diff --git a/java/ql/lib/semmle/code/java/dataflow/DataFlow.qll b/java/ql/lib/semmle/code/java/dataflow/DataFlow.qll
index 54eb809c7b9..c99a8a5a58e 100644
--- a/java/ql/lib/semmle/code/java/dataflow/DataFlow.qll
+++ b/java/ql/lib/semmle/code/java/dataflow/DataFlow.qll
@@ -10,6 +10,6 @@ import java
 module DataFlow {
   private import semmle.code.java.dataflow.internal.DataFlowImplSpecific
   private import codeql.dataflow.DataFlow
-  import DataFlowMake<Location, JavaDataFlow>
+  import DataFlowMakeOverlay<Location, JavaDataFlow>
   import Public
 }
diff --git a/java/ql/lib/semmle/code/java/dataflow/TaintTracking.qll b/java/ql/lib/semmle/code/java/dataflow/TaintTracking.qll
index 159604a95bd..34c7b717428 100644
--- a/java/ql/lib/semmle/code/java/dataflow/TaintTracking.qll
+++ b/java/ql/lib/semmle/code/java/dataflow/TaintTracking.qll
@@ -13,5 +13,5 @@ module TaintTracking {
   private import semmle.code.java.dataflow.internal.DataFlowImplSpecific
   private import semmle.code.java.dataflow.internal.TaintTrackingImplSpecific
   private import codeql.dataflow.TaintTracking
-  import TaintFlowMake<Location, JavaDataFlow, JavaTaintTracking>
+  import TaintFlowMakeOverlay<Location, JavaDataFlow, JavaTaintTracking>
 }
diff --git a/java/ql/lib/semmle/code/java/dataflow/internal/DataFlowImplSpecific.qll b/java/ql/lib/semmle/code/java/dataflow/internal/DataFlowImplSpecific.qll
index 65034ee08b9..97f5020142e 100644
--- a/java/ql/lib/semmle/code/java/dataflow/internal/DataFlowImplSpecific.qll
+++ b/java/ql/lib/semmle/code/java/dataflow/internal/DataFlowImplSpecific.qll
@@ -6,6 +6,7 @@ module;
 
 private import semmle.code.Location
 private import codeql.dataflow.DataFlow
+private import semmle.code.java.Overlay
 
 module Private {
   import DataFlowPrivate
@@ -29,4 +30,6 @@ module JavaDataFlow implements InputSig<Location> {
   predicate mayBenefitFromCallContext = Private::mayBenefitFromCallContext/1;
 
   predicate viableImplInCallContext = Private::viableImplInCallContext/2;
+
+  predicate isEvaluatingInOverlay = isOverlay/0;
 }
diff --git a/shared/dataflow/codeql/dataflow/DataFlow.qll b/shared/dataflow/codeql/dataflow/DataFlow.qll
index 3483287e3b3..49f84d45b2a 100644
--- a/shared/dataflow/codeql/dataflow/DataFlow.qll
+++ b/shared/dataflow/codeql/dataflow/DataFlow.qll
@@ -349,6 +349,18 @@ signature module InputSig<LocationSig Location> {
 
   /** Holds if `fieldFlowBranchLimit` should be ignored for flow going into/out of `c`. */
   default predicate ignoreFieldFlowBranchLimit(DataFlowCallable c) { none() }
+
+  /**
+   * Holds if the evaluator is currently evaluating with an overlay. The
+   * implementation of this predicate needs to be `overlay[local]`. For a
+   * language with no overlay support, `none()` is a valid implementation.
+   *
+   * When called from a local predicate, this predicate holds if we are in the
+   * overlay-only local evaluation. When called from a global predicate, this
+   * predicate holds if we are evaluating globally with overlay and base both
+   * visible.
+   */
+  default predicate isEvaluatingInOverlay() { none() }
 }
 
 module Configs<LocationSig Location, InputSig<Location> Lang> {
@@ -645,10 +657,8 @@ private module PathGraphSigMod {
   }
 }
 
