Symptom (prod, hongmingwang tenant, 2026-04-22):
PUT /workspaces/:id/files/config.yaml → 500
{"error":"failed to write file: docker not available"}
Root cause: WriteFile + ReplaceFiles always reached for the tenant's
Docker client, but SaaS workspaces run as EC2 VMs (no Docker on the
tenant to cp into). There was no SaaS code path, so Save/Save&Restart
in the Config tab silently 500'd for every SaaS user.
Fix: add writeFileViaEIC — same ephemeral-keypair + EIC-tunnel dance
that the Terminal tab already uses (terminal.go). Flow:
1. ssh-keygen ephemeral ed25519 pair
2. aws ec2-instance-connect send-ssh-public-key (60s validity)
3. aws ec2-instance-connect open-tunnel (TLS → :22)
4. ssh ... "install -D -m 0644 /dev/stdin <abs path>"
install -D creates missing parent dirs atomically
5. Kill tunnel + wipe keydir
Runtime → base-path map (new table workspaceFilePathPrefix):
hermes → /home/ubuntu/.hermes
langgraph → /opt/configs
external → /opt/configs
unknown → /opt/configs
Both WriteFile (single file) and ReplaceFiles (bulk) detect
`workspaces.instance_id != ''` and route to EIC instead of Docker.
Local/self-hosted Docker path is unchanged.
Security: the only variable piece in the remote ssh command is the
absolute path, which is built via map lookup + filepath.Clean so
traversal is blocked. shellQuote() wraps it as defence-in-depth.
validateRelPath rejects absolute paths and surviving `..` segments
up-front; tests assert traversal rejection.
Follow-ups tracked separately:
- Reload hook after save (hermes gateway restart via SSH)
- Per-tunnel batching for ReplaceFiles with many files
- Runtime-specific base paths should be declared in the runtime
manifest, not hardcoded in the handler
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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