Record which embedder wrote the stored vectors and warn on a swap (#378)

The embedder moved from paraphrase-multilingual-MiniLM-L12-v2 to
multilingual-e5-small. Both are 384-dimensional, so nothing in the code
noticed: cosine between an old stored vector and a new query vector is
noise, and recall degrades silently.

So the DB now records the embedder that wrote its vectors. One value for
the whole DB (migration #11, a small `meta` key/value table) rather than a
column on every vector row: the backfill re-embeds every note and fact in
one pass, so a per-row marker would hold the same string in every row and
cost a column on two tables for nothing.

The identity comes from the embedder itself via a new optional ID() method
("multilingual-e5-small@384", model file name plus dimension), so pointing
the config at another model changes the string without anyone editing a
constant. mavend logs a loud WARNING at startup naming both the stored and
the configured embedder when they differ.

Detection only — recall behaviour is unchanged. TODO(#378) in
store.CheckEmbedder marks where the backfill will hook in.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CGeSZxh1DCtRxmFVSYVGvJ

internal/store/meta.go

@@ -0,0 +1,65 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"fmt"
+)
+
+// metaKeyEmbedderID names the embedder that wrote the stored vectors.
+//
+// Why one value for the whole DB and not a column on every vector row: the
+// vectors are only ever rewritten all at once (one backfill re-embeds every
+// note and fact together), so a per-row marker would hold the same string in
+// every row and cost a column on two tables for nothing.
+const metaKeyEmbedderID = "embedder_id"
+
+// Meta reads a single value from the meta table. Missing key ⇒ empty string.
+func (s *Store) Meta(ctx context.Context, key string) (string, error) {
+	var v string
+	err := s.db.QueryRowContext(ctx, `SELECT value FROM meta WHERE key = ?`, key).Scan(&v)
+	if errors.Is(err, sql.ErrNoRows) {
+		return "", nil
+	}
+	if err != nil {
+		return "", fmt.Errorf("read meta %s: %w", key, err)
+	}
+	return v, nil
+}
+
+// SetMeta writes (or overwrites) a single meta value.
+func (s *Store) SetMeta(ctx context.Context, key, value string) error {
+	_, err := s.db.ExecContext(ctx,
+		`INSERT INTO meta (key, value) VALUES (?,?)
+		 ON CONFLICT(key) DO UPDATE SET value = excluded.value`, key, value)
+	if err != nil {
+		return fmt.Errorf("write meta %s: %w", key, err)
+	}
+	return nil
+}
+
+// CheckEmbedder compares the embedder now configured against the one that
+// wrote the stored vectors.
+//
+// A DB that has never recorded one is claimed for the current embedder: either
+// it is fresh (nothing stored yet, nothing to fix) or it predates this marker.
+// Returns the stored id and whether it differs from the current one.
+//
+// Vectors from two different models live in different spaces, so cosine
+// between them is noise rather than a low score — and both of our models are
+// 384-dimensional, so nothing else catches it.
+//
+// TODO(#378): on a mismatch, run the one-shot backfill here — re-embed every
+// stored note and fact text with the current embedder (EmbedPassage side),
+// write the vectors back, then SetMeta the current id.
+func (s *Store) CheckEmbedder(ctx context.Context, currentID string) (stored string, mismatch bool, err error) {
+	stored, err = s.Meta(ctx, metaKeyEmbedderID)
+	if err != nil {
+		return "", false, err
+	}
+	if stored == "" {
+		return currentID, false, s.SetMeta(ctx, metaKeyEmbedderID, currentID)
+	}
+	return stored, stored != currentID, nil
+}

internal/store/meta_test.go

@@ -0,0 +1,67 @@
+package store
+
+import (
+	"context"
+	"testing"
+)
+
+// A fresh DB has no marker yet, so the current embedder is recorded and
+// nothing is flagged.
+func TestCheckEmbedderFreshDBRecords(t *testing.T) {
+	s := newTestStore(t)
+	ctx := context.Background()
+
+	stored, mismatch, err := s.CheckEmbedder(ctx, "multilingual-e5-small@384")
+	if err != nil {
+		t.Fatalf("CheckEmbedder: %v", err)
+	}
+	if mismatch {
+		t.Fatal("fresh DB reported a mismatch")
+	}
+	if stored != "multilingual-e5-small@384" {
+		t.Fatalf("stored = %q", stored)
+	}
+	got, err := s.Meta(ctx, metaKeyEmbedderID)
+	if err != nil {
+		t.Fatalf("Meta: %v", err)
+	}
+	if got != "multilingual-e5-small@384" {
+		t.Fatalf("marker not persisted, got %q", got)
+	}
+}
+
+// Both models are 384-dim, so this is the only thing that catches the swap.
+func TestCheckEmbedderDifferentModelMismatch(t *testing.T) {
+	s := newTestStore(t)
+	ctx := context.Background()
+
+	if err := s.SetMeta(ctx, metaKeyEmbedderID, "paraphrase-multilingual-MiniLM-L12-v2@384"); err != nil {
+		t.Fatalf("SetMeta: %v", err)
+	}
+	stored, mismatch, err := s.CheckEmbedder(ctx, "multilingual-e5-small@384")
+	if err != nil {
+		t.Fatalf("CheckEmbedder: %v", err)
+	}
+	if !mismatch {
+		t.Fatal("different embedder not detected")
+	}
+	if stored != "paraphrase-multilingual-MiniLM-L12-v2@384" {
+		t.Fatalf("stored = %q", stored)
+	}
+}
+
+// The same embedder must never raise a false alarm, including on re-check.
+func TestCheckEmbedderSameModelNoAlarm(t *testing.T) {
+	s := newTestStore(t)
+	ctx := context.Background()
+
+	for i := 0; i < 2; i++ {
+		_, mismatch, err := s.CheckEmbedder(ctx, "multilingual-e5-small@384")
+		if err != nil {
+			t.Fatalf("CheckEmbedder: %v", err)
+		}
+		if mismatch {
+			t.Fatalf("false alarm on pass %d", i)
+		}
+	}
+}

internal/store/migrations.go

@@ -83,6 +83,11 @@ ALTER TABLE reminders ADD COLUMN next_fire_ts INTEGER;`, // #2
 		expires_ts INTEGER NOT NULL
 	);
 	CREATE INDEX IF NOT EXISTS idx_dialogue_sessions_expires ON dialogue_sessions (expires_ts);`, // #10 — the follow-up session survives a restart (Vikunja #363); small, TTL-pruned table, not a history log
+
+	`CREATE TABLE IF NOT EXISTS meta (
+		key   TEXT PRIMARY KEY,
+		value TEXT NOT NULL
+	);`, // #11 — small key/value table for facts about the DB itself; first key is embedder_id (Vikunja #378)
 }
 
 // migrate applies every migration with a number greater than the DB's current