op-will · op-will · May 6, 2026
diff --git a/crates/cli/src/default_scenarios/erc20.rs b/crates/cli/src/default_scenarios/erc20.rs
@@ -119,6 +119,7 @@ impl ToTestConfig for Erc20Args {
                     func_def = func_def.with_fuzz(&[FuzzParam {
                         param: Some("guy".to_string()),
                         value: None,
+                        tx_field: None,
                         min: Some(U256::from(1)),
                         max: Some(
                             U256::from_str("0x0000000000ffffffffffffffffffffffffffffffff").unwrap(),

diff --git a/crates/cli/src/server/static/openrpc.json b/crates/cli/src/server/static/openrpc.json
@@ -15,7 +15,7 @@
       "BlobsCliArgs": {"file":"crates/cli/src/default_scenarios/blobs.rs","line":10},
       "BuilderParams": {"file":"crates/cli/src/server/rpc_server/types.rs","line":227},
       "BuiltinScenarioCli": {"file":"crates/cli/src/default_scenarios/builtin.rs","line":33},
-      "BundleCallDefinition": {"file":"crates/core/src/generator/function_def.rs","line":53},
+      "BundleCallDefinition": {"file":"crates/core/src/generator/function_def.rs","line":58},
       "BundleTypeCli": {"file":"crates/cli/src/commands/common.rs","line":426},
       "CompiledContract": {"file":"crates/core/src/generator/create_def.rs","line":8},
       "ContenderSessionInfo": {"file":"crates/cli/src/server/sessions.rs","line":127},
@@ -29,7 +29,7 @@
       "FillBlockCliArgs": {"file":"crates/cli/src/default_scenarios/fill_block.rs","line":17},
       "FunctionCallDefinition": {"file":"crates/core/src/generator/function_def.rs","line":12},
       "FundAccountsParams": {"file":"crates/cli/src/server/rpc_server/types.rs","line":273},
-      "FuzzParam": {"file":"crates/core/src/generator/function_def.rs","line":162},
+      "FuzzParam": {"file":"crates/core/src/generator/function_def.rs","line":182},
       "RevertCliArgs": {"file":"crates/cli/src/default_scenarios/revert.rs","line":8},
       "ServerStatus": {"file":"crates/cli/src/server/rpc_server/types.rs","line":33},
       "SessionOptions": {"file":"crates/cli/src/server/rpc_server/types.rs","line":234},
@@ -45,6 +45,7 @@
       "TestConfig": {"file":"crates/cli/src/server/rpc_server/types.rs","line":83},
       "TestConfigSource": {"file":"crates/cli/src/server/rpc_server/types.rs","line":83},
       "TransferStressCliArgs": {"file":"crates/cli/src/default_scenarios/transfers.rs","line":12},
+      "TxField": {"file":"crates/core/src/generator/function_def.rs","line":176},
       "TxTypeCli": {"file":"crates/cli/src/commands/common.rs","line":395},
       "UniV2CliArgs": {"file":"crates/cli/src/default_scenarios/uni_v2.rs","line":15}
     }
@@ -188,9 +189,9 @@
       "EthereumOpcode": {"type":"string","enum":["Stop","Add","Mul","Sub","Div","Sdiv","Mod","Smod","Addmod","Mulmod","Exp","Signextend","Lt","Gt","Slt","Sgt","Eq","Iszero","And","Or","Xor","Not","Byte","Shl","Shr","Sar","Sha3","Keccak256","Address","Balance","Origin","Caller","Callvalue","Calldataload","Calldatasize","Calldatacopy","Codesize","Codecopy","Gasprice","Extcodesize","Extcodecopy","Returndatasize","Returndatacopy","Extcodehash","Blockhash","Coinbase","Timestamp","Number","Prevrandao","Gaslimit","Chainid","Selfbalance","Basefee","Pop","Mload","Mstore","Mstore8","Sload","Sstore","Msize","Gas","Log0","Log1","Log2","Log3","Log4","Create","Call","Callcode","Return","Delegatecall","Create2","Staticcall","Revert","Invalid","Selfdestruct"]},
       "EthereumPrecompile": {"type":"string","enum":["HashSha256","HashRipemd160","Identity","ModExp","EcAdd","EcMul","EcPairing","Blake2f"]},
       "FillBlockCliArgs": {"description":"Taken from the CLI, this is used to fill a block with transactions.","type":"object","properties":{"max_gas_per_block":{"type":"integer"}}},
-      "FunctionCallDefinition": {"description":"User-facing definition of a function call to be executed.","type":"object","properties":{"to":{"description":"Address of the contract to call.","type":"string"},"from":{"description":"Address of the tx sender.","type":"string"},"from_pool":{"description":"Get a `from` address from the pool of signers specified here.","type":"string"},"signature":{"description":"Name of the function to call.","type":"string"},"args":{"description":"Parameters to pass to the function.","type":"array","items":{"type":"string"}},"value":{"description":"Value in wei to send with the tx.","type":"string"},"fuzz":{"description":"Parameters to fuzz during the test.","type":"array","items":{"$ref":"#/components/schemas/FuzzParam"}},"kind":{"description":"Optional type of the spam transaction for categorization.","type":"string"},"gas_limit":{"description":"Optional gas limit, which will skip gas estimation. This allows reverting txs to be sent.","type":"integer"},"blob_data":{"description":"Optional blob data; tx type must be set to EIP4844 by spammer","type":"string"},"authorization_address":{"description":"Optional setCode data; tx type must be set to EIP7702 by spammer","type":"string"},"for_all_accounts":{"description":"If true and `from_pool` is set, run this setup transaction for all accounts in the pool. Defaults to false (only runs for the first account).","type":"boolean"}},"required":["to","for_all_accounts"]},
