diff --git a/crates/Cargo.lock b/crates/Cargo.lock
index cdaceb3ae9..871b6bab8a 100644
--- a/crates/Cargo.lock
+++ b/crates/Cargo.lock
@@ -303,10 +303,18 @@ dependencies = [
 [[package]]
 name = "xrpl-host-functions"
 version = "0.1.0"
+dependencies = [
+ "xrpl-host-functions-macros",
+]
 
 [[package]]
 name = "xrpl-host-functions-macros"
 version = "0.1.0"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 3.0.3",
+]
 
 [[package]]
 name = "xrpl-wasm-vm"
diff --git a/crates/xrpl-host-functions-macros/Cargo.toml b/crates/xrpl-host-functions-macros/Cargo.toml
index ad785f2543..e5efeda8ea 100644
--- a/crates/xrpl-host-functions-macros/Cargo.toml
+++ b/crates/xrpl-host-functions-macros/Cargo.toml
@@ -3,4 +3,10 @@ name = "xrpl-host-functions-macros"
 version = "0.1.0"
 edition.workspace = true
 
+[lib]
+proc-macro = true
+
 [dependencies]
+syn = { version = "3", features = ["full"] }
+quote = "1"
+proc-macro2 = "1"
diff --git a/crates/xrpl-host-functions-macros/src/lib.rs b/crates/xrpl-host-functions-macros/src/lib.rs
index b93cf3ffd9..6d40738670 100644
--- a/crates/xrpl-host-functions-macros/src/lib.rs
+++ b/crates/xrpl-host-functions-macros/src/lib.rs
@@ -1,5 +1,170 @@
-pub fn add(left: u64, right: u64) -> u64 {
-    left + right
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{
+    Attribute, Expr, ExprLit, Lit, Signature, TraitItemFn,
+    parse::{Parse, ParseStream},
+    parse2,
+};
+
+#[proc_macro]
+pub fn host_functions(input: TokenStream) -> TokenStream {
+    expand(input.into())
+        .unwrap_or_else(syn::Error::into_compile_error)
+        .into()
+}
+
+fn expand(input: proc_macro2::TokenStream) -> syn::Result<proc_macro2::TokenStream> {
+    let HostFunctionsInput { functions } = parse2(input)?;
+
+    // let mut errors = Vec::new();
+
+    for f in functions {}
+
+    Ok(quote! {
+        trait HostFunctions {
+
+        }
+
+        enum HostFunctionSpec {
+
+        }
+
+        impl HostFunctionSpec
+    }
+    .into())
+}
+
+struct HostFunctionsInput {
+    functions: Vec<TraitItemFn>,
+}
+
+impl Parse for HostFunctionsInput {
+    fn parse(input: ParseStream) -> syn::Result<Self> {
+        let mut functions = Vec::new();
+        while !input.is_empty() {
+            functions.push(input.parse()?);
+        }
+        Ok(HostFunctionsInput { functions })
+    }
+}
+
+struct ParsedHostFunction {
+    gas: usize,
+    wasm_name: String,
+    docs: Vec<Attribute>,
+    signature: Signature,
+}
+
+impl ParsedHostFunction {
+    const GAS_PATH: &str = "gas";
+    const WASM_NAME_PATH: &str = "wasm_name";
+
+    fn parse(value: TraitItemFn) -> Result<Self, syn::Error> {
+        let mut gas = None;
+        let mut wasm_name = None;
+        let mut docs = Vec::new();
+        let mut errors = Vec::new();
+
+        for attr in &value.attrs {
+            let named = match attr.meta.require_name_value() {
+                Ok(n) => n,
+                Err(e) => {
+                    errors.push(e);
+                    continue;
+                }
+            };
+
+            match &named.path {
+                p if p.is_ident(Self::GAS_PATH) => {
+                    let parsed_value = match Self::parse_number(&named.value) {
+                        Ok(n) => n,
+                        Err(e) => {
+                            errors.push(e);
+                            continue;
+                        }
+                    };
+                    if gas.replace(parsed_value).is_some() {
+                        errors.push(syn::Error::new_spanned(
+                            named,
+                            format!("duplicated {} attribute", Self::GAS_PATH),
+                        ));
+                    }
+                }
+                p if p.is_ident(Self::WASM_NAME_PATH) => {
+                    let parsed_value = match Self::parse_string(&named.value) {
+                        Ok(n) => n,
+                        Err(e) => {
+                            errors.push(e);
+                            continue;
+                        }
+                    };
+                    if wasm_name.replace(parsed_value).is_some() {
+                        errors.push(syn::Error::new_spanned(
+                            named,
+                            format!("duplicated {} attribute", Self::WASM_NAME_PATH),
+                        ));
+                    }
+                }
+                p if p.is_ident("doc") => {
+                    docs.push(attr.clone());
+                }
+                _ => {
+                    errors.push(syn::Error::new_spanned(named, "unexpected attribute"));
+                }
+            }
+        }
+
+        if !errors.is_empty() {
+            return Err(errors
+                .into_iter()
+                .reduce(|mut l, r| {
+                    l.combine(r);
+                    l
+                })
+                .unwrap());
+        }
+        if gas.is_none() {
+            return Err(syn::Error::new_spanned(
+                &value.sig,
+                format!("missing {} attribute", Self::GAS_PATH),
+            ));
+        }
+
+        if wasm_name.is_none() {
+            return Err(syn::Error::new_spanned(
+                &value.sig,
+                format!("missing {} attribute", Self::WASM_NAME_PATH),
+            ));
+        }
+
+        Ok(Self {
+            gas: gas.unwrap(),
+            wasm_name: wasm_name.unwrap(),
+            docs,
+            signature: value.sig,
+        })
+    }
+
+    fn parse_number(value: &Expr) -> Result<usize, syn::Error> {
+        match value {
+            Expr::Lit(ExprLit {
+                lit: Lit::Int(i), ..
+            }) => i.base10_parse::<usize>(),
+            other => Err(syn::Error::new_spanned(
+                other,
+                "expected an integer literal",
+            )),
+        }
+    }
+
+    fn parse_string(value: &Expr) -> Result<String, syn::Error> {
+        match value {
+            Expr::Lit(ExprLit {
+                lit: Lit::Str(s), ..
+            }) => Ok(s.value()),
+            other => Err(syn::Error::new_spanned(other, "expected string literal")),
+        }
+    }
 }
 
