diff --git a/docs/changelog/118774.yaml b/docs/changelog/118774.yaml
new file mode 100644
index 0000000000000..cbd1ca82d1c59
--- /dev/null
+++ b/docs/changelog/118774.yaml
@@ -0,0 +1,5 @@
+pr: 118774
+summary: Apply default k for knn query eagerly
+area: Vector Search
+type: bug
+issues: []
diff --git a/docs/reference/query-dsl/knn-query.asciidoc b/docs/reference/query-dsl/knn-query.asciidoc
index daf9e9499a189..e42bd78d9f14a 100644
--- a/docs/reference/query-dsl/knn-query.asciidoc
+++ b/docs/reference/query-dsl/knn-query.asciidoc
@@ -100,7 +100,7 @@ include::{es-ref-dir}/rest-api/common-parms.asciidoc[tag=knn-query-vector-builde
 --
 (Optional, integer) The number of nearest neighbors to return from each shard.
 {es} collects `k` results from each shard, then merges them to find the global top results.
-This value must be less than or equal to `num_candidates`. Defaults to `num_candidates`.
+This value must be less than or equal to `num_candidates`. Defaults to search request size.
 --
 
 `num_candidates`::
diff --git a/rest-api-spec/build.gradle b/rest-api-spec/build.gradle
index ed8a1f147b4fa..a104ec675adc2 100644
--- a/rest-api-spec/build.gradle
+++ b/rest-api-spec/build.gradle
@@ -60,5 +60,23 @@ tasks.named("yamlRestCompatTestTransform").configure ({ task ->
   task.skipTest("cat.aliases/10_basic/Deprecated local parameter", "CAT APIs not covered by compatibility policy")
   task.skipTest("cat.shards/10_basic/Help", "sync_id is removed in 9.0")
   task.skipTest("search/500_date_range/from, to, include_lower, include_upper deprecated", "deprecated parameters are removed in 9.0")
+  task.skipTest("search.vectors/41_knn_search_bbq_hnsw/Test knn search", "Scoring has changed in latest versions")
+  task.skipTest("search.vectors/42_knn_search_bbq_flat/Test knn search", "Scoring has changed in latest versions")
+  task.skipTest("search.vectors/180_update_dense_vector_type/Test create and update dense vector mapping with bulk indexing", "waiting for #118774 backport")
+  task.skipTest("search.vectors/160_knn_query_missing_params/kNN query in a bool clause - missing num_candidates", "waiting for #118774 backport")
+  task.skipTest("search.vectors/110_knn_query_with_filter/Simple knn query", "waiting for #118774 backport")
+  task.skipTest("search.vectors/160_knn_query_missing_params/kNN search used in nested field - missing num_candidates", "waiting for #118774 backport")
+  task.skipTest("search.vectors/180_update_dense_vector_type/Test create and update dense vector mapping to int4 with per-doc indexing and flush", "waiting for #118774 backport")
+  task.skipTest("search.vectors/110_knn_query_with_filter/PRE_FILTER: knn query with internal filter as pre-filter", "waiting for #118774 backport")
+  task.skipTest("search.vectors/180_update_dense_vector_type/Index, update and merge", "waiting for #118774 backport")
+  task.skipTest("search.vectors/160_knn_query_missing_params/kNN query with missing num_candidates param - size provided", "waiting for #118774 backport")
+  task.skipTest("search.vectors/110_knn_query_with_filter/POST_FILTER: knn query with filter from a parent bool query as post-filter", "waiting for #118774 backport")
+  task.skipTest("search.vectors/120_knn_query_multiple_shards/Aggregations with collected number of docs depends on num_candidates", "waiting for #118774 backport")
+  task.skipTest("search.vectors/180_update_dense_vector_type/Test create and update dense vector mapping with per-doc indexing and flush", "waiting for #118774 backport")
+  task.skipTest("search.vectors/110_knn_query_with_filter/PRE_FILTER: knn query with alias filter as pre-filter", "waiting for #118774 backport")
+  task.skipTest("search.vectors/140_knn_query_with_other_queries/Function score query with knn query", "waiting for #118774 backport")
+  task.skipTest("search.vectors/130_knn_query_nested_search/nested kNN search inner_hits size > 1", "waiting for #118774 backport")
+  task.skipTest("search.vectors/110_knn_query_with_filter/PRE_FILTER: pre-filter across multiple aliases", "waiting for #118774 backport")
+  task.skipTest("search.vectors/160_knn_query_missing_params/kNN search in a dis_max query - missing num_candidates", "waiting for #118774 backport")
   task.skipTest("search.highlight/30_max_analyzed_offset/Plain highlighter with max_analyzed_offset < 0 should FAIL", "semantics of test has changed")
 })
diff --git a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/110_knn_query_with_filter.yml b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/110_knn_query_with_filter.yml
index 618951711cffd..3d4841a16d82d 100644
--- a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/110_knn_query_with_filter.yml
+++ b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/110_knn_query_with_filter.yml
@@ -59,7 +59,9 @@ setup:
 
