fix(flow): drop leading orphan tool messages after token trimming (#68)

internal/conversation/flow/resolver.go

@@ -750,6 +750,13 @@ func trimMessagesByTokens(messages []messageWithUsage, maxTokens int) []conversa
 		}
 	}
 
+	// Keep provider-valid message order: a "tool" message must follow a preceding
+	// assistant tool call. When history is head-trimmed, a leading tool message
+	// may become orphaned and cause provider 400 errors.
+	for cutoff < len(messages) && strings.EqualFold(strings.TrimSpace(messages[cutoff].Message.Role), "tool") {
+		cutoff++
+	}
+
 	result := make([]conversation.ModelMessage, 0, len(messages)-cutoff)
 	for _, m := range messages[cutoff:] {
 		result = append(result, m.Message)

internal/conversation/flow/resolver_trim_test.go

@@ -0,0 +1,94 @@
+package flow
+
+import (
+	"testing"
+
+	"github.com/memohai/memoh/internal/conversation"
+)
+
+func TestTrimMessagesByTokens_DropsLeadingOrphanTool(t *testing.T) {
+	t.Parallel()
+
+	messages := []messageWithUsage{
+		{
+			Message: conversation.ModelMessage{
+				Role:    "user",
+				Content: conversation.NewTextContent("1111"),
+			},
+		},
+		{
+			Message: conversation.ModelMessage{
+				Role: "assistant",
+				ToolCalls: []conversation.ToolCall{
+					{
+						ID:   "call-1",
+						Type: "function",
+						Function: conversation.ToolCallFunction{
+							Name:      "calc",
+							Arguments: `{"x":1}`,
+						},
+					},
+				},
+			},
+		},
+		{
+			Message: conversation.ModelMessage{
+				Role:       "tool",
+				ToolCallID: "call-1",
+				Content:    conversation.NewTextContent("2"),
+			},
+		},
+		{
+			Message: conversation.ModelMessage{
+				Role:    "assistant",
+				Content: conversation.NewTextContent("done"),
+			},
+		},
+	}
+
+	trimmed := trimMessagesByTokens(messages, 2)
+	if len(trimmed) == 0 {
+		t.Fatal("expected non-empty trimmed messages")
+	}
+	if trimmed[0].Role == "tool" {
+		t.Fatal("expected first trimmed message not to be tool")
+	}
+}
+
+func TestTrimMessagesByTokens_KeepsToolWhenPaired(t *testing.T) {
+	t.Parallel()
+
+	messages := []messageWithUsage{
+		{
+			Message: conversation.ModelMessage{
+				Role: "assistant",
+				ToolCalls: []conversation.ToolCall{
+					{
+						ID:   "call-1",
+						Type: "function",
+						Function: conversation.ToolCallFunction{
+							Name:      "calc",
+							Arguments: `{"x":1}`,
+						},
+					},
+				},
+			},
+		},
+		{
+			Message: conversation.ModelMessage{
+				Role:       "tool",
+				ToolCallID: "call-1",
+				Content:    conversation.NewTextContent("2"),
+			},
+		},
+	}
+
+	trimmed := trimMessagesByTokens(messages, 100)
+	if len(trimmed) != 2 {
+		t.Fatalf("expected 2 messages, got %d", len(trimmed))
+	}
+	if trimmed[0].Role != "assistant" || trimmed[1].Role != "tool" {
+		t.Fatalf("unexpected role order: %q -> %q", trimmed[0].Role, trimmed[1].Role)
+	}
+}
+