Function newSubSession

pkg/runtime/agent_delegation.go:153–201

newSubSession builds a *session.Session from a SubSessionConfig and a parent session. It consolidates the session options that were previously duplicated across handleTaskTransfer and RunAgent.

(parent *session.Session, cfg SubSessionConfig, childAgent *agent.Agent)

// session. It consolidates the session options that were previously duplicated
// across handleTaskTransfer and RunAgent.
func newSubSession(parent *session.Session, cfg SubSessionConfig, childAgent *agent.Agent) *session.Session {
	// Sub-agents start in a fresh session, so they don't see the user's
	// original messages or attached files. Snapshot the parent's attached
	// files once and propagate them both to the system prompt (so the agent
	// is told about them) and to the child session (so further nested
	// transfers keep inheriting them).
	attachedFiles := parent.AttachedFilesSnapshot()

	sysMsg := cfg.SystemMessage
	if sysMsg == "" {
		sysMsg = buildTaskSystemMessage(cfg.Task, cfg.ExpectedOutput, attachedFiles)
	}

	userMsg := cfg.ImplicitUserMessage
	if userMsg == "" {
		userMsg = "Please proceed."
	}

	opts := []session.Opt{
		session.WithSystemMessage(sysMsg),
		session.WithImplicitUserMessage(userMsg),
		session.WithMaxIterations(childAgent.MaxIterations()),
		session.WithMaxConsecutiveToolCalls(childAgent.MaxConsecutiveToolCalls()),
		session.WithMaxOldToolCallTokens(childAgent.MaxOldToolCallTokens()),
		session.WithTitle(cfg.Title),
		session.WithToolsApproved(cfg.ToolsApproved),
		session.WithNonInteractive(cfg.NonInteractive),
		session.WithSendUserMessage(false),
		session.WithParentID(parent.ID),
		session.WithAttachedFiles(attachedFiles),
	}
	if cfg.PinAgent {
		opts = append(opts, session.WithAgentName(cfg.AgentName))
	}
	// Merge parent's excluded tools with config's excluded tools so that
	// nested sub-sessions (e.g. skill → transfer_task → child) inherit
	// exclusions from all ancestors and don't re-introduce filtered tools.
	excludedTools := mergeExcludedTools(parent.ExcludedTools, cfg.ExcludedTools)
	if len(excludedTools) > 0 {
		opts = append(opts, session.WithExcludedTools(excludedTools))
	}
	if len(cfg.AllowedTools) > 0 {
		opts = append(opts, session.WithAllowedTools(cfg.AllowedTools))
	}
	if len(cfg.ExtraToolSets) > 0 {
		opts = append(opts, session.WithExtraToolSets(cfg.ExtraToolSets))
	}
	return session.New(opts...)
}

// mergeExcludedTools combines two excluded-tool lists, deduplicating entries.
// It returns nil when both inputs are empty.