-module DataFlowMake<LocationSig Location, InputSig<Location> Lang> {
+private module DataFlowMakeCore<LocationSig Location, InputSig<Location> Lang> {
   private import Lang
-  private import internal.DataFlowImpl::MakeImpl<Location, Lang>
-  private import internal.DataFlowImplStage1::MakeImplStage1<Location, Lang>
   import Configs<Location, Lang>
 
   /**
@@ -691,59 +701,6 @@ module DataFlowMake<LocationSig Location, InputSig<Location> Lang> {
     predicate flowToExpr(DataFlowExpr sink);
   }
 
-  /**
-   * Constructs a global data flow computation.
-   */
-  module Global<ConfigSig Config> implements GlobalFlowSig {
-    private module C implements FullStateConfigSig {
-      import DefaultState<Config>
-      import Config
-
-      predicate accessPathLimit = Config::accessPathLimit/0;
-
-      predicate isAdditionalFlowStep(Node node1, Node node2, string model) {
-        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
-      }
-    }
-
-    private module Stage1 = ImplStage1<C>;
-
-    import Stage1::PartialFlow
-
-    private module Flow = Impl<C, Stage1::Stage1NoState>;
-
-    import Flow
-  }
-
-  /**
-   * Constructs a global data flow computation using flow state.
-   */
-  module GlobalWithState<StateConfigSig Config> implements GlobalFlowSig {
-    private module C implements FullStateConfigSig {
-      import Config
-
-      predicate accessPathLimit = Config::accessPathLimit/0;
-
-      predicate isAdditionalFlowStep(Node node1, Node node2, string model) {
-        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
-      }
-
-      predicate isAdditionalFlowStep(
-        Node node1, FlowState state1, Node node2, FlowState state2, string model
-      ) {
-        Config::isAdditionalFlowStep(node1, state1, node2, state2) and model = "Config"
-      }
-    }
-
-    private module Stage1 = ImplStage1<C>;
-
-    import Stage1::PartialFlow
-
-    private module Flow = Impl<C, Stage1::Stage1WithState>;
-
-    import Flow
-  }
-
   signature class PathNodeSig {
     /** Gets a textual representation of this element. */
     string toString();
@@ -1141,3 +1098,135 @@ module DataFlowMake<LocationSig Location, InputSig<Location> Lang> {
     import PathGraph
   }
 }
+
+module DataFlowMake<LocationSig Location, InputSig<Location> Lang> {
+  import DataFlowMakeCore<Location, Lang>
+  private import Lang
+  private import internal.DataFlowImpl::MakeImpl<Location, Lang>
+  private import internal.DataFlowImplStage1::MakeImplStage1<Location, Lang>
+
+  /**
+   * Constructs a global data flow computation.
+   */
+  module Global<ConfigSig Config> implements GlobalFlowSig {
+    private module C implements FullStateConfigSig {
+      import DefaultState<Config>
+      import Config
+
+      predicate accessPathLimit = Config::accessPathLimit/0;
+
+      predicate isAdditionalFlowStep(Node node1, Node node2, string model) {
+        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
+      }
+
+      predicate observeOverlayInformedIncrementalMode() { none() }
+    }
+
+    private module Stage1 = ImplStage1<C>;
+
+    import Stage1::PartialFlow
+
+    private module Flow = Impl<C, Stage1::Stage1NoState>;
+
+    import Flow
+  }
+
+  /**
+   * Constructs a global data flow computation using flow state.
+   */
+  module GlobalWithState<StateConfigSig Config> implements GlobalFlowSig {
+    private module C implements FullStateConfigSig {
+      import Config
+
+      predicate accessPathLimit = Config::accessPathLimit/0;
+
+      predicate isAdditionalFlowStep(Node node1, Node node2, string model) {
+        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
+      }
+
+      predicate isAdditionalFlowStep(
+        Node node1, FlowState state1, Node node2, FlowState state2, string model
+      ) {