 #[cfg(test)]
@@ -7,8 +172,25 @@ mod tests {
     use super::*;
 
     #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
+    fn reads_gas_and_wasm_name() {
+        let f: TraitItemFn = syn::parse_quote! {
+            /// some comment
+            #[gas = 60]
+            #[wasm_name = "ldgr_index"]
+            fn get_ledger_sqn() -> [u8; 4];
+        };
+        let p = ParsedHostFunction::parse(f).unwrap();
+        assert_eq!(p.gas, 60);
+        assert_eq!(p.wasm_name, "ldgr_index");
+    }
+
+    #[test]
+    fn rejects_unknown_attribute() {
+        let f: TraitItemFn = syn::parse_quote! {
+            #[gas = 60]
+            #[wsam_name = "typo"]
+            fn get_ledger_sqn() -> [u8; 4];
+        };
+        assert!(ParsedHostFunction::parse(f).is_err());
     }
 }
diff --git a/crates/xrpl-host-functions/Cargo.toml b/crates/xrpl-host-functions/Cargo.toml
index 67052a9fc2..c08bb7d62f 100644
--- a/crates/xrpl-host-functions/Cargo.toml
+++ b/crates/xrpl-host-functions/Cargo.toml
@@ -4,3 +4,4 @@ version = "0.1.0"
 edition.workspace = true
 
