diff --git a/datasette/extras.py b/datasette/extras.py
index 36014185..fb8c2e06 100644
--- a/datasette/extras.py
+++ b/datasette/extras.py
@@ -5,6 +5,8 @@ from typing import ClassVar
 
 from asyncinject import Registry
 
+from datasette.utils.asgi import BadRequest
+
 
 def extra_names_from_request(request):
     extra_bits = request.args.getlist("_extra")
@@ -113,6 +115,17 @@ class ExtraRegistry:
             self._allowed_names[key] = names
         return names
 
+    def validate_requested(self, requested, scope):
+        """
+        Raise BadRequest if any requested extra name is not a public extra
+        for this scope. Used by data formats such as .json - HTML pages
+        silently ignore unknown names instead.
+        """
+        allowed = self._allowed_names_for_scope(scope, include_internal=False)
+        unknown = sorted(name for name in requested if name not in allowed)
+        if unknown:
+            raise BadRequest("Unknown _extra: {}".format(", ".join(unknown)))
+
     async def resolve(self, requested, context, scope, include_internal=False):
         allowed_names = self._allowed_names_for_scope(scope, include_internal)
         requested_names = [name for name in requested if name in allowed_names]
diff --git a/datasette/views/database.py b/datasette/views/database.py
index 99e85d2b..b7131a05 100644
--- a/datasette/views/database.py
+++ b/datasette/views/database.py
@@ -8,7 +8,7 @@ import markupsafe
 import os
 import textwrap
 
-from datasette.extras import extra_names_from_request
+from datasette.extras import extra_names_from_request, ExtraScope
 from datasette.database import QueryInterrupted
 from datasette.resources import DatabaseResource, QueryResource
 from datasette.stored_queries import StoredQuery, stored_query_to_dict
@@ -855,6 +855,7 @@ class QueryView(View):
                 raise DatasetteError("?sql= is required", status=400)
             data = {"ok": True, "rows": rows, "columns": columns}
             extras = extra_names_from_request(request)
+            table_extra_registry.validate_requested(extras, ExtraScope.QUERY)
             if extras:
                 query_extra_context = QueryExtraContext(
                     datasette=datasette,
diff --git a/datasette/views/row.py b/datasette/views/row.py
index 71539f34..296a0ac1 100644
--- a/datasette/views/row.py
+++ b/datasette/views/row.py
@@ -596,6 +596,9 @@ class RowView(BaseView):
         }
 
         extras = extra_names_from_request(request)
+        if request.url_vars.get("format"):
+            # Data formats reject unknown extras; HTML ignores them
+            table_extra_registry.validate_requested(extras, ExtraScope.ROW)
 
         # Process extras
         row_extra_context = RowExtraContext(
diff --git a/datasette/views/table.py b/datasette/views/table.py
index 44919281..ed3b1276 100644
--- a/datasette/views/table.py
+++ b/datasette/views/table.py
@@ -2254,6 +2254,10 @@ async def table_view_data(
 
     # Resolve extras
     extras = extra_names_from_request(request)
+    if not extra_extras:
+        # Data formats reject unknown extras; the HTML path (which passes
+        # extra_extras={"_html"}) resolves internal extras of its own
+        table_extra_registry.validate_requested(extras, ExtraScope.TABLE)
     if any(k for k in request.args.keys() if k == "_facet" or k.startswith("_facet_")):
         extras.add("facet_results")
     if request.args.get("_shape") == "object":
diff --git a/docs/json_api.rst b/docs/json_api.rst
index 2a859ba2..0069055b 100644
--- a/docs/json_api.rst
+++ b/docs/json_api.rst
@@ -283,6 +283,8 @@ These can be repeated or comma-separated:
 
