diff --git a/openeo_driver/backend.py b/openeo_driver/backend.py
index 26a7e445..ec08d46d 100644
--- a/openeo_driver/backend.py
+++ b/openeo_driver/backend.py
@@ -173,6 +173,12 @@ class LoadParameters(dict):
     temporal_extent = dict_item(default=(None, None))
     spatial_extent = dict_item(default={})
     global_extent = dict_item(default={})  # TODO #441 to be removed
+    # Identifies the source (e.g. `load_collection`/`load_stac` node) these parameters were
+    # extracted for. Useful (e.g. in post-dry-run global extent determination) to look up
+    # per-source information that can not simply be merged across all sources
+    # (e.g. the "mask" branch of a `mask` process potentially needing a different/larger extent
+    # than the data cube it masks).
+    source_id = dict_item(default=None)
     filter_temporal_labels = dict_item(default=None)
     bands = dict_item(default=None)
     properties = dict_item(default={})
@@ -213,6 +219,13 @@ def _copy_for_eq(self):
         copy = super().copy()
         #load_collection with or without data mask should return an 'equivalent' result
         copy["data_mask"] = {}
+        # `source_id` is just an identifier of the process graph node these parameters were
+        # extracted for. It should not influence equality (e.g. for load_collection caching):
+        # identical loads from different process graph nodes should be considered equivalent.
+        # Anything source-specific that influences the actual result (e.g. a per-source
+        # global extent) should be resolved into other (equality-relevant) fields or the
+        # EvalEnv before caching boundaries.
+        copy["source_id"] = None
         return copy
 
     def __eq__(self, o: object) -> bool:
diff --git a/openeo_driver/dry_run.py b/openeo_driver/dry_run.py
index e894134c..45356963 100644
--- a/openeo_driver/dry_run.py
+++ b/openeo_driver/dry_run.py
@@ -146,6 +146,7 @@ def _extract_resample_constraint(operation_name: str, args: Union[dict, tuple])
         ],
     ),
     PropagationRule("aggregate_spatial", operations=["aggregate_spatial"]),
+    PropagationRule("is_mask", operations=["is_mask"]),
     PropagationRule("sar_backscatter", operations=["sar_backscatter"]),
     PropagationRule("process_type", operations=["process_type"]),
     PropagationRule("custom_cloud_mask", operations=["custom_cloud_mask"]),
@@ -658,6 +659,10 @@ def mask(self, mask: "DryRunDataCube", replacement=None) -> "DryRunDataCube":
         # TODO: if mask cube has no temporal or bbox extent: copy from self?
         # TODO: or add reference to the self trace to the mask trace and vice versa?
         mask_resampled = mask._process("resample_cube_spatial", arguments={"target": self, "method": "near"})
+        # Tag the mask branch so that downstream (post-dry-run) logic can distinguish
+        # the "mask" source from the "data" source being masked (e.g. to avoid inflating
+        # the data cube's extent with the (possibly larger) extent of the mask cube).
+        mask_resampled = mask_resampled._process("is_mask", arguments={})
         cube = self._process("mask", {"mask": mask_resampled})
         return DryRunDataCube(
             traces=cube._traces + mask_resampled._traces, data_tracer=cube._data_tracer, metadata=cube.metadata
diff --git a/openeo_driver/processgraph/load_params.py b/openeo_driver/processgraph/load_params.py
index 4b3c950f..bd71afd6 100644
--- a/openeo_driver/processgraph/load_params.py
+++ b/openeo_driver/processgraph/load_params.py
@@ -220,6 +220,7 @@ def _extract_load_parameters(env: EvalEnv, source_id: SourceId) -> LoadParameter
     source_constraints.remove((source_id, constraints))  # Side effect!
 
     params = LoadParameters()
+    params.source_id = source_id
     if temporal_extent := constraints.get("temporal_extent"):
         params.temporal_extent = temporal_extent
     labels_args = constraints.get("filter_labels", {})
diff --git a/tests/test_dry_run.py b/tests/test_dry_run.py
index 99d3d1ee..066167ea 100644
--- a/tests/test_dry_run.py
+++ b/tests/test_dry_run.py
@@ -319,6 +319,7 @@ def test_evaluate_graph_diamond(dry_run_env, dry_run_tracer):
             ("load_collection", ("S2_FOOBAR", ()), "load1"),
             {
                 "bands": ["grass"],
+                'is_mask': {},
                 "resample": {"method": "near", "resolution": (10, 10), "target_crs": CRS_UTM},
                 "spatial_extent": {"west": 1, "east": 2, "south": 51, "north": 52, "crs": "EPSG:4326"},
             },
@@ -2989,7 +2990,7 @@ def test_deduplicate_source_constraints(dry_run_env, dry_run_tracer):
     assert source_constraints == [
         (
             ("load_stac", ("https://stac.test/collection1", (), ()), "loadstac1"),
-            {"temporal_extent": ("2026-01-01", "2026-02-02")},
+            {'is_mask': {}, "temporal_extent": ("2026-01-01", "2026-02-02")},
         ),
         (
             ("load_stac", ("https://stac.test/collection1", (), ()), "loadstac1"),
@@ -3005,8 +3006,12 @@ def test_deduplicate_source_constraints(dry_run_env, dry_run_tracer):
     assert deduped == [
         (
             ("load_stac", ("https://stac.test/collection1", (), ()), "loadstac1"),
-            {"temporal_extent": ("2026-01-01", "2026-02-02")},
+            {'is_mask': {},"temporal_extent": ("2026-01-01", "2026-02-02")},
         ),
+        (
+            ("load_stac", ("https://stac.test/collection1", (), ()), "loadstac1"),
+            {"temporal_extent": ("2026-01-01", "2026-02-02")},
+        )
     ]