diff --git a/xls/codegen/BUILD b/xls/codegen/BUILD
index febba2b6e0..dd16c2c5c7 100644
--- a/xls/codegen/BUILD
+++ b/xls/codegen/BUILD
@@ -734,6 +734,7 @@ cc_library(
         "@com_google_absl//absl/container:flat_hash_map",
         "@com_google_absl//absl/log",
         "@com_google_absl//absl/log:check",
+        "@com_google_absl//absl/status",
         "@com_google_absl//absl/status:statusor",
         "@com_google_absl//absl/strings:str_format",
     ],
@@ -953,7 +954,6 @@ proto_library(
     name = "module_signature_proto",
     srcs = ["module_signature.proto"],
     deps = [
-        ":xls_metrics_proto",
         "//xls/ir:channel_proto",
         "//xls/ir:foreign_function_data_proto",
         "//xls/ir:xls_type_proto",
@@ -2011,6 +2011,7 @@ cc_library(
         "//xls/ir:channel",
         "//xls/passes:pass_base",
         "@com_google_absl//absl/base",
+        "@com_google_absl//absl/status",
         "@com_google_absl//absl/status:statusor",
     ],
 )
diff --git a/xls/codegen/block_stitching_pass.cc b/xls/codegen/block_stitching_pass.cc
index d03c3b9e98..d4c6a2e2fa 100644
--- a/xls/codegen/block_stitching_pass.cc
+++ b/xls/codegen/block_stitching_pass.cc
@@ -490,6 +490,11 @@ absl::Status StitchSingleValueChannel(
 absl::Status StitchStreamingChannel(
     Block* container, StreamingChannel* channel, const ChannelMap& channel_map,
     const absl::flat_hash_map<Block*, ::xls::Instantiation*>& instantiations) {
+  if (channel->flow_control() == FlowControl::kValidData) {
+    return absl::UnimplementedError(
+        "Channels with valid_data flow control are not supported in codegen "
+        "v1.0.");
+  }
   auto input_iter = channel_map.channel_to_streaming_input().find(channel);
   auto output_iter = channel_map.channel_to_streaming_output().find(channel);
   bool has_input = input_iter != channel_map.channel_to_streaming_input().end();
diff --git a/xls/codegen/clone_nodes_into_block_handler.cc b/xls/codegen/clone_nodes_into_block_handler.cc
index 48ec0c26bc..72d8d23ec8 100644
--- a/xls/codegen/clone_nodes_into_block_handler.cc
+++ b/xls/codegen/clone_nodes_into_block_handler.cc
@@ -242,6 +242,10 @@ AddPortsForReceive(ChannelRef channel, Block* block,
   std::optional<Node*> valid;
   std::optional<Node*> ready;
   if (ChannelRefKind(channel) == ChannelKind::kStreaming) {
+    if (ChannelRefFlowControl(channel) == FlowControl::kValidData) {
+      return absl::UnimplementedError(
+          "valid_data flow control not supported in codegen v1.0");
+    }
     XLS_ASSIGN_OR_RETURN(
         valid, block->AddInputPort(
                    absl::StrCat(ChannelRefName(channel),
@@ -290,6 +294,10 @@ absl::StatusOr<CloneNodesIntoBlockHandler::ChannelConnection> AddPortsForSend(
   std::optional<Node*> valid;
   std::optional<Node*> ready;
   if (ChannelRefKind(channel) == ChannelKind::kStreaming) {
+    if (ChannelRefFlowControl(channel) == FlowControl::kValidData) {
+      return absl::UnimplementedError(
+          "valid_data flow control not supported in codegen v1.0");
+    }
     XLS_ASSIGN_OR_RETURN(Node * one, block->MakeNode<xls::Literal>(
                                          SourceInfo(), Value(UBits(1, 1))));
     XLS_ASSIGN_OR_RETURN(
@@ -875,6 +883,10 @@ absl::StatusOr<Node*> CloneNodesIntoBlockHandler::HandleReceiveNode(
   }
 