 ---
 "Simple knn query":
-
+  - requires:
+      cluster_features: "search.vectors.k_param_supported"
+      reason: 'k param for knn as query is required'
   - do:
       search:
         index: my_index
@@ -71,8 +73,9 @@ setup:
               field: my_vector
               query_vector: [1, 1, 1, 1]
               num_candidates: 5
+              k: 5
 
-  - match: { hits.total.value: 5 } # collector sees num_candidates docs
+  - match: { hits.total.value: 5 }
   - length: {hits.hits: 3}
   - match: { hits.hits.0._id: "1" }
   - match: { hits.hits.0.fields.my_name.0: v1 }
@@ -93,8 +96,9 @@ setup:
               field: my_vector
               query_vector: [1, 1, 1, 1]
               num_candidates: 5
+              k: 5
 
-  - match: { hits.total.value: 5 } # collector sees num_candidates docs
+  - match: { hits.total.value: 5 }
   - length: {hits.hits: 3}
   - match: { hits.hits.0._id: "2" }
   - match: { hits.hits.0.fields.my_name.0: v2 }
@@ -140,6 +144,7 @@ setup:
                     field: my_vector
                     query_vector: [1, 1, 1, 1]
                     num_candidates: 5
+                    k: 5
 
   - match: { hits.total.value: 5 }
   - length: { hits.hits: 3 }
@@ -184,6 +189,7 @@ setup:
               field: my_vector
               query_vector: [1, 1, 1, 1]
               num_candidates: 100
+              k: 100
 
   - match: { hits.total.value: 10 } # 5 docs from each alias
   - length: {hits.hits: 6}
@@ -213,6 +219,7 @@ setup:
               field: my_vector
               query_vector: [1, 1, 1, 1]
               num_candidates: 5
+              k: 5
               filter:
                 term:
                   my_name: v2
@@ -243,9 +250,10 @@ setup:
                     field: my_vector
                     query_vector: [1, 1, 1, 1]
                     num_candidates: 5
+                    k: 5
 
   - match: { hits.total.value: 2 }
-  - length: {hits.hits: 2} # knn query returns top 5 docs, but they are post-filtered to 2 docs
+  - length: {hits.hits: 2} # knn query returns top 3 docs, but they are post-filtered to 2 docs
   - match: { hits.hits.0._id: "2" }
   - match: { hits.hits.0.fields.my_name.0: v2 }
   - match: { hits.hits.1._id: "4" }
@@ -271,4 +279,4 @@ setup:
                         my_name: v1
 
