diff --git a/java/ql/lib/qlpack.yml b/java/ql/lib/qlpack.yml
index c7a6a045451e..c27a06994b2f 100644
--- a/java/ql/lib/qlpack.yml
+++ b/java/ql/lib/qlpack.yml
@@ -23,3 +23,4 @@ dataExtensions:
   - ext/generated/*.model.yml
   - ext/experimental/*.model.yml
 warnOnImplicitThis: true
+compileForOverlayEval: true
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 65034ee08b93..97f5020142e0 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 3483287e3b39..efcd54ff696d 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> {
diff --git a/shared/dataflow/codeql/dataflow/internal/DataFlowImpl.qll b/shared/dataflow/codeql/dataflow/internal/DataFlowImpl.qll
index a7e0736432ac..f06faafcf964 100644
--- a/shared/dataflow/codeql/dataflow/internal/DataFlowImpl.qll
+++ b/shared/dataflow/codeql/dataflow/internal/DataFlowImpl.qll
@@ -3376,24 +3376,81 @@ module MakeImpl<LocationSig Location, InputSig<Location> Lang> {
     import S6
 
     /**
-     * Holds if data can flow from `source` to `sink`.
+     * Holds if data can flow from `source` to `sink`. When running in overlay
+     * incremental mode on a supported language, this predicate only has
+     * results where either the source or sink is in the overlay database.
      */
-    predicate flow(Node source, Node sink) {
+    private predicate flowIncremental(Node source, Node sink) {
       exists(PathNode source0, PathNode sink0 |
         flowPath(source0, sink0) and source0.getNode() = source and sink0.getNode() = sink
       )
     }
 
+    overlay[local]
+    private predicate flowLocal(Node source, Node sink) =
+      forceLocal(flowIncremental/2)(source, sink)
+
+    /**
+     * Holds if data can flow from `source` to `sink`.
+     */
+    predicate flow(Node source, Node sink) {
+      flowIncremental(source, sink)
+      or
+      flowLocal(source, sink)
+    }
+
+    /**
+     * Holds if data can flow from some source to `sink`. When running in
+     * overlay incremental mode on a supported language, this predicate only
+     * has results where either the source or sink is in the overlay database.
+     */
+    private predicate flowToIncremental(Node sink) {
+      exists(PathNode n | n.isSink() and n.getNode() = sink)
+    }
+
+    overlay[local]
+    private predicate flowToLocal(Node sink) = forceLocal(flowToIncremental/1)(sink)
+
     /**
      * Holds if data can flow from some source to `sink`.
      */
-    predicate flowTo(Node sink) { exists(PathNode n | n.isSink() and n.getNode() = sink) }
+    predicate flowTo(Node sink) {
+      flowToIncremental(sink)
+      or
+      flowToLocal(sink)
+    }
 
     /**
      * Holds if data can flow from some source to `sink`.
      */
     predicate flowToExpr(Expr sink) { flowTo(exprNode(sink)) }
 
