# LLM-fallback CSV parser — Design Spec

**Date:** 2026-05-27
**Status:** Draft — pending implementation plan

## Context

Today the only supported broker import is Trading 212. `parse_t212_csv` expects
T212's exact column set (`Slice`, `Owned quantity`, etc.) and raises
`CSVImportError` on anything else. Every non-T212 user hits a wall at
onboarding.

Rather than write a hand-rolled parser per broker (IBKR, Vanguard, Fidelity,
Schwab, eToro, Degiro, …) — and chase format drift forever — we use an LLM as
a transparent fallback. The LLM never sees holdings as data; it only looks at
**headers plus a handful of sample rows** and returns a JSON column-mapping.
Our existing Python code does the row iteration.

The first time a broker format appears, the LLM produces a mapping. We
fingerprint the format (sha256 of normalized headers) and cache the mapping
in a new `csv_format_templates` table. Every subsequent upload of the same
format — by any user — replays the cached mapping deterministically, with no
LLM call.

The cache row stores the header row and a single anonymous sample data row
(the first row from the originating upload, verbatim). No user identifier is
recorded — the row is not linked back to whoever uploaded it. The purpose of
the sample is to give the operator material to look at when designing future
native parsers; this collection is **passive learning only**, the system
never attempts to author or modify parser code automatically.

Portfolio import is already advertised as a paid-only feature; we make that
explicit at the route level as part of this work.

## Goals

- Accept CSV exports from any broker, not just T212.
- Pay the LLM cost only once per **format**, not once per user.
- Never persist user holdings on the server (already a system-wide invariant).
- Surface the same response shape to the browser regardless of which parser
  branch ran — no client changes beyond a copy tweak.

## Non-goals

- Per-broker UI customisation. The drop-zone stays generic.
- A human admin queue for reviewing LLM-discovered formats. Operator can
  inspect rows directly in the DB if curious.
- **Auto-promoting learned formats to native parsers.** The operator will
  hand-write any native parser by looking at the collected sample rows. The
  system never writes or modifies code.
- Self-healing or auto-evicting stale cache entries. If a broker silently
  changes their export shape under us, the cached mapping will start
  producing parse errors; the operator deletes the row manually. We do not
  invalidate cache entries automatically.
- Multi-stage / verification LLM passes. One call per first-time format.

## Architecture

```
POST /api/portfolio/parse  (paid-only)
├─ parse_t212_csv(raw)             ── happy path, unchanged
│   └─ CSVImportError ↴
│
├─ parse_with_llm(raw, session)
│   ├─ detect delimiter + preamble offset
│   ├─ fingerprint = sha256(normalised headers)
│   ├─ SELECT csv_format_templates WHERE fingerprint=?
│   │   ├─ HIT  → apply mapping (bump use_count/last_used_at after successful parse)
│   │   └─ MISS → openrouter.call_llm(headers + 3-5 sample rows)
│   │             → validate mapping
│   │             → INSERT csv_format_templates
│   │             → apply mapping
│   └─ returns ParsedPie  (same shape as T212 path)
│
└─ resolve_slice → upsert_tickers → inline Yahoo fetch → JSON response
   (existing pipeline, unchanged)
```

### Why column-mapping, not full extraction

We pass the LLM only **headers plus 3–5 sample rows**, not the full CSV. The
LLM returns column names, not transcribed numbers. Three benefits:

1. **Safety** — LLMs hallucinate digits; they don't hallucinate column names
   that aren't there. Mapping validation can verify every named column exists
   in the actual header row.
2. **Cost** — prompt is ~1 KB regardless of portfolio size.
3. **Cacheability** — the mapping IS the cache. Replay is deterministic Python,
   no LLM in the loop on re-imports.

### Why global cache, not per-user

The column structure of an IBKR Activity Statement is a property of IBKR, not
of any individual user. The cache row contains no user identifier — the
sample data row is stored verbatim but anonymously, with nothing linking it
to the uploader. Global cache is strictly better: faster onboarding for the
second IBKR user, and the collected samples form a small, useful corpus for
hand-writing native parsers later.