   XLS_RET_CHECK_EQ(ChannelRefKind(connection.channel), ChannelKind::kStreaming);
+  if (ChannelRefFlowControl(connection.channel) == FlowControl::kValidData) {
+    return absl::UnimplementedError(
+        "valid_data flow control not supported in codegen v1.0");
+  }
   XLS_RET_CHECK_EQ(ChannelRefFlowControl(connection.channel),
                    FlowControl::kReadyValid)
       << " channel " << ChannelRefToString(connection.channel);
@@ -999,6 +1011,10 @@ absl::StatusOr<Node*> CloneNodesIntoBlockHandler::HandleSendNode(
   }
 
   XLS_RET_CHECK_EQ(ChannelRefKind(connection.channel), ChannelKind::kStreaming);
+  if (ChannelRefFlowControl(connection.channel) == FlowControl::kValidData) {
+    return absl::UnimplementedError(
+        "valid_data flow control not supported in codegen v1.0");
+  }
   XLS_RET_CHECK_EQ(ChannelRefFlowControl(connection.channel),
                    FlowControl::kReadyValid);
 
diff --git a/xls/codegen/conversion_utils.cc b/xls/codegen/conversion_utils.cc
index dafd41e64d..f607c558be 100644
--- a/xls/codegen/conversion_utils.cc
+++ b/xls/codegen/conversion_utils.cc
@@ -69,6 +69,10 @@ absl::Status CheckForMultiplexingCycle(Channel* channel,
   }
   StreamingChannel* streaming_channel =
       absl::down_cast<StreamingChannel*>(channel);
+  if (streaming_channel->flow_control() == FlowControl::kValidData) {
+    return absl::UnimplementedError(
+        "Channels with flow control valid_data are not yet supported.");
+  }
   if (!streaming_channel->channel_config().fifo_config().has_value()) {
     return absl::InvalidArgumentError(
         absl::StrCat("Cannot allow multiple operations from proc `", proc_name,
diff --git a/xls/codegen/mark_channel_fifos_pass.cc b/xls/codegen/mark_channel_fifos_pass.cc
index f3196a15ae..4bed0f598a 100644
--- a/xls/codegen/mark_channel_fifos_pass.cc
+++ b/xls/codegen/mark_channel_fifos_pass.cc
@@ -15,6 +15,7 @@
 #include "xls/codegen/mark_channel_fifos_pass.h"
 
 #include "absl/base/casts.h"
+#include "absl/status/status.h"
 #include "absl/status/statusor.h"
 #include "xls/codegen/codegen_options.h"
 #include "xls/codegen/codegen_pass.h"
@@ -45,6 +46,11 @@ absl::StatusOr<bool> MarkChannelFifosPass::RunInternal(
       continue;
     }
     StreamingChannel* schan = absl::down_cast<StreamingChannel*>(chan);
+    if (schan->flow_control() == FlowControl::kValidData) {
+      return absl::UnimplementedError(
+          "Channels with flow control valid_data are not supported in codegen "
+          "v1.0.");
+    }
     if (!schan->channel_config().input_flop_kind()) {
       schan->SetChannelConfig(schan->channel_config().WithInputFlopKind(
           GetRealFlopKind(options.codegen_options.flop_inputs(),
diff --git a/xls/codegen/module_signature.cc b/xls/codegen/module_signature.cc
index c14f1c47fa..45d91dcf9e 100644
--- a/xls/codegen/module_signature.cc
+++ b/xls/codegen/module_signature.cc
@@ -171,6 +171,8 @@ ModuleSignatureBuilder& ModuleSignatureBuilder::AddStreamingChannel(
   ChannelProto* channel = proto_.add_channels();
   channel->set_name(name);
   channel->set_kind(CHANNEL_KIND_STREAMING);
+  CHECK_NE(flow_control, FlowControl::kValidData)
+      << "valid_data flow control not supported in codegen v1.0";
   if (flow_control == FlowControl::kReadyValid) {
     channel->set_flow_control(CHANNEL_FLOW_CONTROL_READY_VALID);
   } else {
@@ -212,6 +214,8 @@ ModuleSignatureBuilder& ModuleSignatureBuilder::AddStreamingChannelInterface(
   interface->set_direction(direction);
   *interface->mutable_type() = type->ToProto();
   interface->set_kind(CHANNEL_KIND_STREAMING);
+  CHECK_NE(flow_control, FlowControl::kValidData)
+      << "valid_data flow control not supported in codegen v1.0";
   interface->mutable_streaming()->set_flow_control(
       flow_control == FlowControl::kReadyValid
           ? CHANNEL_FLOW_CONTROL_READY_VALID
diff --git a/xls/codegen/module_signature.proto b/xls/codegen/module_signature.proto
index e4aee0a4f8..fbb0b4c3fe 100644
--- a/xls/codegen/module_signature.proto
+++ b/xls/codegen/module_signature.proto
@@ -42,6 +42,7 @@ enum ChannelKindProto {
 enum ChannelFlowControlProto {
   CHANNEL_FLOW_CONTROL_NONE = 0;
   CHANNEL_FLOW_CONTROL_READY_VALID = 1;
+  CHANNEL_FLOW_CONTROL_VALID_DATA = 2;
 }
 
 // Data structure describing a port on a module.
diff --git a/xls/codegen/passes_ng/stage_to_block_conversion.cc b/xls/codegen/passes_ng/stage_to_block_conversion.cc
index 131741bbe1..9fe6b22ce5 100644
--- a/xls/codegen/passes_ng/stage_to_block_conversion.cc
+++ b/xls/codegen/passes_ng/stage_to_block_conversion.cc
@@ -73,6 +73,8 @@ RDVNodeGroupName CreatePortNamesForChannel(
           ? options.streaming_channel_data_suffix()
           : "";
   std::string_view valid_suffix = options.streaming_channel_valid_suffix();
+  CHECK_NE(chan_interface->flow_control(), FlowControl::kValidData)
+      << "valid_data flow control not supported in codegen v2.0";
 
   // Construct names for the ports.
   std::string port_ready_name =
@@ -530,6 +532,13 @@ class TopProcToBlockCloner : public ProcToBlockClonerBase {
   absl::Status CreateWiresForChannel(const Channel* chan, Block* block) {
     Proc* proc = proc_metadata_.proc();
 
+    if (chan->kind() == ChannelKind::kStreaming &&
+        absl::down_cast<const StreamingChannel*>(chan)->flow_control() ==
+            FlowControl::kValidData) {
+      return absl::UnimplementedError(
+          "valid_data flow control not supported in codegen v2.0");
+    }
+
     // Retrieve the suffix for the ports associated with the channel.
     std::string_view data_suffix =
         (chan->kind() == ChannelKind::kStreaming)
diff --git a/xls/codegen/signature_generator.cc b/xls/codegen/signature_generator.cc
index b669894f40..3c1586b7a5 100644
--- a/xls/codegen/signature_generator.cc
+++ b/xls/codegen/signature_generator.cc
@@ -26,6 +26,7 @@
 #include "absl/container/flat_hash_map.h"
 #include "absl/log/check.h"
 #include "absl/log/log.h"
+#include "absl/status/status.h"
 #include "absl/status/statusor.h"
 #include "absl/strings/str_format.h"
 #include "xls/codegen/codegen_options.h"
@@ -150,6 +151,11 @@ absl::StatusOr<ModuleSignature> GenerateSignature(
             : CHANNEL_DIRECTION_RECEIVE;
     switch (metadata.channel_kind) {
       case ChannelKind::kStreaming: {
+        if (metadata.ready_port.has_value() !=
+            metadata.valid_port.has_value()) {
+          return absl::UnimplementedError(
+              "Codegen v1.0 only supports ready_valid or no flow control.");
+        }
         FlowControl flow_control =
             (metadata.ready_port.has_value() && metadata.valid_port.has_value())
                 ? FlowControl::kReadyValid
diff --git a/xls/codegen_v_1_5/BUILD b/xls/codegen_v_1_5/BUILD
index 548fa4aabb..a81a3be1f3 100644
--- a/xls/codegen_v_1_5/BUILD
+++ b/xls/codegen_v_1_5/BUILD
@@ -1060,6 +1060,7 @@ cc_library(
         "@com_google_absl//absl/base",
         "@com_google_absl//absl/log",
         "@com_google_absl//absl/log:check",
+        "@com_google_absl//absl/status",
