Apply suggestion from @Copilot

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: tyler-french

server/remote_cache/capabilities_server/capabilities_server.go

@@ -93,9 +93,19 @@ func (s *CapabilitiesServer) GetCapabilities(ctx context.Context, req *repb.GetC
 		}
 
 		if chunkingEnabled {
-			c.CacheCapabilities.FastCdc_2020Params = &repb.FastCdc2020Params{
-				AvgChunkSizeBytes: uint64(chunking.MaxChunkSizeBytes() / 4),
-				Seed:              0,
+			maxChunkSizeBytes := chunking.MaxChunkSizeBytes()
+			if maxChunkSizeBytes > 0 {
+				avgChunkSizeBytes := maxChunkSizeBytes / 4
+				const (
+					minAvgChunkSizeBytes = 1024        // 1 KiB
+					maxAvgChunkSizeBytes = 1024 * 1024 // 1 MiB
+				)
+				if avgChunkSizeBytes >= minAvgChunkSizeBytes && avgChunkSizeBytes <= maxAvgChunkSizeBytes {
+					c.CacheCapabilities.FastCdc_2020Params = &repb.FastCdc2020Params{
+						AvgChunkSizeBytes: uint64(avgChunkSizeBytes),
+						Seed:              0,
+					}
+				}
 			}
 		}
 	}

server/remote_cache/content_addressable_storage_server/content_addressable_storage_server.go

@@ -1167,8 +1167,16 @@ func (s *ContentAddressableStorageServer) SpliceBlob(ctx context.Context, req *r
 		return nil, status.UnimplementedErrorf("SpliceBlob RPC is not currently enabled")
 	}
 
-	if cf := req.GetChunkingFunction(); cf != repb.ChunkingFunction_FAST_CDC_2020 {
-		return nil, status.InvalidArgumentErrorf("unsupported chunking function %v, only FAST_CDC_2020 is supported", cf)
+	cf := req.GetChunkingFunction()
+	if cf != repb.ChunkingFunction_UNKNOWN && cf != repb.ChunkingFunction_FAST_CDC_2020 {
+		return nil, status.InvalidArgumentErrorf("unsupported chunking function %v", cf)
+	}
+
+	// REAPI requires servers accept UNKNOWN; no-op since we only store FastCDC.
+	if cf == repb.ChunkingFunction_UNKNOWN {
+		return &repb.SpliceBlobResponse{
+			BlobDigest: req.GetBlobDigest(),
+		}, nil
 	}
 
 	if req.GetBlobDigest() == nil {