   - match: { hits.total.value:  0}
-  - length: { hits.hits: 0 } # knn query returns top 5 docs, but they are post-filtered to 0 docs
+  - length: { hits.hits: 0 } # knn query returns top 3 docs, but they are post-filtered to 0 docs
diff --git a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/120_knn_query_multiple_shards.yml b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/120_knn_query_multiple_shards.yml
index c6f3e187f7953..c68565e6629f5 100644
--- a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/120_knn_query_multiple_shards.yml
+++ b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/120_knn_query_multiple_shards.yml
@@ -166,55 +166,3 @@ setup:
   - close_to: { hits.hits.2._score: { value: 120, error: 0.00001 } }
   - close_to: { hits.hits.2.matched_queries.bm25_query: { value: 100.0, error: 0.00001 } }
   - close_to: { hits.hits.2.matched_queries.knn_query: { value: 20.0, error: 0.00001 } }
-
----
-"Aggregations with collected number of docs depends on num_candidates":
-  - do:
-      search:
-        index: my_index
-        body:
-          size: 2
-          query:
-            knn:
-              field: my_vector
-              query_vector: [1, 1, 1, 1]
-              num_candidates: 100 # collect up to 100 candidates from each shard
-          aggs:
-            my_agg:
-              terms:
-                field: my_name
-                order:
-                  _key: asc
-
-  - length: {hits.hits: 2}
-  - match: {hits.total.value: 12}
-  - match: {aggregations.my_agg.buckets.0.key: 'v1'}
-  - match: {aggregations.my_agg.buckets.1.key: 'v2'}
-  - match: {aggregations.my_agg.buckets.0.doc_count: 6}
-  - match: {aggregations.my_agg.buckets.1.doc_count: 6}
-
-  - do:
-      search:
-        index: my_index
-        body:
-          size: 2
-          query:
-            knn:
-              field: my_vector
-              query_vector: [ 1, 1, 1, 1 ]
-              num_candidates: 3 # collect 3 candidates from each shard
-          aggs:
-            my_agg2:
-              terms:
-                field: my_name
-                order:
-                  _key: asc
-            my_sum_buckets:
-              sum_bucket:
-                buckets_path: "my_agg2>_count"
-
-  - length: { hits.hits: 2 }
-  - match: { hits.total.value: 6 }
-  - match: { aggregations.my_agg2.buckets.0.key: 'v1' }
-  - match: { aggregations.my_agg2.buckets.1.key: 'v2' }
-  - match: { aggregations.my_sum_buckets.value: 6.0 }
diff --git a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/130_knn_query_nested_search.yml b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/130_knn_query_nested_search.yml
index 79ff3f61742f8..bf07144975650 100644
--- a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/130_knn_query_nested_search.yml
+++ b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/130_knn_query_nested_search.yml
@@ -273,6 +273,7 @@ setup:
                 knn:
                   field: nested.vector
                   query_vector: [ -0.5, 90.0, -10, 14.8, -156.0 ]
+                  k: 5
                   num_candidates: 5
               inner_hits: { size: 2, "fields": [ "nested.paragraph_id" ], _source: false }
 
@@ -295,6 +296,7 @@ setup:
                 knn:
                   field: nested.vector
                   query_vector: [ -0.5, 90.0, -10, 14.8, -156.0 ]
+                  k: 5
                   num_candidates: 5
               inner_hits: { size: 2, "fields": [ "nested.paragraph_id" ], _source: false }
 
diff --git a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/140_knn_query_with_other_queries.yml b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/140_knn_query_with_other_queries.yml
index d52a5daf22344..1e54e497f286f 100644
--- a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/140_knn_query_with_other_queries.yml
+++ b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/140_knn_query_with_other_queries.yml
@@ -69,6 +69,7 @@ setup:
                   field: my_vector
                   query_vector: [ 1, 1, 1, 1 ]
                   num_candidates: 5
+                  k: 5
               functions:
                 - filter: { match: { my_name: v1 } }
                   weight: 10
diff --git a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/160_knn_query_missing_params.yml b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/160_knn_query_missing_params.yml
index 02962e049e267..26c52060dfb22 100644
--- a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/160_knn_query_missing_params.yml
+++ b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/160_knn_query_missing_params.yml
@@ -100,8 +100,9 @@ setup:
             knn:
               field: vector
               query_vector: [1, 1, 1]
+              k: 2
           size: 1
-  - match: { hits.total: 2 }  # due to num_candidates defined as round(1.5 * size), so we only see 2 results
+  - match: { hits.total: 2 }  # k defaults to size
   - length: { hits.hits: 1 }  # one result is only returned though
 
 ---
@@ -117,6 +118,7 @@ setup:
               field: vector
               query_vector: [-1, -1, -1]
               num_candidates: 1
+              k: 1
           size: 10
 
   - match: { hits.total: 1 }
@@ -137,9 +139,10 @@ setup:
                 - knn:
                     field: vector
                     query_vector: [ 1, 1, 0]
+                    k: 1
           size: 1
 
