[FEATURE] Add prompt_id to PostToolUse and PreToolUse hook payloads

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Claude Code already generates a prompt_id for every user prompt and emits it on OTel events
both claude_code.api_request and claude_code.tool_result carry it. However, prompt_id is
not present in the PostToolUse (or PreToolUse) hook JSON payload, which only exposes
session_id.

This creates an inconsistency: the PostToolUse hook fires at the same lifecycle moment as the
claude_code.tool_result OTel event, but the two outputs don't share a common key below
session_id. I imagine this value exists in the runtime at hook-fire time since it's being attached
to the concurrent OTel events.

Proposed Solution

Add prompt_id to the PostToolUse and PreToolUse hook JSON payloads alongside the existing
session_id. The change is additive and non-breaking.

{
  "session_id": "abc123",
  "prompt_id": "550e8400-e29b-41d4-a716-446655440000",
  "hook_event_name": "PostToolUse",
  "tool_name": "Write",
  "tool_input": { ... },
  "tool_response": { ... },
  "tool_use_id": "..."
}

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

Other

Use Case Example

We're building a cost attribution pipeline to associate Claude Code API spend with
PRs and engineering initiatives. The architecture joins two BigQuery pipelines:

• Hooks pipeline captures which files were modified by which Claude session, via

PostToolUse hooks writing (session_id, file_path) tuples at edit time and reconciling
against committed files at push time.

• OTel pipeline captures token counts and cost_usd per prompt_id from

claude_code.api_request events.

These pipelines currently can only join at session_id grain, which is too coarse since a session
spans hours and dozens of prompts, only some of which are relevant to any given commit. With
prompt_id in the hook payload, we can join at prompt grain and produce accurate per-PR and
per-initiative cost attribution.

Additional Context

_No response_