     ?_extra=columns&_extra=count,next_url
 
+Requesting an ``_extra`` name that does not exist returns a ``400`` error in the :ref:`standard error format <json_api_errors>`, for example ``{"ok": false, "error": "Unknown _extra: nope", ...}``.
+
 .. [[[cog
     from json_api_doc import table_extras
     table_extras(cog)
diff --git a/existing-api.md b/existing-api.md
index 6ce29bf5..cf456921 100644
--- a/existing-api.md
+++ b/existing-api.md
@@ -176,9 +176,10 @@ build JSON directly):
 Table, row and query JSON responses support `?_extra=` (repeatable and/or
 comma-separated, extras.py:9-14) to add keys to the response. Extras are
 scope-registered (`ExtraScope.TABLE` / `ROW` / `QUERY`) and only **public**
-extras are available over JSON (extras.py:73-92). Unknown extra names are
-silently ignored. The available names per scope are listed with the relevant
-endpoints below.
+extras are available over JSON (extras.py:73-92). Unknown extra names (and
+internal HTML-only names) on data formats return 400
+`Unknown _extra: <names>`; HTML pages ignore them. The available names per
+scope are listed with the relevant endpoints below.
 
 ---
 
diff --git a/stable-api-recommendations.md b/stable-api-recommendations.md
index a92fbfbd..1bc2412e 100644
--- a/stable-api-recommendations.md
+++ b/stable-api-recommendations.md
@@ -166,7 +166,11 @@ Endpoints disagree about the success envelope:
   matches the read API).~~ ✅ **Done** — row update now returns
   `rows: [{...}]`.
 
-### 2b. `_extra`/`_shape` support is uneven (P2)
+### 2b. `_extra`/`_shape` support is uneven (P2) — partially implemented
+
+> **Status:** unknown `_extra` names on data formats now return 400
+> `Unknown _extra: <names>` (HTML pages still ignore them). Extending
+> extras/shaping to database/instance scope remains open.
 
 The extras system (`?_extra=`, scope-registered) is the 1.0 mechanism for
 response shaping — but it only exists on table, row and query endpoints. The
diff --git a/tests/test_error_shape.py b/tests/test_error_shape.py
index 55370c18..f0ff0a42 100644
--- a/tests/test_error_shape.py
+++ b/tests/test_error_shape.py
@@ -570,3 +570,37 @@ async def test_schema_endpoints_no_existence_oracle(tmp_path_factory):
 async def test_table_schema_unknown_database_is_404_not_500(ds_client):
     response = await ds_client.get("/no_such_db/some_table/-/schema.json")
     assert_canonical_error(response, 404)
+
+
+# Unknown _extra names are a 400, not silently ignored
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "path",
+    (
+        "/fixtures/facetable.json?_extra=nope",
+        "/fixtures/facetable.json?_extra=count,nope",
+        "/fixtures/simple_primary_key/1.json?_extra=nope",
+        "/fixtures/-/query.json?sql=select+1&_extra=nope",
+    ),
+)
+async def test_unknown_extra_is_400(ds_client, path):
+    response = await ds_client.get(path)
+    data = assert_canonical_error(response, 400)
+    assert data["errors"] == ["Unknown _extra: nope"]
+
+
+@pytest.mark.asyncio
+async def test_html_only_extra_via_json_is_400(ds_client):
+    # display_rows exists for the HTML view but is not part of the JSON API
+    response = await ds_client.get("/fixtures/facetable.json?_extra=display_rows")
+    data = assert_canonical_error(response, 400)
+    assert data["errors"] == ["Unknown _extra: display_rows"]
+
+
+@pytest.mark.asyncio
+async def test_unknown_extra_ignored_on_html_pages(ds_client):
+    response = await ds_client.get("/fixtures/facetable?_extra=nope")
+    assert response.status_code == 200
+    assert response.headers["content-type"].startswith("text/html")
diff --git a/tests/test_table_api.py b/tests/test_table_api.py
index e3737ee6..ff413be8 100644
--- a/tests/test_table_api.py
+++ b/tests/test_table_api.py
@@ -123,8 +123,8 @@ async def test_html_only_extras_are_not_available_via_json(ds_client, extra):
     # These extras exist for the HTML view; their values are not JSON
     # serializable so they are internal, not part of the JSON API
     response = await ds_client.get(f"/fixtures/facetable.json?_extra={extra}")
-    assert response.status_code == 200
-    assert extra not in response.json()
+    assert response.status_code == 400
+    assert response.json()["errors"] == [f"Unknown _extra: {extra}"]
 
 
 @pytest.mark.asyncio