write_file / file tool reports success when writing 0 bytes (empty content)

Affected Component

• Agent Tools / Terminal (file / write_file)

Describe the bug

The file tool with action=write (or inferred write) reports success when content is empty:

// backend/pkg/tools/terminal.go — WriteFile
return fmt.Sprintf("Successfully wrote %d bytes to %s", len(content), path), nil

Empty content is a common LLM failure mode (wrong arg name, omitted body, truncated JSON). The agent then treats the write as done and never retries with real content — leaving a 0-byte artifact on disk.

Inference already maps omitted action + empty content → read, but an explicit write with empty content still creates a 0-byte file and returns success.

Observed: FarVision WSL DinD trial on vxcontrol/pentagi:latest (rev around hcffed94 / Hub digest ~2026-10-22). In a 7-day window, 36 file toolcalls returned Successfully wrote 0 bytes (flows 20/22 among others). Host /work is RW and shell/heredoc writes work — this is not a disk/permission issue.

Expected

• Reject empty-content writes with a clear error (force agent retry / fixer), or
• Treat empty content as failure in WriteFile before CopyToContainer, or
• Require non-empty content in the tool schema + validator (and keep inference for omitted action).

Steps to Reproduce

1. Run a flow with a sandbox terminal.
2. Invoke file with {"action":"write","path":"/work/probe.txt","content":""} (or omit content).
3. Observe: tool result Successfully wrote 0 bytes to /work/probe.txt; file exists with size 0; agent may stop without retrying.

Workaround (operators)

Prefer terminal/cat heredoc for critical artifacts; re-prompt when DB/toolcalls show Successfully wrote 0 bytes.

System Configuration

• PentAGI Version: vxcontrol/pentagi:latest (~2026-10-22)
• Deployment Type: Docker Compose (DinD / WSL trial)
• Host OS: Linux (WSL2)

Verification

• I have checked that this issue hasn't been already reported (searched write_file / 0 bytes / Successfully wrote — no matching open issue as of 2026-10-26)
• I have included relevant logs and error messages
• I am running a recent vxcontrol/pentagi:latest