fix(conversation): correct token trimming edge cases (#207)

- Treat maxTokens=0 as "unconfigured/unlimited" instead of disabling
  trimming for any non-positive value (which masked exhausted budgets)
- Set historyBudget=1 when maxTokens>0 but overhead exceeds the limit,
  ensuring aggressive trimming instead of no trimming
- Estimate token cost for messages without usage data (len/4 fallback)
  so user/tool messages are not free-passed during budget accounting

internal/conversation/flow/resolver.go

@@ -342,7 +342,9 @@ func (r *Resolver) resolve(ctx context.Context, req conversation.ChatRequest) (r
 	overhead += systemPromptReserve
 
 	historyBudget := maxTokens - overhead
-	if historyBudget < 0 {
+	if maxTokens > 0 && historyBudget <= 0 {
+		historyBudget = 1
+	} else if historyBudget < 0 {
 		historyBudget = 0
 	}
 
@@ -1279,7 +1281,7 @@ func estimateMessageTokens(msg conversation.ModelMessage) int {
 }
 
 func trimMessagesByTokens(log *slog.Logger, messages []messageWithUsage, maxTokens int) []conversation.ModelMessage {
-	if maxTokens <= 0 || len(messages) == 0 {
+	if maxTokens == 0 || len(messages) == 0 {
 		result := make([]conversation.ModelMessage, len(messages))
 		for i, m := range messages {
 			result[i] = m.Message
@@ -1287,10 +1289,9 @@ func trimMessagesByTokens(log *slog.Logger, messages []messageWithUsage, maxToke
 		return result
 	}
 
-	// Scan from newest to oldest, accumulating per-message outputTokens from
-	// stored usage data. Messages without usage (user / tool) are included for
-	// free — the outputTokens of surrounding assistant turns already account
-	// for the context they consumed.
+	// Scan from newest to oldest, accumulating per-message token costs.
+	// Messages with stored usage data use that value; others fall back to a
+	// character-based estimate so that user/tool messages are not free-passed.
 	totalTokens := 0
 	cutoff := 0
 	messagesWithUsage := 0
@@ -1298,6 +1299,8 @@ func trimMessagesByTokens(log *slog.Logger, messages []messageWithUsage, maxToke
 		if messages[i].UsageOutputTokens != nil {
 			totalTokens += *messages[i].UsageOutputTokens
 			messagesWithUsage++
+		} else {
+			totalTokens += estimateMessageTokens(messages[i].Message)
 		}
 		if totalTokens > maxTokens {
 			cutoff = i + 1

internal/conversation/flow/resolver_trim_test.go

@@ -112,3 +112,55 @@ func TestTrimMessagesByTokens_NoUsage_KeepsAll(t *testing.T) {
 		t.Fatalf("messages without outputTokens should all be kept, got %d", len(trimmed))
 	}
 }
+
+func TestTrimMessagesByTokens_ZeroMeansNoLimit(t *testing.T) {
+	t.Parallel()
+
+	messages := []messageWithUsage{
+		{Message: conversation.ModelMessage{Role: "user", Content: conversation.NewTextContent("hello")}, UsageOutputTokens: intPtr(10000)},
+		{Message: conversation.ModelMessage{Role: "assistant", Content: conversation.NewTextContent("world")}, UsageOutputTokens: intPtr(10000)},
+	}
+
+	// maxTokens = 0 means "no limit configured", should keep all messages.
+	trimmed := trimMessagesByTokens(nil, messages, 0)
+	if len(trimmed) != 2 {
+		t.Fatalf("maxTokens=0 should keep all messages, got %d", len(trimmed))
+	}
+}
+
+func TestTrimMessagesByTokens_SmallBudgetTrims(t *testing.T) {
+	t.Parallel()
+
+	messages := []messageWithUsage{
+		{Message: conversation.ModelMessage{Role: "user", Content: conversation.NewTextContent("old message")}, UsageOutputTokens: intPtr(100)},
+		{Message: conversation.ModelMessage{Role: "assistant", Content: conversation.NewTextContent("old reply")}, UsageOutputTokens: intPtr(200)},
+		{Message: conversation.ModelMessage{Role: "user", Content: conversation.NewTextContent("new message")}, UsageOutputTokens: intPtr(50)},
+		{Message: conversation.ModelMessage{Role: "assistant", Content: conversation.NewTextContent("new reply")}, UsageOutputTokens: intPtr(60)},
+	}
+
+	// Budget of 1: should trim aggressively, NOT return all messages.
+	trimmed := trimMessagesByTokens(nil, messages, 1)
+	if len(trimmed) >= len(messages) {
+		t.Fatalf("maxTokens=1 should trim history, but got %d messages (same as input)", len(trimmed))
+	}
+}
+
+func TestTrimMessagesByTokens_EstimatesFallback(t *testing.T) {
+	t.Parallel()
+
+	// Long user message without usage data — should be estimated.
+	longText := make([]byte, 400)
+	for i := range longText {
+		longText[i] = 'x'
+	}
+	messages := []messageWithUsage{
+		{Message: conversation.ModelMessage{Role: "user", Content: conversation.NewTextContent(string(longText))}},
+		{Message: conversation.ModelMessage{Role: "assistant", Content: conversation.NewTextContent("ok")}, UsageOutputTokens: intPtr(10)},
+	}
+
+	// Budget of 50: user message is ~100 estimated tokens (400/4), should be trimmed.
+	trimmed := trimMessagesByTokens(nil, messages, 50)
+	if len(trimmed) == 2 {
+		t.Fatalf("expected long user message without usage to be trimmed via estimation, got %d", len(trimmed))
+	}
+}