-  - match: { hits.total: 2 }  # due to num_candidates defined as round(1.5 * size), so we only see 2 results from cat:A
+  - match: { hits.total: 1 }
   - length: { hits.hits: 1 }
 
 ---
@@ -154,6 +157,7 @@ setup:
                 - knn:
                     field: vector
                     query_vector: [1, 1, 0]
+                    k: 2
                 - match:
                     category: B
               tie_breaker: 0.8
@@ -175,6 +179,7 @@ setup:
                 knn:
                   field: nested.vector
                   query_vector: [ -0.5, 90.0, -10, 14.8, -156.0 ]
+                  k: 2
               inner_hits: { size: 1, "fields": [ "nested.paragraph_id" ], _source: false }
           size: 1
 
diff --git a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/180_update_dense_vector_type.yml b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/180_update_dense_vector_type.yml
index 855daeaa7f163..99943ef2671bb 100644
--- a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/180_update_dense_vector_type.yml
+++ b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/180_update_dense_vector_type.yml
@@ -109,6 +109,7 @@ setup:
               field: embedding
               query_vector: [1, 1, 1, 1]
               num_candidates: 10
+              k: 10
 
   - match: { hits.total.value: 10 }
   - length: {hits.hits: 3}
@@ -215,6 +216,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 20
+              k: 20
 
   - match: { hits.total.value: 20 }
   - length: { hits.hits: 3 }
@@ -322,6 +324,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 30
+              k: 30
 
   - match: { hits.total.value: 30 }
   - length: { hits.hits: 4 }
@@ -430,6 +433,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 40
+              k: 40
 
   - match: { hits.total.value: 40 }
   - length: { hits.hits: 5 }
@@ -499,6 +503,7 @@ setup:
               field: embedding
               query_vector: [1, 1, 1, 1]
               num_candidates: 10
+              k: 10
 
   - match: { hits.total.value: 10 }
   - length: {hits.hits: 3}
@@ -559,6 +564,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 20
+              k: 20
 
   - match: { hits.total.value: 20 }
   - length: { hits.hits: 3 }
@@ -620,6 +626,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 30
+              k: 30
 
   - match: { hits.total.value: 30 }
   - length: { hits.hits: 4 }
@@ -682,6 +689,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 40
+              k: 40
 
   - match: { hits.total.value: 40 }
   - length: { hits.hits: 5 }
@@ -751,6 +759,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 10
+              k: 10
 
   - match: { hits.total.value: 10 }
   - length: { hits.hits: 3 }
@@ -791,6 +800,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 10
+              k: 10
 
   - match: { hits.total.value: 10 }
   - length: { hits.hits: 3 }
@@ -833,6 +843,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 20
+              k: 20
 
   - match: { hits.total.value: 20 }
   - length: { hits.hits: 3 }
@@ -869,6 +880,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 20
+              k: 20
 
   - match: { hits.total.value: 20 }
   - length: { hits.hits: 3 }
@@ -911,6 +923,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 30
+              k: 30
 
   - match: { hits.total.value: 30 }
   - length: { hits.hits: 4 }
@@ -933,6 +946,7 @@ setup:
             knn:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
+              k: 30
               num_candidates: 30
 
   - match: { hits.total.value: 30 }
@@ -1769,6 +1783,7 @@ setup:
               field: embedding
               query_vector: [1, 1, 1, 1]
               num_candidates: 10
+              k: 10
 
   - match: { hits.total.value: 10 }
   - length: {hits.hits: 3}
@@ -1875,6 +1890,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 20
+              k: 20
 
   - match: { hits.total.value: 20 }
   - length: { hits.hits: 3 }
@@ -1982,6 +1998,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 30
+              k: 30
 
   - match: { hits.total.value: 30 }
   - length: { hits.hits: 4 }
@@ -2090,6 +2107,7 @@ setup:
               field: embedding
               query_vector: [ 1, 1, 1, 1 ]
               num_candidates: 40
+              k: 40
 
