additionalProperties: false,
        required: ["label", "start_bar", "time_signature", "transition"],
        properties: {
          label:      { type: "string" },
          start_bar:  { type: "integer", minimum: 1 },
          bpm:        { type: "number" },
          bpm_start:  { type: "number" },
          bpm_end:    { type: "number" },
          transition: { type: "string", enum: ["step", "ramp"] },
          time_signature: {
            type: "object",
            additionalProperties: false,
            required: ["numerator", "denominator"],
            properties: {
              numerator:   { type: "integer", minimum: 1, maximum: 32 },
              denominator: { type: "integer", enum: [1, 2, 4, 8, 16, 32] },
            },
          },
        },
      },
    },
  },
};

// ─── System prompt ────────────────────────────────────────────────────────────

export const SYSTEM_PROMPT = `\
You are an expert music producer and session musician assisting cover bands with click tracks.

You will receive information about a song and must generate a CTP (Click Track Protocol) document describing the song's full tempo map.

**Use your training knowledge of the specific song.** If you recognise the title and artist, use what you know about its actual structure: section names, bar counts, time signature, and any tempo changes (ritardando, double-time, key change with tempo shift). Your training data is a valuable source — do not ignore it in favour of generic guesses.

CTP rules:
- "version" must be "1.0"
- sections[0].start_bar must be 1
- sections must be sorted by start_bar ascending, with no gaps
- Step sections have a single "bpm" field; ramp sections have "bpm_start" and "bpm_end" (no "bpm" field)
- All BPM values must be between 20 and 400
- time_signature.denominator must be a power of 2 (1, 2, 4, 8, 16, or 32)
- metadata.verified must be false (this is AI-generated, not human-verified)
- metadata.created_at must be an ISO 8601 datetime string

Guidelines for section layout:
- Use typical pop/rock section names: Intro, Verse, Pre-Chorus, Chorus, Bridge, Outro
- Estimate bar counts based on song duration and BPM (bars = duration_seconds × BPM / 60 / beats_per_bar)
- Most songs are 4/4; use 3/4, 6/8, etc. if you know the song uses that meter
- If the song has a tempo change (ritardando, double-time feel, key change with tempo shift), model it with a ramp or step section
- If unsure about sections, use a single constant-tempo section covering the whole song

**BPM authority:** When a "Confirmed BPM" is provided in the user message, it comes from
MusicBrainz community tags or a reliable reference — treat it as ground truth and use it
for all sections unless you know the song has a significant tempo change. Do not average it
with the detected BPM or discard it. When only a "Detected BPM" is provided, use it as a
starting point but apply your knowledge of the song if you recognise it.

The output is a draft for human review. Add reasonable section structure based on the song's typical arrangement.`;

// ─── Provider implementation ──────────────────────────────────────────────────

export const anthropicProvider: AnalysisProvider = {
  id: "anthropic",
  label: "Claude (Anthropic)",
  type: "cloud-ai",

  async isAvailable() {
    if (process.env.ANTHROPIC_API_KEY) {
      return { available: true };
    }
    return { available: false, reason: "ANTHROPIC_API_KEY not set" };
  },

  async generateCTP(input: AnalysisInput): Promise<CTPDocument> {
    const { bpm, duration, title, artist, mbid, contributed_by, confirmedBpm, confirmedTimeSigNum } = input;
    const model = process.env.ANTHROPIC_MODEL ?? "claude-opus-4-6";
    const effectiveBpm = confirmedBpm ?? bpm;
    const approxBars = Math.round((duration * effectiveBpm) / 60 / 4);

    const bpmLine = confirmedBpm
      ? `Confirmed BPM (from MusicBrainz community tags — treat as authoritative): ${confirmedBpm}\nDetected BPM (audio analysis): ${bpm}`
      : `Detected BPM (audio analysis): ${bpm}`;

    const timeSigHint = confirmedTimeSigNum
      ? `\nConfirmed time signature numerator: ${confirmedTimeSigNum}`
      : "";

    const userMessage = `\
Generate a CTP document for the following song:

Title: ${title ?? "Unknown Title"}
Artist: ${artist ?? "Unknown Artist"}
MusicBrainz ID: ${mbid ?? "unknown"}
${bpmLine}${timeSigHint}
Duration: ${duration.toFixed(1)} seconds (~${approxBars} bars at 4/4)
Contributed by: ${contributed_by}

If you recognise this song, use your training knowledge of its actual arrangement — section names, bar counts, time signature, and any tempo changes. If you do not recognise it, use a sensible generic structure based on the BPM and duration above.`;

    // thinking and output_config are not yet in the SDK type definitions;
    // cast through the base param type to avoid type errors.
    type ExtendedParams = Parameters<typeof client.messages.create>[0] & {
      thinking?: { type: string };
      output_config?: { format: { type: string; schema: unknown } };
    };

    const params: ExtendedParams = {
      model,
      max_tokens: 2048,
      thinking: { type: "adaptive" },
      system: SYSTEM_PROMPT,
      messages: [{ role: "user", content: userMessage }],
      output_config: {
        format: {
          type: "json_schema",
          schema: CTP_SCHEMA,
        },
      },
    };

    const response = (await client.messages.create(
      params as Parameters<typeof client.messages.create>[0]
    )) as AnthropicMessage;

    const textBlock = response.content.find((b) => b.type === "text");
    if (!textBlock || textBlock.type !== "text") {
      throw new Error("Claude did not return a text block");
    }

    let parsed: unknown;
    try {
      parsed = JSON.parse(textBlock.text);
    } catch {
      throw new Error(`Claude returned invalid JSON: ${textBlock.text.slice(0, 200)}`);
    }

    const doc = parsed as CTPDocument;
    if (!doc.metadata.created_at || doc.metadata.created_at.includes("placeholder")) {
      doc.metadata.created_at = new Date().toISOString();
    }

    return doc;
  },
};