Claude Code Subagent Observability: v2.1.139 Adds x-claude-code-agent-id Headers and OTEL Span Attributes for Multi-Agent Tracing

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Claude Code v2.1.139 adds observability primitives for multi-agent workflows: API requests from subagents now carry x-claude-code-agent-id and x-claude-code-parent-agent-id HTTP headers, and claude_code.llm_request OpenTelemetry spans include agent_id and parent_agent_id attributes. This enables teams to trace token usage, latency, and cost across parent-child agent hierarchies — critical for debugging complex agent orchestrations and understanding which subagent is driving costs. Combined with the new `claude plugin details <n>` command showing per-session token cost projections, this is the most complete agent cost observability Claude Code has shipped.