Stop the router repetition loop

Route now sets RepeatPenalty on the request, and the grammar's string rule is
capped at 120 characters. Two of 76 fixture cases looped one sentence inside
the text field until MaxTokens, which cut the JSON in half.
Reviewers: the new constant and the grammar string rule.

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

internal/router/llmrouter.go

@@ -23,14 +23,16 @@ func NewLLMRouter(c Completer) *LLMRouter { return &LLMRouter{c: c} }
 
 // routeGrammar — GBNF constraining the model to a JSON ARRAY of fixed-shape
 // action objects (one per ask; compound utterances → multiple). Enum + key set
-// prevent free-form drift from a sub-1B model.
+// prevent free-form drift from a sub-1B model. The string rule is length-bounded
+// so a repetition loop cannot fill the whole token budget with one field and
+// truncate the JSON.
 const routeGrammar = `
 root   ::= "[" ws action ("," ws action)* ws "]"
 action ::= "{" ws "\"intent\"" ws ":" ws intent ("," ws field)* ws "}"
 intent ::= "\"fact\"" | "\"reminder\"" | "\"note\"" | "\"query\"" | "\"act\"" | "\"chat\"" | "\"system\""
 field  ::= key ws ":" ws string
 key    ::= "\"key\"" | "\"value\"" | "\"text\"" | "\"verb\""
-string ::= "\"" ([^"\\] | "\\" .)* "\""
+string ::= "\"" ([^"\\] | "\\" .){0,120} "\""
 ws     ::= [ \t\n]*
 `
 
@@ -77,6 +79,11 @@ const routeSystem = `Классифицируй ровно одно сообще
 
 Ответ — JSON-массив: по одному объекту на каждую просьбу. Обычно один. Если в реплике несколько просьб — по объекту на каждую. "напомни купить молоко, и запиши что кофе кончился" → [{"intent":"reminder","text":"купить молоко"},{"intent":"note","text":"кофе кончился"}]. Только JSON, без пояснений.`
 
+// routeRepeatPenalty — the sub-1B model loops one sentence inside the text field
+// until it runs out of tokens, which truncates the JSON. 1.15 is enough to break
+// the loop without hurting short slot values.
+const routeRepeatPenalty = 1.15
+
 type routeAction struct {
 	Intent string `json:"intent"`
 	Key    string `json:"key"`
@@ -86,7 +93,13 @@ type routeAction struct {
 }
 
 func (lr *LLMRouter) Route(ctx context.Context, utterance string, now time.Time) (Decision, bool, error) {
-	raw, err := lr.c.Complete(ctx, llm.Req{System: routeSystem, User: utterance, Grammar: routeGrammar, MaxTokens: 128})
+	raw, err := lr.c.Complete(ctx, llm.Req{
+		System:        routeSystem,
+		User:          utterance,
+		Grammar:       routeGrammar,
+		MaxTokens:     128,
+		RepeatPenalty: routeRepeatPenalty,
+	})
 	if err != nil {
 		return Decision{}, false, err
 	}

internal/router/llmrouter_test.go

@@ -13,9 +13,35 @@ import (
 type mockLLM struct {
 	out string
 	err error
+	got *llm.Req // last request, when the test wants to inspect it
 }
 
-func (m mockLLM) Complete(_ context.Context, _ llm.Req) (string, error) { return m.out, m.err }
+func (m mockLLM) Complete(_ context.Context, r llm.Req) (string, error) {
+	if m.got != nil {
+		*m.got = r
+	}
+	return m.out, m.err
+}
+
+// Without a repeat penalty the model loops inside the text field until MaxTokens
+// and the truncated JSON fails to parse.
+func TestLLMRouterSetsRepeatPenalty(t *testing.T) {
+	var got llm.Req
+	lr := NewLLMRouter(mockLLM{out: `{"intent":"chat","text":"привет"}`, got: &got})
+	if _, _, err := lr.Route(context.Background(), "привет", time.Now()); err != nil {
+		t.Fatalf("route: %v", err)
+	}
+	if got.RepeatPenalty <= 1 {
+		t.Fatalf("want repeat penalty above 1, got %v", got.RepeatPenalty)
+	}
+}
+
+// An unbounded string rule lets one field eat the whole token budget.
+func TestRouteGrammarBoundsStrings(t *testing.T) {
+	if !strings.Contains(routeGrammar, `string ::= "\"" ([^"\\] | "\\" .){0,120} "\""`) {
+		t.Fatal("grammar string rule lost its length bound")
+	}
+}
 
 // A question naming a fact key used to be stored as a fact because the fact rule
 // was tested first. Keep the query rule above it.