diff --git a/src/loop/__init__.py b/src/loop/__init__.py
new file mode 100644
index 0000000..985c3dc
--- /dev/null
+++ b/src/loop/__init__.py
@@ -0,0 +1 @@
+"""Three-phase agent loop: Gather → Reason → Act."""
diff --git a/src/loop/phase1_gather.py b/src/loop/phase1_gather.py
new file mode 100644
index 0000000..185b1d6
--- /dev/null
+++ b/src/loop/phase1_gather.py
@@ -0,0 +1,37 @@
+"""Phase 1 — Gather: accept raw input, produce structured context.
+
+This is the sensory phase. It receives a raw ContextPayload and enriches
+it with whatever context Timmy needs before reasoning. In the stub form,
+it simply passes the payload through with a phase marker.
+"""
+
+from __future__ import annotations
+
+import logging
+
+from loop.schema import ContextPayload
+
+logger = logging.getLogger(__name__)
+
+
+def gather(payload: ContextPayload) -> ContextPayload:
+    """Accept raw input and return structured context for reasoning.
+
+    Stub: tags the payload with phase=gather and logs transit.
+    Timmy will flesh this out with context selection, memory lookup,
+    adapter polling, and attention-residual weighting.
+    """
+    logger.info(
+        "Phase 1 (Gather) received: source=%s content_len=%d tokens=%d",
+        payload.source,
+        len(payload.content),
+        payload.token_count,
+    )
+
+    result = payload.with_metadata(phase="gather", gathered=True)
+
+    logger.info(
+        "Phase 1 (Gather) produced: metadata_keys=%s",
+        sorted(result.metadata.keys()),
+    )
+    return result
diff --git a/src/loop/phase2_reason.py b/src/loop/phase2_reason.py
new file mode 100644
index 0000000..1e6f3b6
--- /dev/null
+++ b/src/loop/phase2_reason.py
@@ -0,0 +1,36 @@
+"""Phase 2 — Reason: accept gathered context, produce reasoning output.
+
+This is the deliberation phase. It receives enriched context from Phase 1
+and decides what to do. In the stub form, it passes the payload through
+with a phase marker.
+"""
+
+from __future__ import annotations
+
+import logging
+
+from loop.schema import ContextPayload
+
+logger = logging.getLogger(__name__)
+
+
+def reason(payload: ContextPayload) -> ContextPayload:
+    """Accept gathered context and return a reasoning result.
+
+    Stub: tags the payload with phase=reason and logs transit.
+    Timmy will flesh this out with LLM calls, confidence scoring,
+    plan generation, and judgment logic.
+    """
+    logger.info(
+        "Phase 2 (Reason) received: source=%s gathered=%s",
+        payload.source,
+        payload.metadata.get("gathered", False),
+    )
+
+    result = payload.with_metadata(phase="reason", reasoned=True)
+
+    logger.info(
+        "Phase 2 (Reason) produced: metadata_keys=%s",
+        sorted(result.metadata.keys()),
+    )
+    return result
diff --git a/src/loop/phase3_act.py b/src/loop/phase3_act.py
new file mode 100644
index 0000000..446238a
--- /dev/null
+++ b/src/loop/phase3_act.py
@@ -0,0 +1,36 @@
+"""Phase 3 — Act: accept reasoning output, execute and produce feedback.
+
+This is the command phase. It receives the reasoning result from Phase 2
+and takes action. In the stub form, it passes the payload through with a
+phase marker and produces feedback for the next cycle.
+"""
+
+from __future__ import annotations
+
+import logging
+
+from loop.schema import ContextPayload
+
+logger = logging.getLogger(__name__)
+
+
+def act(payload: ContextPayload) -> ContextPayload:
+    """Accept reasoning result and return action output + feedback.
+
+    Stub: tags the payload with phase=act and logs transit.
+    Timmy will flesh this out with tool execution, delegation,
+    response generation, and feedback construction.
