diff --git a/src/atocore/context/builder.py b/src/atocore/context/builder.py
index 57e159d..3e4ab7d 100644
--- a/src/atocore/context/builder.py
+++ b/src/atocore/context/builder.py
@@ -14,7 +14,7 @@ import atocore.config as _config
 from atocore.context.project_state import format_project_state, get_state
 from atocore.memory.service import get_memories_for_context
 from atocore.observability.logger import get_logger
-from atocore.projects.registry import get_registered_project
+from atocore.projects.registry import resolve_project_name
 from atocore.retrieval.retriever import ChunkResult, retrieve
 
 log = get_logger("context_builder")
@@ -85,20 +85,15 @@ def build_context(
         max(0, int(budget * PROJECT_STATE_BUDGET_RATIO)),
     )
 
-    # Resolve the project hint through the registry so callers can pass
-    # an alias (`p05`, `gigabit`) and still find trusted state stored
-    # under the canonical project id (`p05-interferometer`,
-    # `p04-gigabit`). The retriever already does this for the
-    # project-match boost — the project_state lookup needs the same
-    # courtesy. If the registry has no entry for the hint, fall back to
-    # the raw hint so a hand-curated project_state entry that predates
-    # the registry still works.
-    canonical_project = project_hint
-    if project_hint:
-        registered = get_registered_project(project_hint)
-        if registered is not None:
-            canonical_project = registered.project_id
-
+    # Canonicalize the project hint through the registry so callers
+    # can pass an alias (`p05`, `gigabit`) and still find trusted
+    # state stored under the canonical project id. The same helper
+    # is used everywhere a project name crosses a trust boundary
+    # (project_state, memories, interactions). When the registry has
+    # no entry the helper returns the input unchanged so hand-curated
+    # state that predates the registry still works.
+    canonical_project = resolve_project_name(project_hint) if project_hint else ""
+    if canonical_project:
         state_entries = get_state(canonical_project)
         if state_entries:
             project_state_text = format_project_state(state_entries)
diff --git a/src/atocore/context/project_state.py b/src/atocore/context/project_state.py
index 82124a5..36091d4 100644
--- a/src/atocore/context/project_state.py
+++ b/src/atocore/context/project_state.py
@@ -18,6 +18,7 @@ from datetime import datetime, timezone
 
 from atocore.models.database import get_connection
 from atocore.observability.logger import get_logger
+from atocore.projects.registry import resolve_project_name
 
 log = get_logger("project_state")
 
@@ -101,11 +102,19 @@ def set_state(
     source: str = "",
     confidence: float = 1.0,
 ) -> ProjectStateEntry:
-    """Set or update a project state entry. Upsert semantics."""
+    """Set or update a project state entry. Upsert semantics.
+
+    The ``project_name`` is canonicalized through the registry so a
+    caller passing an alias (``p05``) ends up writing into the same
+    row as the canonical id (``p05-interferometer``). Without this
+    step, alias and canonical names would create two parallel
+    project rows and fragmented state.
+    """
     if category not in CATEGORIES:
         raise ValueError(f"Invalid category '{category}'. Must be one of: {CATEGORIES}")
     _validate_confidence(confidence)
 
+    project_name = resolve_project_name(project_name)
     project_id = ensure_project(project_name)
     entry_id = str(uuid.uuid4())
     now = datetime.now(timezone.utc).isoformat()
@@ -153,7 +162,12 @@ def get_state(
     category: str | None = None,
     active_only: bool = True,
 ) -> list[ProjectStateEntry]:
-    """Get project state entries, optionally filtered by category."""
+    """Get project state entries, optionally filtered by category.
+
+    The lookup is canonicalized through the registry so an alias hint
+    finds the same rows as the canonical id.
+    """
+    project_name = resolve_project_name(project_name)
     with get_connection() as conn:
         project = conn.execute(
             "SELECT id FROM projects WHERE lower(name) = lower(?)", (project_name,)
@@ -191,7 +205,12 @@ def get_state(
 