+        Config::isAdditionalFlowStep(node1, state1, node2, state2) and model = "Config"
+      }
+
+      predicate observeOverlayInformedIncrementalMode() { none() }
+    }
+
+    private module Stage1 = ImplStage1<C>;
+
+    import Stage1::PartialFlow
+
+    private module Flow = Impl<C, Stage1::Stage1WithState>;
+
+    import Flow
+  }
+}
+
+module DataFlowMakeOverlay<LocationSig Location, InputSig<Location> Lang> {
+  import DataFlowMake<Location, Lang>
+  private import Lang
+  private import internal.DataFlowImpl::MakeImpl<Location, Lang>
+  private import internal.DataFlowImplStage1::MakeImplStage1<Location, Lang>
+
+  /**
+   * Constructs a global data flow computation.
+   */
+  module Global<ConfigSig Config> implements GlobalFlowSig {
+    private module C implements FullStateConfigSig {
+      import DefaultState<Config>
+      import Config
+
+      predicate accessPathLimit = Config::accessPathLimit/0;
+
+      predicate isAdditionalFlowStep(Node node1, Node node2, string model) {
+        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
+      }
+
+      predicate observeOverlayInformedIncrementalMode() {
+        not Config::observeDiffInformedIncrementalMode()
+      }
+    }
+
+    private module Stage1 = ImplStage1<C>;
+
+    import Stage1::PartialFlow
+
+    private module Flow = OverlayImpl<C, Stage1::Stage1NoState>;
+
+    import Flow
+  }
+
+  /**
+   * Constructs a global data flow computation using flow state.
+   */
+  module GlobalWithState<StateConfigSig Config> implements GlobalFlowSig {
+    private module C implements FullStateConfigSig {
+      import Config
+
+      predicate accessPathLimit = Config::accessPathLimit/0;
+
+      predicate isAdditionalFlowStep(Node node1, Node node2, string model) {
+        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
+      }
+
+      predicate isAdditionalFlowStep(
+        Node node1, FlowState state1, Node node2, FlowState state2, string model
+      ) {
+        Config::isAdditionalFlowStep(node1, state1, node2, state2) and model = "Config"
+      }
+
+      predicate observeOverlayInformedIncrementalMode() {
+        not Config::observeDiffInformedIncrementalMode()
+      }
+    }
+
+    private module Stage1 = ImplStage1<C>;
+
+    import Stage1::PartialFlow
+
+    private module Flow = OverlayImpl<C, Stage1::Stage1WithState>;
+
+    import Flow
+  }
+}
diff --git a/shared/dataflow/codeql/dataflow/TaintTracking.qll b/shared/dataflow/codeql/dataflow/TaintTracking.qll
index bd4b4ecd6ca..7bb9535d096 100644
--- a/shared/dataflow/codeql/dataflow/TaintTracking.qll
+++ b/shared/dataflow/codeql/dataflow/TaintTracking.qll
@@ -47,16 +47,16 @@ signature module InputSig<LocationSig Location, DF::InputSig<Location> Lang> {
 /**
  * Construct the modules for taint-tracking analyses.
  */
-module TaintFlowMake<
+private module TaintFlowMakeCore<
   LocationSig Location, DF::InputSig<Location> DataFlowLang,
   InputSig<Location, DataFlowLang> TaintTrackingLang>
 {
-  private import TaintTrackingLang
-  private import DF::DataFlowMake<Location, DataFlowLang> as DataFlow
-  private import MakeImpl<Location, DataFlowLang> as DataFlowInternal
-  private import MakeImplStage1<Location, DataFlowLang> as DataFlowInternalStage1
+  import TaintTrackingLang
+  import DF::DataFlowMake<Location, DataFlowLang> as DataFlow
+  import MakeImpl<Location, DataFlowLang> as DataFlowInternal
+  import MakeImplStage1<Location, DataFlowLang> as DataFlowInternalStage1
 
-  private module AddTaintDefaults<DataFlowInternal::FullStateConfigSig Config> implements
+  module AddTaintDefaults<DataFlowInternal::FullStateConfigSig Config> implements
     DataFlowInternal::FullStateConfigSig
   {
     import Config
@@ -83,71 +83,9 @@ module TaintFlowMake<
     }
   }
 