   - match: { hits.total.value: 40 }
   - length: { hits.hits: 5 }
diff --git a/server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java b/server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java
index a99f21803556f..b2b23baacc4db 100644
--- a/server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java
+++ b/server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java
@@ -2019,7 +2019,7 @@ && isNotUnitVector(squaredMagnitude)) {
 
         public Query createKnnQuery(
             VectorData queryVector,
-            Integer k,
+            int k,
             int numCands,
             Float numCandsFactor,
             Query filter,
@@ -2052,7 +2052,7 @@ private boolean needsRescore(Float rescoreOversample) {
 
         private Query createKnnBitQuery(
             byte[] queryVector,
-            Integer k,
+            int k,
             int numCands,
             Query filter,
             Float similarityThreshold,
@@ -2074,7 +2074,7 @@ private Query createKnnBitQuery(
 
         private Query createKnnByteQuery(
             byte[] queryVector,
-            Integer k,
+            int k,
             int numCands,
             Query filter,
             Float similarityThreshold,
@@ -2101,7 +2101,7 @@ private Query createKnnByteQuery(
 
         private Query createKnnFloatQuery(
             float[] queryVector,
-            Integer k,
+            int k,
             int numCands,
             Float numCandsFactor,
             Query filter,
diff --git a/server/src/main/java/org/elasticsearch/rest/action/search/SearchCapabilities.java b/server/src/main/java/org/elasticsearch/rest/action/search/SearchCapabilities.java
index 1548a62d2b3ef..c343141490799 100644
--- a/server/src/main/java/org/elasticsearch/rest/action/search/SearchCapabilities.java
+++ b/server/src/main/java/org/elasticsearch/rest/action/search/SearchCapabilities.java
@@ -37,6 +37,8 @@ private SearchCapabilities() {}
     private static final String NESTED_RETRIEVER_INNER_HITS_SUPPORT = "nested_retriever_inner_hits_support";
     /** Fixed the math in {@code moving_fn}'s {@code linearWeightedAvg}. */
     private static final String MOVING_FN_RIGHT_MATH = "moving_fn_right_math";
+    /** knn query where k defaults to the request size. */
+    private static final String K_DEFAULT_TO_SIZE = "k_default_to_size";
 
     private static final String RANDOM_SAMPLER_WITH_SCORED_SUBAGGS = "random_sampler_with_scored_subaggs";
     private static final String OPTIMIZED_SCALAR_QUANTIZATION_BBQ = "optimized_scalar_quantization_bbq";
@@ -57,6 +59,7 @@ private SearchCapabilities() {}
         capabilities.add(OPTIMIZED_SCALAR_QUANTIZATION_BBQ);
         capabilities.add(KNN_QUANTIZED_VECTOR_RESCORE);
         capabilities.add(MOVING_FN_RIGHT_MATH);
+        capabilities.add(K_DEFAULT_TO_SIZE);
         if (Build.current().isSnapshot()) {
             capabilities.add(KQL_QUERY_SUPPORTED);
         }
diff --git a/server/src/main/java/org/elasticsearch/search/vectors/KnnSearchBuilder.java b/server/src/main/java/org/elasticsearch/search/vectors/KnnSearchBuilder.java
index b18ce2dff65cb..9b9718efcf523 100644
--- a/server/src/main/java/org/elasticsearch/search/vectors/KnnSearchBuilder.java
+++ b/server/src/main/java/org/elasticsearch/search/vectors/KnnSearchBuilder.java
@@ -465,7 +465,7 @@ public KnnVectorQueryBuilder toQueryBuilder() {
         if (queryVectorBuilder != null) {
             throw new IllegalArgumentException("missing rewrite");
         }
-        return new KnnVectorQueryBuilder(field, queryVector, null, numCands, rescoreVectorBuilder, similarity).boost(boost)
+        return new KnnVectorQueryBuilder(field, queryVector, numCands, numCands, rescoreVectorBuilder, similarity).boost(boost)
             .queryName(queryName)
             .addFilterQueries(filterQueries);
     }
diff --git a/server/src/main/java/org/elasticsearch/search/vectors/KnnSearchRequestParser.java b/server/src/main/java/org/elasticsearch/search/vectors/KnnSearchRequestParser.java
index 81b00f1329591..12573d5ad496e 100644
--- a/server/src/main/java/org/elasticsearch/search/vectors/KnnSearchRequestParser.java
+++ b/server/src/main/java/org/elasticsearch/search/vectors/KnnSearchRequestParser.java
@@ -256,7 +256,7 @@ public KnnVectorQueryBuilder toQueryBuilder() {
             if (numCands > NUM_CANDS_LIMIT) {
                 throw new IllegalArgumentException("[" + NUM_CANDS_FIELD.getPreferredName() + "] cannot exceed [" + NUM_CANDS_LIMIT + "]");
             }
-            return new KnnVectorQueryBuilder(field, queryVector, null, numCands, null, null);
+            return new KnnVectorQueryBuilder(field, queryVector, numCands, numCands, null, null);
         }
 