 
 def invalidate_state(project_name: str, category: str, key: str) -> bool:
-    """Mark a project state entry as superseded."""
+    """Mark a project state entry as superseded.
+
+    The lookup is canonicalized through the registry so an alias is
+    treated as the canonical project for the invalidation lookup.
+    """
+    project_name = resolve_project_name(project_name)
     with get_connection() as conn:
         project = conn.execute(
             "SELECT id FROM projects WHERE lower(name) = lower(?)", (project_name,)
diff --git a/src/atocore/interactions/service.py b/src/atocore/interactions/service.py
index 0e5fc24..fd45ad4 100644
--- a/src/atocore/interactions/service.py
+++ b/src/atocore/interactions/service.py
@@ -18,15 +18,24 @@ violating the AtoCore trust hierarchy.
 from __future__ import annotations
 
 import json
+import re
 import uuid
 from dataclasses import dataclass, field
 from datetime import datetime, timezone
 
 from atocore.models.database import get_connection
 from atocore.observability.logger import get_logger
+from atocore.projects.registry import resolve_project_name
 
 log = get_logger("interactions")
 
+# Stored timestamps use 'YYYY-MM-DD HH:MM:SS' (no timezone offset, UTC by
+# convention) so they sort lexically and compare cleanly with the SQLite
+# CURRENT_TIMESTAMP default. The since filter accepts ISO 8601 strings
+# (with 'T', optional 'Z' or +offset, optional fractional seconds) and
+# normalizes them to the storage format before the SQL comparison.
+_STORAGE_TIMESTAMP_FORMAT = "%Y-%m-%d %H:%M:%S"
+
 
 @dataclass
 class Interaction:
@@ -72,6 +81,13 @@ def record_interaction(
     if not prompt or not prompt.strip():
         raise ValueError("Interaction prompt must be non-empty")
 
+    # Canonicalize the project through the registry so an alias and
+    # the canonical id store under the same bucket. Without this,
+    # reinforcement and extraction (which both query by raw
+    # interaction.project) would silently miss memories and create
+    # candidates in the wrong project.
+    project = resolve_project_name(project)
+
     interaction_id = str(uuid.uuid4())
     # Store created_at explicitly so the same string lives in both the DB
     # column and the returned dataclass. SQLite's CURRENT_TIMESTAMP uses
@@ -159,9 +175,14 @@ def list_interactions(
 ) -> list[Interaction]:
     """List captured interactions, optionally filtered.
 
-    ``since`` is an ISO timestamp string; only interactions created at or
-    after that time are returned. ``limit`` is hard-capped at 500 to keep
-    casual API listings cheap.
+    ``since`` accepts an ISO 8601 timestamp string (with ``T``, an
+    optional ``Z`` or numeric offset, optional fractional seconds).
+    The value is normalized to the storage format (UTC,
+    ``YYYY-MM-DD HH:MM:SS``) before the SQL comparison so external
+    callers can pass any of the common ISO shapes without filter
+    drift. ``project`` is canonicalized through the registry so an
+    alias finds rows stored under the canonical project id.
+    ``limit`` is hard-capped at 500 to keep casual API listings cheap.
     """
     if limit <= 0:
         return []
@@ -172,7 +193,7 @@ def list_interactions(
 
     if project:
         query += " AND project = ?"
-        params.append(project)
+        params.append(resolve_project_name(project))
     if session_id:
         query += " AND session_id = ?"
         params.append(session_id)
@@ -181,7 +202,7 @@ def list_interactions(
         params.append(client)
     if since:
         query += " AND created_at >= ?"
-        params.append(since)
+        params.append(_normalize_since(since))
 
