Curated against English had quietly become curated against writing

Seen live: "Esta manhã acordei cedo e fui correr ao longo da marginal.
O ar estava fresco e havia poucas pessoas na rua." — unremarkable
Portuguese, two marker hits, zero English hits, and a verdict of
English. Corrected as English, read aloud in an American voice.

The list was missing the ordinary machinery of the language: the
contractions (ao, à, num), the tenses a diary is written in (estava,
havia, fomos), the words that join two clauses (até, depois, então,
onde). Each clears the bar the list already set — an English sentence
has no reason to contain them — so their absence bought nothing.

The floor stays at three. What changed is that three is now reachable
by prose rather than only by a paragraph that argues its own case. fr
and es get the same additions by analogy; neither has an account yet to
catch it live, which is exactly how this one survived. "sin" and "tan"
stay out of the es list: both are English words.

Two regression tests, pointed in opposite directions — ordinary
Portuguese must read as hers, and English about Portugal, English
quoting Portuguese, and a plain English diary must all still read as
English, held verdict included.

Claude-Session: https://claude.ai/code/session_01GJHNvirh7Hzhc9RL3HAvz7

internal/suggestions/doclang_test.go

@@ -446,3 +446,55 @@ func TestPassAnnouncesItsVerdict(t *testing.T) {
 		t.Fatal("empty document answered with no verdict header at all")
 	}
 }
+
+// TestOrdinaryProseIsEnoughEvidence is the regression for what the marker lists
+// were caught doing on 2026-07-29, live, in a browser: unremarkable Portuguese
+// read as English, because the list was curated against English so tightly that
+// it had also been curated against ordinary writing. The document below scored
+// two pair markers and zero English ones, and two is below the corroboration
+// floor — so a paragraph with no evidence of English in it at all came back
+// English, and was corrected and read aloud as English.
+//
+// Every sample here is prose a person might actually write, not prose chosen to
+// contain markers. That is the whole point of the test: the failure was invisible
+// to a suite whose fixtures all argued their own case.
+func TestOrdinaryProseIsEnoughEvidence(t *testing.T) {
+	samples := []struct{ name, text string }{
+		{"the one seen live", "Esta manhã acordei cedo e fui correr ao longo da marginal. O ar estava fresco e havia poucas pessoas na rua. Depois comprei um jornal e li-o sentado num banco ao sol."},
+		{"an afternoon out", "Hoje o céu estava limpo e fomos até ao jardim junto ao rio. A minha mãe trouxe uma manta velha e sentámos-nos debaixo de uma árvore."},
+		{"plans", "Amanhã vamos ao cinema depois do trabalho. Ontem estava demasiado cansada para sair de casa."},
+	}
+	for _, s := range samples {
+		if got := documentLang(s.text, "pt-PT", ""); got != docLangPair {
+			t.Errorf("%s: documentLang = %q, want %q — ordinary Portuguese must not read as English\n%s",
+				s.name, got, docLangPair, s.text)
+		}
+	}
+}
+
+// TestEnglishDidNotGetEasierToMistake is the other half, and the reason the
+// additions were held to "a word an English sentence has no reason to contain".
+// Widening a marker list is only safe if it widens in one direction: these are
+// English documents, including ones about Portugal and ones quoting Portuguese,
+// and every one of them must still come back English.
+func TestEnglishDidNotGetEasierToMistake(t *testing.T) {
+	samples := []struct{ name, text string }{
+		{"plain English", "This morning I woke up early and went for a run along the seafront. The air was fresh and there were few people about. Afterwards I bought a newspaper and read it on a bench."},
+		{"English about Portugal", "We spent a week in Lisbon last summer. The trams were crowded but the food was wonderful, and we walked up to the castle every evening."},
+		{"English quoting her", "My mother always says \"até amanhã\" when she leaves, never goodbye. I asked her why once and she said it sounded less final to her."},
+		{"an English diary", "Today was long. I had two meetings before lunch and another one after, and by the time I got home I could not think straight. Tomorrow should be quieter."},
+	}
+	for _, s := range samples {
+		if got := documentLang(s.text, "pt-PT", ""); got != docLangEnglish {
+			t.Errorf("%s: documentLang = %q, want %q — the widened list must not pull English across\n%s",
+				s.name, got, docLangEnglish, s.text)
+		}
+	}
+	// And the same document must not flip once it is already sitting in English:
+	// the hysteresis band is only a safety net if the low side holds too.
+	for _, s := range samples {
+		if got := documentLang(s.text, "pt-PT", docLangEnglish); got != docLangEnglish {
+			t.Errorf("%s: held verdict flipped to %q", s.name, got)
+		}
+	}
+}