 [dependencies]
+xrpl-host-functions-macros.path = "../xrpl-host-functions-macros"
diff --git a/crates/xrpl-host-functions/src/lib.rs b/crates/xrpl-host-functions/src/lib.rs
index b93cf3ffd9..73a2deecbc 100644
--- a/crates/xrpl-host-functions/src/lib.rs
+++ b/crates/xrpl-host-functions/src/lib.rs
@@ -1,14 +1,110 @@
-pub fn add(left: u64, right: u64) -> u64 {
-    left + right
+#![no_std]
+use xrpl_host_functions_macros::host_abi;
+
+/// Error codes a host function may return.
+///
+/// The discriminants mirror `HostFunctionError` in
+/// `include/xrpl/tx/wasm/WasmCommon.h`, so a negative `i32` crossing the wasm
+/// boundary means the same thing to the guest, the Rust host, and the existing
+/// C++ code. The full set is kept (not just the ones the PoC uses today) to
+/// preserve that shared meaning.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(i32)]
+pub enum HostError {
+    Internal = -1,
+    FieldNotFound = -2,
+    BufferTooSmall = -3,
+    NoArray = -4,
+    NotLeafField = -5,
+    LocatorMalformed = -6,
+    SlotOutRange = -7,
+    SlotsFull = -8,
+    EmptySlot = -9,
+    LedgerObjNotFound = -10,
+    Decoding = -11,
+    DataFieldTooLarge = -12,
+    PointerOutOfBounds = -13,
+    NoMemExported = -14,
+    InvalidParams = -15,
+    InvalidAccount = -16,
+    InvalidField = -17,
+    IndexOutOfBounds = -18,
+    FloatInputMalformed = -19,
+    FloatComputationError = -20,
+    NoRuntime = -21,
+    OutOfGas = -22,
+    OutOfTransferLimit = -23,
 }
 
-#[cfg(test)]
-mod tests {
-    use super::*;
+impl HostError {
+    /// The negative wire value the guest sees as the function's return code.
+    #[inline]
+    pub const fn code(self) -> i32 {
+        self as i32
+    }
 
-    #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
+    /// Reconstruct a `HostError` from its wire code; unknown/positive values map to `Internal`.
+    pub const fn from_code(code: i32) -> HostError {
+        match code {
+            -1 => HostError::Internal,
+            -2 => HostError::FieldNotFound,
+            -3 => HostError::BufferTooSmall,
+            -4 => HostError::NoArray,
+            -5 => HostError::NotLeafField,
+            -6 => HostError::LocatorMalformed,
+            -7 => HostError::SlotOutRange,
+            -8 => HostError::SlotsFull,
+            -9 => HostError::EmptySlot,
+            -10 => HostError::LedgerObjNotFound,
+            -11 => HostError::Decoding,
+            -12 => HostError::DataFieldTooLarge,
+            -13 => HostError::PointerOutOfBounds,
+            -14 => HostError::NoMemExported,
+            -15 => HostError::InvalidParams,
+            -16 => HostError::InvalidAccount,
+            -17 => HostError::InvalidField,
+            -18 => HostError::IndexOutOfBounds,
+            -19 => HostError::FloatInputMalformed,
+            -20 => HostError::FloatComputationError,
+            -21 => HostError::NoRuntime,
+            -22 => HostError::OutOfGas,
+            -23 => HostError::OutOfTransferLimit,
+            _ => HostError::Internal,
+        }
     }
 }
+
+/// Convenience alias for the trait's fallible returns.
+pub type HostResult<T> = Result<T, HostError>;
+
+/// A `sha512Half` digest: the first 32 bytes of a SHA-512, as XRPL uses it.
+pub const HASH_LEN: usize = 32;
+
+/// Per-function ABI metadata: the wasm import name and the consensus-fixed base gas cost.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct HostFnSpec {
+    pub name: &'static str,
+    pub base_gas: u64,
+}
+
+host_functions! {
+    #[gas = 60]
+    #[wasm_name = "ldgr_index"]
+    fn get_ledger_sqn() -> [u8; 4];
+
+    #[gas = 70]
+    #[wasm_name = "home_le_field"]
+    fn get_current_ledger_obj_field(field: i32) -> Vec<u8>;
+
+    #[gas = 2000]
+    #[wasm_name = "sha512_half"]
+    fn sha512_half(data: &[u8]) -> [u8; 32];
+
+    #[gas = 500]
+    #[wasm_name = "trace"]
+    fn trace(msg: &str, data: &[u8], as_hex: bool);
+
+    #[gas = 500]
+    #[wasm_name = "trace_num"]
+    fn trace_num(msg: &str, number: i64);
+}