         "@com_google_absl//absl/status:statusor",
         "@com_google_absl//absl/strings:str_format",
     ],
diff --git a/xls/codegen_v_1_5/channel_to_port_io_lowering_pass.cc b/xls/codegen_v_1_5/channel_to_port_io_lowering_pass.cc
index 6c73fe86f5..4108e35ddb 100644
--- a/xls/codegen_v_1_5/channel_to_port_io_lowering_pass.cc
+++ b/xls/codegen_v_1_5/channel_to_port_io_lowering_pass.cc
@@ -352,6 +352,10 @@ absl::StatusOr<Connector> AddPortsForSend(
   std::optional<Node*> valid;
   std::optional<Node*> ready;
   if (ChannelRefKind(channel) == ChannelKind::kStreaming) {
+    if (ChannelRefFlowControl(channel) == FlowControl::kValidData) {
+      return absl::UnimplementedError(
+          "valid_data flow control is not yet supported.");
+    }
     XLS_ASSIGN_OR_RETURN(
         Node * placeholder_valid,
         block->MakeNode<xls::Literal>(SourceInfo(), Value(UBits(1, 1))));
@@ -416,6 +420,10 @@ absl::StatusOr<Connector> AddPortsForReceive(
   std::optional<Node*> valid;
   std::optional<Node*> ready;
   if (ChannelRefKind(channel) == ChannelKind::kStreaming) {
+    if (ChannelRefFlowControl(channel) == FlowControl::kValidData) {
+      return absl::UnimplementedError(
+          "valid_data flow control is not yet supported.");
+    }
     XLS_ASSIGN_OR_RETURN(
         valid,
         block->AddInputPort(
@@ -1076,6 +1084,10 @@ absl::Status AddOneShotLogic(Connector& connector, ScheduledBlock* block,
                              const BlockConversionPassOptions& options,
                              std::string_view channel_name = "") {
   XLS_RET_CHECK_EQ(connector.direction, ChannelDirection::kSend);
+  if (connector.valid.has_value() && !connector.ready.has_value()) {
+    return absl::UnimplementedError(
+        "valid_data flow control is not yet supported.");
+  }
   XLS_RET_CHECK(connector.ready.has_value());
   XLS_RET_CHECK(connector.valid.has_value());
 
@@ -1505,6 +1517,10 @@ absl::Status AddIOFlopsForReceive(Connector& connector, FlopKind flop_kind,
     return absl::OkStatus();
   }
   CHECK_EQ(ChannelRefKind(channel), ChannelKind::kStreaming);
+  if (ChannelRefFlowControl(channel) == FlowControl::kValidData) {
+    return absl::UnimplementedError(
+        "Valid-data flow control is not yet supported.");
+  }
 
   if (flop_kind == FlopKind::kNone) {
     return absl::OkStatus();
@@ -1558,6 +1574,10 @@ absl::Status AddIOFlopsForSend(Connector& connector, FlopKind flop_kind,
         .status();
   }
   CHECK_EQ(ChannelRefKind(channel), ChannelKind::kStreaming);
+  if (ChannelRefFlowControl(channel) == FlowControl::kValidData) {
+    return absl::UnimplementedError(
+        "Valid-data flow control is not yet supported.");
+  }
   CHECK(connector.valid.has_value());
   CHECK(connector.ready.has_value());
 