         @Override
diff --git a/server/src/main/java/org/elasticsearch/search/vectors/KnnVectorQueryBuilder.java b/server/src/main/java/org/elasticsearch/search/vectors/KnnVectorQueryBuilder.java
index 88f6312fa7e6f..a65757cc25876 100644
--- a/server/src/main/java/org/elasticsearch/search/vectors/KnnVectorQueryBuilder.java
+++ b/server/src/main/java/org/elasticsearch/search/vectors/KnnVectorQueryBuilder.java
@@ -495,15 +495,16 @@ protected QueryBuilder doIndexMetadataRewrite(QueryRewriteContext context) throw
     @Override
     protected Query doToQuery(SearchExecutionContext context) throws IOException {
         MappedFieldType fieldType = context.getFieldType(fieldName);
-        int requestSize;
-        if (k != null) {
-            requestSize = k;
+        int k;
+        if (this.k != null) {
+            k = this.k;
         } else {
-            requestSize = context.requestSize() == null || context.requestSize() < 0 ? DEFAULT_SIZE : context.requestSize();
+            k = context.requestSize() == null || context.requestSize() < 0 ? DEFAULT_SIZE : context.requestSize();
+            if (numCands != null) {
+                k = Math.min(k, numCands);
+            }
         }
-        int adjustedNumCands = numCands == null
-            ? Math.round(Math.min(NUM_CANDS_MULTIPLICATIVE_FACTOR * requestSize, NUM_CANDS_LIMIT))
-            : numCands;
+        int adjustedNumCands = numCands == null ? Math.round(Math.min(NUM_CANDS_MULTIPLICATIVE_FACTOR * k, NUM_CANDS_LIMIT)) : numCands;
         if (fieldType == null) {
             return new MatchNoDocsQuery();
         }
diff --git a/server/src/test/java/org/elasticsearch/action/search/KnnSearchSingleNodeTests.java b/server/src/test/java/org/elasticsearch/action/search/KnnSearchSingleNodeTests.java
index 353188af8be3c..568186e0cae5b 100644
--- a/server/src/test/java/org/elasticsearch/action/search/KnnSearchSingleNodeTests.java
+++ b/server/src/test/java/org/elasticsearch/action/search/KnnSearchSingleNodeTests.java
@@ -418,7 +418,7 @@ public void testKnnSearchAction() throws IOException {
         float[] queryVector = randomVector();
         assertResponse(
             client().prepareSearch("index1", "index2")
-                .setQuery(new KnnVectorQueryBuilder("vector", queryVector, null, 5, null, null))
+                .setQuery(new KnnVectorQueryBuilder("vector", queryVector, 5, 5, null, null))
                 .setSize(2),
             response -> {
                 // The total hits is num_cands * num_shards, since the query gathers num_cands hits from each shard
diff --git a/server/src/test/java/org/elasticsearch/search/vectors/AbstractKnnVectorQueryBuilderTestCase.java b/server/src/test/java/org/elasticsearch/search/vectors/AbstractKnnVectorQueryBuilderTestCase.java
index 375712ee60861..244d539403315 100644
--- a/server/src/test/java/org/elasticsearch/search/vectors/AbstractKnnVectorQueryBuilderTestCase.java
+++ b/server/src/test/java/org/elasticsearch/search/vectors/AbstractKnnVectorQueryBuilderTestCase.java
@@ -47,7 +47,6 @@
 import java.util.stream.Stream;
 