-  /**
-   * Constructs a global taint tracking computation.
-   */
-  module Global<DataFlow::ConfigSig Config> implements DataFlow::GlobalFlowSig {
-    private module Config0 implements DataFlowInternal::FullStateConfigSig {
-      import DataFlowInternal::DefaultState<Config>
-      import Config
-
-      predicate isAdditionalFlowStep(
-        DataFlowLang::Node node1, DataFlowLang::Node node2, string model
-      ) {
-        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
-      }
-    }
-
-    private module C implements DataFlowInternal::FullStateConfigSig {
-      import AddTaintDefaults<Config0>
-    }
-
-    private module Stage1 = DataFlowInternalStage1::ImplStage1<C>;
-
-    import Stage1::PartialFlow
-
-    private module Flow = DataFlowInternal::Impl<C, Stage1::Stage1NoState>;
-
-    import Flow
-  }
-
-  /**
-   * Constructs a global taint tracking computation using flow state.
-   */
-  module GlobalWithState<DataFlow::StateConfigSig Config> implements DataFlow::GlobalFlowSig {
-    private module Config0 implements DataFlowInternal::FullStateConfigSig {
-      import Config
-
-      predicate isAdditionalFlowStep(
-        DataFlowLang::Node node1, DataFlowLang::Node node2, string model
-      ) {
-        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
-      }
-
-      predicate isAdditionalFlowStep(
-        DataFlowLang::Node node1, FlowState state1, DataFlowLang::Node node2, FlowState state2,
-        string model
-      ) {
-        Config::isAdditionalFlowStep(node1, state1, node2, state2) and model = "Config"
-      }
-    }
-
-    private module C implements DataFlowInternal::FullStateConfigSig {
-      import AddTaintDefaults<Config0>
-    }
-
-    private module Stage1 = DataFlowInternalStage1::ImplStage1<C>;
-
-    import Stage1::PartialFlow
-
-    private module Flow = DataFlowInternal::Impl<C, Stage1::Stage1WithState>;
-
-    import Flow
-  }
-
   signature int speculationLimitSig();
 
-  private module AddSpeculativeTaintSteps<
+  module AddSpeculativeTaintSteps<
     DataFlowInternal::FullStateConfigSig Config, speculationLimitSig/0 speculationLimit> implements
     DataFlowInternal::FullStateConfigSig
   {
@@ -215,6 +153,79 @@ module TaintFlowMake<
       state1.getState() = state2.getState()
     }
   }
+}
+
+module TaintFlowMake<
+  LocationSig Location, DF::InputSig<Location> DataFlowLang,
+  InputSig<Location, DataFlowLang> TaintTrackingLang>
+{
+  private import TaintFlowMakeCore<Location, DataFlowLang, TaintTrackingLang>
+
+  /**
+   * Constructs a global taint tracking computation.
+   */
+  module Global<DataFlow::ConfigSig Config> implements DataFlow::GlobalFlowSig {
+    private module Config0 implements DataFlowInternal::FullStateConfigSig {
+      import DataFlowInternal::DefaultState<Config>
+      import Config
+
+      predicate isAdditionalFlowStep(
+        DataFlowLang::Node node1, DataFlowLang::Node node2, string model
+      ) {
+        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
+      }
+
+      predicate observeOverlayInformedIncrementalMode() { none() }
+    }
+
+    private module C implements DataFlowInternal::FullStateConfigSig {
+      import AddTaintDefaults<Config0>
+    }
+
+    private module Stage1 = DataFlowInternalStage1::ImplStage1<C>;
+
+    import Stage1::PartialFlow
+
+    private module Flow = DataFlowInternal::Impl<C, Stage1::Stage1NoState>;
+
+    import Flow
+  }
+
+  /**
+   * Constructs a global taint tracking computation using flow state.
+   */
+  module GlobalWithState<DataFlow::StateConfigSig Config> implements DataFlow::GlobalFlowSig {
+    private module Config0 implements DataFlowInternal::FullStateConfigSig {
+      import Config
+
+      predicate isAdditionalFlowStep(
+        DataFlowLang::Node node1, DataFlowLang::Node node2, string model
+      ) {
+        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
+      }
+
+      predicate isAdditionalFlowStep(
+        DataFlowLang::Node node1, FlowState state1, DataFlowLang::Node node2, FlowState state2,
+        string model
+      ) {
+        Config::isAdditionalFlowStep(node1, state1, node2, state2) and model = "Config"
+      }
+
+      predicate observeOverlayInformedIncrementalMode() { none() }
+    }
+
+    private module C implements DataFlowInternal::FullStateConfigSig {
+      import AddTaintDefaults<Config0>
+    }
+
+    private module Stage1 = DataFlowInternalStage1::ImplStage1<C>;
+
+    import Stage1::PartialFlow
+
+    private module Flow = DataFlowInternal::Impl<C, Stage1::Stage1WithState>;
+
+    import Flow
+  }
 
