Feature request: Expose session_id to the conversation context

Problem

Claude Code's session_id is available in the JSON piped to statusLine commands and hooks, but is not accessible from within the conversation itself — not as an environment variable, not through any tool, and not in any system context.

This makes it difficult for tools and plugins that need Claude to write session-keyed data (e.g., task status files, session-specific caches) because Claude cannot discover its own session ID without a workaround.

Current Workaround

The workaround we implemented for the ccstatusline task-objective widget:

1. The statusLine command receives session_id in its stdin JSON
2. It walks up the process tree (ps -o ppid=,comm=) to find the Claude CLI PID
3. It writes the session ID to ~/.cache/ccstatusline/sessions/<claude-pid>
4. Claude runs echo $PPID to get the CLI PID, then reads the file to discover its session ID

This works but is fragile (Unix-only, requires process tree walking) and adds latency to every statusLine render.

Proposed Solution

Expose session_id as an environment variable (e.g., CLAUDE_SESSION_ID) in the shell environment used by the Bash tool. This would allow Claude to directly reference it without any discovery mechanism.

Alternatively, exposing it in a system context field that Claude can read would also work.

Use Cases

• Task tracking widgets: Status line tools that show what Claude is working on, keyed by session
• Session-specific caches: Tools that need to store per-session state
• Multi-session coordination: When multiple Claude sessions run concurrently in the same project, session ID is the only reliable differentiator
• Session resume: The session ID is stable across --resume, making it better than PID for keying persistent data