 import static org.elasticsearch.index.mapper.vectors.DenseVectorFieldMapper.NUM_CANDS_OVERSAMPLE_LIMIT;
-import static org.elasticsearch.search.SearchService.DEFAULT_SIZE;
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.hasSize;
@@ -82,7 +81,7 @@ private void checkIndexTypeAndDimensions() {
 
     abstract KnnVectorQueryBuilder createKnnVectorQueryBuilder(
         String fieldName,
-        Integer k,
+        int k,
         int numCands,
         RescoreVectorBuilder rescoreVectorBuilder,
         Float similarity
@@ -138,8 +137,8 @@ protected void initializeAdditionalMappings(MapperService mapperService) throws
     @Override
     protected KnnVectorQueryBuilder doCreateTestQueryBuilder() {
         String fieldName = randomBoolean() ? VECTOR_FIELD : VECTOR_ALIAS_FIELD;
-        Integer k = randomBoolean() ? null : randomIntBetween(1, 100);
-        int numCands = randomIntBetween(k == null ? DEFAULT_SIZE : k + 20, 1000);
+        int k = randomIntBetween(1, 100);
+        int numCands = randomIntBetween(k + 20, 1000);
         KnnVectorQueryBuilder queryBuilder = createKnnVectorQueryBuilder(
             fieldName,
             k,
diff --git a/server/src/test/java/org/elasticsearch/search/vectors/KnnByteVectorQueryBuilderTests.java b/server/src/test/java/org/elasticsearch/search/vectors/KnnByteVectorQueryBuilderTests.java
index f6c2e754cec63..26066389c63f1 100644
--- a/server/src/test/java/org/elasticsearch/search/vectors/KnnByteVectorQueryBuilderTests.java
+++ b/server/src/test/java/org/elasticsearch/search/vectors/KnnByteVectorQueryBuilderTests.java
@@ -20,7 +20,7 @@ DenseVectorFieldMapper.ElementType elementType() {
     @Override
     protected KnnVectorQueryBuilder createKnnVectorQueryBuilder(
         String fieldName,
-        Integer k,
+        int k,
         int numCands,
         RescoreVectorBuilder rescoreVectorBuilder,
         Float similarity
diff --git a/server/src/test/java/org/elasticsearch/search/vectors/KnnFloatVectorQueryBuilderTests.java b/server/src/test/java/org/elasticsearch/search/vectors/KnnFloatVectorQueryBuilderTests.java
index 6f67e4be29a06..70d29ab525ef1 100644
--- a/server/src/test/java/org/elasticsearch/search/vectors/KnnFloatVectorQueryBuilderTests.java
+++ b/server/src/test/java/org/elasticsearch/search/vectors/KnnFloatVectorQueryBuilderTests.java
@@ -20,7 +20,7 @@ DenseVectorFieldMapper.ElementType elementType() {
     @Override
     KnnVectorQueryBuilder createKnnVectorQueryBuilder(
         String fieldName,
-        Integer k,
+        int k,
         int numCands,
         RescoreVectorBuilder rescoreVectorBuilder,
         Float similarity
diff --git a/server/src/test/java/org/elasticsearch/search/vectors/KnnSearchBuilderTests.java b/server/src/test/java/org/elasticsearch/search/vectors/KnnSearchBuilderTests.java
index a39438af5b72a..108dc60e2ee3b 100644
--- a/server/src/test/java/org/elasticsearch/search/vectors/KnnSearchBuilderTests.java
+++ b/server/src/test/java/org/elasticsearch/search/vectors/KnnSearchBuilderTests.java
@@ -224,9 +224,9 @@ public void testToQueryBuilder() {
             builder.addFilterQuery(filter);
         }
 
-        QueryBuilder expected = new KnnVectorQueryBuilder(field, vector, null, numCands, rescoreVectorBuilder, similarity).addFilterQueries(
-            filterQueries
-        ).boost(boost);
+        QueryBuilder expected = new KnnVectorQueryBuilder(field, vector, numCands, numCands, rescoreVectorBuilder, similarity)
+            .addFilterQueries(filterQueries)
+            .boost(boost);
         assertEquals(expected, builder.toQueryBuilder());
     }