building-workflows

Building workflows

A PostHog workflow is a directed graph: a list of action nodes (actions) wired by edges (edges), with exactly one trigger node that starts every run. You author that graph as JSON and ship it over MCP. Always call it a "workflow" to the user. "Hog flow" is the internal code name (HogFlow), not a user-facing term.

The single biggest failure mode is getting the graph JSON structurally wrong. The backend stores actions/config as loose JSON, but the visual editor parses every node against a strict schema, so a malformed node saves but then breaks the editor view for the whole workflow. Before composing or editing any graph, read references/graph-schema.md. It is the contract; do not improvise node shapes from these examples alone.

The lifecycle

Work the workflow through these stages. Don't jump straight to enabling it.

1. Compose the graph. Build actions + edges per references/graph-schema.md. For any function node, don't guess the template: list the live catalog with cdp-function-templates-list and read its required inputs with cdp-function-templates-retrieve.
2. Create as a draft. workflows-create. Every workflow is created draft; it does not execute yet.
3. Test-run it. workflows-test-run runs one step at a time. Start at the first step (omit current_action_id, or point it at the trigger) with sample globals ({event, person, groups}); the result includes the next step's id (nextActionId). Feed that back as current_action_id and run again, walking step by step to the end. Skip delay nodes by jumping to the action after them (delays aren't simulated). Async side effects (HTTP/email/SMS) are mocked unless you set mock_async_functions=false. Read each step's trace to confirm the path taken.
4. Read logs while iterating. workflows-logs shows the per-step execution trace (levels DEBUG to ERROR). This is how you see why a step skipped, branched, or errored.
5. Edit the draft, then re-test. Patch the graph with workflows-patch-graph (see Editing a draft). Every edit invalidates your earlier test — re-run the affected path before moving on. Drafts only; active workflows are read-only over MCP.
6. Enable (one-way door, needs the user's explicit sign-off). workflows-enable flips it to active and an event/webhook/manual trigger starts firing on matching activity. You can't edit a live workflow over MCP (to change it you recreate it as a new draft), so treat enabling as effectively irreversible: finish testing, then get the user's explicit go before enabling. Don't enable on your own initiative.
7. Dispatch (batch/schedule only). A batch workflow does not fire on enable alone. Send a one-off broadcast with workflows-run-batch, or attach a recurring schedule with workflows-schedule-create. Confirm with workflows-get that status=='active' and its read-only schedules field has an active entry.
8. Monitor. Drill down: workflows-global-stats (which workflows are failing) to workflows-stats (one workflow's trend) to workflows-list-invocations (who it failed for) to workflows-get-invocation (the triggering payload) to workflows-logs (the failing step).

Full tool catalog, grouped by job: references/lifecycle-and-debugging.md.

Editing a draft

Patch, don't replace. Edit a draft with workflows-patch-graph: a small, ordered list of id-addressed operations (update_action, add_action, remove_action, add_edge, remove_edge, replace_action_edges). update_action deep-merges its patch, so changing one email subject is a few lines, not the whole graph. The ops apply atomically server-side (read, apply in order, validate, save only if valid), and the response echoes the full updated graph, so you never re-fetch before the next edit. This keeps each round-trip tiny instead of re-transmitting every action and edge.

workflows-update is the fallback, for two cases: top-level metadata a graph patch can't express (for example renaming the workflow), or as an escape hatch when you genuinely can't get workflows-patch-graph to land a change (send the whole corrected workflow rather than keep fighting the op list). For everything else, patch.

After any patch, re-test the path you changed (step 3). A patch that validates structurally can still route the wrong way.

Email templates follow the same rule: edit a template's design with workflows-patch-email-template (surgical, id-addressed ops over the Unlayer blocks), not workflows-update-email-template, which resends the entire design JSON. Compose and edit templates with the designing-email-templates skill.

Changing a live workflow

Active workflows cannot be edited over MCP. To change a live workflow, create a new draft (workflows-create) with the updated graph, test it, and enable it. If the user wants to edit a live workflow in place, tell them that isn't supported yet and offer the new-draft path.

What the server owns, never send it

The server compiles and manages these. Authoring them by hand is the fastest way to a broken workflow:

• bytecode on any filter, trigger, condition, conversion, or masking. Compiled server-side from the human-readable properties/hash. Omit it; send filters: {...}, not bytecode.
• trigger (top-level). Derived from the trigger action in actions. Read-only. Set the trigger by adding the trigger node, not by setting this field.
• billable_action_types, version, id, created_*. Computed/managed.

Minimal worked example

Event trigger, wait 1 day, send email, exit. Note: exactly one trigger, every non-exit node has an outgoing edge, ids are referenced consistently by edges, and no bytecode is sent.

{
  "name": "Nudge after signup",
  "description": "One day after signup, send a reminder.",
  "exit_condition": "exit_only_at_end",
  "actions": [
    {
      "id": "trigger_node",
      "name": "Signed up",
      "type": "trigger",
      "config": {
        "type": "event",
        "filters": { "events": [{ "id": "user signed up", "name": "user signed up", "type": "events", "order": 0 }] }
      }
    },
    {
      "id": "delay_1",
      "name": "Wait 1 day",
      "type": "delay",
      "config": { "delay_duration": "1d" }
    },
    {
      "id": "email_1",
      "name": "Reminder email",
      "type": "function_email",
      "config": {
        "template_id": "template-email",
        "message_category_type": "marketing",
        "inputs": {
          "email": {
            "value": {
              "to": { "email": "{person.properties.email}", "name": "" },
              "from": { "email": "[email protected]", "name": "Example" },
              "subject": "Don't forget to finish setting up",
              "html": "<p>Hi {person.properties.first_name}, …</p>"
            }
          }
        }
      }
    },
    {
      "id": "exit_node",
      "name": "Exit",
      "type": "exit",
      "config": { "reason": "Done" }
    }
  ],
  "edges": [
    { "from": "trigger_node", "to": "delay_1", "type": "continue" },
    { "from": "delay_1", "to": "email_1", "type": "continue" },
    { "from": "email_1", "to": "exit_node", "type": "continue" }
  ]
}

For anything beyond a placeholder email body, author the design with the designing-email-templates skill and reference the template. Don't hand-write production email HTML here.

Hard rules to surface to the user, not work around

• Behavioral targeting is unsupported. "Did event X at least N times over the last M days" can't be expressed as a trigger or a batch/schedule audience. If asked, reject it and explain; don't approximate it with a broken filter. (The backend rejects behavioral cohorts in batch audiences outright.)
• Batch audiences target who a person is, not what they did. Person properties and/or static/property-based cohorts only. Event/action filters in a batch audience are silently dropped, so they're rejected.
• Prefer re-evaluating audiences. For batch, inline person-property conditions or a dynamic (filter-based) cohort re-evaluate as people qualify; a static cohort is a frozen list, use only for an explicit given set.