Option B identifies the missing executable capability. Defining specialized agents makes their configurations available, but the coordinator must still be permitted to call the tool that invokes them. Without that tool, Claude can describe an intended delegation in ordinary text but cannot create a subagent execution.

The current Claude Agent SDK documentation requires " Agent " in allowedTools to auto-approve subagent invocations. The tool was renamed from " Task " to " Agent " in Claude Code 2.1.63, so the terminology in the original candidate question required correction. Older integrations may still expose " Task " in initialization or permission records.

Option A is unlikely because properly written AgentDefinition.description values already tell Claude when each agent should be used. Option C misinterprets context isolation: the parent supplies the subagent’s assignment through the Agent tool’s prompt, and no additional automatic forwarding setting is required. Option D would normally produce truncation evidence or a max_tokens stop reason rather than silent absence of every invocation. The coordinator needs both agent definitions and permission to use the invocation tool.

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