From 9cbb4e7338c62abe9345fe70cc81694373be87e2 Mon Sep 17 00:00:00 2001
From: Fabiana Cecin <fabiana@waku.org>
Date: Mon, 20 Apr 2026 08:48:27 -0300
Subject: [PATCH] fix: prefer --num-shards-in-network over preset (#3816)

* fill numShardsInCluster from preset when builder slot is none
* add regression tests
---
 tests/factory/test_waku_conf.nim              | 48 +++++++++++++++++++
 .../conf_builder/waku_conf_builder.nim        | 13 +++--
 2 files changed, 54 insertions(+), 7 deletions(-)

diff --git a/tests/factory/test_waku_conf.nim b/tests/factory/test_waku_conf.nim
index 9d05f7fb5..eeacf791b 100644
--- a/tests/factory/test_waku_conf.nim
+++ b/tests/factory/test_waku_conf.nim
@@ -213,6 +213,54 @@ suite "Waku Conf - build with cluster conf":
       check rlnRelayConf.epochSizeSec == networkConf.rlnEpochSizeSec
       check rlnRelayConf.userMessageLimit == userMessageLimit.uint
 
+  test "num-shards-in-network > 0 overrides preset":
+    ## Setup
+    let networkConf = NetworkConf.LogosDevConf()
+    var builder = WakuConfBuilder.init()
+
+    # Sanity check
+    check networkConf.shardingConf.kind == AutoSharding
+    check networkConf.shardingConf.numShardsInCluster > 1
+
+    ## Given: preset says >1 shards but user explicitly sets 1
+    builder.withNetworkConf(networkConf)
+    builder.withNumShardsInCluster(1)
+    builder.withShardingConf(AutoSharding)
+
+    ## When
+    let conf = builder.build().expect("build should succeed")
+
+    ## Then: user value wins, not preset
+    conf.validate().expect("conf should validate")
+    check conf.shardingConf.kind == AutoSharding
+    check conf.shardingConf.numShardsInCluster == 1
+
+  test "num-shards-in-network == 0 does not override preset":
+    ## Passing an AutoSharding preset and trying to override with
+    ## --num-shards-in-network=0 (which is StaticSharding) doesn't work.
+    ## Note that --num-shards-in-network=0 and omitting the switch are
+    ## internally the same. Promoting the config to an Option[uint16] is
+    ## probably not worth it since overriding an AutoSharding preset with
+    ## StaticSharding shouldn't make any sense (that is, no use case).
+
+    ## Given: emulate --preset=logos.dev --num-shards-in-network=0
+    let networkConf = NetworkConf.LogosDevConf()
+    var builder = WakuConfBuilder.init()
+    builder.withNetworkConf(networkConf)
+    # Note: builder.withNumShardsInCluster() is not called when the
+    # value that comes from the CLI path is 0 (which means it was
+    # either set to 0 or was left unset).
+    builder.withShardingConf(StaticSharding)
+
+    ## When
+    let conf = builder.build().expect("build should succeed")
+
+    ## Then: preset wins and StaticSharding user intent is lost
+    conf.validate().expect("conf should validate")
+    check conf.shardingConf.kind == networkConf.shardingConf.kind
+    check conf.shardingConf.numShardsInCluster ==
+      networkConf.shardingConf.numShardsInCluster
+
 suite "Waku Conf - node key":
   test "Node key is generated":
     ## Setup
diff --git a/waku/factory/conf_builder/waku_conf_builder.nim b/waku/factory/conf_builder/waku_conf_builder.nim
index 956d733d3..78dbd9eb9 100644
--- a/waku/factory/conf_builder/waku_conf_builder.nim
+++ b/waku/factory/conf_builder/waku_conf_builder.nim
@@ -299,7 +299,6 @@ proc buildShardingConf(
     bNumShardsInCluster: Option[uint16],
     bSubscribeShards: Option[seq[uint16]],
 ): (ShardingConf, seq[uint16]) =
-  echo "bSubscribeShards: ", bSubscribeShards
   case bShardingConfKind.get(AutoSharding)
   of StaticSharding:
     (ShardingConf(kind: StaticSharding), bSubscribeShards.get(@[]))
@@ -374,17 +373,17 @@ proc applyNetworkConf(builder: var WakuConfBuilder) =
     warn "Sharding Conf was provided alongside a network conf",
       used = networkConf.shardingConf.kind, discarded = builder.shardingConf
 
-  if builder.numShardsInCluster.isSome():
-    warn "Num Shards In Cluster was provided alongside a network conf",
-      used = networkConf.shardingConf.numShardsInCluster,
-      discarded = builder.numShardsInCluster
-
   case networkConf.shardingConf.kind
   of StaticSharding:
     builder.shardingConf = some(StaticSharding)
   of AutoSharding:
     builder.shardingConf = some(AutoSharding)
-    builder.numShardsInCluster = some(networkConf.shardingConf.numShardsInCluster)
+    if builder.numShardsInCluster.isSome():
+      warn "Num Shards In Cluster overrides network conf preset",
+        used = builder.numShardsInCluster.get(),
+        ignored = networkConf.shardingConf.numShardsInCluster
+    else:
+      builder.numShardsInCluster = some(networkConf.shardingConf.numShardsInCluster)
 
   if networkConf.discv5Discovery:
     if builder.discv5Conf.enabled.isNone: