diff --git a/lib/compute-at-edge-abi/compute-at-edge.witx b/lib/compute-at-edge-abi/compute-at-edge.witx
index 48022a81..554d2d11 100644
--- a/lib/compute-at-edge-abi/compute-at-edge.witx
+++ b/lib/compute-at-edge-abi/compute-at-edge.witx
@@ -110,6 +110,23 @@
         (param $nwritten_out (@witx pointer (@witx usize)))
         (result $err (expected (error $fastly_status)))
     )
+
+    ;;; Returns a u64 body length if the length of a body is known, or `FastlyStatus::None`
+    ;;; otherwise.
+    ;;;
+    ;;; If the length is unknown, it is likely due to the body arising from an HTTP/1.1 message with
+    ;;; chunked encoding, an HTTP/2 or later message with no `content-length`, or being a streaming
+    ;;; body.
+    ;;;
+    ;;; Note that receiving a length from this function does not guarantee that the full number of
+    ;;; bytes can actually be read from the body. For example, when proxying a response from a
+    ;;; backend, this length may reflect the `content-length` promised in the response, but if the
+    ;;; backend connection is closed prematurely, fewer bytes may be delivered before this body
+    ;;; handle can no longer be read.
+    (@interface func (export "known_length")
+        (param $h $body_handle)
+        (result $err (expected $body_length (error $fastly_status)))
+    )
 )
 
 (module $fastly_log
@@ -214,6 +231,13 @@
         (result $err (expected $num_bytes (error $fastly_status)))
     )
 
+    (@interface func (export "downstream_tls_ja4")
+         (param $ja4_out (@witx pointer (@witx char8)))
+         (param $ja4_max_len (@witx usize))
+         (param $nwritten_out (@witx pointer (@witx usize)))
+         (result $err (expected (error $fastly_status)))
+     )
+
     (@interface func (export "new")
         (result $err (expected $request_handle (error $fastly_status)))
     )
diff --git a/lib/compute-at-edge-abi/typenames.witx b/lib/compute-at-edge-abi/typenames.witx
index bec555b0..f3d5752d 100644
--- a/lib/compute-at-edge-abi/typenames.witx
+++ b/lib/compute-at-edge-abi/typenames.witx
@@ -356,3 +356,5 @@
 (typename $rate u32)
 (typename $count u32)
 (typename $has u32)
+
+(typename $body_length u64)
diff --git a/lib/src/body.rs b/lib/src/body.rs
index 58f6ee2e..42419c5f 100644
--- a/lib/src/body.rs
+++ b/lib/src/body.rs
@@ -145,6 +145,20 @@ impl Body {
             self.chunks.push_front(chunk.into())
         }
     }
+
+    pub fn len(&self) -> Option<u64> {
+        let mut len = 0u64;
+
+        for chunk in &self.chunks {
+            if let Chunk::HttpBody(body) = chunk {
+                len = len.checked_add(body.size_hint().exact()?)?;
+            } else {
+                return None;
+            }
+        }
+
+        Some(len)
+    }
 }
 
 impl<T: Into<Chunk>> From<T> for Body {
diff --git a/lib/src/wiggle_abi/body_impl.rs b/lib/src/wiggle_abi/body_impl.rs
index d2d85e16..2c5005d6 100644
--- a/lib/src/wiggle_abi/body_impl.rs
+++ b/lib/src/wiggle_abi/body_impl.rs
@@ -11,7 +11,9 @@ use {
         session::Session,
         wiggle_abi::{
             fastly_http_body::FastlyHttpBody,
-            types::{BodyHandle, BodyWriteEnd, MultiValueCursor, MultiValueCursorResult},
+            types::{
+                BodyHandle, BodyLength, BodyWriteEnd, MultiValueCursor, MultiValueCursorResult,
+            },
         },
     },
     http_body::Body as HttpBody,
@@ -215,4 +217,14 @@ impl FastlyHttpBody for Session {
         }
         Err(Error::Again)
     }
+
+    fn known_length(&mut self, body_handle: BodyHandle) -> Result<BodyLength, Error> {
+        if self.is_streaming_body(body_handle) {
+            Err(Error::ValueAbsent)
+        } else if let Some(len) = self.body_mut(body_handle)?.len() {
+            Ok(len)
+        } else {
+            Err(Error::ValueAbsent)
+        }
+    }
 }
diff --git a/lib/src/wiggle_abi/req_impl.rs b/lib/src/wiggle_abi/req_impl.rs
index 3d6737a7..29b80a5e 100644
--- a/lib/src/wiggle_abi/req_impl.rs
+++ b/lib/src/wiggle_abi/req_impl.rs
@@ -226,6 +226,16 @@ impl FastlyHttpReq for Session {
         Err(Error::NotAvailable("Client TLS JA3 hash"))
     }
 
+    #[allow(unused_variables)] // FIXME UFSM 2024-02-19: Remove this directive once implemented.
+    fn downstream_tls_ja4(
+        &mut self,
+        ja4_out: &GuestPtr<u8>,
+        ja4_max_len: u32,
+        nwritten_out: &GuestPtr<u32>,
+    ) -> Result<(), Error> {
+        Err(Error::NotAvailable("Client TLS JA4 hash"))
+    }
+
     fn framing_headers_mode_set(
         &mut self,
         _h: RequestHandle,