fix: update env template to use LANGFUSE_BASE_URL

[milanagm]

Closes LFE-10249

What does this PR do?

A contributor who follows CONTRIBUTING.md set up langfuse-python and to run the E2E test suite hits a KeyError before any test runs, because .env.template does not include LANGFUSE_BASE_URL, which the test infrastructure hard-requires.

Root cause

LANGFUSE_BASE_URL is the canonical env var in v4; LANGFUSE_HOST is deprecated (AGENTS.md:131, code_review.md:14).

The SDK client itself works with either — reads LANGFUSE_BASE_URL first, falls back to LANGFUSE_HOST (langfuse/_client/client.py:258-263).

But the test infra has no fallback: tests/support/api_wrapper.py:19 uses os.environ["LANGFUSE_BASE_URL"] (subscript → KeyError if unset). tests/support/utils.py:72 passes os.environ.get("LANGFUSE_BASE_URL") (→ None), which then hits that subscript.

.env.template ships only LANGFUSE_HOST, never LANGFUSE_BASE_URL.

CONTRIBUTING.md:39 tells contributors to set up E2E env vars "based on .env.template".

5 of 8 files in tests/e2e/ call get_api() / LangfuseAPI, so all of them fail without the var.

Steps to reproduce

cp .env.template .env and fill in keys (per CONTRIBUTING).

Start a local Langfuse server.

Run uv run --frozen pytest -n 4 --dist worksteal tests/e2e -m "not serial_e2e".

Result: errors/failures originating from KeyError('LANGFUSE_BASE_URL').

Minimal repro (no server needed):

$ env -i LANGFUSE_HOST=http://localhost:3000/ \

LANGFUSE_PUBLIC_KEY=pk-lf-... LANGFUSE_SECRET_KEY=sk-lf-... \
python -c "from tests.support.api_wrapper import LangfuseAPI; LangfuseAPI(base_url=None)"

KeyError: 'LANGFUSE_BASE_URL'

Greptile Summary

This PR fixes a contributor-facing DX issue: .env.template was missing LANGFUSE_BASE_URL (the canonical v4 env var), causing a KeyError before any E2E test ran when following the CONTRIBUTING.md setup steps. The fix adds LANGFUSE_BASE_URL as the primary entry and retains LANGFUSE_HOST with a deprecation comment.

• Adds LANGFUSE_BASE_URL=http://localhost:3000 to .env.template, matching what tests/support/api_wrapper.py:19 requires via os.environ["LANGFUSE_BASE_URL"].
• Keeps LANGFUSE_HOST in the template with an inline comment marking it deprecated, which preserves backward-compat awareness but introduces a minor risk (see comment).

Confidence Score: 5/5

Safe to merge — a one-line template addition that unblocks E2E tests for new contributors without touching any runtime code.

The change is confined to .env.template, adds exactly the missing variable that tests/support/api_wrapper.py hard-requires, and introduces no logic changes. The only minor concern is the inline comment on the deprecated LANGFUSE_HOST line, which could confuse non-python-dotenv tooling.

No files require special attention; .env.template is the only changed file and the fix is straightforward.

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
.env.template:3
Inline comments in `.env` files are handled inconsistently across tooling. `python-dotenv` strips them correctly, but `source .env` in bash or `export $(cat .env | xargs)` would set `LANGFUSE_HOST` to the literal string `http://localhost:3000  # deprecated alias for LANGFUSE_BASE_URL`, breaking any fallback path that reads `LANGFUSE_HOST`. Since `LANGFUSE_BASE_URL` is now in the template and is the canonical var, moving the deprecation note to its own comment line avoids this entirely.

```suggestion
# LANGFUSE_HOST is a deprecated alias for LANGFUSE_BASE_URL
LANGFUSE_HOST=http://localhost:3000
```

_{Reviews (1): Last reviewed commit: "add LANGFUSE_BASE_URL to .env.template s..." | Re-trigger Greptile}

Greptile also left 1 inline comment on this PR.

[claude]

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[hassiebp]

Thanks Milana! Could you please update the PR title to follow semantic commit convention? See also for other commits in this repo

[hassiebp]

Thanks for your contribution, @milanagm 🥳