diff --git a/xls/codegen_v_1_5/global_channel_block_stitching_pass.cc b/xls/codegen_v_1_5/global_channel_block_stitching_pass.cc
index 19efd1efd7..39a10c6c10 100644
--- a/xls/codegen_v_1_5/global_channel_block_stitching_pass.cc
+++ b/xls/codegen_v_1_5/global_channel_block_stitching_pass.cc
@@ -146,6 +146,11 @@ absl::Status StitchStreamingOutputToFifo(
                               SourceInfo(), block_valid, fifo,
                               FifoInstantiation::kPushValidPortName)
                           .status());
+  if (!output.ready_port.has_value()) {
+    return absl::UnimplementedError(
+        "Channels with flow control other than ready_valid are not yet "
+        "supported.");
+  }
   XLS_RET_CHECK(output.ready_port.has_value());
   XLS_RETURN_IF_ERROR(caller
                           ->MakeNode<xls::InstantiationInput>(
@@ -168,6 +173,11 @@ absl::Status StitchStreamingInputToFifo(
       xls::Node * fifo_valid,
       caller->MakeNode<xls::InstantiationOutput>(
           SourceInfo(), fifo, FifoInstantiation::kPopValidPortName));
+  if (!input.ready_port.has_value()) {
+    return absl::UnimplementedError(
+        "Channels with flow control other than ready_valid are not yet "
+        "supported.");
+  }
   XLS_RET_CHECK(input.ready_port.has_value());
 
   XLS_ASSIGN_OR_RETURN(
@@ -205,6 +215,11 @@ absl::Status ExposeStreamingOutput(
   VLOG(5) << "Exposing output port valid: " << name_or_none(output.valid_port);
   VLOG(5) << "Exposing output port ready: " << name_or_none(output.ready_port);
 