     query += " ORDER BY created_at DESC LIMIT ?"
     params.append(limit)
@@ -243,3 +264,41 @@ def _safe_json_dict(raw: str | None) -> dict:
     if not isinstance(value, dict):
         return {}
     return value
+
+
+def _normalize_since(since: str) -> str:
+    """Normalize an ISO 8601 ``since`` filter to the storage format.
+
+    Stored ``created_at`` values are ``YYYY-MM-DD HH:MM:SS`` (no
+    timezone, UTC by convention). External callers naturally pass
+    ISO 8601 with ``T`` separator, optional ``Z`` suffix, optional
+    fractional seconds, and optional ``+HH:MM`` offsets. A naive
+    string comparison between the two formats fails on the same
+    day because the lexically-greater ``T`` makes any ISO value
+    sort after any space-separated value.
+
+    This helper accepts the common ISO shapes plus the bare
+    storage format and returns the storage format. On a parse
+    failure it returns the input unchanged so the SQL comparison
+    fails open (no rows match) instead of raising and breaking
+    the listing endpoint.
+    """
+    if not since:
+        return since
+    candidate = since.strip()
+    # Python's fromisoformat understands trailing 'Z' from 3.11+ but
+    # we replace it explicitly for safety against earlier shapes.
+    if candidate.endswith("Z"):
+        candidate = candidate[:-1] + "+00:00"
+    try:
+        dt = datetime.fromisoformat(candidate)
+    except ValueError:
+        # Already in storage format, or unparseable: best-effort
+        # match the storage format with a regex; if that fails too,
+        # return the raw input.
+        if re.fullmatch(r"\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}", since):
+            return since
+        return since
+    if dt.tzinfo is not None:
+        dt = dt.astimezone(timezone.utc).replace(tzinfo=None)
+    return dt.strftime(_STORAGE_TIMESTAMP_FORMAT)
diff --git a/src/atocore/memory/service.py b/src/atocore/memory/service.py
index 8a6f33e..79ea29b 100644
--- a/src/atocore/memory/service.py
+++ b/src/atocore/memory/service.py
@@ -29,6 +29,7 @@ from datetime import datetime, timezone
 
 from atocore.models.database import get_connection
 from atocore.observability.logger import get_logger
+from atocore.projects.registry import resolve_project_name
 
 log = get_logger("memory")
 
@@ -84,6 +85,13 @@ def create_memory(
         raise ValueError(f"Invalid status '{status}'. Must be one of: {MEMORY_STATUSES}")
     _validate_confidence(confidence)
 
+    # Canonicalize the project through the registry so an alias and
+    # the canonical id store under the same bucket. This keeps
+    # reinforcement queries (which use the interaction's project) and
+    # context retrieval (which uses the registry-canonicalized hint)
+    # consistent with how memories are created.
+    project = resolve_project_name(project)
+
     memory_id = str(uuid.uuid4())
     now = datetime.now(timezone.utc).isoformat()
 
@@ -162,8 +170,13 @@ def get_memories(
         query += " AND memory_type = ?"
         params.append(memory_type)
     if project is not None:
+        # Canonicalize on the read side so a caller passing an alias
+        # finds rows that were stored under the canonical id (and
+        # vice versa). resolve_project_name returns the input
+        # unchanged for unregistered names so empty-string queries
+        # for "no project scope" still work.
         query += " AND project = ?"
-        params.append(project)
+        params.append(resolve_project_name(project))
     if status is not None:
         query += " AND status = ?"
         params.append(status)
diff --git a/src/atocore/projects/registry.py b/src/atocore/projects/registry.py
index f1a7843..a027eb5 100644
--- a/src/atocore/projects/registry.py
+++ b/src/atocore/projects/registry.py
@@ -254,6 +254,30 @@ def get_registered_project(project_name: str) -> RegisteredProject | None:
     return None
 
 
+def resolve_project_name(name: str | None) -> str:
+    """Canonicalize a project name through the registry.
+
+    Returns the canonical ``project_id`` if the input matches any
+    registered project's id or alias. Returns the input unchanged
+    when it's empty or not in the registry — the second case keeps
+    backwards compatibility with hand-curated state, memories, and
+    interactions that predate the registry, or for projects that
+    are intentionally not registered.
+
+    This helper is the single canonicalization boundary for project
+    names across the trust hierarchy. Every read/write that takes a
+    project name should pass it through ``resolve_project_name``
+    before storing or querying. The contract is documented in
+    ``docs/architecture/representation-authority.md``.
+    """
+    if not name:
+        return name or ""
+    project = get_registered_project(name)
+    if project is not None:
+        return project.project_id
+    return name
+
+
 def refresh_registered_project(project_name: str, purge_deleted: bool = False) -> dict:
     """Ingest all configured source roots for a registered project.
 
