fix: multi-tool-call sessions drop 2nd+ tool results

sanitizeHistoryForProvider only allowed tool messages when the
preceding message was an assistant with tool_calls. For assistant
turns with 2+ tool calls, the 2nd tool result's predecessor was
the 1st tool result (role "tool"), causing it to be dropped.

This made the API see an assistant with N tool_calls but <N
results, triggering "tool call result does not follow tool call".

Fix: also allow tool messages after sibling tool messages.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/agent/context.go

@@ -259,11 +259,12 @@ func sanitizeHistoryForProvider(history []providers.Message) []providers.Message
 				continue
 			}
 			last := sanitized[len(sanitized)-1]
-			if last.Role != "assistant" || len(last.ToolCalls) == 0 {
-				logger.DebugCF("agent", "Dropping orphaned tool message", map[string]any{})
-				continue
-			}
+			// Allow tool results after their assistant or after sibling tool results
+			if last.Role == "tool" || (last.Role == "assistant" && len(last.ToolCalls) > 0) {
 				sanitized = append(sanitized, msg)
+			} else {
+				logger.DebugCF("agent", "Dropping orphaned tool message", map[string]any{})
+			}
 
 		case "assistant":
 			if len(msg.ToolCalls) > 0 {

pkg/agent/loop_test.go

@@ -1679,3 +1679,39 @@ func TestBuildRichStatus_LatestEntryNoInlineResult(t *testing.T) {
 		t.Errorf("expected exactly 1 separator, got %d in:\n%s", sepCount, got)
 	}
 }
+
+func TestSanitizeHistoryForProvider_MultiToolCall(t *testing.T) {
+	// Regression: assistant with 2+ tool_calls had 2nd+ tool results dropped
+	// because the check only allowed tool after assistant, not after sibling tool.
+	history := []providers.Message{
+		{Role: "user", Content: "hello"},
+		{Role: "assistant", Content: "", ToolCalls: []providers.ToolCall{
+			{ID: "a", Function: &providers.FunctionCall{Name: "exec"}},
+			{ID: "b", Function: &providers.FunctionCall{Name: "read_file"}},
+		}},
+		{Role: "tool", Content: "ok", ToolCallID: "a"},
+		{Role: "tool", Content: "ok", ToolCallID: "b"},
+		{Role: "assistant", Content: "done"},
+	}
+
+	got := sanitizeHistoryForProvider(history)
+
+	// All 5 messages must survive
+	if len(got) != 5 {
+		roles := make([]string, len(got))
+		for i, m := range got {
+			roles[i] = m.Role
+		}
+		t.Fatalf("expected 5 messages, got %d: %v", len(got), roles)
+	}
+	// Verify both tool results present
+	toolCount := 0
+	for _, m := range got {
+		if m.Role == "tool" {
+			toolCount++
+		}
+	}
+	if toolCount != 2 {
+		t.Errorf("expected 2 tool results, got %d", toolCount)
+	}
+}