# Design: Algorithm Rewrite — Per-Article Classification, No Rewrite Pass

**Date**: 2026-03-25
**Scope**: Complete rewrite of synthesis generation pipeline. Per-article LLM classify/summarize, source rotation, remove rewrite pass, remove deprecated settings.

---

## Context

The current pipeline has grown complex: batch classification, separate rewrite pass, URL restoration, "Autre" fill-up, LLM article extraction, source diversity window. The new algorithm simplifies to: scrape each article → LLM classify/summarize per article → stop when categories are full. No batch steps, no rewrite pass.

## New Algorithm

See `docs/algorithm.md` for the complete specification.

### Key Changes from Current Code

1. **Per-article LLM call** replaces batch classification + batch rewrite. Each article gets one LLM call returning `{title, summary, category}`.
2. **Source rotation** — rolling window based on last source used in article_history.
3. **No rewrite pass** — summaries are final from the per-article call.
4. **No URL restoration** — no rewrite means no hallucinated URLs to fix.
5. **No "Autre" fill-up** — the 75% target logic is removed.
6. **No LLM article extraction** — removed, per-article LLM call handles content directly.
7. **Phase 2 summaries are final** — search LLM returns title/url/summary, scraping is validation only.

## Removed Settings (Destructive Migration)

```sql
ALTER TABLE settings DROP COLUMN source_diversity_window;
ALTER TABLE settings DROP COLUMN use_llm_for_article_extraction;
```

## Removed Code

- `SYNTHESIS_MIN_FILL_RATIO` constant + fill-up logic
- `scrape_single_article_with_llm` function
- `scrape_article_dispatch` / LLM extraction branching in `scrape_flat_urls` and `scrape_articles`
- `build_rewrite_prompt` / `build_rewrite_schema`
- `build_classification_prompt` / `build_classification_schema`
- `build_article_extraction_prompt` / `build_article_extraction_schema`
- `parse_classification_response`
- `filter_empty_scraped_articles`
- `restore_scraped_urls`
- `limit_articles_per_source` / `dedup_by_url` / `filter_homepage_urls` (dedup happens inline)
- `build_rewrite_schema` / `build_final_sections` (no rewrite pass)
- `scrape_articles` (replaced by per-article scraping in the main loop)
- `scrape_flat_urls` (replaced by per-article scraping in the main loop)
- `head_html` field from `ScrapedContent` (LLM extraction removed)

## New Code

### New prompt: `build_article_classify_prompt`

```rust
pub fn build_article_classify_prompt(
    title: &str,
    body_snippet: &str,
    categories: &[String], // includes "Autre"
) -> (String, String)
```

Asks the LLM to classify the article into a category and generate a title + 4-5 line summary.

### New schema: `build_article_classify_schema`

```json
{
  "type": "object",
  "properties": {
    "title": { "type": "string" },
    "summary": { "type": "string" },
    "category": { "type": "string" }
  },
  "required": ["title", "summary", "category"],
  "additionalProperties": false
}
```

### New DB query: `get_last_source_url`

```rust
pub async fn get_last_source_url(pool: &PgPool, user_id: Uuid) -> Result<Option<String>, AppError>
```

Returns the `source_url` from the most recent `used` entry for source rotation.

### Source rotation

Rotate the user's source list so that the source following the last-used source comes first.

## Rewritten `run_generation_inner` Flow

```
Initialization:
  Load settings, sources, provider, models, rate limiter
  Cleanup article history + LLM logs
  Build classification categories (user categories + "Autre")
  Initialize: article_scraped, source_counts, url_source, filled_counts, seen_urls

Phase 1 (if sources not empty):
  1a. Rotate sources (rolling window via get_last_source_url)
  For each source:
    Extract article links (LLM or heuristic, max 10)
    Deduplicate + filter against article history
    Track url → source mapping

  1b. For each candidate URL:
    Check source_counts → skip if exceeded max_articles_per_source (trace: filtered_diversity)
    Scrape article (heuristic only — title, date, body, soft-404)
    Skip if empty/failed (trace: filtered_empty)
    LLM call: classify + summarize → {title, summary, category} (logged)
    If category full → assign to "Autre"
    Add to article_scraped + update filled_counts + source_counts
    If total >= (num_categories_including_autre) × max_items_per_category → break

Phase 2 (if any user category under-filled):
  2a. Compute gaps per category
  2b. LLM search call → {category_0: [{title, url, summary}], ...} (logged)
  2c. Filter: homepage, cross-phase dedup, url dedup, source limit, history (all traced)
  Scrape for validation (filter empty, trace drops)
  Merge into article_scraped

Save:
  Sanitize null bytes
  Build sections from article_scraped (group by category key → NewsSection)
  Save synthesis with job_id
  Record used articles in article_history
```

## ScrapedContent Simplification

Remove `head_html: String` field (LLM extraction removed). Keep `url: String` (redirect-resolved URL).

## Files to Modify

**Migration:**
- **Create:** migration to drop `source_diversity_window` and `use_llm_for_article_extraction`

**Backend — rewrite:**
- **Rewrite:** `backend/src/services/synthesis.rs` — new `run_generation_inner`, remove all batch/rewrite/fill-up code
- **Simplify:** `backend/src/services/scraper.rs` — remove `head_html` from `ScrapedContent`
- **Modify:** `backend/src/services/prompts.rs` — remove old prompts, add `build_article_classify_prompt`
- **Modify:** `backend/src/services/llm/schema.rs` — remove old schemas, add `build_article_classify_schema`
- **Modify:** `backend/src/models/settings.rs` — remove 2 fields from all structs + Default + validation
- **Modify:** `backend/src/db/settings.rs` — remove from queries
- **Modify:** `backend/src/db/article_history.rs` — add `get_last_source_url`
- **Modify:** `backend/src/models/synthesis.rs` — remove `source_url` from `ScrapedNewsItem` if no longer needed, or keep for tracing

**Frontend:**
- **Modify:** `frontend/src/types.ts` — remove 2 fields
- **Modify:** `frontend/src/pages/Settings.tsx` — remove diversity window input + LLM extraction checkbox
- **Modify:** `frontend/src/i18n/fr.ts` — remove labels
- **Modify:** `e2e/tests/generation-live.spec.ts` — update settings payload

## What Does NOT Change

- `source_scraper.rs` — link extraction (both heuristic and LLM) stays as-is
- `article_history` table + tracing — stays, used for dedup + provenance
- `llm_call_log` table — stays, logging per LLM call
- `use_llm_for_source_links` setting — stays
- `max_articles_per_source` setting — stays
- `article_history_days` setting — stays
- LLM provider trait (`call_llm`) — stays
- Frontend: ArticleHistory page, LlmLogs page, provenance section — all stay