## Data model

New table `csv_format_templates`:

| Column | Type | Notes |
|---|---|---|
| `id` | int PK | |
| `fingerprint` | `VARCHAR(64) UNIQUE NOT NULL` | sha256 hex of normalised header tuple |
| `headers` | JSON | List of strings — actual header row from the upload |
| `sample_row` | JSON | First data row from the originating upload, verbatim. Not linked to any user. |
| `mapping` | JSON | `{ticker_col, qty_col, name_col, cost_col, currency_col}` |
| `preamble_rows` | INT NOT NULL DEFAULT 0 | Non-data lines before the header row |
| `delimiter` | CHAR(1) NOT NULL DEFAULT ',' | |
| `broker_label` | VARCHAR(128) | LLM-identified label, e.g. "Interactive Brokers Activity Statement" |
| `first_seen_at` | DATETIME(tz) NOT NULL | When the format was first cached |
| `use_count` | INT NOT NULL DEFAULT 1 | Bumped on each successful cache hit |
| `last_used_at` | DATETIME(tz) NOT NULL | |
| `llm_model` | VARCHAR(64) | Provenance of the initial extraction |
| `llm_cost_usd` | FLOAT | Same |

Migration: `alembic/versions/0021_csv_format_template.py` (based on `0020`).

The full uploaded CSV is **not** stored — only the header row plus a single
data row (`sample_row`). No `user_id` column exists on this table; the sample
is anonymous by construction. This is a deliberate, narrow exception to the
otherwise-strict "no holdings persisted" invariant: we keep one row per
format so the operator has concrete material to look at when hand-writing a
future native parser. One anonymous row carries no portfolio context (no
totals, no other positions) and cannot be linked back to an account.

## Components

### `app/services/llm_csv_parser.py` — new

Public surface:

```python
async def parse_with_llm(
    raw: bytes,
    session: AsyncSession,
) -> ParsedPie:
    """LLM-fallback CSV parser.

    Decodes raw bytes, detects delimiter and preamble offset, fingerprints
    the header row, hits the csv_format_templates cache. On miss, calls
    openrouter.call_llm with headers + 3-5 sample rows to extract a
    column-mapping, validates it, persists a new template, and applies the
    mapping. Returns the same ParsedPie shape as parse_t212_csv.
    """

class LLMParseError(ValueError):
    """Raised when the LLM call fails or returns an unusable mapping."""
```

Internal helpers (not exported):

- `_detect_dialect(raw: bytes) -> tuple[str, int]` — returns `(delimiter, preamble_rows)`. Uses Python's `csv.Sniffer` for delimiter, then walks rows until the first row whose tokens look like column headers (heuristic: all-strings, none parse as numbers).
- `_fingerprint(headers: list[str]) -> str` — lowercases, strips whitespace, joins with `|`, returns sha256 hex.
- `_extract_mapping_via_llm(client, headers, samples) -> dict` — builds the system prompt, calls `openrouter.call_llm`, parses the JSON envelope, raises `LLMParseError` on malformed output.
- `_validate_mapping(mapping, headers, first_row) -> None` — every named column must exist in `headers`; `qty_col`'s value on `first_row` must parse as a positive number; `cost_col` (if present) must parse as a number. Raises `LLMParseError` on failure.
- `_apply_mapping(rows, mapping) -> ParsedPie` — iterates remaining rows, builds `ParsedPosition` instances, computes totals from `qty * avg_cost` when explicit totals aren't present.

Reuses without modification:

- `app/services/openrouter.py::call_llm` — provider fallback chain + AICall ledger logging
- `app/services/csv_import.py::ParsedPie, ParsedPosition, CSVImportError` — same return type, same error hierarchy. `LLMParseError` inherits from `CSVImportError` so the route can catch both as one.

### `app/routers/universe.py::parse_portfolio` — modified

Two small changes:

1. Add `Depends(require_paid)` to the route decorator. (Portfolio import has always been advertised as paid; this aligns the implementation.)
2. Wrap the existing `parse_t212_csv` call in a try/except that falls through to `parse_with_llm` on `CSVImportError`:

```python
try:
    pie = parse_t212_csv(raw)
except CSVImportError:
    from app.services.llm_csv_parser import parse_with_llm, LLMParseError
    try:
        pie = await parse_with_llm(raw, session)
    except LLMParseError as e:
        raise HTTPException(status_code=400, detail=str(e))
```

Everything below this point in the function — resolve_slice loop, upsert_tickers, inline Yahoo fetch, response build — is unchanged. `pie` has the same shape regardless of branch.

### `app/models.py` — new model

`CsvFormatTemplate` declared alongside the other tables. Columns as in the data model table above.

