# Writing E2e Tests

> Use when a developer wants to add, write, or create an end-to-end test for an Opik feature, page, or branch — e.g. "add an e2e test for the experiments comparison page", "write a test for the feature I just built", "e2e test for this branch", "cover the dataset items flow with a test". Runs the full loop in tests_end_to_end/e2e/ — analyze the feature and frontend code, explore the live UI with the Playwright MCP, write the Page Object Model + spec, and run it locally until green.

- Skill: `comet-ml/writing-e2e-tests` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add comet-ml/writing-e2e-tests`
- Raw SKILL.md: https://api.skillmd.com/api/skills/comet-ml/writing-e2e-tests/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Web & Frontend
- Author: comet-ml (https://skillmd.com/u/comet-ml)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/comet-ml/writing-e2e-tests

---


# Writing E2E Tests

This skill is how we add an end-to-end test to the Opik E2E suite. You give it a feature, page, or branch; it runs a proven loop end-to-end and leaves you with a working, locally-verified Playwright test.

**Announce at start:** "I'm using the writing-e2e-tests skill to add an E2E test for X."

## Where tests live

The suite is at `tests_end_to_end/e2e/`. Inside it:

- **Specs:** `tests/<feature>/<name>.spec.ts` — one feature directory per page family (`datasets`, `trace-explore`, `experiments`, `test-suites`, `online-evaluation`, …).
- **Page Object Models:** `pom/<name>.page.ts` — one class per page, methods for the interactions a test needs.
- **Fixtures:** `fixtures/<name>.fixture.ts` — seed entities (project, dataset, trace, experiment, testSuite) and tear them down. Composed in a chain; re-exported from `fixtures/index.ts`.
- **SDK clients:** `core/sdk/` — `sdkClient.python` (HTTP wrapper over the bridge) and `sdkClient.typescript` (direct `new Opik({...})`) for seeding. `core/backend/` holds the typed REST client for inspection + teardown.
- **Bridge:** `services/opik-sdk-driver/` — a FastAPI app (run with `uv`) wrapping the Python SDK, exposing routes the TS clients call. Playwright's `webServer` directive auto-spawns it during a test run; you don't start it by hand.

Specs and POMs import through path aliases: `import { test, expect } from '@e2e/fixtures'` and `import { LogsPage } from '@e2e/pom/logs.page'`.

## Tooling — already set up

- The **`Playwright` MCP** (live-UI exploration) and the **`playwright-test` MCP** (`browser_generate_locator`) are already configured in the repo's `.mcp.json`. No setup step.
- Tests run via the plain Playwright CLI from `tests_end_to_end/e2e/`. The `webServer` directive spawns the bridge automatically.

## Conventions

Read [conventions.md](conventions.md) before writing any POM or spec. It carries the rules that keep tests legible and stable: mandatory `test.step()` wrapping, UI-first assertions, selector preference, public-SDK-only seeding, fixture seed shapes, and the tag taxonomy. They aren't optional polish — each prevents a class of failure.

## The loop

```dot
digraph writing_e2e {
    rankdir=TB;
    "1. Scope (GATE)" [shape=box];
    "2. Analyze feature + FE code" [shape=box];
    "3. Discover live UI (GATE)" [shape=box];
    "4. Write POM + spec" [shape=box];
    "5. Run until green" [shape=box];
    "Green?" [shape=diamond];

    "1. Scope (GATE)" -> "2. Analyze feature + FE code";
    "2. Analyze feature + FE code" -> "3. Discover live UI (GATE)";
    "3. Discover live UI (GATE)" -> "4. Write POM + spec";
    "4. Write POM + spec" -> "5. Run until green";
    "5. Run until green" -> "Green?";
    "Green?" -> "4. Write POM + spec" [label="no — fix"];
    "Green?" -> "done" [label="yes"];
}
```

### Step 1 — Scope (gate, lightweight)

Work out, from the request:

- **What flow / feature** the test covers, and **which page** it lives on. If the dev pointed at a branch or PR, read the diff to find what changed.
- **The target.** Default is local OSS at `http://localhost:5173` (`OPIK_DEPLOYMENT=oss`, workspace `default`) — the natural target for "test the feature I just built." Only use another target if the dev asks.
- **Tags** — pick a tier (`@t1-smoke` / `@t2-cuj` / `@t3-nightly`) and a feature tag, per [conventions.md](conventions.md).

Run the **safety check** (below) before any seeding. Then confirm the scope in one short message — feature, page, target, tags — and proceed. Don't write a formal spec document.

### Step 2 — Analyze the feature and frontend code

Before touching the browser:

- Read the page's FE source under `apps/opik-frontend/src/v2/pages/<Page>/` — the route it renders at, the components it composes, and any `data-testid` attributes already present. The route shape is what your POM's `goto()` will use.
- Identify the **entity preconditions**: what must exist for the page to render real data (an empty project shows only the empty state). Decide how to seed it — which fixture fits, or which bridge/SDK call. Seed via the SDK/bridge, never by click-creating through the UI.
- Check `fixtures/` for an existing fixture that already seeds the shape you need; reuse it before writing a new one.

### Step 3 — Discover the live UI (gate, lightweight)

**Invoke the `playwright-pom-discovery` skill** (via the Skill tool). It walks the live page with the Playwright MCP: seed state, navigate authed, snapshot the accessibility tree, enumerate `data-testid`s, pick the most stable selector for each element you'll target, and flag any element that has no stable selector (needs a FE `data-testid` added in this change).