+      "FunctionCallDefinition": {"description":"User-facing definition of a function call to be executed.","type":"object","properties":{"to":{"description":"Address of the contract to call.","type":"string"},"from":{"description":"Address of the tx sender.","type":"string"},"from_pool":{"description":"Get a `from` address from the pool of signers specified here.","type":"string"},"signature":{"description":"Name of the function to call.","type":"string"},"args":{"description":"Parameters to pass to the function.","type":"array","items":{"type":"string"}},"value":{"description":"Value in wei to send with the tx.","type":"string"},"fuzz":{"description":"Parameters to fuzz during the test.","type":"array","items":{"$ref":"#/components/schemas/FuzzParam"}},"kind":{"description":"Optional type of the spam transaction for categorization.","type":"string"},"gas_limit":{"description":"Optional gas limit, which will skip gas estimation. This allows reverting txs to be sent.","type":"integer"},"blob_data":{"description":"Optional blob data; tx type must be set to EIP4844 by spammer","type":"string"},"authorization_address":{"description":"Optional setCode data; tx type must be set to EIP7702 by spammer","type":"string"},"for_all_accounts":{"description":"If true and `from_pool` is set, run this setup transaction for all accounts in the pool. Defaults to false (only runs for the first account).","type":"boolean"},"max_priority_fee_per_gas":{"description":"Optional EIP-1559 priority fee (wei) for this tx. May be a `{placeholder}`. If unset, the spammer falls back to its default (`gas_price / 10`). This field is also fuzzable via `FuzzParam::tx_field = \"max_priority_fee_per_gas\"`.","type":"string"}},"required":["to","for_all_accounts"]},
       "FundAccountsParams": {"type":"object","properties":{"sessionId":{"type":"integer"},"agentClass":{"$ref":"#/components/schemas/AgentClass"},"amount":{"type":"string"}},"required":["sessionId","amount"]},
-      "FuzzParam": {"type":"object","properties":{"param":{"description":"Name of the parameter to fuzz.","type":"string"},"value":{"description":"Fuzz the `value` field of the tx (ETH sent with the tx).","type":"boolean"},"min":{"description":"Minimum value fuzzer will use.","type":"string"},"max":{"description":"Maximum value fuzzer will use.","type":"string"}}},
+      "FuzzParam": {"type":"object","properties":{"param":{"description":"Name of the parameter to fuzz.","type":"string"},"value":{"description":"Fuzz the `value` field of the tx (ETH sent with the tx).","type":"boolean"},"tx_field":{"description":"Fuzz a tx-level field (e.g. `max_priority_fee_per_gas`). Mutually exclusive with `param` and `value`.","$ref":"#/components/schemas/TxField"},"min":{"description":"Minimum value fuzzer will use.","type":"string"},"max":{"description":"Maximum value fuzzer will use.","type":"string"}}},
       "RevertCliArgs": {"type":"object","properties":{"gas_use":{"description":"Amount of gas to use before reverting.","type":"integer"}},"required":["gas_use"]},
       "ServerStatus": {"description":"Data returned from the `status` endpoint, containing general info about the server.","type":"object","properties":{"numSessions":{"type":"integer"}},"required":["numSessions"]},
       "SessionOptions": {"type":"object","properties":{"auth":{"$ref":"#/components/schemas/AuthParams"},"builder":{"$ref":"#/components/schemas/BuilderParams"},"minBalance":{"type":"string"},"timeoutSecs":{"type":"object","properties":{"secs":{"type":"integer"},"nanos":{"type":"integer"}}},"txType":{"$ref":"#/components/schemas/TxTypeCli"},"privateKeys":{"type":"array","items":{"type":"string"}},"agents":{"$ref":"#/components/schemas/AgentParams"},"env":{"type":"object","additionalProperties":{"type":"string"}}}},
@@ -206,6 +207,7 @@
       "TestConfig": {"description":"Configuration to run a test scenario; used to generate PlanConfigs. Defines TOML schema for scenario files.","type":"object","properties":{"env":{"description":"Template variables","type":"object","additionalProperties":{"type":"string"}},"create":{"description":"Contract deployments; array of hex-encoded bytecode strings.","type":"array","items":{"$ref":"#/components/schemas/CreateDefinition"}},"setup":{"description":"Setup steps to run before spamming.","type":"array","items":{"$ref":"#/components/schemas/FunctionCallDefinition"}},"spam":{"description":"Function to call in spam txs.","type":"array","items":{"$ref":"#/components/schemas/SpamRequest"}}}},
       "TestConfigSource": {"oneOf":[{"type":"object","properties":{"TomlBase64": {"type":"string"}},"required":["TomlBase64"]},{"type":"object","properties":{"Json": {"$ref":"#/components/schemas/TestConfig"}},"required":["Json"]},{"type":"object","properties":{"Builtin": {"$ref":"#/components/schemas/BuiltinScenarioCli"}},"required":["Builtin"]}]},
       "TransferStressCliArgs": {"type":"object","properties":{"amount":{"type":"string"},"recipient":{"type":"string"}},"required":["amount"]},
+      "TxField": {"description":"Routes a `FuzzParam` to a specific tx-level field (as opposed to a function-call argument or the `value` field).","type":"string","enum":["MaxPriorityFeePerGas"]},
       "TxTypeCli": {"type":"string","enum":["legacy","eip1559","eip4844","eip7702"]},
       "UniV2CliArgs": {"type":"object","properties":{"num_tokens":{"type":"integer"},"weth_per_token":{"type":"string"},"initial_token_supply":{"type":"string"},"weth_trade_amount":{"type":"string"},"token_trade_amount":{"type":"string"}},"required":["num_tokens","weth_per_token","initial_token_supply"]}
     }

