diff --git a/platform/indexing-impl/src/com/intellij/find/ngrams/TrigramIndex.java b/platform/indexing-impl/src/com/intellij/find/ngrams/TrigramIndex.java
index 6a37c2e3f474..1dc89421d4e5 100644
--- a/platform/indexing-impl/src/com/intellij/find/ngrams/TrigramIndex.java
+++ b/platform/indexing-impl/src/com/intellij/find/ngrams/TrigramIndex.java
@@ -91,9 +91,15 @@ public final class TrigramIndex extends ScalarIndexExtension<Integer> implements
     return true;
   }
 
+  @Override
+  @Internal
+  public int shardlessVersion() {
+    return 4;
+  }
+
   @Override
   public int getVersion() {
-    return 4 + (SHARDS - 1);
+    return shardlessVersion() + (SHARDS - 1);
   }
 
   @Override
diff --git a/platform/indexing-impl/src/com/intellij/util/indexing/storage/sharding/ShardableIndexExtension.java b/platform/indexing-impl/src/com/intellij/util/indexing/storage/sharding/ShardableIndexExtension.java
index b4e48249a633..4ca2ce9135a1 100644
--- a/platform/indexing-impl/src/com/intellij/util/indexing/storage/sharding/ShardableIndexExtension.java
+++ b/platform/indexing-impl/src/com/intellij/util/indexing/storage/sharding/ShardableIndexExtension.java
@@ -1,6 +1,7 @@
 // Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
 package com.intellij.util.indexing.storage.sharding;
 
+import com.intellij.util.indexing.IndexExtension;
 import org.jetbrains.annotations.ApiStatus;
 
 /**
@@ -48,4 +49,14 @@ public interface ShardableIndexExtension {
       return 3;
     }
   }
+
+  /**
+   * Implementation version, without taking shards# into account.
+   * Motivation: generally, shards count should be a part of {@link IndexExtension#getVersion()} -- e.g. change in shardsCount _must_
+   * trigger re-indexing -- but in some cases shards count is not important. It all comes down to the exact interpretation of
+   * {@link IndexExtension#getVersion()}: is it a version of current index layout, or it is a version of index layout that could be
+   * _open-and-read_ by current code? This method returns "version that could be open-and-read", while {@link IndexExtension#getVersion()}
+   * returns a current layout version.
+   */
+  int shardlessVersion();
 }
diff --git a/platform/lang-impl/src/com/intellij/psi/impl/cache/impl/id/IdIndexImpl.java b/platform/lang-impl/src/com/intellij/psi/impl/cache/impl/id/IdIndexImpl.java
index ef0d02b23491..52e43935d692 100644
--- a/platform/lang-impl/src/com/intellij/psi/impl/cache/impl/id/IdIndexImpl.java
+++ b/platform/lang-impl/src/com/intellij/psi/impl/cache/impl/id/IdIndexImpl.java
@@ -43,8 +43,13 @@ public final class IdIndexImpl extends IdIndex implements CustomInputsIndexFileB
     return SHARDS;
   }
 
+  @Override
+  public int shardlessVersion() {
+    return super.getVersion();
+  }
+
   @Override
   public int getVersion() {
-    return super.getVersion() + (SHARDS - 1);
+    return shardlessVersion() + (SHARDS - 1);
   }
 }
diff --git a/platform/platform-tests/benchmarks/src/com/intellij/openapi/vfs/newvfs/persistent/indexes/IndexStorageLayoutBenchmark.java b/platform/platform-tests/benchmarks/src/com/intellij/openapi/vfs/newvfs/persistent/indexes/IndexStorageLayoutBenchmark.java
index 5564777fa8fd..fef16f978adf 100644
--- a/platform/platform-tests/benchmarks/src/com/intellij/openapi/vfs/newvfs/persistent/indexes/IndexStorageLayoutBenchmark.java
+++ b/platform/platform-tests/benchmarks/src/com/intellij/openapi/vfs/newvfs/persistent/indexes/IndexStorageLayoutBenchmark.java
@@ -1,4 +1,4 @@
-// Copyright 2000-2024 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+// Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
 package com.intellij.openapi.vfs.newvfs.persistent.indexes;
 
 import com.intellij.openapi.module.Module;
@@ -157,6 +157,11 @@ public class IndexStorageLayoutBenchmark {
       public int shardsCount() {
         return 2;
       }
+
+      @Override
+      public int shardlessVersion() {
+        return super.getVersion();
+      }
     }
   }
 
diff --git a/platform/util/src/com/intellij/util/indexing/IndexExtension.java b/platform/util/src/com/intellij/util/indexing/IndexExtension.java
index 5b6d509898b7..15a9e31f804a 100644
--- a/platform/util/src/com/intellij/util/indexing/IndexExtension.java
+++ b/platform/util/src/com/intellij/util/indexing/IndexExtension.java
@@ -1,4 +1,4 @@
-// Copyright 2000-2023 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+// Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
 
 package com.intellij.util.indexing;
 
@@ -9,7 +9,7 @@ import org.jetbrains.annotations.NotNull;
 /**
  * Represents index data format specification, namely serialization format for keys & values, and a mapping from input to
  * indexing data.
- *
+ * <p>
  * To create index corresponding to any extension one could use {@link com.intellij.util.indexing.impl.MapReduceIndex}.
  */
 public abstract class IndexExtension<Key, Value, Input> {
@@ -25,6 +25,15 @@ public abstract class IndexExtension<Key, Value, Input> {
 
   public abstract @NotNull DataExternalizer<Value> getValueExternalizer();
 
-  /** Version of index format/algo. Generally, if the version is changed -- index must be rebuilt. */
+  /**
+   * Version of index format/algo.
+   * Generally, if the version is changed -- index must be rebuilt.
+   * <p>
+   * Discussion: "index version" is quite ill-defined concept, because really it should be not a single version, but a few separate
+   * versions for "binary layout to be written version", "binary layout compatible to read version", "indexer algo version", etc.
+   * Currently, all those 'versions' are merged into a single version -- which simplifies things a bit, but causes issues from time to time.
+   *
+   * @see ShardableIndexExtension#shardlessVersion
+   */
   public abstract int getVersion();
 }
diff --git a/platform/util/testSrc/com/intellij/util/indexing/IndexStorageLayoutProviderTestBase.java b/platform/util/testSrc/com/intellij/util/indexing/IndexStorageLayoutProviderTestBase.java
index f0596194d7fb..e4a1d84d91ba 100644
--- a/platform/util/testSrc/com/intellij/util/indexing/IndexStorageLayoutProviderTestBase.java
+++ b/platform/util/testSrc/com/intellij/util/indexing/IndexStorageLayoutProviderTestBase.java
@@ -1,4 +1,4 @@
-// Copyright 2000-2024 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+// Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
 package com.intellij.util.indexing;
 
 import com.intellij.openapi.util.io.ByteArraySequence;
@@ -538,6 +538,11 @@ public abstract class IndexStorageLayoutProviderTestBase {
       return VERSION;
     }
 
+    @Override
+    public int shardlessVersion() {
+      return VERSION + (shardsCount() - 1) * 10;
+    }
+
     @Override
     public int shardsCount() {
       return 3;