fix(anvil): coerce decoder hardfork to executed network family

.changelog/anvil-decoder-executed-hardfork.md

@@ -0,0 +1,5 @@
+---
+anvil: patch
+---
+
+Fixed anvil trace decoding to follow the executed network hardfork when a cross-namespace hardfork override is configured.

crates/anvil/src/config.rs

@@ -1422,7 +1422,7 @@ impl NodeConfig {
             .unwrap_or_else(|| self.get_hardfork());
         let mut decoder_builder = CallTraceDecoderBuilder::new()
             .with_networks(self.networks)
-            .with_hardfork(Some(active_hardfork));
+            .with_hardfork(Some(self.networks.executed_hardfork(active_hardfork)));
         if self.print_traces {
             // if traces should get printed we configure the decoder with the signatures cache
             if let Ok(identifier) = SignaturesIdentifier::new(false) {

crates/anvil/src/eth/backend/mem/mod.rs

@@ -1490,7 +1490,7 @@ impl<N: Network> Backend<N> {
 
     /// Returns a trace decoder configured for the currently resolved hardfork.
     fn call_trace_decoder(&self) -> Arc<CallTraceDecoder> {
-        let hardfork = Some(self.hardfork());
+        let hardfork = Some(self.networks.executed_hardfork(self.hardfork()));
         let decoder = self.call_trace_decoder.read();
         if decoder.hardfork() == hardfork {
             return Arc::clone(&decoder);
@@ -9883,6 +9883,20 @@ mod tests {
         assert_eq!(outcome.block_number, original_best_number + 1);
     }
 
+    #[tokio::test]
+    async fn trace_decoder_follows_executed_hardfork_for_cross_namespace_override() {
+        let (api, _) = spawn(
+            NodeConfig::test_tempo()
+                .with_hardfork(Some(FoundryHardfork::Ethereum(EthereumHardfork::Prague))),
+        )
+        .await;
+
+        let decoder = api.backend.call_trace_decoder();
+        assert_eq!(decoder.hardfork(), Some(FoundryHardfork::Tempo(api.backend.tempo_hardfork())));
+        // The refresh compares the same coerced value, so repeated calls stay stable.
+        assert!(Arc::ptr_eq(&decoder, &api.backend.call_trace_decoder()));
+    }
+
     #[cfg(feature = "monad")]
     #[tokio::test]
     async fn monad_trace_decoder_follows_resolved_hardfork() {

crates/evm/networks/src/lib.rs

@@ -461,6 +461,23 @@ impl NetworkConfigs {
         false
     }
 
+    /// Coerces `hardfork` into this network's family.
+    ///
+    /// Execution and fee rules apply the same lossy `From` conversions, so a cross-namespace
+    /// override such as `--hardfork prague` on a Tempo node runs as a Tempo hardfork. Callers that
+    /// need to describe what actually executed, like trace decoding, go through here rather than
+    /// carrying the configured value.
+    pub fn executed_hardfork(&self, hardfork: FoundryHardfork) -> FoundryHardfork {
+        if self.is_tempo() {
+            return TempoHardfork::from(hardfork).into();
+        }
+        #[cfg(feature = "monad")]
+        if self.is_monad() {
+            return MonadHardfork::from(hardfork).into();
+        }
+        hardfork
+    }
+
     /// Returns additional cheatcode contract addresses for the active network.
     pub const fn extra_cheatcode_addresses(&self) -> &'static [Address] {
         #[cfg(feature = "monad")]
@@ -1789,4 +1806,16 @@ mod tests {
             assert!(cfg_optimism.is_optimism());
         }
     }
+
+    #[test]
+    fn executed_hardfork_follows_the_network_family() {
+        // A cross-namespace override runs as the configured network's hardfork, so the value used
+        // to describe execution has to be coerced the same way.
+        let prague = FoundryHardfork::Ethereum(EthereumHardfork::Prague);
+        assert_eq!(NetworkConfigs::default().executed_hardfork(prague), prague);
+        assert_eq!(
+            NetworkConfigs::with_tempo().executed_hardfork(prague),
+            FoundryHardfork::Tempo(TempoHardfork::from(prague))
+        );
+    }
 }