claude -p subprocess produces empty stdout when called from Bash tool

[WesleyMFrederick]

Problem

claude -p called from the Bash tool inside a Claude Code session exits 0 but produces zero bytes of stdout. The Seatbelt nesting bypass (CLAUDE_SANDBOX_ACTIVE + unset CLAUDECODE) works correctly — no sandbox or nesting errors. But output is lost.

Context

Another session needs claude -p as a semantic layer inside a UserPromptSubmit hook (learning injection via haiku). The sandbox fix on branch fix/nested-seatbelt-bypass resolved the kernel-level nesting error, but stdout capture remains broken.

Definition of Success

From inside a running Claude Code session, this Bash tool call:

echo '{"prompt":"test"}' | claude -p --model haiku --max-turns 1 --output-format text

Must:

1. Exit 0
2. Print non-empty text to stdout (any response from haiku)
3. Complete in under 15 seconds
4. Work without manual env overrides (no env -u CLAUDECODE, no unset prefix)

Evidence

Tested on branch fix/nested-seatbelt-bypass with CLAUDE_SANDBOX_ACTIVE=1 confirmed in env:

Criteria	Result
Exit 0	✅
Non-empty stdout	❌ Empty (0 bytes)
Complete in <15s	✅
No manual env overrides	✅

Hypothesis

Claude Code may redirect or suppress child claude process stdout when invoked from the Bash tool. This is separate from the Seatbelt sandbox issue — it's Claude Code internal output handling.

Acceptance Criteria

• claude -p produces non-empty stdout when called from Bash tool
• No manual env variable workarounds required
• Works under Seatbelt sandbox (branch sessions)