feat(messaging): store a sender and reply-to on transactional templates by mayteio · Pull Request #121871 · PostHog/posthog

mayteio · 2026-10-22T07:36:47Z

Problem

A transactional email can't choose who it comes from. Every template sends from the sender picked when it was made transactional, with no reply-to.
Moving PostHog's own emails off Customer.io needs this. A security email should come from a security address and take replies there.
The send path also accepts any text as a custom sender address, then quietly sends from the sender's own address instead.

This is the API half of the feature. The editor is #121079, stacked on this PR.

Changes

Template content takes from (sender, custom name and address) and replyTo. The API and the MCP email template tools accept both.
Making a template transactional stores the sender it was made with. Each publish copies the sender, name, address and reply-to into the hidden workflow, so the sender can change after conversion.
A custom address must be one address, at most 254 characters, on the sender's verified domain. The API rejects any other address on publish. Conversion rejects an address left over from a sender on another domain. Drafts still save, and a Liquid address is checked only at send time, as today.
A string or empty from means no sender, on reads and writes. That covers blank templates, templates saved before this change, and templates saved from a destination's email editor.
A sender without an integration ID is a 400.
Mechanical: regenerated frontend types and MCP schemas and snapshots, and a facade function that resolves the default sender.

How did you test this code?

The full feature was tested manually on a local stack, with the editor from #121079. A transactional template got a custom sender, display name, address and reply-to, published, and was sent through the API. MailDev received From: Hedgebox Security Team <security-team@posthog.com> and Reply-To: security@posthog.com.

Test rationale:

test_saving_a_transactional_template_writes_a_new_workflow_version switches the sender and sets a name, a Liquid address and a reply-to. It fails if the serializer drops a field or the sync ignores one.
test_rejects_an_update_that_would_stop_a_transactional_template_sending gains these cases: another project's sender, an unverified sender, an off-domain address, a non-address, an address that is too long, and a sender without an ID.
test_rejects_making_a_template_transactional_it_cannot_send gains a custom address left on another sender's domain.
test_retrieve_message_template lists and reads a template stored with each legacy from shape. Each one crashed the list before.
test_create_email_template_with_subject_succeeds creates with the editor's blank from and with a destination's sender string.
Every new case fails without this change.

Release status

This change is behind a feature flag and is not available to users

Transactional templates sit behind workflows-transactional-email-api.

Docs update

None. No existing doc covers transactional template settings.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Agent: PostHog Desktop with Claude Code, Claude Opus 5.5 (claude-opus-5-5)

Skills: improving-drf-endpoints, writing-tests, stacking-prs, writing-pr-descriptions.
Split out of feat(workflows): edit a transactional template's sender from the editor #121079, which was over stamphog's line cap. The MailDev reply-to change is its own PR.
Two PostHog Review rounds on feat(workflows): edit a transactional template's sender from the editor #121079 shaped the legacy-sender handling, the length bound and the conversion check.

Created with PostHog Desktop

🤖 Generated with Claude Code

- Template content takes `from` (sender, custom name and address) and `replyTo`. Each publish copies them into the transactional workflow, and making a template transactional stores the sender it was made with. - A custom address must be one address of at most 254 characters on the sender's verified domain. The API rejects it on publish, and conversion rejects one left over from another sender. - A sender string or empty value, from blank templates, older templates or a destination's editor, means no sender, on reads and writes. - A sender without an integration ID is a 400. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Generated-By: PostHog Desktop Task-Id: 90d9a51d-beba-47ba-8f50-b351327676f9

trunk-io · 2026-10-22T07:36:51Z

😎 This pull request was merged.

github-actions · 2026-10-22T07:37:08Z

🤖 CI report

⚠️

Trunk lane — backend Python lane

This PR is assigned to the backend Python lane. It runs backend Python tests and may merge in parallel with PRs in other lanes.

✅ Complexity (TypeScript) — clean

Cyclomatic complexity above the limit in changed typescript files (10 for production files, 15 for test files). Warn only: worth simplifying when you next touch these functions.

✅ Duplication (Python) — clean

New Python code duplication introduced by this branch. Fails at 70+ tokens in app code, or 150+ tokens when both copies live in test files. Advisory while the gate proves itself: extract a shared helper instead of copying.

✅ Duplication (TypeScript) — clean

New TypeScript code duplication introduced by this branch. Fails at 70+ tokens in app code, or 150+ tokens when both copies live in test files. Advisory while the gate proves itself: extract a shared helper instead of copying.