+    """
+    logger.info(
+        "Phase 3 (Act) received: source=%s reasoned=%s",
+        payload.source,
+        payload.metadata.get("reasoned", False),
+    )
+
+    result = payload.with_metadata(phase="act", acted=True)
+
+    logger.info(
+        "Phase 3 (Act) produced: metadata_keys=%s",
+        sorted(result.metadata.keys()),
+    )
+    return result
diff --git a/src/loop/runner.py b/src/loop/runner.py
new file mode 100644
index 0000000..1a872d8
--- /dev/null
+++ b/src/loop/runner.py
@@ -0,0 +1,40 @@
+"""Loop runner — orchestrates the three phases in sequence.
+
+Runs Gather → Reason → Act as a single cycle, passing output from each
+phase as input to the next. The Act output feeds back as input to the
+next Gather call.
+"""
+
+from __future__ import annotations
+
+import logging
+
+from loop.phase1_gather import gather
+from loop.phase2_reason import reason
+from loop.phase3_act import act
+from loop.schema import ContextPayload
+
+logger = logging.getLogger(__name__)
+
+
+def run_cycle(payload: ContextPayload) -> ContextPayload:
+    """Execute one full Gather → Reason → Act cycle.
+
+    Returns the Act phase output, which can be fed back as input
+    to the next cycle.
+    """
+    logger.info("=== Loop cycle start: source=%s ===", payload.source)
+
+    gathered = gather(payload)
+    reasoned = reason(gathered)
+    acted = act(reasoned)
+
+    logger.info(
+        "=== Loop cycle complete: phases=%s ===",
+        [
+            gathered.metadata.get("phase"),
+            reasoned.metadata.get("phase"),
+            acted.metadata.get("phase"),
+        ],
+    )
+    return acted
diff --git a/src/loop/schema.py b/src/loop/schema.py
new file mode 100644
index 0000000..dff176a
--- /dev/null
+++ b/src/loop/schema.py
@@ -0,0 +1,43 @@
+"""Data schema for the three-phase loop.
+
+Each phase passes a ContextPayload forward. The schema is intentionally
+minimal — Timmy decides what fields matter as the loop matures.
+"""
+
+from __future__ import annotations
+
+import logging
+from dataclasses import dataclass, field
+from datetime import UTC, datetime
+
+logger = logging.getLogger(__name__)
+
+
+@dataclass
+class ContextPayload:
+    """Immutable context packet passed between loop phases.
+
+    Attributes:
+        source: Where this payload originated (e.g. "user", "timer", "event").
+        content: The raw content string to process.
+        timestamp: When the payload was created.
+        token_count: Estimated token count for budget tracking. -1 = unknown.
+        metadata: Arbitrary key-value pairs for phase-specific data.
+    """
+
+    source: str
+    content: str
+    timestamp: datetime = field(default_factory=lambda: datetime.now(UTC))
+    token_count: int = -1
+    metadata: dict = field(default_factory=dict)
+
+    def with_metadata(self, **kwargs: object) -> ContextPayload:
+        """Return a new payload with additional metadata merged in."""
+        merged = {**self.metadata, **kwargs}
+        return ContextPayload(
+            source=self.source,
+            content=self.content,
+            timestamp=self.timestamp,
+            token_count=self.token_count,
+            metadata=merged,
+        )
diff --git a/tests/loop/__init__.py b/tests/loop/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/loop/test_three_phase.py b/tests/loop/test_three_phase.py
new file mode 100644
index 0000000..dd1f28f
--- /dev/null
+++ b/tests/loop/test_three_phase.py
@@ -0,0 +1,133 @@
+"""Tests for the three-phase loop scaffold.
