diff --git a/modules/twap-monitor/Cargo.toml b/modules/twap-monitor/Cargo.toml
index bafc696..6cde093 100644
--- a/modules/twap-monitor/Cargo.toml
+++ b/modules/twap-monitor/Cargo.toml
@@ -10,5 +10,6 @@ crate-type = ["cdylib"]
 
 [dependencies]
 cowprotocol = { version = "1.0.0-alpha.3", default-features = false }
-alloy-sol-types = { version = "1.5", default-features = false }
+alloy-primitives = { version = "1.5", default-features = false, features = ["std"] }
+alloy-sol-types = { version = "1.5", default-features = false, features = ["std"] }
 wit-bindgen = { version = "0.57", default-features = false, features = ["macros", "realloc"] }
diff --git a/modules/twap-monitor/src/lib.rs b/modules/twap-monitor/src/lib.rs
index 72c0763..fbf0515 100644
--- a/modules/twap-monitor/src/lib.rs
+++ b/modules/twap-monitor/src/lib.rs
@@ -8,8 +8,10 @@ wit_bindgen::generate!({
     generate_all,
 });
 
-use nexum::host::logging;
-use nexum::host::types;
+use alloy_primitives::{Address, B256, keccak256};
+use alloy_sol_types::{SolEvent, SolValue};
+use cowprotocol::{ComposableCoW::ConditionalOrderCreated, ConditionalOrderParams};
+use nexum::host::{local_store, logging, types};
 
 struct TwapMonitor;
 
@@ -19,11 +21,98 @@ impl Guest for TwapMonitor {
         Ok(())
     }
 
-    fn on_event(_event: types::Event) -> Result<(), HostError> {
-        // Dispatch on Event::Log (ConditionalOrderCreated) and Event::Block
-        // (TWAP poll tick) lands in BLEU-826 / BLEU-827.
+    fn on_event(event: types::Event) -> Result<(), HostError> {
+        if let types::Event::Logs(logs) = event {
+            for log in &logs {
+                if let Some((owner, params)) =
+                    decode_conditional_order_created(&log.topics, &log.data)
+                {
+                    persist_watch(owner, &params)?;
+                }
+            }
+        }
+        // Event::Block (TWAP poll) lands in BLEU-827; Tick / Message are not
+        // used by this module.
         Ok(())
     }
 }
 
+/// Decode a raw event log against `ComposableCoW.ConditionalOrderCreated`.
+///
+/// Returns `None` when topic0 does not match the event signature or the
+/// payload fails ABI decoding — both are non-fatal for an indexer that
+/// shares a subscription with adjacent events. Kept on plain slices so
+/// the host-free unit tests under `#[cfg(test)]` can call it without
+/// wit-bindgen scaffolding.
+fn decode_conditional_order_created(
+    topics: &[Vec<u8>],
+    data: &[u8],
+) -> Option<(Address, ConditionalOrderParams)> {
+    let topic0 = topics.first()?;
+    if topic0.len() != 32 || B256::from_slice(topic0) != ConditionalOrderCreated::SIGNATURE_HASH {
+        return None;
+    }
+    let words: Vec<B256> = topics
+        .iter()
+        .filter(|t| t.len() == 32)
+        .map(|t| B256::from_slice(t))
+        .collect();
+    let decoded = ConditionalOrderCreated::decode_raw_log(words, data).ok()?;
+    Some((decoded.owner, decoded.params))
+}
+
+/// Persist a watch entry. `set` overwrites in place, so re-indexing the
+/// same log (re-org replay, overlapping subscription windows) produces no
+/// observable side effect — the idempotency the issue asks for.
+fn persist_watch(owner: Address, params: &ConditionalOrderParams) -> Result<(), HostError> {
+    let encoded = params.abi_encode();
+    let params_hash = keccak256(&encoded);
+    let key = format!("watch:{owner:#x}:{params_hash:#x}");
+    local_store::set(&key, &encoded)?;
+    logging::log(logging::Level::Info, &format!("indexed {key}"));
+    Ok(())
+}
+
 export!(TwapMonitor);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use alloy_primitives::{address, b256, hex};
+
+    #[test]
+    fn decodes_well_formed_log() {
+        let owner = address!("00112233445566778899aabbccddeeff00112233");
+        let params = ConditionalOrderParams {
+            handler: address!("ffeeddccbbaa00998877665544332211ffeeddcc"),
+            salt: b256!("0101010101010101010101010101010101010101010101010101010101010101"),
+            staticInput: hex!("deadbeef").to_vec().into(),
+        };
+        // address indexed: 20-byte address left-padded to 32 bytes.
+        let owner_topic = {
+            let mut t = vec![0u8; 12];
+            t.extend_from_slice(owner.as_slice());
+            t
+        };
+        let topics = vec![ConditionalOrderCreated::SIGNATURE_HASH.to_vec(), owner_topic];
+        let data = params.abi_encode();
+
+        let (decoded_owner, decoded_params) =
+            decode_conditional_order_created(&topics, &data).expect("decode succeeds");
+        assert_eq!(decoded_owner, owner);
+        assert_eq!(decoded_params, params);
+    }
+
+    #[test]
+    fn rejects_wrong_topic() {
+        let topics =
+            vec![b256!("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa").to_vec()];
+        let data = vec![];
+        assert!(decode_conditional_order_created(&topics, &data).is_none());
+    }
+
+    #[test]
+    fn rejects_empty_topics() {
+        assert!(decode_conditional_order_created(&[], &[]).is_none());
+    }
+}