diff --git a/README.md b/README.md
index 0ed686f..66ca661 100644
--- a/README.md
+++ b/README.md
@@ -6,10 +6,14 @@ Access pre-computed derivative and microstructure metrics with parallel download
 
 ## Installation
 
+Install it with the data frame library you use. The extra also brings the HTTP client that fetching data needs:
+
 ```bash
-pip install aperiodic
+pip install "aperiodic[polars]"   # or "aperiodic[pandas]"
 ```
 
+A plain `pip install aperiodic` is enough to sign in with `aperiodic.login()`, but not to fetch data.
+
 Install from source:
 
 ```bash
diff --git a/src/aperiodic/_backends/_pyfetch_transport.py b/src/aperiodic/_backends/_pyfetch_transport.py
index f7e1d91..8e86830 100644
--- a/src/aperiodic/_backends/_pyfetch_transport.py
+++ b/src/aperiodic/_backends/_pyfetch_transport.py
@@ -22,6 +22,25 @@
 
 T = TypeVar("T")
 
+NO_HTTP_CLIENT = (
+    "Fetching data needs an HTTP client, which a plain `pip install aperiodic` "
+    "leaves out. Install one with the data frame library you use: "
+    'pip install "aperiodic[polars]" or pip install "aperiodic[pandas]".'
+)
+
+
+def _in_browser_runtime() -> bool:
+    """Pyodide, or a test that fakes it by putting its modules in ``sys.modules``."""
+    return sys.platform == "emscripten" or any(
+        name in sys.modules for name in ("pyodide", "pyodide.ffi", "pyodide.http")
+    )
+
+
+def _require_browser_runtime() -> None:
+    """Outside Pyodide this transport is only chosen because httpx is missing."""
+    if not _in_browser_runtime():
+        raise ImportError(NO_HTTP_CLIENT)
+
 
 def _has_pyodide_ffi() -> bool:
     """Detect ``pyodide.ffi`` without importing it or crashing on test mocks.
@@ -38,7 +57,11 @@ def _has_pyodide_ffi() -> bool:
     """
     if "pyodide.ffi" in sys.modules:
         return True
-    return find_spec("pyodide.ffi") is not None
+    try:
+        return find_spec("pyodide.ffi") is not None
+    except (ModuleNotFoundError, ValueError):
+        # find_spec imports the parent package, which is not there outside Pyodide.
+        return False
 
 
 def run_async(coro: Coroutine[None, None, T]) -> T:
@@ -61,7 +84,11 @@ def run_async(coro: Coroutine[None, None, T]) -> T:
     runtime), raise a clear error pointing users to the ``_async``
     counterparts rather than silently returning a ``Task``.
     """
+    if not _in_browser_runtime():
+        coro.close()
+        raise ImportError(NO_HTTP_CLIENT)
     if not _has_pyodide_ffi():
+        coro.close()
         raise RuntimeError(
             "Synchronous Aperiodic client calls require Pyodide ≥ 0.26 with "
             "JavaScript Promise Integration (stack switching). Use the "
@@ -143,6 +170,7 @@ async def fetch_json(
     retried with exponential backoff. Every other response is handed to the
     caller as data or an ``APIError`` on the first attempt.
     """
+    _require_browser_runtime()
     full_url = _build_url(url, params)
 
     for attempt in range(max_retries):
@@ -221,6 +249,7 @@ async def download_parquet_bytes(
     Returns:
         Tuple of (year, month, raw_bytes)
     """
+    _require_browser_runtime()
     from pyodide.http import pyfetch  # type: ignore[import-not-found]
 
     async with semaphore:
diff --git a/tests/test_pyfetch_transport.py b/tests/test_pyfetch_transport.py
index 649a0bd..efeaa4c 100644
--- a/tests/test_pyfetch_transport.py
+++ b/tests/test_pyfetch_transport.py
@@ -2,7 +2,9 @@
 
 from __future__ import annotations
 
+import asyncio
 import json
+import sys
 from unittest import mock
 
 import pytest
@@ -11,6 +13,8 @@
 pa = pytest.importorskip("pyarrow")
 pq = pytest.importorskip("pyarrow.parquet")
 
+from aperiodic._backends import _pyfetch_transport as pyfetch_transport  # noqa: E402
+
 
 class FakeResponse:
     """Minimal mock for pyfetch response."""
@@ -265,3 +269,28 @@ async def test_raises_download_error_after_retries(self):
                     semaphore=asyncio.Semaphore(10),
                     max_retries=0,
                 )
+
+
+class TestPlainInstall:
+    """Regular Python with no httpx: the extras are named, never a missing pyodide."""
+
+    @pytest.fixture(autouse=True)
+    def _no_pyodide(self, monkeypatch):
+        for name in ("pyodide", "pyodide.ffi", "pyodide.http"):
+            monkeypatch.delitem(sys.modules, name, raising=False)
+
+    def test_pyodide_detection_is_false_rather_than_an_error(self):
+        assert pyfetch_transport._has_pyodide_ffi() is False
+
+    def test_a_sync_call_names_the_extras_to_install(self):
+        async def fetch():
+            return 1
+
+        coro = fetch()
+        with pytest.raises(ImportError, match=r"aperiodic\[polars\]"):
+            pyfetch_transport.run_async(coro)
+        assert coro.cr_frame is None  # closed, so no "never awaited" warning
+
+    def test_an_async_call_names_the_extras_to_install(self):
+        with pytest.raises(ImportError, match=r"aperiodic\[pandas\]"):
+            asyncio.run(pyfetch_transport.fetch_json("https://x.test", {}, {}))