diff --git a/.github/workflows/compare-encoding-ethereum.yml b/.github/workflows/compare-encoding-ethereum.yml
index 1b562e4..b92bbb5 100644
--- a/.github/workflows/compare-encoding-ethereum.yml
+++ b/.github/workflows/compare-encoding-ethereum.yml
@@ -19,6 +19,11 @@ concurrency:
 
 permissions: read-all
 
+env:
+  # DEBUG/retest: encode this exact block in both encoders so the comparison is
+  # deterministic (no live-block race between the alloy and ethers jobs).
+  RETEST_BLOCK: '25463089'
+
 jobs:
   setup:
     runs-on: ubuntu-24.04
@@ -65,13 +70,14 @@ jobs:
         run: |
           cargo build --release
 
-      - name: Execute for ${{ needs.setup.outputs.timeout-minutes }} minutes
+      - name: Encode target block ${{ env.RETEST_BLOCK }}
         timeout-minutes: ${{ fromJSON(needs.setup.outputs.step-timeout) }}
         working-directory: rust
         run: |
           ./target/release/encode-blocks \
             --eth-rpc-url wss://mainnet.infura.io/ws/v3/${{ secrets.INFURA_ETHEREUM_RPC_KEY }} \
-            --path-to-store-json /var/tmp/encoded-data/alloy/
+            --path-to-store-json /var/tmp/encoded-data/alloy/ \
+            --target-block "$RETEST_BLOCK"
         env:
           TIMEOUT_MINUTES: ${{ needs.setup.outputs.timeout-minutes }}
 
@@ -114,12 +120,13 @@ jobs:
       - run: npm ci
       - run: npm run build --if-present
 
-      - name: Execute for ${{ needs.setup.outputs.timeout-minutes }} minutes
+      - name: Encode target block ${{ env.RETEST_BLOCK }}
         timeout-minutes: ${{ fromJSON(needs.setup.outputs.step-timeout) }}
         run: |
           node dist/bin/encode-blocks.js \
             wss://blockchain.googleapis.com/v1/projects/creditcoin-test/locations/us-central1/endpoints/ethereum-mainnet/rpc?key=${{ secrets.GOOGLE_ETHEREUM_RPC_KEY }} \
-            /var/tmp/encoded-data/ethers/
+            /var/tmp/encoded-data/ethers/ \
+            "$RETEST_BLOCK"
         env:
           TIMEOUT_MINUTES: ${{ needs.setup.outputs.timeout-minutes }}
 
@@ -190,9 +197,9 @@ jobs:
           git status
 
           set -eo pipefail
-          # filter only modified content b/c between the 2 CI jobs we may not have
-          # encoded exactly the same set of blocks. Especially if cargo needs to
-          # recompile from scratch
+          # Both encoders were pointed at the SAME fixed block (${{ env.RETEST_BLOCK }}),
+          # so the modified-only filter now compares like-for-like tx files
+          # instead of racing potentially non-overlapping live blocks.
           git diff --diff-filter=M | colordiff
 
           # will exit non-zero in case of differences
diff --git a/rust/bin/encode_blocks.rs b/rust/bin/encode_blocks.rs
index a366ecd..fb3fc6e 100644
--- a/rust/bin/encode_blocks.rs
+++ b/rust/bin/encode_blocks.rs
@@ -25,6 +25,13 @@ pub struct CliArguments {
 
     #[arg(long, help = "Directory path to store JSON files", required = true)]
     pub path_to_store_json: String,
+
+    #[arg(
+        long,
+        help = "Optional: encode exactly this block number then exit (deterministic retest mode). \
+                When omitted, subscribe to live blocks for TIMEOUT_MINUTES."
+    )]
+    pub target_block: Option<u64>,
 }
 
 async fn encode_transaction(
@@ -215,6 +222,20 @@ async fn main() -> Result<()> {
         .on_ws(WsConnect::new(args.eth_rpc_url))
         .await?;
 
+    // Deterministic retest mode: encode exactly one fixed block then exit. This
+    // lets both CI encoders (alloy + ethers) operate on the identical block
+    // instead of racing live blocks that may not overlap between the two jobs.
+    if let Some(block_number) = args.target_block {
+        println!("=== encoding single block {block_number} then exiting ...");
+        block_handler(
+            provider.clone(),
+            block_number,
+            args.path_to_store_json.clone(),
+        )
+        .await?;
+        return Ok(());
+    }
+
     let subscriber = provider.subscribe_blocks().await?;
     let mut stream = subscriber.into_stream();
     let mut last_seen_block: Option<u64> = None;
diff --git a/src/bin/encode-blocks.ts b/src/bin/encode-blocks.ts
index eb4f0fe..2990b3d 100644
--- a/src/bin/encode-blocks.ts
+++ b/src/bin/encode-blocks.ts
@@ -142,15 +142,41 @@ async function encodeBlocks(rpcUrl: string, pathToStoreJson: string): Promise<vo
   });
 }
 
+/**
+ * Encode a single, fixed block then exit. Used to deterministically re-test
+ * alloy-vs-ethers encoding on the exact same block across both CI jobs, rather
+ * than racing live blocks that may not overlap between the two encoders.
+ */
+async function encodeSingleBlock(rpcUrl: string, pathToStoreJson: string, blockNumber: number): Promise<void> {
+  console.log(`=== encoding single block ${blockNumber} then exiting ...`);
+
+  mkdirSync(pathToStoreJson, { recursive: true });
+
+  const provider = new WebSocketProvider(rpcUrl);
+  try {
+    await blockHandler('block', provider, blockNumber, pathToStoreJson);
+  } finally {
+    await provider.destroy();
+  }
+}
+
 if (process.argv.length < 4) {
-  console.error('node dist/bin/encode-blocks.js <ws://ethRpcUrl> <pathToStoreJson>');
+  console.error('node dist/bin/encode-blocks.js <ws://ethRpcUrl> <pathToStoreJson> [targetBlockNumber]');
   process.exit(1);
 }
 
 const rpcUrl = process.argv[2] || 'ws://127.0.0.1:8545';
 const pathToStoreJson = process.argv[3];
+// Optional: when provided, encode exactly this one block and exit (deterministic
+// retest mode). When omitted, subscribe to live blocks for TIMEOUT_MINUTES.
+const targetBlockArg = process.argv[4];
+
+const run =
+  targetBlockArg !== undefined
+    ? encodeSingleBlock(rpcUrl, pathToStoreJson, parseInt(targetBlockArg, 10))
+    : encodeBlocks(rpcUrl, pathToStoreJson);
 
-encodeBlocks(rpcUrl, pathToStoreJson).catch((reason) => {
+run.catch((reason) => {
   console.error(reason);
   process.exit(1);
 });