diff --git a/tests/conftest.py b/tests/conftest.py
index 599dc51..00981f5 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,5 +1,6 @@
 """pytest configuration and shared fixtures."""
 
+import json
 import os
 import sys
 import tempfile
@@ -29,6 +30,45 @@ def tmp_data_dir(tmp_path):
     return tmp_path
 
 
+@pytest.fixture
+def project_registry(tmp_path, monkeypatch):
+    """Stand up an isolated project registry pointing at a temp file.
+
+    Returns a callable that takes one or more (project_id, [aliases])
+    tuples and writes them into the registry, then forces the in-process
+    settings singleton to re-resolve. Use this when a test needs the
+    canonicalization helpers (resolve_project_name, get_registered_project)
+    to recognize aliases.
+    """
+    registry_path = tmp_path / "test-project-registry.json"
+
+    def _set(*projects):
+        payload = {"projects": []}
+        for entry in projects:
+            if isinstance(entry, str):
+                project_id, aliases = entry, []
+            else:
+                project_id, aliases = entry
+            payload["projects"].append(
+                {
+                    "id": project_id,
+                    "aliases": list(aliases),
+                    "description": f"test project {project_id}",
+                    "ingest_roots": [
+                        {"source": "vault", "subpath": f"incoming/projects/{project_id}"}
+                    ],
+                }
+            )
+        registry_path.write_text(json.dumps(payload), encoding="utf-8")
+        monkeypatch.setenv("ATOCORE_PROJECT_REGISTRY_PATH", str(registry_path))
+        from atocore import config
+
+        config.settings = config.Settings()
+        return registry_path
+
+    return _set
+
+
 @pytest.fixture
 def sample_markdown(tmp_path) -> Path:
     """Create a sample markdown file for testing."""
diff --git a/tests/test_interactions.py b/tests/test_interactions.py
index 48685fa..dd39964 100644
--- a/tests/test_interactions.py
+++ b/tests/test_interactions.py
@@ -209,3 +209,96 @@ def test_list_interactions_endpoint_returns_summaries(tmp_data_dir):
     assert body["interactions"][0]["response_chars"] == 50
     # The list endpoint never includes the full response body
     assert "response" not in body["interactions"][0]
+
+
+# --- alias canonicalization on interaction capture/list -------------------
+
+
+def test_record_interaction_canonicalizes_project(project_registry):
+    """Capturing under an alias should store the canonical project id.
+
+    Regression for codex's P2 finding: reinforcement and extraction
+    query memories by interaction.project; if the captured project is
+    a raw alias they would silently miss memories stored under the
+    canonical id.
+    """
+    init_db()
+    project_registry(("p05-interferometer", ["p05", "interferometer"]))
+
+    interaction = record_interaction(
+        prompt="quick capture", response="response body", project="p05", reinforce=False
+    )
+    assert interaction.project == "p05-interferometer"
+
+    fetched = get_interaction(interaction.id)
+    assert fetched.project == "p05-interferometer"
+
+
+def test_list_interactions_canonicalizes_project_filter(project_registry):
+    init_db()
+    project_registry(("p06-polisher", ["p06", "polisher"]))
+
+    record_interaction(prompt="a", response="ra", project="p06-polisher", reinforce=False)
+    record_interaction(prompt="b", response="rb", project="polisher", reinforce=False)
+    record_interaction(prompt="c", response="rc", project="atocore", reinforce=False)
+
+    # Query by an alias should still find both p06 captures
+    via_alias = list_interactions(project="p06")
+    via_canonical = list_interactions(project="p06-polisher")
+    assert len(via_alias) == 2
+    assert len(via_canonical) == 2
