email template renderer broke and onboarding agent failed on missing merge tag

Fix email template renderer breaking the onboarding agent on a missing merge tag

TL;DR

The renderer fails the whole send when one merge tag has no value. Give every tag a default or make missing tags non-fatal, and validate the data before rendering. One empty field should not kill a welcome email.

The error

Onboarding agent failed
Email template renderer broke on missing merge tag: first_name.

Use this skill

Agent? Search Vectle for this error:

curl -s "https://vectle.com/api/v1/search?q=email template renderer broke and onboarding agent failed on missing merge tag"

Fix it

Step 1: Identify the missing tag

Read the error for the tag name and check the user record.

Expected: You confirm the field is empty for this user.

Step 2: Add defaults for all tags

Give every merge tag a sensible fallback in the template.

Expected: Missing data renders as the default instead of failing.

Step 3: Validate data before rendering

Check required fields exist before calling the renderer.

Expected: Bad records are quarantined with a clear reason.

Step 4: Make the renderer non-fatal on missing tags

Configure the renderer to warn and continue rather than throw.

Expected: One missing tag never kills a send again.

Step 5: Re-run the welcome send

Retry the failed email.

Expected: It sends with defaults in place of the missing value.

When this applies

• Template rendering fails on missing merge tags
• Welcome emails die for users with incomplete profiles
• You are building templated email agents

When it doesn't

• The template syntax itself is broken (fix the template)
• All tags are missing (check the data source)
• The email sends but looks wrong (check the template logic)

Compatibility

Template renderers generally: Handlebars, Liquid, Jinja-style, and others.

Variant phrasings

missing merge tag email render failed

Same failure. Defaults plus non-fatal rendering fixes it.

template variable undefined email agent

Undefined variables should degrade, not crash. Set that behavior explicitly.

welcome email failed empty field

Empty profile fields are normal for new users. Templates must handle them.

Why it happens

Templates reference user fields that are not always populated, especially for brand-new users. Strict renderers throw on the first missing value, and the agent treats it as a fatal send failure. The data gap is expected; the renderer's strictness is the bug.

Edge cases

• Defaults must be sensible per tag; a wrong default is worse than a blank
• Quarantined records still need the email eventually; alert, do not just skip
• Test templates against a minimal user record with every optional field empty

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/pst1srJ0lHxvfkoBI6SYkR_g