   /**
    * Constructs a global taint tracking computation that also allows a given
@@ -232,6 +243,8 @@ module TaintFlowMake<
       ) {
         Config::isAdditionalFlowStep(node1, node2) and model = "Config"
       }
+
+      predicate observeOverlayInformedIncrementalMode() { none() }
     }
 
     private module C implements DataFlowInternal::FullStateConfigSig {
@@ -270,6 +283,8 @@ module TaintFlowMake<
       ) {
         Config::isAdditionalFlowStep(node1, state1, node2, state2) and model = "Config"
       }
+
+      predicate observeOverlayInformedIncrementalMode() { none() }
     }
 
     private module C implements DataFlowInternal::FullStateConfigSig {
@@ -285,3 +300,157 @@ module TaintFlowMake<
     import Flow
   }
 }
+
+module TaintFlowMakeOverlay<
+  LocationSig Location, DF::InputSig<Location> DataFlowLang,
+  InputSig<Location, DataFlowLang> TaintTrackingLang>
+{
+  private import TaintFlowMakeCore<Location, DataFlowLang, TaintTrackingLang>
+
+  /**
+   * Constructs a global taint tracking computation.
+   */
+  module Global<DataFlow::ConfigSig Config> implements DataFlow::GlobalFlowSig {
+    private module Config0 implements DataFlowInternal::FullStateConfigSig {
+      import DataFlowInternal::DefaultState<Config>
+      import Config
+
+      predicate isAdditionalFlowStep(
+        DataFlowLang::Node node1, DataFlowLang::Node node2, string model
+      ) {
+        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
+      }
+
+      predicate observeOverlayInformedIncrementalMode() {
+        not Config::observeDiffInformedIncrementalMode()
+      }
+    }
+
+    private module C implements DataFlowInternal::FullStateConfigSig {
+      import AddTaintDefaults<Config0>
+    }
+
+    private module Stage1 = DataFlowInternalStage1::ImplStage1<C>;
+
+    import Stage1::PartialFlow
+
+    private module Flow = DataFlowInternal::OverlayImpl<C, Stage1::Stage1NoState>;
+
+    import Flow
+  }
+
+  /**
+   * Constructs a global taint tracking computation using flow state.
+   */
+  module GlobalWithState<DataFlow::StateConfigSig Config> implements DataFlow::GlobalFlowSig {
+    private module Config0 implements DataFlowInternal::FullStateConfigSig {
+      import Config
+
+      predicate isAdditionalFlowStep(
+        DataFlowLang::Node node1, DataFlowLang::Node node2, string model
+      ) {
+        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
+      }
+
+      predicate isAdditionalFlowStep(
+        DataFlowLang::Node node1, FlowState state1, DataFlowLang::Node node2, FlowState state2,
+        string model
+      ) {
+        Config::isAdditionalFlowStep(node1, state1, node2, state2) and model = "Config"
+      }
+
+      predicate observeOverlayInformedIncrementalMode() {
+        not Config::observeDiffInformedIncrementalMode()
+      }
+    }
+
+    private module C implements DataFlowInternal::FullStateConfigSig {
+      import AddTaintDefaults<Config0>
+    }
+
+    private module Stage1 = DataFlowInternalStage1::ImplStage1<C>;
+
+    import Stage1::PartialFlow
+
+    private module Flow = DataFlowInternal::OverlayImpl<C, Stage1::Stage1WithState>;
+
+    import Flow
+  }
+
+  /**
+   * Constructs a global taint tracking computation that also allows a given
+   * maximum number of speculative taint steps.
+   */