+
+Validates the acceptance criteria from issue #324:
+1. Loop accepts context payload as input to Phase 1
+2. Phase 1 output feeds into Phase 2 without manual intervention
+3. Phase 2 output feeds into Phase 3 without manual intervention
+4. Phase 3 output feeds back into Phase 1
+5. Full cycle completes without crash
+6. No state leaks between cycles
+7. Each phase logs what it received and what it produced
+"""
+
+from datetime import datetime
+
+from loop.phase1_gather import gather
+from loop.phase2_reason import reason
+from loop.phase3_act import act
+from loop.runner import run_cycle
+from loop.schema import ContextPayload
+
+
+def _make_payload(source: str = "test", content: str = "hello") -> ContextPayload:
+    return ContextPayload(source=source, content=content, token_count=5)
+
+
+# --- Schema ---
+
+
+def test_context_payload_defaults():
+    p = ContextPayload(source="user", content="hi")
+    assert p.source == "user"
+    assert p.content == "hi"
+    assert p.token_count == -1
+    assert p.metadata == {}
+    assert isinstance(p.timestamp, datetime)
+
+
+def test_with_metadata_returns_new_payload():
+    p = _make_payload()
+    p2 = p.with_metadata(foo="bar")
+    assert p2.metadata == {"foo": "bar"}
+    assert p.metadata == {}  # original unchanged
+
+
+def test_with_metadata_merges():
+    p = _make_payload().with_metadata(a=1)
+    p2 = p.with_metadata(b=2)
+    assert p2.metadata == {"a": 1, "b": 2}
+
+
+# --- Individual phases ---
+
+
+def test_gather_marks_phase():
+    result = gather(_make_payload())
+    assert result.metadata["phase"] == "gather"
+    assert result.metadata["gathered"] is True
+
+
+def test_reason_marks_phase():
+    gathered = gather(_make_payload())
+    result = reason(gathered)
+    assert result.metadata["phase"] == "reason"
+    assert result.metadata["reasoned"] is True
+
+
+def test_act_marks_phase():
+    gathered = gather(_make_payload())
+    reasoned = reason(gathered)
+    result = act(reasoned)
+    assert result.metadata["phase"] == "act"
+    assert result.metadata["acted"] is True
+
+
+# --- Full cycle ---
+
+
+def test_full_cycle_completes():
+    """Acceptance criterion 5: full cycle completes without crash."""
+    payload = _make_payload(source="user", content="What is sovereignty?")
+    result = run_cycle(payload)
+    assert result.metadata["gathered"] is True
+    assert result.metadata["reasoned"] is True
+    assert result.metadata["acted"] is True
+
+
+def test_full_cycle_preserves_source():
+    """Source field survives the full pipeline."""
+    result = run_cycle(_make_payload(source="timer"))
+    assert result.source == "timer"
+
+
+def test_full_cycle_preserves_content():
+    """Content field survives the full pipeline."""
+    result = run_cycle(_make_payload(content="test data"))
+    assert result.content == "test data"
+
+
+def test_no_state_leaks_between_cycles():
+    """Acceptance criterion 6: no state leaks between cycles."""
+    r1 = run_cycle(_make_payload(source="cycle1", content="first"))
+    r2 = run_cycle(_make_payload(source="cycle2", content="second"))
+    assert r1.source == "cycle1"
+    assert r2.source == "cycle2"
+    assert r1.content == "first"
+    assert r2.content == "second"
+
+
+def test_cycle_output_feeds_back_as_input():
+    """Acceptance criterion 4: Phase 3 output feeds back into Phase 1."""
+    first = run_cycle(_make_payload(source="initial"))
+    second = run_cycle(first)
+    # Second cycle should still work — no crash, metadata accumulates
+    assert second.metadata["gathered"] is True
+    assert second.metadata["acted"] is True
+
+
+def test_phases_log(caplog):
+    """Acceptance criterion 7: each phase logs what it received and produced."""
+    import logging
+
+    with caplog.at_level(logging.INFO):
+        run_cycle(_make_payload())
+
+    messages = caplog.text
+    assert "Phase 1 (Gather) received" in messages
+    assert "Phase 1 (Gather) produced" in messages
+    assert "Phase 2 (Reason) received" in messages
+    assert "Phase 2 (Reason) produced" in messages
+    assert "Phase 3 (Act) received" in messages
+    assert "Phase 3 (Act) produced" in messages
+    assert "Loop cycle start" in messages
+    assert "Loop cycle complete" in messages