diff --git a/crates/core/src/generator/constants.rs b/crates/core/src/generator/constants.rs
@@ -1,4 +1,5 @@
 pub const VALUE_KEY: &str = "__tx_value_contender__";
+pub const MAX_PRIORITY_FEE_KEY: &str = "__tx_max_priority_fee_contender__";
 pub const SENDER_KEY: &str = "_sender";
 pub const SETCODE_KEY: &str = "_setCodeSender";
 

diff --git a/crates/core/src/generator/function_def.rs b/crates/core/src/generator/function_def.rs
@@ -46,6 +46,11 @@ pub struct FunctionCallDefinition {
     /// Defaults to false (only runs for the first account).
     #[serde(default, skip_serializing_if = "std::ops::Not::not")]
     pub for_all_accounts: bool,
+    /// Optional EIP-1559 priority fee (wei) for this tx. May be a `{placeholder}`.
+    /// If unset, the spammer falls back to its default (`gas_price / 10`).
+    /// This field is also fuzzable via `FuzzParam::tx_field = "max_priority_fee_per_gas"`.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub max_priority_fee_per_gas: Option<String>,
 }
 
 /// User-facing definition of a function call to be executed.
@@ -70,6 +75,7 @@ impl FunctionCallDefinition {
             blob_data: None,
             authorization_address: None,
             for_all_accounts: false,
+            max_priority_fee_per_gas: None,
         }
     }
 
@@ -122,6 +128,10 @@ impl FunctionCallDefinition {
         self.for_all_accounts = for_all_accounts;
         self
     }
+    pub fn with_max_priority_fee_per_gas(mut self, fee_wei: impl AsRef<str>) -> Self {
+        self.max_priority_fee_per_gas = Some(fee_wei.as_ref().to_owned());
+        self
+    }
 
     pub fn sidecar_data(&self) -> Result<Option<BlobTransactionSidecar>, GeneratorError> {
         let sidecar_data = if let Some(data) = self.blob_data.as_ref() {
@@ -154,16 +164,29 @@ pub struct FunctionCallDefinitionStrict {
     pub fuzz: Vec<FuzzParam>,
     pub kind: Option<String>,
     pub gas_limit: Option<u64>,
+    pub max_priority_fee_per_gas: Option<String>, // may be a placeholder, so we can't use u128
     pub sidecar: Option<BlobTransactionSidecar>,
     pub authorization: Option<Vec<SignedAuthorization>>,
 }
 