+  module SpeculativeGlobal<DataFlow::ConfigSig Config, speculationLimitSig/0 speculationLimit>
+    implements DataFlow::GlobalFlowSig
+  {
+    private module Config0 implements DataFlowInternal::FullStateConfigSig {
+      import DataFlowInternal::DefaultState<Config>
+      import Config
+
+      predicate isAdditionalFlowStep(
+        DataFlowLang::Node node1, DataFlowLang::Node node2, string model
+      ) {
+        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
+      }
+
+      predicate observeOverlayInformedIncrementalMode() {
+        not Config::observeDiffInformedIncrementalMode()
+      }
+    }
+
+    private module C implements DataFlowInternal::FullStateConfigSig {
+      import AddTaintDefaults<AddSpeculativeTaintSteps<Config0, speculationLimit/0>>
+    }
+
+    private module Stage1 = DataFlowInternalStage1::ImplStage1<C>;
+
+    import Stage1::PartialFlow
+
+    private module Flow = DataFlowInternal::OverlayImpl<C, Stage1::Stage1WithState>;
+
+    import Flow
+  }
+
+  /**
+   * Constructs a global taint tracking computation using flow state that also
+   * allows a given maximum number of speculative taint steps.
+   */
+  module SpeculativeGlobalWithState<
+    DataFlow::StateConfigSig Config, speculationLimitSig/0 speculationLimit> implements
+    DataFlow::GlobalFlowSig
+  {
+    private module Config0 implements DataFlowInternal::FullStateConfigSig {
+      import Config
+
+      predicate isAdditionalFlowStep(
+        DataFlowLang::Node node1, DataFlowLang::Node node2, string model
+      ) {
+        Config::isAdditionalFlowStep(node1, node2) and model = "Config"
+      }
+
+      predicate isAdditionalFlowStep(
+        DataFlowLang::Node node1, FlowState state1, DataFlowLang::Node node2, FlowState state2,
+        string model
+      ) {
+        Config::isAdditionalFlowStep(node1, state1, node2, state2) and model = "Config"
+      }
+
+      predicate observeOverlayInformedIncrementalMode() {
+        not Config::observeDiffInformedIncrementalMode()
+      }
+    }
+
+    private module C implements DataFlowInternal::FullStateConfigSig {
+      import AddTaintDefaults<AddSpeculativeTaintSteps<Config0, speculationLimit/0>>
+    }
+
+    private module Stage1 = DataFlowInternalStage1::ImplStage1<C>;
+
+    import Stage1::PartialFlow
+
+    private module Flow = DataFlowInternal::OverlayImpl<C, Stage1::Stage1WithState>;
+
+    import Flow
+  }
+}
diff --git a/shared/dataflow/codeql/dataflow/internal/DataFlowImpl.qll b/shared/dataflow/codeql/dataflow/internal/DataFlowImpl.qll
index a7e0736432a..099866ab6bd 100644
--- a/shared/dataflow/codeql/dataflow/internal/DataFlowImpl.qll
+++ b/shared/dataflow/codeql/dataflow/internal/DataFlowImpl.qll
@@ -143,6 +143,15 @@ module MakeImpl<LocationSig Location, InputSig<Location> Lang> {
      */
     predicate observeDiffInformedIncrementalMode();
 
+    /**
+     * Holds if sources and sinks should be filtered to only include those that
+     * may lead to a flow path with either a source or a sink in the overlay database.
+     * This only has an effect when running
+     * in overlay-informed incremental mode. This should be used in conjunction
+     * with the `OverlayImpl` implementation to merge the base results back in.
+     */
+    predicate observeOverlayInformedIncrementalMode();
+
     Location getASelectedSourceLocation(Node source);
 