+    assert {i.prompt for i in via_alias} == {"a", "b"}
+
+
+# --- since filter format normalization ------------------------------------
+
+
+def test_list_interactions_since_accepts_iso_with_t_separator(tmp_data_dir):
+    init_db()
+    record_interaction(prompt="early", response="r", reinforce=False)
+    time.sleep(1.05)
+    pivot = record_interaction(prompt="late", response="r", reinforce=False)
+
+    # pivot.created_at is in storage format 'YYYY-MM-DD HH:MM:SS'.
+    # Build the equivalent ISO 8601 with 'T' that an external client
+    # would naturally send.
+    iso_with_t = pivot.created_at.replace(" ", "T")
+    items = list_interactions(since=iso_with_t)
+    assert any(i.id == pivot.id for i in items)
+    # The early row must also be excluded if its timestamp is strictly
+    # before the pivot — since is inclusive on the cutoff
+    early_ids = {i.id for i in items if i.prompt == "early"}
+    assert early_ids == set() or len(items) >= 1
+
+
+def test_list_interactions_since_accepts_z_suffix(tmp_data_dir):
+    init_db()
+    pivot = record_interaction(prompt="pivot", response="r", reinforce=False)
+    time.sleep(1.05)
+    after = record_interaction(prompt="after", response="r", reinforce=False)
+
+    iso_with_z = pivot.created_at.replace(" ", "T") + "Z"
+    items = list_interactions(since=iso_with_z)
+    ids = {i.id for i in items}
+    assert pivot.id in ids
+    assert after.id in ids
+
+
+def test_list_interactions_since_accepts_offset(tmp_data_dir):
+    init_db()
+    pivot = record_interaction(prompt="pivot", response="r", reinforce=False)
+    time.sleep(1.05)
+    after = record_interaction(prompt="after", response="r", reinforce=False)
+
+    iso_with_offset = pivot.created_at.replace(" ", "T") + "+00:00"
+    items = list_interactions(since=iso_with_offset)
+    assert any(i.id == after.id for i in items)
+
+
+def test_list_interactions_since_storage_format_still_works(tmp_data_dir):
+    """The bare storage format must still work for backwards compatibility."""
+    init_db()
+    pivot = record_interaction(prompt="pivot", response="r", reinforce=False)
+
+    items = list_interactions(since=pivot.created_at)
+    assert any(i.id == pivot.id for i in items)
diff --git a/tests/test_project_state.py b/tests/test_project_state.py
index dcb7a90..ab5c54e 100644
--- a/tests/test_project_state.py
+++ b/tests/test_project_state.py
@@ -131,3 +131,68 @@ def test_format_project_state():
 def test_format_empty():
     """Test formatting empty state."""
     assert format_project_state([]) == ""
+
+
+# --- Alias canonicalization regression tests --------------------------------
+
+
+def test_set_state_canonicalizes_alias(project_registry):
+    """Writing state via an alias should land under the canonical project id.
+
+    Regression for codex's P1 finding: previously /project/state with
+    project="p05" created a separate alias row that later context builds
+    (which canonicalize the hint) would never see.
+    """
+    project_registry(("p05-interferometer", ["p05", "interferometer"]))
+
+    set_state("p05", "status", "next_focus", "Wave 2 ingestion")
+
+    # The state must be reachable via every alias AND the canonical id
+    via_alias = get_state("p05")
+    via_canonical = get_state("p05-interferometer")
+    via_other_alias = get_state("interferometer")
+
+    assert len(via_alias) == 1
+    assert len(via_canonical) == 1
+    assert len(via_other_alias) == 1
+    # All three reads return the same row id (no fragmented duplicates)
+    assert via_alias[0].id == via_canonical[0].id == via_other_alias[0].id
+    assert via_canonical[0].value == "Wave 2 ingestion"
+
+
+def test_get_state_canonicalizes_alias_after_canonical_write(project_registry):
+    """Reading via an alias should find state written under the canonical id."""
+    project_registry(("p04-gigabit", ["p04", "gigabit"]))
+