+    /**
+     * Holds if data can flow from `source` to some sink. When running in
+     * overlay incremental mode on a supported language, this predicate only
+     * has results where either the source or sink is in the overlay database.
+     */
+    private predicate flowFromIncremental(Node source) {
+      exists(PathNode n | n.isSource() and n.getNode() = source)
+    }
+
+    overlay[local]
+    private predicate flowFromLocal(Node source) = forceLocal(flowFromIncremental/1)(source)
+
+    /**
+     * Holds if data can flow from `source` to some sink.
+     */
+    predicate flowFrom(Node source) {
+      flowFromIncremental(source)
+      or
+      flowFromLocal(source)
+    }
+
+    /**
+     * Holds if data can flow from `source` to some sink.
+     */
+    predicate flowFromExpr(Expr source) { flowFrom(exprNode(source)) }
+
     /**
      * INTERNAL: Only for debugging.
      *
diff --git a/shared/dataflow/codeql/dataflow/internal/DataFlowImplStage1.qll b/shared/dataflow/codeql/dataflow/internal/DataFlowImplStage1.qll
index c7883df0de18..6e716b74bc33 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 +129,95 @@ 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)
+      }
+
+      /**
+       * Holds if `node` comes from a file that was in the diff for which we
+       * are producing results.
+       */
+      overlay[global]
+      private predicate isDiffFileNode(Node node) {
+        exists(string filePath |
+          node.getLocation().hasLocationInfo(filePath, _, _, _, _) and
+          AlertFiltering::fileIsInDiff(filePath)
+        )
+      }
+
+      overlay[global]
       pragma[nomagic]
       private predicate isFilteredSource(Node source) {
         Config::isSource(source, _) and
-        if Config::observeDiffInformedIncrementalMode()
-        then AlertFiltering::filterByLocation(Config::getASelectedSourceLocation(source))
-        else any()
+        // Data flow is always incremental in one of two ways.
+        // 1. If the configuration is diff-informed and diff information is
+        //    available, we filter to only include nodes in the diff, which
+        //    gives the smallest set of nodes.
+        // 2. If not, in global evaluation with overlay, we filter to only
+        //    include nodes from files in the overlay or the diff; flow from
+        //    other nodes will be added back later. There can be two reasons
+        //    why we are in this case:
+        //    1. This could be the primary configuration for a query that
+        //       hasn't yet become diff-informed. In that case, the
+        //       `getASelectedSourceLocation` information is probably just the
+        //       default, and it's a fairly safe overapproximation to
+        //       effectively expand to all nodes in the file (via
+        //       `isDiffFileNode`).
+        //    2. This could be a secondary configuration, like a helper
+        //       configuration for finding sources or sinks of a primary
+        //       configuration. In that case, the results of this configuration
+        //       are typically in the same file as the final alert.
+        if
+          Config::observeDiffInformedIncrementalMode() and
+          AlertFiltering::diffInformationAvailable()
+        then AlertFiltering::locationIsInDiff(Config::getASelectedSourceLocation(source))
+        else (
+          // If we are in base-only global evaluation, do not filter out any sources.
+          not isEvaluatingInOverlay()
+          or
+          // If we are in global evaluation with an overlay present, restrict
+          // sources to those visible in the overlay or
+          isOverlayNode(source)
+          or
+          // those from files in the diff. The diff is a subset of the overlay
+          // in the common case, but this is not guaranteed. Including the diff here
+          // ensures that we re-evaluate flow that passes from a file in the
+          // diff (but in the base), through a file in the overlay with
+          // possibly important changes, back to the base.
+          isDiffFileNode(source)
+        )
       }
 
+      overlay[global]
       pragma[nomagic]
       private predicate isFilteredSink(Node sink) {
         (
           Config::isSink(sink, _) or
           Config::isSink(sink)
         ) and
-        if Config::observeDiffInformedIncrementalMode()
-        then AlertFiltering::filterByLocation(Config::getASelectedSinkLocation(sink))
-        else any()
+        // See the comments in `isFilteredSource` for the reasoning behind the following.
+        if
+          Config::observeDiffInformedIncrementalMode() and
+          AlertFiltering::diffInformationAvailable()
+        then AlertFiltering::locationIsInDiff(Config::getASelectedSinkLocation(sink))
+        else (
+          not isEvaluatingInOverlay()
+          or
+          isOverlayNode(sink)
+          or
+          isDiffFileNode(sink)
+        )
       }
 
       private predicate hasFilteredSource() { isFilteredSource(_) }
diff --git a/shared/util/codeql/util/AlertFiltering.qll b/shared/util/codeql/util/AlertFiltering.qll
index bfc9d901b2fa..aabd42c7dbb2 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 intersects the diff range. When that is the
    * case, ensuring that alerts mentioning this location are included in the
@@ -103,8 +118,17 @@ 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, _, _, _, _)

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