diff --git a/cache/core/core.go b/cache/core/core.go
index 9261668..3373ac9 100644
--- a/cache/core/core.go
+++ b/cache/core/core.go
@@ -258,6 +258,27 @@ type LookupOptions struct {
 	// To indicates the ending offset to read from the cached object.  A
 	// value of 0 means to read to the end of the object.
 	To uint64
+
+	// LegacyReturnWholeBody restores a legacy behavior around range requests.
+	//
+	// In SDK v1.4.2 and earlier, under certain circumstances the requested range
+	// (From/To) would be ignored. Specifically, if:
+	// - The lookup (reader) is concurrent with the writer of the body
+	// - The size of the cached item's body was not provided by the writer
+	// - The reader requests a specific range of the cached item's body
+	//   (`From` and `To` are provided in LookupOptions or GetBodyOptions)
+	// then the core cache API would ignore the requested range and provide
+	// the entire body, instead of providing just the requested range.
+	//
+	// In SDK v1.4.3, the default behavior changed. In a concurrent read/write:
+	// - If From and To are nonzero, the reader will block until the start of
+	//   the requested range has been provided by the writer.
+	// - Only the requested range will be returned to the reader.
+	//
+	// Note that the full body is still provided if the range is invalid.
+	//
+	// LegacyReturnWholeBody restores the v1.4.2 behavior.
+	LegacyReturnWholeBody bool
 }
 
 func abiLookupOptions(opts LookupOptions) (fastly.CacheLookupOptions, error) {
@@ -272,6 +293,8 @@ func abiLookupOptions(opts LookupOptions) (fastly.CacheLookupOptions, error) {
 		abiOpts.SetRequest(req)
 	}
 
+	abiOpts.SetAlwaysUseRequestedRange(!opts.LegacyReturnWholeBody)
+
 	return abiOpts, nil
 }
 
diff --git a/cache/core/core_test.go b/cache/core/core_test.go
index c359eec..871dda2 100644
--- a/cache/core/core_test.go
+++ b/cache/core/core_test.go
@@ -208,3 +208,64 @@ func ExampleFound_GetRange() {
 
 	fmt.Printf("The cached value was: %s", cachedStr)
 }
+
+func ExampleLookupOptions_LegacyReturnWholeBody() {
+	const (
+		key      = "my_key"
+		contents = "my cached object"
+	)
+
+	// Start an insert; this will be concurrent with the read.
+	// Data written to this handle is streamed into the Fastly cache.
+	w, err := core.Insert([]byte(key), core.WriteOptions{
+		TTL:           time.Hour,
+		SurrogateKeys: []string{key},
+		Length:        uint64(len(contents)),
+	})
+	if err != nil {
+		panic(err)
+	}
+
+	// With the write still outstanding, start a lookup for a specific range.
+	legacy, err := core.Lookup([]byte(key), core.LookupOptions{
+		From:                  3,
+		To:                    8,
+		LegacyReturnWholeBody: true,
+	})
+	if err != nil {
+		panic(err)
+	}
+	// With the write still outstanding, start a lookup for a specific range.
+	updated, err := core.Lookup([]byte(key), core.LookupOptions{
+		From: 3,
+		To:   8,
+	})
+	if err != nil {
+		panic(err)
+	}
+
+	// The read and write are concurrent, and with an unknown length.
+	// In the legacy mode, we'll see the whole body;
+	// in the updated mode, we'll see just the range.
+
+	// Finish writing the body:
+	if _, err := io.WriteString(w, contents); err != nil {
+		panic(err)
+	}
+
+	legacyStr, err := io.ReadAll(legacy.Body)
+	if err != nil {
+		panic(err)
+	}
+	updatedStr, err := io.ReadAll(updated.Body)
+	if err != nil {
+		panic(err)
+	}
+
+	if got, want := string(legacyStr), "my cached object"; got != want {
+		panic(fmt.Sprintf("got: %q, want: %q", got, want))
+	}
+	if got, want := string(updatedStr), "cached"; got != want {
+		panic(fmt.Sprintf("got: %q, want: %q", got, want))
+	}
+}
diff --git a/internal/abi/fastly/cache_guest.go b/internal/abi/fastly/cache_guest.go
index 12ecccd..26e06f0 100644
--- a/internal/abi/fastly/cache_guest.go
+++ b/internal/abi/fastly/cache_guest.go
@@ -21,6 +21,14 @@ func (o *CacheLookupOptions) SetRequest(req *HTTPRequest) {
 	o.mask |= cacheLookupOptionsMaskRequestHeaders
 }
 
+func (o *CacheLookupOptions) SetAlwaysUseRequestedRange(alwaysUseRequestedRange bool) {
+	if alwaysUseRequestedRange {
+		o.mask |= cacheLookupOptionsMaskAlwaysUseRequestedRange
+	} else {
+		o.mask &= ^cacheLookupOptionsMaskAlwaysUseRequestedRange
+	}
+}
+
 type CacheGetBodyOptions struct {
 	opts cacheGetBodyOptions
 	mask cacheGetBodyOptionsMask
diff --git a/internal/abi/fastly/hostcalls_noguest.go b/internal/abi/fastly/hostcalls_noguest.go
index 0e1d515..8772710 100644
--- a/internal/abi/fastly/hostcalls_noguest.go
+++ b/internal/abi/fastly/hostcalls_noguest.go
@@ -425,6 +425,10 @@ func (o *CacheLookupOptions) SetRequest(req *HTTPRequest) error {
 	return fmt.Errorf("not implemented")
 }
 
+func (o *CacheLookupOptions) SetAlwaysUseRequestedRange(alwaysUseRequestedRange bool) error {
+	return fmt.Errorf("not implemented")
+}
+
 func (o *CacheGetBodyOptions) From(from uint64) error {
 	return fmt.Errorf("not implemented")
 }
diff --git a/internal/abi/fastly/types.go b/internal/abi/fastly/types.go
index 49bbe5e..c0ae0a8 100644
--- a/internal/abi/fastly/types.go
+++ b/internal/abi/fastly/types.go
@@ -721,13 +721,16 @@ type cacheLookupOptions struct {
 //	    (flags (@witx repr u32)
 //	        $reserved
 //	        $request_headers
+//	        $service_id
+//	        $always_use_requested_range
 //	    )
 //	)
 type cacheLookupOptionsMask prim.U32
 
 const (
-	cacheLookupOptionsMaskReserved       cacheLookupOptionsMask = 0b0000_0001 // $reserved
-	cacheLookupOptionsMaskRequestHeaders cacheLookupOptionsMask = 0b0000_0010 // $request_headers
+	cacheLookupOptionsMaskReserved                cacheLookupOptionsMask = 0b0000_0001 // $reserved
+	cacheLookupOptionsMaskRequestHeaders          cacheLookupOptionsMask = 0b0000_0010 // $request_headers
+	cacheLookupOptionsMaskAlwaysUseRequestedRange cacheLookupOptionsMask = 0b0000_1000 // $always_use_requested_range
 )
 
 // witx: