diff --git a/packages/api/alembic/versions/27f37d3cf8d2_add_dataset_prompt_addendum.py b/packages/api/alembic/versions/27f37d3cf8d2_add_dataset_prompt_addendum.py
new file mode 100644
index 0000000..79d65ad
--- /dev/null
+++ b/packages/api/alembic/versions/27f37d3cf8d2_add_dataset_prompt_addendum.py
@@ -0,0 +1,28 @@
+"""add dataset prompt_addendum
+
+Revision ID: 27f37d3cf8d2
+Revises: 092e2aa153ce
+Create Date: 2026-05-21 10:09:30.488887
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '27f37d3cf8d2'
+down_revision: Union[str, Sequence[str], None] = '092e2aa153ce'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    op.add_column('datasets', sa.Column('prompt_addendum', sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    op.drop_column('datasets', 'prompt_addendum')
diff --git a/packages/api/src/cell_explorer_api/db/models.py b/packages/api/src/cell_explorer_api/db/models.py
index 2e31a51..8cd2d3a 100644
--- a/packages/api/src/cell_explorer_api/db/models.py
+++ b/packages/api/src/cell_explorer_api/db/models.py
@@ -55,6 +55,7 @@ class Dataset(SQLModel, table=True):
     description: str | None = None
     is_public: bool = Field(default=False)
     required_roles: list[str] = Field(default_factory=list, sa_column=Column(JSON))
+    prompt_addendum: str | None = Field(default=None)
     chat_enabled: bool = Field(default=False)
     created_at: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))
     updated_at: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))
diff --git a/packages/api/src/cell_explorer_api/routes/admin.py b/packages/api/src/cell_explorer_api/routes/admin.py
index 308503d..bb3cc6e 100644
--- a/packages/api/src/cell_explorer_api/routes/admin.py
+++ b/packages/api/src/cell_explorer_api/routes/admin.py
@@ -58,6 +58,7 @@ class DatasetCreate(BaseModel):
     description: str | None = None
     is_public: bool = False
     required_roles: list[str] = []
+    prompt_addendum: str | None = None
     chat_enabled: bool = False
 
 
@@ -67,6 +68,7 @@ class DatasetUpdate(BaseModel):
     description: str | None = None
     is_public: bool | None = None
     required_roles: list[str] | None = None
+    prompt_addendum: str | None = None
     chat_enabled: bool | None = None
 
 
@@ -79,6 +81,7 @@ class DatasetAdminResponse(BaseModel):
     description: str | None
     is_public: bool
     required_roles: list[str]
+    prompt_addendum: str | None
     chat_enabled: bool
 
 
@@ -175,6 +178,7 @@ async def list_datasets_admin(
             description=dataset.description,
             is_public=dataset.is_public,
             required_roles=dataset.required_roles,
+            prompt_addendum=dataset.prompt_addendum,
             chat_enabled=dataset.chat_enabled,
         )
         for dataset in result.all()
@@ -209,6 +213,7 @@ async def create_dataset(
         description=dataset.description,
         is_public=dataset.is_public,
         required_roles=dataset.required_roles,
+        prompt_addendum=dataset.prompt_addendum,
         chat_enabled=dataset.chat_enabled,
     )
 
@@ -239,6 +244,7 @@ async def update_dataset(
         description=dataset.description,
         is_public=dataset.is_public,
         required_roles=dataset.required_roles,
+        prompt_addendum=dataset.prompt_addendum,
         chat_enabled=dataset.chat_enabled,
     )
 
diff --git a/packages/api/src/cell_explorer_api/services/chat_session.py b/packages/api/src/cell_explorer_api/services/chat_session.py
index e737157..4be5f58 100644
--- a/packages/api/src/cell_explorer_api/services/chat_session.py
+++ b/packages/api/src/cell_explorer_api/services/chat_session.py
@@ -32,11 +32,13 @@
 
 
 # In-process cache of DatasetContext keyed by (slug, updated_at). The Dataset
-# row's updated_at bumps on every admin PUT (see routes/admin.py), so this
-# self-invalidates without an explicit hook: admin edits change the key,
-# subsequent requests miss the cache and rebuild. Process restart clears the
-# cache (uvicorn --reload covers dev). Stale entries accumulate on edits but
-# the leak is bounded by edit frequency and dataset count.
+# row's updated_at bumps unconditionally on every admin PUT (see
+# routes/admin.py update_dataset, line ~234), so this self-invalidates for any
+# DB-sourced DatasetContext field — including prompt_addendum — without an
+# explicit hook: admin edits change the key, subsequent requests miss the
+# cache and rebuild. Process restart clears the cache (uvicorn --reload covers
+# dev). Stale entries accumulate on edits but the leak is bounded by edit
+# frequency and dataset count.
 #
 # See issue #101.
 _dataset_ctx_cache: dict[tuple[str, datetime], DatasetContext] = {}
@@ -62,6 +64,7 @@ async def _build_dataset_context_cached(
             slug=dataset.slug,
             name=dataset.name,
             description=dataset.description or "",
+            prompt_addendum=dataset.prompt_addendum,
         )
     return _dataset_ctx_cache[key]
 
diff --git a/packages/api/tests/services/test_chat_session.py b/packages/api/tests/services/test_chat_session.py
index 41772a7..f0f8801 100644
--- a/packages/api/tests/services/test_chat_session.py
+++ b/packages/api/tests/services/test_chat_session.py
@@ -358,6 +358,43 @@ async def _spy_build(*args, **kwargs):
         assert call_count == 2, f"build_dataset_context called {call_count}x; expected 2 after updated_at bump"
 
 
+@pytest.mark.asyncio
+async def test_make_chat_agent_forwards_prompt_addendum_to_dataset_context():
+    """When the Dataset row has prompt_addendum set, make_chat_agent forwards
+    it to build_dataset_context (via _build_dataset_context_cached)."""
+    dataset = _public_dataset()
+    dataset.prompt_addendum = "Test curator note: cells were sorted on CD45."
+    datasource = MagicMock(base_url="https://example.com", type="HTTP_TOKEN", credential_ref=None)
+    db = await _mk_db_session(_make_db_row(dataset, datasource))
+
+    fake_anndata = MagicMock(n_obs=10, n_vars=20, obsm_keys=[], obs_columns=[])
+
+    captured: dict = {}
+    from cell_explorer_agent import build_dataset_context as _real
+
+    async def _spy_build(*args, **kwargs):
+        captured.update(kwargs)
+        return await _real(*args, **kwargs)
+
+    with patch("cell_explorer_api.services.chat_session.ZarrStore") as MockZS, \
+         patch("cell_explorer_api.services.chat_session.AnnDataStore") as MockADS, \
+         patch("cell_explorer_api.services.chat_session.StrataStore") as MockSS, \
+         patch("cell_explorer_api.services.chat_session.build_dataset_context", _spy_build):
+        MockZS.open = AsyncMock(return_value=MagicMock())
+        MockADS.open = AsyncMock(return_value=fake_anndata)
+        MockSS.open = AsyncMock(return_value=MagicMock())
+
+        user = _FakeUser(roles=[])
+        llm = FakeLLMClient(scripts=[])
+        settings = MagicMock()
+        agent = await make_chat_agent(
+            user=user, dataset_slug="pbmc3k", db=db, settings=settings, llm=llm,
+        )
+
+    assert captured.get("prompt_addendum") == "Test curator note: cells were sorted on CD45."
+    assert agent.dataset_ctx.prompt_addendum == "Test curator note: cells were sorted on CD45."
+
+
 @pytest.mark.asyncio
 async def test_dataset_ctx_cache_is_independent_per_slug():
     """Two datasets with different slugs cache independently."""
diff --git a/packages/api/tests/test_admin_routes.py b/packages/api/tests/test_admin_routes.py
index 3a6ed28..d34675c 100644
--- a/packages/api/tests/test_admin_routes.py
+++ b/packages/api/tests/test_admin_routes.py
@@ -302,3 +302,102 @@ def test_admin_create_dataset_chat_enabled_defaults_false(seeded_app):
     )
     assert response.status_code == 201
     assert response.json()["chat_enabled"] is False
+
+
+# --- prompt_addendum field ---
+
+
+def test_create_dataset_with_prompt_addendum(seeded_app):
+    """POST /admin/datasets stores prompt_addendum and surfaces it on response."""
+    ds_id = seeded_app.state.test_datasource_id
+    client = TestClient(seeded_app)
+    response = client.post(
+        "/api/admin/datasets",
+        json={
+            "datasource_id": ds_id,
+            "name": "Test",
+            "slug": "test-with-addendum",
+            "path": "test.zarr",
+            "prompt_addendum": "Important: cells were sorted on CD45 first.",
+        },
+        headers=AUTH_HEADER,
+    )
+    assert response.status_code == 201, response.text
+    body = response.json()
+    assert body["prompt_addendum"] == "Important: cells were sorted on CD45 first."
+
+
+def test_create_dataset_prompt_addendum_defaults_null(seeded_app):
+    """prompt_addendum is null when not supplied on create."""
+    ds_id = seeded_app.state.test_datasource_id
+    client = TestClient(seeded_app)
+    response = client.post(
+        "/api/admin/datasets",
+        json={
+            "datasource_id": ds_id,
+            "name": "No-Addendum-Default",
+            "slug": "no-addendum-default",
+            "path": "no-addendum.zarr",
+        },
+        headers=AUTH_HEADER,
+    )
+    assert response.status_code == 201, response.text
+    assert response.json()["prompt_addendum"] is None
+
+
+def test_update_dataset_prompt_addendum(seeded_app):
+    """PUT updates only prompt_addendum; other fields unchanged."""
+    ds_id = seeded_app.state.test_datasource_id
+    client = TestClient(seeded_app)
+    # Create a dataset with no addendum first
+    create_resp = client.post(
+        "/api/admin/datasets",
+        json={
+            "datasource_id": ds_id,
+            "name": "No-Addendum",
+            "slug": "no-addendum",
+            "path": "no-addendum.zarr",
+        },
+        headers=AUTH_HEADER,
+    )
+    assert create_resp.status_code == 201
+    created = create_resp.json()
+    assert created["prompt_addendum"] is None
+    # Update only prompt_addendum
+    response = client.put(
+        "/api/admin/datasets/no-addendum",
+        json={"prompt_addendum": "Curator notes here."},
+        headers=AUTH_HEADER,
+    )
+    assert response.status_code == 200, response.text
+    body = response.json()
+    assert body["prompt_addendum"] == "Curator notes here."
+    # Other fields kept their previous values
+    assert body["name"] == created["name"]
+    assert body["path"] == created["path"]
+
+
+def test_get_datasets_includes_prompt_addendum(seeded_app):
+    """GET /admin/datasets surfaces prompt_addendum on each row."""
+    ds_id = seeded_app.state.test_datasource_id
+    client = TestClient(seeded_app)
+    # Create a dataset with an addendum
+    create_resp = client.post(
+        "/api/admin/datasets",
+        json={
+            "datasource_id": ds_id,
+            "name": "Has-Addendum",
+            "slug": "has-addendum",
+            "path": "has-addendum.zarr",
+            "prompt_addendum": "Sample curator note.",
+        },
+        headers=AUTH_HEADER,
+    )
+    assert create_resp.status_code == 201
+    response = client.get("/api/admin/datasets", headers=AUTH_HEADER)
+    assert response.status_code == 200
+    body = response.json()
+    rows = {d["slug"]: d for d in body["datasets"]}
+    assert "has-addendum" in rows
+    assert "prompt_addendum" in rows["has-addendum"]
+    assert rows["has-addendum"]["prompt_addendum"] == "Sample curator note."
diff --git a/packages/cell-explorer-agent/src/cell_explorer_agent/prompt/dataset_context.py b/packages/cell-explorer-agent/src/cell_explorer_agent/prompt/dataset_context.py
index ca945e5..0ad12be 100644
--- a/packages/cell-explorer-agent/src/cell_explorer_agent/prompt/dataset_context.py
+++ b/packages/cell-explorer-agent/src/cell_explorer_agent/prompt/dataset_context.py
@@ -23,10 +23,16 @@ class DatasetContext:
     n_var: int
     obs_columns: list[ObsColumnInfo]
     embedding_keys: list[str]
+    prompt_addendum: str | None = None
 
