diff --git a/changelog.d/economy-query-ignore-unknown.fixed.md b/changelog.d/economy-query-ignore-unknown.fixed.md
new file mode 100644
index 000000000..c99b8b486
--- /dev/null
+++ b/changelog.d/economy-query-ignore-unknown.fixed.md
@@ -0,0 +1 @@
+Ignore undeclared query parameters on the legacy economy routes instead of returning HTTP 400; declared parameters and the SPM selection object stay strictly validated. The release gate's live suite appends a staging_probe parameter that the typed parser had started rejecting.
diff --git a/docs/canonical-spm.md b/docs/canonical-spm.md
index 43a96ceee..524746218 100644
--- a/docs/canonical-spm.md
+++ b/docs/canonical-spm.md
@@ -244,8 +244,10 @@ query string. For example, before URL encoding:
 ```
 
 Use the client's query-encoding support to encode that JSON once. All query
-parameters are scalar: repeated keys, unknown parameters, malformed JSON and
-duplicate fields inside the SPM object return HTTP 400. `region` is required;
+parameters are scalar: repeated declared keys, malformed JSON and duplicate or
+unknown fields inside the SPM object return HTTP 400. Query parameters the route does
+not declare are ignored, as the legacy routes always did; an omitted or
+misspelled `spm` therefore inherits the certified default measurement. `region` is required;
 annual requests require `time_period`, while budget-window requests require
 `start_year` and `window_size` (1–75, ending no later than 2099). Years use four
 digits. Optional fields are `dataset` (default `default`), `version` (installed
diff --git a/docs/engineering/skills/api-routes.md b/docs/engineering/skills/api-routes.md
index 1b6a4b91a..2f34ec6b7 100644
--- a/docs/engineering/skills/api-routes.md
+++ b/docs/engineering/skills/api-routes.md
@@ -48,7 +48,11 @@ uniformity.
 
 ## Parsing Rules
 
-- Reject unknown query parameters rather than ignoring misspellings.
+- Reject unknown query parameters rather than ignoring misspellings. The one
+  deliberate exception is the legacy economy GET family (`EconomyQuery` and
+  its subclasses), which ignores undeclared parameters because callers append
+  parameters those routes never read, such as the release gate's
+  `staging_probe`; its declared parameters and the `spm` object stay strict.
 - Reject a scalar query parameter supplied more than once rather than selecting
   an arbitrary value.
 - Accept repeated keys only when the canonical field is explicitly list-valued.
@@ -109,7 +113,7 @@ For each new or changed query contract, cover the applicable cases:
 - optional parameters use their canonical defaults;
 - valid values receive canonical normalization;
 - invalid types and out-of-range values are rejected;
-- unknown parameters are rejected;
+- unknown parameters are rejected (ignored on the legacy economy GET family);
 - duplicate scalar parameters are rejected;
 - explicitly list-valued parameters accept the documented repeated form;
 - OpenAPI declares the runtime name, type, required status, default, and bounds;
diff --git a/policyengine_api/query_parameters.py b/policyengine_api/query_parameters.py
index 60af041be..01a45ed11 100644
--- a/policyengine_api/query_parameters.py
+++ b/policyengine_api/query_parameters.py
@@ -93,7 +93,11 @@ def validate_legacy_user_id(value: str) -> str:
 
 
 class StrictQueryParameters(BaseModel):
-    """Base for query contracts that reject every undeclared field."""
+    """Base for query contracts that reject every undeclared field.
+
+    ``EconomyQuery`` and its subclasses are the one family that overrides this
+    to ignore undeclared parameters; see its docstring for why.
+    """
 
     model_config = ConfigDict(extra="forbid", frozen=True)
 
@@ -129,7 +133,19 @@ def parse_integer_query_value(value: Any) -> Any:
 
 
 class EconomyQuery(StrictQueryParameters):
-    """Common legacy economy query options; identifiers remain in the path."""
+    """Common legacy economy query options; identifiers remain in the path.