🚨 Comment density — 7% of added code lines are comments (18 of 241)

This section warns when comments are more than 3% of the code lines a PR adds, and alerts above 6%. Before agent-assisted PRs, the typical share was about 2%. Only full-line comments count. Docstrings, generated files, snapshots, migrations, and workflow files are left out.

Comments that restate the code, record how the change came about, or narrate the next line add noise for the next reader. Keep the comments that explain a reason the code cannot show, and remove the rest. See .agents/skills/writing-code-comments/SKILL.md for the house rules.

Files with the most added comment lines:

File	Comment lines	Added lines
`products/messaging/backend/services/transactional_email.py`	8	53
`products/messaging/backend/presentation/views/message_templates.py`	6	63
`products/messaging/backend/tests/api/test_message_templates.py`	2	26
`products/messaging/backend/tests/api/test_transactional_email.py`	1	78
`products/workflows/backend/facade/contracts.py`	1	4

This check does not block merging. It updates on every push and clears when the share drops.

✅ Bundle size — no change

Uncompressed size of every built .js bundle, compared against the base branch.

Total: 75.83 MiB · no change

No file changed by more than 1000 B.

_{Posted automatically by build-bundle-size-report · uncompressed bytes from dist-report}

⚠️

Eager graph — 2 over budget

How much code each root ships on the eager path — downloaded and parsed before the surface is interactive. Measured from the esbuild output chunks (post-tree-shake, static imports only); lazy import() / React.lazy chunks are not counted.

Root	Eager (shipped)	Δ vs base	Budget
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🟢 src/scenes/web-analytics/LiveMetricsDashboard/LiveWebAnalyticsMetrics.tsx stays out of src/scenes/web-analytics/WebAnalyticsScene.tsx
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✅ Toolbar bundle — eager 2.28 MiB within budget

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Metric	Size	Δ vs base	Budget
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✅ Dist folder size — no change

Total size of the built frontend/dist folder (all assets), compared against the base branch.

Total: 1016.05 MiB · no change

ℹ️ MCP UI apps size — 34 app(s), 27836.6 KB JS

Built size of each MCP UI app (main.js + styles.css).

App	JS	CSS
debug	840.2 KB	209.0 KB
action	764.3 KB	209.0 KB
action-list	831.2 KB	209.0 KB
cohort	763.3 KB	209.0 KB
cohort-list	830.2 KB	209.0 KB
email-template	763.1 KB	209.0 KB
error-details	768.1 KB	209.0 KB
error-issue	764.6 KB	209.0 KB
error-issue-list	831.8 KB	209.0 KB
experiment	828.3 KB	209.0 KB
experiment-list	832.0 KB	209.0 KB
experiment-results	830.8 KB	209.0 KB
feature-flag	830.6 KB	209.0 KB
feature-flag-list	834.3 KB	209.0 KB
feature-flag-testing	766.5 KB	209.0 KB
inline-scan	763.7 KB	209.0 KB
insight-actors	829.7 KB	209.0 KB
llm-costs	827.0 KB	209.0 KB
session-recording	765.5 KB	209.0 KB
survey	764.9 KB	209.0 KB
survey-global-stats	827.0 KB	209.0 KB
survey-list	831.9 KB	209.0 KB
survey-stats	827.0 KB	209.0 KB
trace-span	763.6 KB	209.0 KB
trace-span-list	831.1 KB	209.0 KB
vision-observation-list	830.3 KB	209.0 KB
workflow	763.6 KB	209.0 KB
workflow-list	830.6 KB	209.0 KB
workflow-setup-status	763.3 KB	209.0 KB
query-results	1082.2 KB	209.0 KB
render-ui	1157.0 KB	209.0 KB
visual-review-snapshots	768.1 KB	209.0 KB
workflow-data-suggestions	765.9 KB	209.0 KB
workflow-sending-rules	764.6 KB	209.0 KB

ℹ️ MCP agent API — agent-facing tool changes

What this PR changes for agents, from the tool schema snapshots and tool definitions.

Tools changed (2):

Tool	Params	Scopes	Annotations	Schema chars
`workflows-create-email-template`				3,354 -> 4,347
`workflows-update-email-template`				3,440 -> 4,433

✅ Playwright — all passed

All tests passed.

View test results →

⚠️

Backend coverage — 97.0% of changed backend lines covered — 3 uncovered

🧪 Backend test coverage

Patch coverage — changed backend lines (products + core): ███████████████████░ 97.0% (113 / 116)

File	Patch	Uncovered changed lines
`products/messaging/backend/services/transactional_email.py`	90.6%	79, 85, 95

🤖 Agents: add a test only if an uncovered line exposes a realistic regression that existing tests miss. Otherwise explain why no new test is needed under "How did you test this code?". Gap list: the patch-coverage artifact on this run (gh run download 502843202383296 -n patch-coverage), or the coverage-data block at the end of this comment.