### `app/templates/settings.html` — copy tweak

- Section heading: "Import portfolio (Trading 212 CSV)" → "Import portfolio (CSV)"
- Drop-zone label: "Drop a T212 pie CSV here" → "Drop your broker's portfolio CSV here"
- Drop-zone hint: append " · T212, IBKR, and others auto-detected" after the size limit
- The "Export your pie from T212" instructions paragraph stays as a help link — T212 is still the best-documented happy path — but its phrasing softens to "If you use Trading 212…"

## LLM prompt shape

System prompt fixes the schema. User message contains headers + samples.

```
SYSTEM: You are an expert at recognising broker portfolio CSV formats.
You will be given the header row and 3-5 sample data rows from a CSV.
Identify which column contains each field. Return ONLY JSON, no prose.

Schema:
{
  "ticker_col": "<header name or null>",
  "qty_col":    "<header name or null>",
  "name_col":   "<header name or null>",
  "cost_col":   "<header name or null>",    // average price per share or unit cost
  "currency_col": "<header name or null>",
  "broker_label": "<short identifier like 'IBKR Activity Statement' or null>"
}

Rules:
- Use null when no column is a good match.
- ticker_col and qty_col are required; if either is missing return all nulls.
- Use the EXACT header string as it appears in the input.

USER: headers: ["Symbol","Position","Avg Price","Currency"]
samples:
  AAPL,100,150.00,USD
  MSFT,50,300.00,USD
  ...
```

The LLM never sees the entire file; it sees only the first ~5 data rows.
Token cost is bounded and uniform regardless of portfolio size.

## Error handling

| Failure | Response | Ledger |
|---|---|---|
| LLM provider down | 502 "couldn't parse — try again later" | AICall status=failed |
| LLM returns non-JSON | 400 "couldn't recognise as portfolio CSV" | AICall status=ok, no template stored |
| Mapping missing required columns (ticker/qty) | 400 same | AICall status=ok, no template stored |
| Mapping references non-existent column | 400 same | AICall status=ok, no template stored |
| Mapping validates but row parse fails on numerics | 400 same | template NOT stored |
| Cache hit but row parse fails (format drifted under us) | 400 with parse error | — |

If a broker quietly changes their CSV shape such that a previously-good
cached mapping starts producing parse failures, the user sees an error and
the operator deletes the offending `csv_format_templates` row by hand. No
automatic eviction, no automatic retry. The cache is a learning store, not
a self-managing system.

## Testing

`tests/test_llm_csv_parser.py`:

- **Fingerprint stability** — case/whitespace/BOM variants of the same headers hash to the same fingerprint.
- **Cache hit path** — pre-populate a `CsvFormatTemplate` row, mock `call_llm` to fail loudly, assert it is NOT called, assert positions come out correct, assert `use_count` is incremented.
- **Cache miss path** — mock `call_llm` to return a valid mapping JSON, assert a row is inserted with the upload's actual first data row as `sample_row` and no user_id anywhere, assert positions come out correct.
- **LLM returns malformed JSON** — raises `LLMParseError`, no template stored.
- **LLM maps to non-existent column** — raises `LLMParseError`, no template stored.
- **LLM maps qty to a non-numeric column** — raises `LLMParseError` on validation.
- **Stale cached mapping on parse failure** — pre-populate a template whose mapping no longer matches the file content, assert a 400 is returned and the template is NOT deleted automatically (operator owns eviction).
- **Integration** — POST a fabricated IBKR-shaped fixture to `/api/portfolio/parse`, assert ParsedPie round-trips, assert no second LLM call on a repeat upload.

Existing `tests/test_csv_import.py` must still pass — the T212 happy path is unchanged.

## Verification

End-to-end manual check after deploy:

1. Upload a T212 fixture → exists path stays unchanged (same dashboard load behaviour).
2. Upload a fabricated IBKR CSV → first upload calls LLM, returns positions, template row created in DB.
3. Re-upload the same IBKR CSV → second call has zero LLM cost (verify by counting `ai_calls` rows before/after), `use_count` increments to 2.
4. Inspect `csv_format_templates` row: confirm `headers` matches the upload's headers, `sample_row` is the first real data row, no `user_id` column exists on the table.
5. Upload random garbage (e.g. a screenshot renamed `.csv`) → 400 with clean error, no template stored, AICall row logged.
6. Free-tier account attempts import → 402 (paid gating).

## Open questions for the implementation plan

- Whether to read sample rows with `csv.reader` and re-encode them as text for the LLM (safer for embedded commas/quotes), or pass the raw first-N-lines verbatim. Default: the safer reader path.
- Whether to cap LLM-parsed portfolios at the same 1 MB limit as T212 (yes) and whether to add a separate cap on number-of-rows fed to the LLM as samples (yes, 5).
- Whether to log the fingerprint to the request log on cache hit/miss for operability. Default: yes, at INFO level, with `event_type="csv.format.cache_hit"` / `"csv.format.cache_miss"`.