fix(ratelimit): initialise state on invalid decides

Co-Authored-By: Virgil <virgil@lethean.io>

ratelimit.go

@@ -590,15 +590,6 @@ func (rl *RateLimiter) AllStats() map[string]ModelStats {
 // Decide returns structured allow/deny information for an estimated request.
 // It never records usage; call RecordUsage after a successful decision.
 func (rl *RateLimiter) Decide(model string, estimatedTokens int) Decision {
-	if estimatedTokens < 0 {
-		return Decision{
-			Allowed: false,
-			Code:    DecisionInvalidTokens,
-			Reason:  "estimated tokens must be non-negative",
-			Stats:   rl.Stats(model),
-		}
-	}
-
 	rl.mu.Lock()
 	defer rl.mu.Unlock()
 
@@ -615,6 +606,13 @@ func (rl *RateLimiter) Decide(model string, estimatedTokens int) Decision {
 	}
 
 	if quota.MaxRPM == 0 && quota.MaxTPM == 0 && quota.MaxRPD == 0 {
+		if estimatedTokens < 0 {
+			decision.Allowed = false
+			decision.Code = DecisionInvalidTokens
+			decision.Reason = "estimated tokens must be non-negative"
+			decision.Stats = rl.snapshotLocked(model)
+			return decision
+		}
 		decision.Allowed = true
 		decision.Code = DecisionUnlimited
 		decision.Reason = "all limits are unlimited"
@@ -629,6 +627,14 @@ func (rl *RateLimiter) Decide(model string, estimatedTokens int) Decision {
 		rl.State[model] = stats
 	}
 
+	if estimatedTokens < 0 {
+		decision.Allowed = false
+		decision.Code = DecisionInvalidTokens
+		decision.Reason = "estimated tokens must be non-negative"
+		decision.Stats = rl.snapshotLocked(model)
+		return decision
+	}
+
 	decision.Stats = rl.snapshotLocked(model)
 
 	if quota.MaxRPD > 0 && stats.DayCount >= quota.MaxRPD {

ratelimit_test.go

@@ -363,12 +363,17 @@ func TestRatelimit_Decide_Good(t *testing.T) {
 
 	t.Run("negative estimate returns invalid decision", func(t *testing.T) {
 		rl := newTestLimiter(t)
+		model := "neg"
+		rl.Quotas[model] = ModelQuota{MaxRPM: 5, MaxTPM: 50, MaxRPD: 5}
 
-		decision := rl.Decide("neg", -5)
+		decision := rl.Decide(model, -5)
 
 		assert.False(t, decision.Allowed)
 		assert.Equal(t, DecisionInvalidTokens, decision.Code)
 		assert.Zero(t, decision.RetryAfter)
+		require.Contains(t, rl.State, model)
+		require.NotNil(t, rl.State[model])
+		assert.Equal(t, 0, rl.State[model].DayCount)
 	})
 }