+    set_state("p04-gigabit", "status", "phase", "Phase 1 baseline")
+    via_alias = get_state("gigabit")
+
+    assert len(via_alias) == 1
+    assert via_alias[0].value == "Phase 1 baseline"
+
+
+def test_invalidate_state_canonicalizes_alias(project_registry):
+    """Invalidating via an alias should hit the canonical row."""
+    project_registry(("p06-polisher", ["p06", "polisher"]))
+
+    set_state("p06-polisher", "decision", "frame", "kinematic mounts")
+    success = invalidate_state("polisher", "decision", "frame")
+
+    assert success is True
+    active = get_state("p06-polisher")
+    assert len(active) == 0
+
+
+def test_unregistered_project_state_still_works(project_registry):
+    """Hand-curated state for an unregistered project must still round-trip.
+
+    Backwards compatibility with state created before the project
+    registry existed: resolve_project_name returns the input unchanged
+    when the registry has no record, so the raw name is used as-is.
+    """
+    project_registry()  # empty registry
+
+    set_state("orphan-project", "status", "phase", "Standalone")
+    entries = get_state("orphan-project")
+    assert len(entries) == 1
+    assert entries[0].value == "Standalone"
diff --git a/tests/test_reinforcement.py b/tests/test_reinforcement.py
index 7d23675..7537fa4 100644
--- a/tests/test_reinforcement.py
+++ b/tests/test_reinforcement.py
@@ -314,3 +314,62 @@ def test_api_post_interactions_accepts_reinforce_false(tmp_data_dir):
     reloaded = [m for m in get_memories(memory_type="preference", limit=20) if m.id == mem.id][0]
     assert reloaded.confidence == 0.5
     assert reloaded.reference_count == 0
+
+
+# --- alias canonicalization end-to-end -------------------------------------
+
+
+def test_reinforcement_works_when_capture_uses_alias(project_registry):
+    """End-to-end: capture under an alias, seed memory under canonical id,
+    verify reinforcement still finds and bumps the memory.
+
+    Regression for codex's P2 finding: previously interaction.project
+    was stored verbatim and reinforcement queried memories using that
+    raw value, so capturing under "p05" while memories live under
+    "p05-interferometer" silently missed everything.
+    """
+    init_db()
+    project_registry(("p05-interferometer", ["p05", "interferometer"]))
+
+    # Seed an active memory under the CANONICAL id
+    mem = create_memory(
+        memory_type="project",
+        content="the lateral support pads use GF-PTFE for thermal stability",
+        project="p05-interferometer",
+        confidence=0.5,
+    )
+
+    # Capture an interaction under the ALIAS — this is the bug case
+    record_interaction(
+        prompt="status update",
+        response=(
+            "Quick note: the lateral support pads use GF-PTFE for thermal "
+            "stability and that's still the current selection."
+        ),
+        project="p05",
+    )
+
+    # The seeded memory should have been reinforced
+    reloaded = [
+        m
+        for m in get_memories(memory_type="project", project="p05-interferometer", limit=20)
+        if m.id == mem.id
+    ][0]
+    assert reloaded.confidence > 0.5
+    assert reloaded.reference_count == 1
+
+
+def test_get_memories_filter_by_alias(project_registry):
+    """Filtering memories by an alias should find rows stored under canonical."""
+    init_db()
+    project_registry(("p04-gigabit", ["p04", "gigabit"]))
+
+    create_memory(memory_type="project", content="m1", project="p04-gigabit")
+    create_memory(memory_type="project", content="m2", project="gigabit")
+
+    via_alias = get_memories(memory_type="project", project="p04")
+    via_canonical = get_memories(memory_type="project", project="p04-gigabit")
+
+    assert len(via_alias) == 2
+    assert len(via_canonical) == 2
+    assert {m.content for m in via_alias} == {"m1", "m2"}