     Location getASelectedSinkLocation(Node sink);
@@ -171,6 +180,56 @@ module MakeImpl<LocationSig Location, InputSig<Location> Lang> {
     }
   }
 
+  /**
+   * Constructs a data flow computation given a full input configuration, and
+   * an initial stage 1 pruning with merging of overlay and base results.
+   */
+  module OverlayImpl<FullStateConfigSig Config, Stage1Output<Config::FlowState> Stage1> {
+    private module Flow = Impl<Config, Stage1>;
+
+    import Flow
+
+    /**
+     * Holds if data can flow from `source` to `sink`.
+     *
+     * This is a local predicate that only has results local to the overlay/base database.
+     */
+    private predicate flowLocal(Node source, Node sink) = forceLocal(Flow::flow/2)(source, sink)
+
+    /**
+     * Holds if data can flow from `source` to `sink`.
+     */
+    predicate flow(Node source, Node sink) {
+      Flow::flow(source, sink)
+      or
+      // If we are overlay informed (i.e. we are not diff-informed), we
+      // merge in the local results which includes the base database results.
+      flowLocal(source, sink) and Config::observeOverlayInformedIncrementalMode()
+    }
+
+    /**
+     * Holds if data can flow from some source to `sink`.
+     * This is a local predicate that only has results local to the overlay/base database.
+     */
+    predicate flowToLocal(Node sink) = forceLocal(Flow::flowTo/1)(sink)
+
+    /**
+     * Holds if data can flow from some source to `sink`.
+     */
+    predicate flowTo(Node sink) {
+      Flow::flowTo(sink)
+      or
+      // If we are overlay informed (i.e. we are not diff-informed), we
+      // merge in the local results which includes the base database results.
+      flowToLocal(sink) and Config::observeOverlayInformedIncrementalMode()
+    }
+
+    /**
+     * Holds if data can flow from some source to `sink`.
+     */
+    predicate flowToExpr(Lang::DataFlowExpr sink) { flowTo(exprNode(sink)) }
+  }
+
   /**
    * Constructs a data flow computation given a full input configuration, and
    * an initial stage 1 pruning.
diff --git a/shared/dataflow/codeql/dataflow/internal/DataFlowImplStage1.qll b/shared/dataflow/codeql/dataflow/internal/DataFlowImplStage1.qll
index c7883df0de1..bb79ff62f5b 100644
--- a/shared/dataflow/codeql/dataflow/internal/DataFlowImplStage1.qll
+++ b/shared/dataflow/codeql/dataflow/internal/DataFlowImplStage1.qll
@@ -4,7 +4,7 @@
  * Provides an implementation of a fast initial pruning of global
  * (interprocedural) data flow reachability (Stage 1).
  */
-overlay[local?]
+overlay[local?] // when this is removed, put `overlay[local?]` on `isOverlayNode`.
 module;
 
 private import codeql.util.Unit
@@ -129,23 +129,76 @@ module MakeImplStage1<LocationSig Location, InputSig<Location> Lang> {
 
       private module AlertFiltering = AlertFilteringImpl<Location>;
 
