Remote sandbox triggers: outbound curl to external APIs (slack.com) blocked since ~March 28

Summary

Outbound curl requests to slack.com from remote sandbox triggers (scheduled agents) are failing with exit code 56 (network receive error). This was working as recently as Friday, March 27, 2026.

Setup

• A scheduled trigger runs on a cron schedule
• The agent uses Slack MCP to read/search messages, then uses curl with a Slack bot token to send a DM (so the message comes from a bot identity, not the authenticated user)
• The bot token is valid and works locally (auth.test returns ok: true)

Expected Behavior

curl -s -X POST https://slack.com/api/chat.postMessage ... should succeed from within the remote sandbox, as it did before March 28.

Actual Behavior

curl fails with exit code 56 (network receive error). The agent cannot reach slack.com via direct HTTP.

MCP connections (which route through Anthropic's managed infrastructure) continue to work fine.

Workaround

Using Slack MCP tools to send the message works, but sends as the authenticated user rather than the bot — which defeats the purpose of using a custom Slack app.

Impact

Any trigger that relies on outbound HTTP to external APIs (not just Slack) is likely affected.

Environment

• Trigger IDs: trig_017c3ALpvhfYy5NenErhFeFn, trig_017qmmbKtyUQLTBxjnVoJFsD
• Environment: Default (Anthropic Cloud)
• Model: claude-sonnet-4-6