bug: use_subagent returns EmptyResponse in --no-interactive mode

Checks

• I have searched github.com/aws/amazon-q-developer-cli/issues and there are no duplicates of my issue
• I have run q doctor in the affected terminal session
• I have run q restart and replicated the issue again

Operating system

• kiro-cli 2.6.0 - Linux (WSL2 Ubuntu 24.04, glibc 2.39)

Expected behaviour

Orchestrating Agent call subagent successfully

Actual behaviour

While building an agent evaluation system (parent agent orchestrating specialized sub-agents), we discovered that sub-agent responses are lost when running in --no-interactive mode. The sub-agent executes fully (reads files, writes results), but its final text response is never returned to the parent agent.

Steps to reproduce

Reproduce

kiro-cli chat --agent repro-parent --no-interactive "call the child agent to process /tmp/repro_input.txt"

Expected

Parent receives child's text response (analysis + confirmation)

Actual

Parent reports EmptyResponse. But /tmp/repro_output/result.json IS written correctly by the child.

See repro_output.log for full output.

Key Findings

Mode	Child Response Type	Result
--no-interactive	Multi-line text after fs_write	❌ EmptyResponse (100%)
--no-interactive	Single short line after fs_write	✅ Works (0% failure)
Interactive (tty)	Multi-line text after fs_write	✅ Works (0% failure)
--no-interactive	Multi-line text without fs_write	✅ Works (0% failure)

The bug requires ALL of:

1. --no-interactive mode (no tty)
2. Sub-agent uses fs_write tool
3. Sub-agent responds with multi-line text after writing

Workaround

Change child prompt ending from:

IMPORTANT: You MUST write the result file AND respond with a natural language confirmation. Do not respond with only JSON.

To:

IMPORTANT: After writing the result file, respond with ONLY this exact text: 'DONE:case-001:PASS'. Nothing else.

See repro_output_short_reply.log for successful output with workaround.

Files

• repro-parent.json — minimal parent agent (only use_subagent tool)
• repro-child.json — child agent that triggers bug (fs_read + fs_write + multi-line response)
• repro_output.log — output showing EmptyResponse (bug)
• repro_output_short_reply.log — output showing success (workaround)

Environment

Führe zunächst den Reproduktionsbefehl mit repro-parent.json und repro-child.json aus und vergleiche anschließend repro_output.log mit repro_output_short_reply.log. Verfolge, wie --no-interactive die Antwort des untergeordneten Prozesses nach fs_write verarbeitet; abgeschlossen ist die Änderung, wenn der übergeordnete Prozess den mehrzeiligen Text des untergeordneten Prozesses statt EmptyResponse erhält und die Ergebnisdatei weiterhin geschrieben wird.