+      /**
+       * Holds if the given node is visible in overlay-only local evaluation.
+       *
+       * This predicate needs to be `overlay[local?]`, either directly or
+       * through annotations from an outer scope. If `Node` is global for the
+       * language under analysis, then every node is considered an overlay
+       * node, which means there will effectively be no overlay-based
+       * filtering of sources and sinks.
+       */
+      private predicate isOverlayNode(Node node) {
+        isEvaluatingInOverlay() and
+        // Any local node is an overlay node if we are evaluating in overlay mode
+        exists(node)
+      }
+
+      /**
+       * The filtering if we aren't meant to be diff-informed.
+       *
+       * Shared between sources and sinks.
+       */
+      overlay[global]
+      pragma[inline]
+      private predicate nonDiffInformedFilter(Node node) {
+        // If we are in base-only global evaluation, do not filter out any sources/sinks.
+        not isEvaluatingInOverlay()
+        or
+        // If the configuration doesn't merge overlays, do not filter out any sources/sinks.
+        not Config::observeOverlayInformedIncrementalMode()
+        or
+        // If we are in global evaluation with an overlay present, restrict
+        // sources/sinks to those visible in the overlay.
+        isOverlayNode(node)
+      }
+
+      overlay[global]
       pragma[nomagic]
       private predicate isFilteredSource(Node source) {
         Config::isSource(source, _) and
+        // Data flow is always incremental in one of two ways.
+        // 1. If the configuration is diff-informed, we filter to only include nodes in the diff,
+        //    which gives the smallest set of nodes.
+        //    If diff information is not available, we do not filter at all.
+        // 2. If not, in global evaluation with overlay, we filter to only
+        //    include nodes from files in the overlay; flow from
+        //    other nodes will be added back later.
+        // We start by seeing if we should be in case 1.
         if Config::observeDiffInformedIncrementalMode()
-        then AlertFiltering::filterByLocation(Config::getASelectedSourceLocation(source))
-        else any()
+        then
+          // Case 1: We are meant to be diff-informed.
+          // We still only filter if we have diff information.
+          AlertFiltering::diffInformationAvailable()
+          implies
+          AlertFiltering::locationIsInDiff(Config::getASelectedSourceLocation(source))
+        else nonDiffInformedFilter(source)
       }
 
+      overlay[global]
       pragma[nomagic]
       private predicate isFilteredSink(Node sink) {
         (
           Config::isSink(sink, _) or
           Config::isSink(sink)
         ) and
+        // See the comments in `isFilteredSource` for the reasoning behind the following.
         if Config::observeDiffInformedIncrementalMode()
-        then AlertFiltering::filterByLocation(Config::getASelectedSinkLocation(sink))
-        else any()
+        then
+          AlertFiltering::diffInformationAvailable()
+          implies
+          AlertFiltering::locationIsInDiff(Config::getASelectedSinkLocation(sink))
+        else nonDiffInformedFilter(sink)
       }
 
       private predicate hasFilteredSource() { isFilteredSource(_) }
diff --git a/shared/util/codeql/util/AlertFiltering.qll b/shared/util/codeql/util/AlertFiltering.qll
index 1480421ae59..60597327c3b 100644
--- a/shared/util/codeql/util/AlertFiltering.qll
+++ b/shared/util/codeql/util/AlertFiltering.qll
@@ -82,6 +82,21 @@ module AlertFilteringImpl<LocationSig Location> {
     )
   }
 
+  /** Holds if diff information is available in this evaluation. */
+  predicate diffInformationAvailable() {
+    restrictAlertsTo(_, _, _) or restrictAlertsToExactLocation(_, _, _, _, _)
+  }
+
+  /**
+   * Holds if diff information is available, and `filePath` is in the diff
+   * range.
+   */
+  predicate fileIsInDiff(string filePath) {
+    restrictAlertsTo(filePath, _, _)
+    or
+    restrictAlertsToExactLocation(filePath, _, _, _, _)
+  }
+
   /**
    * Holds if the given location is a match for one of the active filtering
    * predicates in this module, or if all filtering predicates are inactive
@@ -92,8 +107,17 @@ module AlertFilteringImpl<LocationSig Location> {
    */
   bindingset[location]
   predicate filterByLocation(Location location) {
-    not restrictAlertsTo(_, _, _) and not restrictAlertsToExactLocation(_, _, _, _, _)
+    not diffInformationAvailable()
     or
+    locationIsInDiff(location)
+  }
+
+  /**
+   * Like `filterByLocation`, except that if there is no diff range, this
+   * predicate never holds.
+   */
+  bindingset[location]
+  predicate locationIsInDiff(Location location) {
     exists(string filePath |
       restrictAlertsToEntireFile(filePath) and
       location.hasLocationInfo(filePath, _, _, _, _)