 
 async def build_dataset_context(
-    z: ZarrAccess, *, slug: str, name: str, description: str
+    z: ZarrAccess,
+    *,
+    slug: str,
+    name: str,
+    description: str,
+    prompt_addendum: str | None = None,
 ) -> DatasetContext:
     n_obs, n_var = await z.shape()
     obs = await z.obs_columns()
@@ -47,4 +53,5 @@ async def build_dataset_context(
             for c in obs
         ],
         embedding_keys=list(emb),
+        prompt_addendum=prompt_addendum,
     )
diff --git a/packages/cell-explorer-agent/src/cell_explorer_agent/prompt/system.py b/packages/cell-explorer-agent/src/cell_explorer_agent/prompt/system.py
index bda9f15..d812ea5 100644
--- a/packages/cell-explorer-agent/src/cell_explorer_agent/prompt/system.py
+++ b/packages/cell-explorer-agent/src/cell_explorer_agent/prompt/system.py
@@ -24,6 +24,12 @@ def build_system_prompt(ctx: DatasetContext) -> str:
     lines.append(f"Dataset: {ctx.slug} — {ctx.name}")
     if ctx.description:
         lines.append(f"Description: {ctx.description}")
+    if ctx.prompt_addendum and ctx.prompt_addendum.strip():
+        lines.append("")
+        lines.append("=== Curator notes (authoritative dataset context) ===")
+        lines.append(ctx.prompt_addendum)
+        lines.append("=== end curator notes ===")
+        lines.append("")
     lines.append(f"Shape: {ctx.n_obs} cells × {ctx.n_var} genes.")
     lines.append("")
     lines.append("Obs columns:")
diff --git a/packages/cell-explorer-agent/tests/test_prompt_system.py b/packages/cell-explorer-agent/tests/test_prompt_system.py
index 20f3b57..c4df9cd 100644
--- a/packages/cell-explorer-agent/tests/test_prompt_system.py
+++ b/packages/cell-explorer-agent/tests/test_prompt_system.py
@@ -1,3 +1,6 @@
+import pytest
+
+
 def test_system_prompt_mentions_chart_awareness():
     """The agent is told to write concise summaries when a chart is present
     in the tool result, instead of re-enumerating every row."""
@@ -13,3 +16,60 @@ def test_system_prompt_mentions_chart_awareness():
     assert "chart" in prompt.lower()
     # gene_panel_by_obs is registered in the tool-use policy.
     assert "gene_panel_by_obs" in prompt
+
+
+def _minimal_ctx(**kwargs):
+    from cell_explorer_agent.prompt.dataset_context import DatasetContext, ObsColumnInfo
+    return DatasetContext(
+        slug="test-ds",
+        name="Test Dataset",
+        description="A test description.",
+        n_obs=1000,
+        n_var=200,
+        obs_columns=[ObsColumnInfo(name="cell_type", dtype="categorical", cardinality=5)],
+        embedding_keys=["X_umap"],
+        **kwargs,
+    )
+
+
+def test_build_system_prompt_includes_curator_notes_when_present():
+    from cell_explorer_agent.prompt.system import build_system_prompt
+    addendum = "There is a subtle but important consideration: cells were sorted on CD45."
+    ctx = _minimal_ctx(prompt_addendum=addendum)
+    prompt = build_system_prompt(ctx)
+
+    assert "=== Curator notes (authoritative dataset context) ===" in prompt
+    assert addendum in prompt
+    assert "=== end curator notes ===" in prompt
+
+    # Opening fence appears before the shape line.
+    idx_fence = prompt.index("=== Curator notes (authoritative dataset context) ===")
+    idx_shape = prompt.index("Shape:")
+    assert idx_fence < idx_shape
+
+    # Opening fence appears after description.
+    idx_desc = prompt.index("A test description.")
+    assert idx_desc < idx_fence
+
+    # The block is paragraph-isolated: a blank line follows the closing
+    # fence so it doesn't visually bleed into the shape metadata.
+    assert "=== end curator notes ===\n\nShape:" in prompt
+
+
+def test_build_system_prompt_omits_curator_notes_when_none():
+    from cell_explorer_agent.prompt.system import build_system_prompt
+    ctx = _minimal_ctx(prompt_addendum=None)
+    prompt = build_system_prompt(ctx)
+    assert "Curator notes" not in prompt
+
+    # Byte-identical to prompt built from a context without the field set (default None).
+    ctx2 = _minimal_ctx()
+    assert prompt == build_system_prompt(ctx2)
+
+
+@pytest.mark.parametrize("addendum", ["", "   \n  \t  "])
+def test_build_system_prompt_omits_curator_notes_when_empty(addendum):
+    from cell_explorer_agent.prompt.system import build_system_prompt
+    ctx = _minimal_ctx(prompt_addendum=addendum)
+    prompt = build_system_prompt(ctx)
+    assert "Curator notes" not in prompt