diff --git a/crates/adapter/src/fastly/core.rs b/crates/adapter/src/fastly/core.rs
index e613905e..493d3cc0 100644
--- a/crates/adapter/src/fastly/core.rs
+++ b/crates/adapter/src/fastly/core.rs
@@ -995,7 +995,9 @@ pub mod fastly_http_req {
         nwritten: *mut usize,
     ) -> FastlyStatus {
         alloc_result!(region_out, region_max_len, nwritten, {
-            fastly::api::http_req::downstream_compliance_region()
+            fastly::api::http_req::downstream_compliance_region(
+                u64::try_from(region_max_len).trapping_unwrap(),
+            )
         })
     }
 
diff --git a/lib/src/component/http_req.rs b/lib/src/component/http_req.rs
index 902e9fd9..32fb6841 100644
--- a/lib/src/component/http_req.rs
+++ b/lib/src/component/http_req.rs
@@ -808,8 +808,17 @@ impl http_req::Host for Session {
         Err(Error::NotAvailable("Client TLS JA4 hash").into())
     }
 
-    async fn downstream_compliance_region(&mut self) -> Result<Vec<u8>, types::Error> {
-        Ok(Vec::from(b"none"))
+    async fn downstream_compliance_region(
+        &mut self,
+        region_max_len: u64,
+    ) -> Result<Vec<u8>, types::Error> {
+        let region = Session::downstream_compliance_region(self);
+        let region_len = region.len();
+
+        match u64::try_from(region_len) {
+            Ok(region_len) if region_len <= region_max_len => Ok(region.into()),
+            too_large => Err(types::Error::BufferLen(too_large.unwrap_or(0))),
+        }
     }
 
     async fn original_header_names_get(
diff --git a/lib/src/session.rs b/lib/src/session.rs
index 754d8f2e..8b1b5615 100644
--- a/lib/src/session.rs
+++ b/lib/src/session.rs
@@ -38,6 +38,8 @@ use {
     tokio::sync::oneshot::Sender,
 };
 
+const REGION_NONE: &[u8] = b"none";
+
 /// Data specific to an individual request, including any host-side
 /// allocations on behalf of the guest processing the request.
 pub struct Session {
@@ -45,6 +47,11 @@ pub struct Session {
     downstream_client_addr: SocketAddr,
     /// The IP address and port that received this session.
     downstream_server_addr: SocketAddr,
+    /// The compliance region that this request was received in.
+    ///
+    /// For now this is just always `"none"`, but we place the field in the session
+    /// to make it easier to implement custom configuration values later on.
+    downstream_compliance_region: Vec<u8>,
     /// Handle for the downstream request "parts". NB the backing parts data can be mutated
     /// or even removed from the relevant map.
     downstream_req_handle: RequestHandle,
@@ -165,6 +172,7 @@ impl Session {
         Session {
             downstream_server_addr: server_addr,
             downstream_client_addr: client_addr,
+            downstream_compliance_region: Vec::from(REGION_NONE),
             downstream_req_handle,
             downstream_req_body_handle,
             downstream_req_original_headers,
@@ -204,6 +212,11 @@ impl Session {
         self.downstream_server_addr.ip()
     }
 
+    /// Retrieve the compliance region that received the request for this session.
+    pub fn downstream_compliance_region(&self) -> &[u8] {
+        self.downstream_compliance_region.as_slice()
+    }
+
     /// Retrieve the handle corresponding to the downstream request.
     pub fn downstream_request(&self) -> RequestHandle {
         self.downstream_req_handle
diff --git a/lib/src/wiggle_abi/req_impl.rs b/lib/src/wiggle_abi/req_impl.rs
index 954923bc..23cd1ce8 100644
--- a/lib/src/wiggle_abi/req_impl.rs
+++ b/lib/src/wiggle_abi/req_impl.rs
@@ -308,23 +308,25 @@ impl FastlyHttpReq for Session {
         region_max_len: u32,
         nwritten_out: GuestPtr<u32>,
     ) -> Result<(), Error> {
-        const REGION_NONE: &[u8] = b"none";
-        const REGION_NONE_LEN: u32 = 4;
+        let region = Session::downstream_compliance_region(self);
+        let region_len = region.len();
 
-        if region_max_len < REGION_NONE_LEN {
-            // Let the guest know how much we want to write.
-            memory.write(nwritten_out, REGION_NONE_LEN)?;
+        match u32::try_from(region_len) {
+            Ok(region_len) if region_len <= region_max_len => {
+                memory.copy_from_slice(region, region_out.as_array(region_max_len))?;
+                memory.write(nwritten_out, region_len.try_into().unwrap_or(0))?;
 
-            return Err(Error::BufferLengthError {
-                buf: "region_out",
-                len: "region_max_len",
-            });
-        }
-
-        memory.copy_from_slice(REGION_NONE, region_out.as_array(region_max_len))?;
-        memory.write(nwritten_out, REGION_NONE_LEN)?;
+                Ok(())
+            }
+            too_large => {
+                memory.write(nwritten_out, too_large.unwrap_or(0))?;
 
-        Ok(())
+                Err(Error::BufferLengthError {
+                    buf: "region_out",
+                    len: "region_max_len",
+                })
+            }
+        }
     }
 
     fn framing_headers_mode_set(
diff --git a/lib/wit/deps/fastly/compute.wit b/lib/wit/deps/fastly/compute.wit
index d52a8be3..a7dcb7ce 100644
--- a/lib/wit/deps/fastly/compute.wit
+++ b/lib/wit/deps/fastly/compute.wit
@@ -323,7 +323,7 @@ interface http-req {
 
   downstream-tls-ja4: func(max-len: u64) -> result<list<u8>, error>;
 
-  downstream-compliance-region: func() -> result<list<u8>, error>;
+  downstream-compliance-region: func(max-len: u64) -> result<list<u8>, error>;
 
   new: func() -> result<request-handle, error>;