agent loop detected re-sending the same welcome email after sendgrid send failed

Fix agent loop re-sending the same welcome email after SendGrid send failed

TL;DR

The agent re-sends because it never recorded the attempt, so every retry looks like the first try. Record each send attempt with its result before retrying, and cap retries. The loop is a missing idempotency record, not a SendGrid problem.

The error

Agent loop detected
Welcome email re-sent 12 times to the same user after SendGrid send failures.

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Agent? Search Vectle for this error:

curl -s "https://vectle.com/api/v1/search?q=agent loop detected re-sending the same welcome email after sendgrid send failed"

Fix it

Step 1: Stop the agent loop first

Pause the agent or the welcome sequence job.

Expected: No more duplicate emails go out.

Step 2: Check what SendGrid actually did

Look up the recipient in SendGrid activity to see which sends succeeded.

Expected: You know which attempts really failed versus which just looked like failures.

Step 3: Record attempts before retrying

Write each send attempt to a log or state store with the message id and result before any retry.

Expected: Retries consult the log and never resend a recorded attempt.

Step 4: Cap retries with backoff

Limit retries per user and back off between them.

Expected: A failing send gives up gracefully instead of looping forever.

Step 5: Resume and verify

Resume the agent and watch one failing send end to end.

Expected: It retries the capped number of times, then stops and alerts.

When this applies

• Agents re-send welcome emails in a loop
• Users report duplicate onboarding emails
• You are building email-sending agents

When it doesn't

• SendGrid rejects every send (fix the API key or sender)
• Different users get each other's emails (that is a templating bug)
• The email content is wrong (fix the template)

Compatibility

SendGrid mail send API. Any agent framework with retry logic.

Variant phrasings

agent resending welcome email loop

Same loop. The missing piece is always the attempt record.

sendgrid send failed agent retry storm

Retry storms need both idempotency and a cap. Either alone still misbehaves.

duplicate welcome emails onboarding agent

Duplicates erode trust fast. Pause first, then fix the loop.

Why it happens

The agent's retry logic treats every attempt as new because nothing persists the attempt history. A send that fails ambiguously, timeout with unknown outcome, gets retried as if unsent, and the cycle repeats. Idempotency records break the cycle by making retries aware of history.

Edge cases

• SendGrid may have accepted a send your agent thinks failed; reconcile before declaring failure
• Message-id dedup on your side catches replays the provider cannot
• Alert on retry exhaustion so a human sees the stuck user instead of silence

If it still fails

• Reproduce with a minimal run: one user, one file, one step.
• Read the agent's full trace, not just the final error; the failure is usually upstream.
• Check the underlying API or tool directly, outside the agent, to separate agent bugs from service bugs.
• Reduce concurrency to one and see if the failure persists; races hide as flakes.
• If the run is business-critical, add a human checkpoint before the destructive steps.

Prevention

• Checkpoint long runs so any failure resumes instead of restarting.
• Cap and back off every retry loop; unbounded retries are outages waiting to happen.
• Validate inputs at each pipeline stage; fail fast with clear errors.
• Log enough context per step that a timeout is diagnosable without rerunning.
• Give destructive steps a human checkpoint or a dry-run mode.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_zsf8FR44dgKhYOQ-jl6dKw