Per-product line coverage (touched products)

Product	Coverage	Lines
`demo`	`███████████░░░░░░░░░` 56.4%	1,542 / 2,736
`batch_exports`	`████████████████░░░░` 81.6%	22,832 / 27,996
`engineering_analytics`	`█████████████████░░░` 85.0%	11,463 / 13,482
`commands`	`█████████████████░░░` 85.5%	1,618 / 1,893
`mcp_analytics`	`█████████████████░░░` 86.1%	5,699 / 6,617
`warehouse_suggestions`	`██████████████████░░` 87.7%	2,422 / 2,762
`posthog_ai`	`██████████████████░░` 87.9%	4,287 / 4,875
`dashboards`	`██████████████████░░` 88.3%	7,327 / 8,300
`notebooks`	`██████████████████░░` 88.5%	15,287 / 17,281
`mcp_registry`	`██████████████████░░` 89.0%	1,596 / 1,794
`cohorts`	`██████████████████░░` 89.6%	8,568 / 9,559
`product_tours`	`██████████████████░░` 89.7%	1,355 / 1,510
`signals`	`██████████████████░░` 90.6%	80,008 / 88,328
`tasks`	`██████████████████░░` 90.7%	79,609 / 87,743
`data_warehouse`	`██████████████████░░` 90.8%	14,722 / 16,216
`canvas`	`██████████████████░░` 91.1%	7,998 / 8,776
`today`	`██████████████████░░` 91.2%	3,798 / 4,163
`cdp`	`██████████████████░░` 91.4%	10,166 / 11,119
`autoresearch`	`██████████████████░░` 91.4%	11,606 / 12,693
`slack_app`	`██████████████████░░` 91.6%	20,010 / 21,852
`metrics`	`██████████████████░░` 91.6%	10,231 / 11,171
`business_knowledge`	`██████████████████░░` 91.6%	9,965 / 10,878
`data_modeling`	`██████████████████░░` 91.7%	10,072 / 10,985
`wizard`	`██████████████████░░` 91.7%	6,870 / 7,490
`managed_warehouse`	`██████████████████░░` 91.8%	11,923 / 12,995
`error_tracking`	`██████████████████░░` 91.9%	17,572 / 19,120
`streamlit_apps`	`██████████████████░░` 91.9%	3,139 / 3,415
`conversations`	`██████████████████░░` 92.2%	30,218 / 32,778
`review_hog`	`██████████████████░░` 92.2%	17,726 / 19,221
`stamphog`	`███████████████████░` 92.5%	8,776 / 9,483
`early_access_features`	`███████████████████░` 92.6%	1,339 / 1,446
`alerts_platform`	`███████████████████░` 92.8%	7,386 / 7,962
`notifications`	`███████████████████░` 93.0%	1,199 / 1,289
`web_analytics`	`███████████████████░` 93.2%	25,607 / 27,465
`alerts`	`███████████████████░` 93.3%	9,442 / 10,118
`surveys`	`███████████████████░` 93.4%	6,918 / 7,406
`cross_project_dashboards`	`███████████████████░` 93.4%	880 / 942
`approvals`	`███████████████████░` 93.4%	4,576 / 4,898
`exports`	`███████████████████░` 93.6%	4,125 / 4,406
`billing_alerts`	`███████████████████░` 93.7%	2,093 / 2,233
`context_layer`	`███████████████████░` 93.8%	3,946 / 4,208
`messaging`	`███████████████████░` 93.8%	4,692 / 5,002
`mcp_store`	`███████████████████░` 93.9%	9,084 / 9,676
`marketing_analytics`	`███████████████████░` 93.9%	21,083 / 22,454
`customer_analytics`	`███████████████████░` 94.0%	27,699 / 29,453
`workflows`	`███████████████████░` 94.1%	22,330 / 23,719
`ai_observability`	`███████████████████░` 94.2%	27,744 / 29,447
`logs`	`███████████████████░` 94.3%	19,216 / 20,369
`replay_vision`	`███████████████████░` 94.5%	31,999 / 33,861
`actions`	`███████████████████░` 94.7%	1,004 / 1,060
`reminders`	`███████████████████░` 94.8%	760 / 802
`growth`	`███████████████████░` 94.9%	11,347 / 11,959
`endpoints`	`███████████████████░` 95.1%	9,658 / 10,160
`annotations`	`███████████████████░` 95.1%	1,095 / 1,151
`skills`	`███████████████████░` 95.2%	7,336 / 7,707
`experiments`	`███████████████████░` 95.3%	38,842 / 40,739
`access_control`	`███████████████████░` 95.5%	8,079 / 8,459
`data_catalog`	`███████████████████░` 95.6%	4,513 / 4,721
`product_analytics`	`███████████████████░` 95.8%	29,661 / 30,966
`revenue_analytics`	`███████████████████░` 95.9%	1,877 / 1,958
`data_quality`	`███████████████████░` 96.0%	8,523 / 8,878
`feature_flags`	`███████████████████░` 96.2%	27,805 / 28,914
`subscriptions`	`███████████████████░` 96.3%	12,031 / 12,494
`tracing`	`███████████████████░` 96.4%	4,360 / 4,524
`warehouse_sources`	`███████████████████░` 96.5%	467,816 / 484,734
`account_audits`	`███████████████████░` 96.7%	768 / 794
`pulse`	`███████████████████░` 97.1%	2,023 / 2,083
`analytics_platform`	`████████████████████` 98.0%	2,798 / 2,855
`groups`	`████████████████████` 98.2%	1,823 / 1,856