+  if (!output.ready_port.has_value()) {
+    return absl::UnimplementedError(
+        "Channels with flow control other than ready_valid are not yet "
+        "supported.");
+  }
   XLS_RET_CHECK(output.data_port.has_value());
   XLS_RET_CHECK(output.ready_port.has_value());
   XLS_RET_CHECK(output.valid_port.has_value());
@@ -246,6 +261,11 @@ absl::Status ExposeStreamingOutput(
 absl::Status ExposeStreamingInput(
     StreamingChannel* channel, const ChannelPortMetadata& input, Block* block,
     ::xls::BlockInstantiation* block_instantiation) {
+  if (!input.ready_port.has_value()) {
+    return absl::UnimplementedError(
+        "Channels with flow control other than ready_valid are not yet "
+        "supported.");
+  }
   XLS_RET_CHECK(input.data_port.has_value());
   XLS_RET_CHECK(input.ready_port.has_value());
   XLS_RET_CHECK(input.valid_port.has_value());
diff --git a/xls/codegen_v_1_5/proc_lowering_block_eval_psc_test.cc b/xls/codegen_v_1_5/proc_lowering_block_eval_psc_test.cc
index f39632c42a..244a3cf60e 100644
--- a/xls/codegen_v_1_5/proc_lowering_block_eval_psc_test.cc
+++ b/xls/codegen_v_1_5/proc_lowering_block_eval_psc_test.cc
@@ -798,6 +798,12 @@ absl::StatusOr<BlockEvaluationResults> EvalBlock(
       // Only process streaming channels in this block
       continue;
     }
+    if (!metadata.data_port.has_value() || !metadata.valid_port.has_value() ||
+        !metadata.ready_port.has_value()) {
+      return absl::UnimplementedError(
+          "Channels with flow control other than ready_valid are not yet "
+          "supported.");
+    }
     // If this channel isn't in the input, skip it.
     if (!inputs.contains(channel_name)) {
       continue;
@@ -845,6 +851,12 @@ absl::StatusOr<BlockEvaluationResults> EvalBlock(
       // Only process streaming channels in this block
       continue;
     }
+    if (!metadata.data_port.has_value() || !metadata.valid_port.has_value() ||
+        !metadata.ready_port.has_value()) {
+      return absl::UnimplementedError(
+          "Channels with flow control other than ready_valid are not yet "
+          "supported.");
+    }
     sinks.push_back(ChannelSink(
         *metadata.data_port, *metadata.valid_port, *metadata.ready_port, 0.5,
         block, ChannelSink::BehaviorDuringReset::kIgnoreValid));
@@ -880,6 +892,12 @@ absl::StatusOr<BlockEvaluationResults> EvalBlock(
       // Only process streaming channels in this block
       continue;
     }
+    if (!metadata.data_port.has_value() || !metadata.valid_port.has_value() ||
+        !metadata.ready_port.has_value()) {
+      return absl::UnimplementedError(
+          "Channels with flow control other than ready_valid are not yet "
+          "supported.");
+    }
     if (!actual_outputs.contains(*metadata.data_port)) {
       continue;
     }
diff --git a/xls/codegen_v_1_5/signature_generation_pass.cc b/xls/codegen_v_1_5/signature_generation_pass.cc
index 239b06124f..e9a0d9552a 100644
--- a/xls/codegen_v_1_5/signature_generation_pass.cc
+++ b/xls/codegen_v_1_5/signature_generation_pass.cc
@@ -25,6 +25,7 @@
 #include "absl/base/casts.h"
 #include "absl/log/check.h"
 #include "absl/log/log.h"
+#include "absl/status/status.h"
 #include "absl/status/statusor.h"
 #include "absl/strings/str_format.h"
 #include "xls/codegen/codegen_options.h"
@@ -151,6 +152,12 @@ absl::StatusOr<verilog::ModuleSignature> GenerateSignature(
             : verilog::ChannelDirectionProto::CHANNEL_DIRECTION_RECEIVE;
     switch (metadata.channel_kind) {
       case ChannelKind::kStreaming: {
+        if (metadata.ready_port.has_value() !=
+            metadata.valid_port.has_value()) {
+          return absl::UnimplementedError(
+              "Channels with flow control other than ready_valid or none are "
+              "not yet supported.");
+        }
         FlowControl flow_control =
             (metadata.ready_port.has_value() && metadata.valid_port.has_value())
                 ? FlowControl::kReadyValid
diff --git a/xls/ir/channel.cc b/xls/ir/channel.cc
index 273408980a..9409b24139 100644
--- a/xls/ir/channel.cc
+++ b/xls/ir/channel.cc
@@ -262,6 +262,8 @@ std::string FlowControlToString(FlowControl fc) {
       return "none";
     case FlowControl::kReadyValid:
       return "ready_valid";
+    case FlowControl::kValidData:
+      return "valid_data";
   }
   LOG(FATAL) << "Invalid flow control value: " << static_cast<int64_t>(fc);
 }
@@ -273,8 +275,11 @@ absl::StatusOr<FlowControl> StringToFlowControl(std::string_view str) {
   if (str == "ready_valid") {
     return FlowControl::kReadyValid;
   }
+  if (str == "valid_data") {
+    return FlowControl::kValidData;
+  }
   return absl::InvalidArgumentError(
-      absl::StrFormat("Invalid channel kind '%s'", str));
+      absl::StrFormat("Invalid flow control '%s'", str));
 }
 
 std::ostream& operator<<(std::ostream& os, FlowControl fc) {
diff --git a/xls/ir/channel.h b/xls/ir/channel.h
index c1d2dc25e5..385451bb5e 100644
--- a/xls/ir/channel.h
+++ b/xls/ir/channel.h
@@ -301,6 +301,15 @@ enum class FlowControl : uint8_t {
   // signal is valid. When both ready and valid are asserted a transaction
   // occurs.
   kReadyValid,
+
+  // The channel uses a valid-data handshake. A valid signal indicates that the
+  // data is valid, and the receiver is presumed to always be ready to accept
+  // data. A transaction occurs whenever valid is asserted.
+  //
+  // This is used when the channel doesn't want to support backpressure. Safe
+  // use requires ensuring that the receiver can always accept data when the
+  // sender is sending, and can optionally be checked with an assert.
+  kValidData,
 };
 
 std::string FlowControlToString(FlowControl fc);