When discovery is done, report a short summary — the selectors you'll use per element, and any missing testids you'll add — and confirm before writing code. Don't write anything under `pom/` before this step.

### Step 4 — Write the POM + spec

- Write or extend the POM in `pom/<name>.page.ts` using the selectors from discovery. Each method wraps its body in `test.step()` and returns through the callback (see [conventions.md](conventions.md)).
- Write the spec in `tests/<feature>/<name>.spec.ts`: tier + `@area:` on the describe block, a `@cap:` per test, coarse `test.step()` phases, UI-first assertions.
- **Take the `@area:`/`@cap:` values from `tests_end_to_end/coverage/taxonomy.yaml` — never invent them.** Grep it for the feature first; the capability you're covering is usually already reserved as `covered: false`. Update the taxonomy in the same change: add the spec to the area's `specs:` list and flip each covered capability to `covered: true` with its tier. See the Tags section of [conventions.md](conventions.md).
- If discovery flagged a missing/brittle selector, add a descriptive `data-testid` to the FE component in the **same change**.

#### Rebuilding the FE after adding a `data-testid`

The local OSS deployment serves the frontend from a **Docker image** — file changes to `apps/opik-frontend/` are not picked up automatically. After adding a `data-testid`, you must rebuild and restart the container before the test can find it.

From `deployment/docker-compose/`:

```bash
# 1. Build a new image from the updated source
docker compose --profile opik build frontend

# 2. Recreate the container using the locally built image
#    (pull_policy defaults to "always" — override it so Docker uses the local build)
docker stop opik-frontend-1 && docker rm opik-frontend-1
OPIK_FRONTEND_PULL_POLICY=never docker compose --profile opik up -d --no-deps frontend
```

Verify the new `data-testid` is live before running the test:

```bash
docker exec opik-frontend-1 sh -c 'grep -r "your-testid" /usr/share/nginx/html/ | wc -l'
# should print a non-zero number
```

> **Network note:** if the rebuilt container loses connectivity to the backend (502 errors), the container may have ended up on the wrong Docker network. Fix it:
> ```bash
> docker network disconnect opik-opik_default opik-frontend-1
> docker network connect opik-opik_default opik-frontend-1
> ```

### Step 5 — Run until green

From `tests_end_to_end/e2e/`:

```bash
npx playwright test tests/<feature>/<name>.spec.ts --reporter=list
```

The bridge auto-spawns (you'll see its startup line in the output). If a test fails, **read the failure trace** (`npx playwright show-trace`) rather than adjusting selectors blindly — see "verify the test render before blaming the backend" in [conventions.md](conventions.md). Fix and re-run until green. Report the actual run output.

Three checks before you call it done — each catches something the single-spec run can't:

```bash
# 1. Tags resolve in the taxonomy (this is the CI `tag-lint` job — run it locally, it's instant)
python3 tests_end_to_end/coverage/tag_lint.py --taxonomy tests_end_to_end/coverage/taxonomy.yaml --estate tests_end_to_end

# 2. If you touched a shared POM, the whole feature directory still passes
npx playwright test tests/<feature>/ --reporter=list

# 3. Types
npx tsc --noEmit
```

"My spec passes" is not "I didn't break anything": a shared POM is used by sibling specs, and
tag-lint failures never surface in a Playwright run at all.

## Safety: verify local config before seeding

The Python SDK behind the bridge reads `~/.opik.config`. If it points at a cloud environment, seeding would create real data there. Before any seed against a local target:

```bash
cat ~/.opik.config
```

If `url_override` is anything other than `http://localhost:5173/api`, back it up and point it local:

```bash
cp ~/.opik.config ~/.opik.config.bak 2>/dev/null || true
cat > ~/.opik.config << 'EOF'
[opik]
url_override = http://localhost:5173/api
workspace = default
EOF
```

When the work is done, remind the dev to restore: `cp ~/.opik.config.bak ~/.opik.config`. If it already points local, skip this.

## Anti-patterns

| Symptom | What you skipped |
|---|---|
| "Let me read the FE source to find the selector" | Discovery — snapshot the rendered DOM. What renders is the only source of truth for selectors. |
| "I'll explore the empty page and figure out the rows later" | Seeding — an empty-state-only POM never exercises the row template or open-detail actions. |
| "I'll write the POM and find out if it works when the whole suite runs" | Run-until-green in isolation — iterate on the one spec, don't debug it inside a full suite run. |
| "`page.locator('tbody tr:nth-child(3)')` is fine" | Flagging the missing testid — brittle structural selectors are the top source of flake; add a `data-testid`. |
| "I'll create the dataset through the UI so the page has data" | SDK/bridge seeding — UI-create is what the test exercises, not how you set up. |
| "`@cap:traces.bulk-delete-traces` describes what my test does" | Checking the taxonomy — `@area:`/`@cap:` values are a fixed vocabulary in `taxonomy.yaml`, not free text. CI's `tag-lint` rejects invented names, and the capability you want is usually already reserved there. |
| "I'll clean up in a `try`/`finally` at the end of the test" | Fixture teardown — it already runs on pass, fail and timeout, and keeps cleanup out of the assertions (where it also escapes `test.step()` and never reaches the trace). Add a fixture, or a register-callback one when the id only appears mid-test. |
| "Adding a fixture would touch files outside my ticket" | Nothing — `fixtures/` and `core/backend/client.ts` are where seeding and teardown belong. Reuse first, but adding one is normal, not scope creep. |
| "The spec passes, so I'm done" | `tag_lint.py`, the feature-directory run, and `tsc` — a green single spec hides tag errors and sibling breakage from a shared POM. |