Report-only. Patch coverage = changed backend lines covered vs origin/master. Sorted lowest first.
Known gaps: lines covered only by Temporal tests show as uncovered; core line numbers may drift if master changed the same file.

mypy flagged a possibly missing integration ID in the sender lookup. Without a sender there is no domain to check against, and the sender field already rejects that case. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Generated-By: PostHog Desktop Task-Id: 90d9a51d-beba-47ba-8f50-b351327676f9

parameterai · 2026-10-22T08:20:27Z

Risk: No findings

This PR adds a per-template email sender (integration ID, display name, custom from address) and reply-to to transactional email templates, copies them into the backing workflow on conversion and every publish, and validates the custom address against the sender's verified domain. I traced the authoring API, the conversion/publish service, and the nodejs send path end to end and found the new fields fail closed at each boundary: integration lookups are team-scoped, unverified senders are rejected at publish and send, and an off-domain or malformed custom address (including one produced by Liquid at send time) is discarded in favor of the sender's own address.

_{Sentinel reviewed f799de5 · Review settings}

stamphog

Approved.

The disclosed API, write-path and sender-validation changes enter risky territory, with assurance supplied by owning-team authorship and STRONG familiarity. Regression tests cover sender changes, invalid integrations and legacy content; there are no outstanding review concerns or identified refusal grounds.

Author wrote 92% of the modified lines and has 38 merged PRs in these paths (familiarity STRONG).

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	163L, 6F substantive, 623L/14F incl. docs/generated/snapshots — within ceiling
tier	✓	T1-agent / T1d-complex (623L, 14F, two-areas, feat)
stamphog 2.6.0		`.stamphog/policy.yml` @ `f799de5` · reviewed head `f799de5`

trunk-io · 2026-10-22T08:40:13Z

_{View Full Report ↗︎ ⋅ Docs}

trunk-io · 2026-10-22T09:21:56Z

This pull request is queued for merge as part of 121079, which will merge 121871, 121079.

trunk-io · 2026-10-22T09:43:12Z

This pull request was merged into master as part of stacked PR 121079.

deployment-status-posthog · 2026-10-22T10:07:00Z

Deploy status

Environment	Status	Deployed At	Workflow
dev	✅ Deployed	2026-10-22 10:06 UTC	Run
prod-us	✅ Deployed	2026-10-22 10:20 UTC	Run
prod-eu	✅ Deployed	2026-10-22 10:22 UTC	Run

mayteio self-assigned this Oct 22, 2026

mayteio mentioned this pull request Oct 22, 2026

fix(cdp): pass reply-to to the local maildev transport #121872

Merged

mayteio added this pull request to stack #121873 October 22, 2026 07:37

mayteio mentioned this pull request Oct 22, 2026

feat(workflows): edit a transactional template's sender from the editor #121079

Merged

mayteio marked this pull request as ready for review October 22, 2026 08:20

mayteio added the stamphog Request AI approval (no full review) label Oct 22, 2026 — with PostHog

pr-assigner-resolver-posthog Bot requested a review from a team October 22, 2026 08:21

stamphog Bot approved these changes Oct 22, 2026

View reviewed changes

trunk-io Bot merged commit 705811c into master Oct 22, 2026
406 checks passed

trunk-io Bot deleted the posthog/transactional-email-sender-api branch October 22, 2026 09:43

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feat(messaging): store a sender and reply-to on transactional templates - #121871

feat(messaging): store a sender and reply-to on transactional templates#121871
trunk-io[bot] merged 2 commits into
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posthog/transactional-email-sender-api

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