+/// Routes a `FuzzParam` to a specific tx-level field (as opposed to a
+/// function-call argument or the `value` field).
+#[derive(Clone, Copy, Deserialize, Debug, Serialize, PartialEq, Eq)]
+#[serde(rename_all = "snake_case")]
+pub enum TxField {
+    /// EIP-1559 priority fee (wei).
+    MaxPriorityFeePerGas,
+}
+
 #[derive(Clone, Deserialize, Debug, Serialize)]
 pub struct FuzzParam {
     /// Name of the parameter to fuzz.
     pub param: Option<String>,
     /// Fuzz the `value` field of the tx (ETH sent with the tx).
     pub value: Option<bool>,
+    /// Fuzz a tx-level field (e.g. `max_priority_fee_per_gas`).
+    /// Mutually exclusive with `param` and `value`.
+    pub tx_field: Option<TxField>,
     /// Minimum value fuzzer will use.
     pub min: Option<U256>,
     /// Maximum value fuzzer will use.
@@ -221,4 +244,52 @@ mod tests {
             .with_for_all_accounts(false);
         assert!(!def.for_all_accounts);
     }
+
+    #[test]
+    fn parses_max_priority_fee_per_gas_field() {
+        let toml = r#"
+            to = "0x1234567890123456789012345678901234567890"
+            from_pool = "p"
+            signature = "burn(uint256)"
+            max_priority_fee_per_gas = "10000000000"
+        "#;
+        let def: FunctionCallDefinition = toml::from_str(toml).unwrap();
+        assert_eq!(
+            def.max_priority_fee_per_gas.as_deref(),
+            Some("10000000000")
+        );
+    }
+
+    #[test]
+    fn parses_fuzz_with_tx_field_routing() {
+        let toml = r#"
+            to = "0x1234567890123456789012345678901234567890"
+            from_pool = "p"
+            signature = "burn(uint256)"
+            fuzz = [{ tx_field = "max_priority_fee_per_gas", min = "0x2540be400", max = "0x4a817c800" }]
+        "#;
+        let def: FunctionCallDefinition = toml::from_str(toml).unwrap();
+        let fuzz = def.fuzz.expect("fuzz must parse");
+        assert_eq!(fuzz.len(), 1);
+        assert_eq!(fuzz[0].tx_field, Some(TxField::MaxPriorityFeePerGas));
+        assert!(fuzz[0].param.is_none());
+        assert!(fuzz[0].value.is_none());
+    }
+
+    #[test]
+    fn rejects_fuzz_with_tx_field_and_param_both_set() {
+        let toml = r#"
+            to = "0x1234567890123456789012345678901234567890"
+            from_pool = "p"
+            signature = "burn(uint256 n)"
+            fuzz = [{ param = "n", tx_field = "max_priority_fee_per_gas", min = "0x1", max = "0x2" }]
+        "#;
+        // Deserialization itself succeeds; conflict is caught later by
+        // `parse_map_key` when the fuzz map is built.
+        let def: FunctionCallDefinition =
+            toml::from_str(toml).expect("toml parses; conflict caught at runtime");
+        let fuzz = def.fuzz.expect("fuzz must parse");
+        assert!(fuzz[0].param.is_some());
+        assert!(fuzz[0].tx_field.is_some());
+    }
 }
diff --git a/crates/core/src/generator/templater.rs b/crates/core/src/generator/templater.rs
@@ -225,13 +225,19 @@ where
             .as_ref()
             .map(|x| self.replace_placeholders(x, placeholder_map))
             .and_then(|s| s.parse::<U256>().ok());
+        let max_priority_fee_per_gas = funcdef
+            .max_priority_fee_per_gas
+            .as_ref()
+            .map(|x| self.replace_placeholders(x, placeholder_map))
+            .and_then(|s| s.parse::<u128>().ok());
 
         Ok(TransactionRequest {
             to: Some(TxKind::Call(to)),
             input: alloy::rpc::types::TransactionInput::both(input.into()),
             from: Some(funcdef.from),
             value,
             gas: funcdef.gas_limit,
+            max_priority_fee_per_gas,
             sidecar: funcdef.sidecar.to_owned(),
             authorization_list: funcdef.authorization.to_owned(),
             ..Default::default()