+
+    These legacy GET routes accepted any query string before the typed parser,
+    and callers still append parameters the routes never read, such as the
+    ``staging_probe`` correlation id the release gate's live suite sends. An
+    undeclared parameter is therefore ignored rather than rejected: an omitted
+    ``spm`` selection already dispatches the certified default measurement, so
+    a misspelled one cannot select anything else. Declared parameters keep
+    their validation, repeated scalars are still rejected, and the ``spm``
+    object itself stays strict.
+    """
+
+    model_config = ConfigDict(extra="ignore", frozen=True)
 
     region: EconomyRegion
     dataset: str = Field(default="default", description="Dataset selection")
diff --git a/tests/unit/routes/test_economy_spm_query.py b/tests/unit/routes/test_economy_spm_query.py
index aae3f21c8..792ae41af 100644
--- a/tests/unit/routes/test_economy_spm_query.py
+++ b/tests/unit/routes/test_economy_spm_query.py
@@ -125,12 +125,30 @@ def test_duplicate_scalar_economy_queries_never_dispatch(economy_http, field):
     gateway.get_spm_capability.assert_not_called()
 
 
-def test_unknown_economy_query_never_dispatches(economy_http):
-    client, path, query, dispatch, _, _ = economy_http
-    response = client.get(path, query_string=query + [("spmm", "{}")])
-    assert response.status_code == 400
-    assert "spmm" in response.get_json()["message"]
-    dispatch.assert_not_called()
+@pytest.mark.parametrize(
+    "extra",
+    [("spmm", "{}"), ("staging_probe", "cloud-run-stg-1-abcdef-utah")],
+    ids=["misspelled-selection", "release-gate-probe"],
+)
+def test_undeclared_economy_query_is_ignored_and_dispatches_defaults(
+    economy_http, extra
+):
+    """Undeclared parameters never reach the service and never block dispatch.
+
+    The release gate's live suite appends ``staging_probe`` to every economy
+    request, and legacy callers append parameters these GET routes never read.
+    A misspelled selection cannot choose a measurement the omitted selection
+    would not: both inherit the certified defaults.
+    """
+    client, path, query, dispatch, _, setups = economy_http
+    response = client.get(path, query_string=query + [extra])
+    assert response.status_code == 200, response.get_json()
+    assert response.get_json()["status"] == "computing"
+    dispatch.assert_called_once()
+    assert extra[0] not in dispatch.call_args.kwargs
+    assert dispatch.call_args.kwargs["options"] == {}
+    assert len(setups) == 1
+    assert setups[0].options["spm"]["geography_kind"] == "county"
 
 
 def test_economy_required_query_fields_are_rejected_before_dispatch(economy_http):
diff --git a/tests/unit/test_query_parameters.py b/tests/unit/test_query_parameters.py
index 75ca8f07e..5947d6eae 100644
--- a/tests/unit/test_query_parameters.py
+++ b/tests/unit/test_query_parameters.py
@@ -348,3 +348,28 @@ def households(
     }
     assert parameters["offset"]["schema"]["default"] == 0
     assert parameters["limit"]["schema"]["default"] == 100
+
+
+def _strict_query_subclasses(base: type) -> set[type]:
+    found: set[type] = set()
+    for subclass in base.__subclasses__():
+        found.add(subclass)
+        found |= _strict_query_subclasses(subclass)
+    return found
+
+
+def test_only_the_economy_query_family_ignores_undeclared_parameters() -> None:
+    """The economy tolerance must not leak into any other query contract."""
+    from policyengine_api import query_parameters
+
+    tolerant = {
+        model
+        for model in _strict_query_subclasses(query_parameters.StrictQueryParameters)
+        if model.model_config.get("extra") != "forbid"
+    }
+    economy_family = {query_parameters.EconomyQuery} | _strict_query_subclasses(
+        query_parameters.EconomyQuery
+    )
+    assert tolerant == economy_family
+    assert all(model.model_config.get("extra") == "ignore" for model in tolerant)
+    assert query_parameters.StrictQueryParameters.model_config.get("extra") == "forbid"