diff --git a/focus_project/focus_project/sources/__init__.py b/focus_project/focus_project/sources/__init__.py
new file mode 100644
index 0000000..b43b6b1
--- /dev/null
+++ b/focus_project/focus_project/sources/__init__.py
@@ -0,0 +1,87 @@
+# Copyright (c) 2026 Krzysztof Rudnicki
+"""Evidence adapters: each turns one record of work into time intervals.
+
+An adapter is a module ``focus_project.sources.<name>`` exposing one pure,
+read-only function::
+
+    def intervals(day: date, project: Project) -> list[Interval]
+
+``Interval`` is ``(start, end)`` in unix seconds, ``start <= end``. Adapters
+may return spans outside ``day``; :func:`collect` clips them. To add a source,
+add the module and its name to :data:`ADAPTERS`; a name whose module does not
+exist yet is skipped as inactive.
+
+Failure mode (fail closed, named): an adapter that raises is logged and
+contributes nothing; it never stops the others or the harvester.
+"""
+
+from __future__ import annotations
+
+from datetime import date, datetime, time, timedelta
+import importlib
+import logging
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from focus_project._model import Project
+
+Interval = tuple[float, float]
+
+# Each heartbeat vouches for the five minutes before it.
+HEARTBEAT_WINDOW_S = 300.0
+ADAPTERS = ("activity", "leetcode", "browser")
+_LOG = logging.getLogger(__name__)
+
+
+def heartbeat_interval(ts: float) -> Interval:
+    """The span a heartbeat at ``ts`` covers: ``[ts - 300, ts]``."""
+    return (ts - HEARTBEAT_WINDOW_S, ts)
+
+
+def day_bounds(day: date) -> Interval:
+    """Local midnight to the next local midnight, as unix seconds."""
+    start = datetime.combine(day, time.min).astimezone()
+    end = datetime.combine(day + timedelta(days=1), time.min).astimezone()
+    return (start.timestamp(), end.timestamp())
+
+
+def clip(spans: list[Interval], bounds: Interval) -> list[Interval]:
+    """``spans`` limited to ``bounds``; empty and inverted spans dropped."""
+    lo, hi = bounds
+    out = [(max(a, lo), min(b, hi)) for a, b in spans]
+    return [(a, b) for a, b in out if b > a]
+
+
+def collect_by_source(day: date, project: Project) -> dict[str, list[Interval]]:
+    """Every adapter's clipped intervals for ``day``, keyed by adapter name.
+
+    Missing adapters are absent from the result; failing ones map to ``[]``.
+    """
+    bounds = day_bounds(day)
+    found: dict[str, list[Interval]] = {}
+    for name in ADAPTERS:
+        module_name = f"{__name__}.{name}"
+        try:
+            module = importlib.import_module(module_name)
+        except ModuleNotFoundError as exc:
+            if exc.name == module_name:
+                _LOG.info("source %s: not installed (inactive)", name)
+                continue
+            _LOG.exception("source %s: import failed, contributes 0", name)
+            found[name] = []
+            continue
+        except Exception:  # noqa: BLE001 (any broken adapter must contribute 0)
+            _LOG.exception("source %s: import failed, contributes 0", name)
+            found[name] = []
+            continue
+        try:
+            found[name] = clip(list(module.intervals(day, project)), bounds)
+        except Exception:  # noqa: BLE001 (fail closed: this source adds 0)
+            _LOG.exception("source %s: failed, contributes 0", name)
+            found[name] = []
+    return found
+
+
+def collect(day: date, project: Project) -> list[Interval]:
+    """All adapters' clipped intervals for ``day``, concatenated."""
+    return [s for spans in collect_by_source(day, project).values() for s in spans]
diff --git a/focus_project/focus_project/tests/test_sources.py b/focus_project/focus_project/tests/test_sources.py
new file mode 100644
index 0000000..9f3aafa
--- /dev/null
+++ b/focus_project/focus_project/tests/test_sources.py
@@ -0,0 +1,94 @@
+# Copyright (c) 2026 Krzysztof Rudnicki
+"""The adapter registry: clipping, inactive modules, fail-closed errors."""
+
+from __future__ import annotations
+
+from datetime import date
+import logging
+import sys
+import types
+from typing import TYPE_CHECKING
+
+import pytest
+
+from focus_project import sources
+from focus_project._model import Project
+
+if TYPE_CHECKING:
+    from collections.abc import Callable
+
+DAY = date(2026, 11, 2)
+PROJECT = Project(
+    id="p",
+    name="P",
+    start=DAY,
+    end=DAY,
+    replaces=("leetcode",),
+    daily_minutes=120,
+    unlock_minutes=30,
+)
+
+
+def _fake(
+    monkeypatch: pytest.MonkeyPatch,
+    name: str,
+    fn: Callable[[date, Project], list[sources.Interval]],
+) -> None:
+    module = types.ModuleType(f"focus_project.sources.{name}")
+    module.intervals = fn  # type: ignore[attr-defined]
+    monkeypatch.setitem(sys.modules, f"focus_project.sources.{name}", module)
+
+
+def _none(_day: date, _project: Project) -> list[sources.Interval]:
+    return []
+
+
+def test_day_bounds_is_24h_local() -> None:
+    lo, hi = sources.day_bounds(DAY)
+    assert hi - lo == 86400
+
+
+def test_clip_limits_and_drops_empty() -> None:
+    assert sources.clip([(0, 5), (4, 20), (30, 31), (9, 9)], (3, 10)) == [
+        (3, 5),
+        (4, 10),
+    ]
+
+
+def test_heartbeat_covers_previous_five_minutes() -> None:
+    assert sources.heartbeat_interval(1000.0) == (700.0, 1000.0)
+
+
+def test_collect_clips_to_the_day(monkeypatch: pytest.MonkeyPatch) -> None:
+    lo, hi = sources.day_bounds(DAY)
+    _fake(monkeypatch, "activity", lambda _d, _p: [(lo - 60, lo + 60), (hi, hi + 9)])
+    _fake(monkeypatch, "leetcode", _none)
+    _fake(monkeypatch, "browser", _none)
+    assert sources.collect(DAY, PROJECT) == [(lo, lo + 60)]
+
+
+def test_failing_source_contributes_zero_and_is_logged(
+    monkeypatch: pytest.MonkeyPatch, caplog: pytest.LogCaptureFixture
+) -> None:
+    lo, _ = sources.day_bounds(DAY)
+
+    def boom(_d: date, _p: Project) -> list[sources.Interval]:
+        msg = "db locked"
+        raise OSError(msg)
+
+    _fake(monkeypatch, "activity", boom)
+    _fake(monkeypatch, "leetcode", lambda _d, _p: [(lo, lo + 60)])
+    _fake(monkeypatch, "browser", _none)
+    with caplog.at_level(logging.ERROR):
+        result = sources.collect_by_source(DAY, PROJECT)
+    assert result["activity"] == []
+    assert result["leetcode"] == [(lo, lo + 60)]
+    assert "source activity: failed" in caplog.text
+
+
+def test_missing_adapter_is_inactive_not_an_error(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    _fake(monkeypatch, "activity", _none)
+    monkeypatch.setattr(sources, "ADAPTERS", ("activity", "nonexistent"))
+    assert "nonexistent" not in sources.collect_by_source(DAY, PROJECT)