diff --git a/sql/connect/common/src/main/protobuf/spark/connect/base.proto b/sql/connect/common/src/main/protobuf/spark/connect/base.proto
index 7f317defd47b5..0a2bb4c3eef27 100644
--- a/sql/connect/common/src/main/protobuf/spark/connect/base.proto
+++ b/sql/connect/common/src/main/protobuf/spark/connect/base.proto
@@ -43,8 +43,6 @@ message Plan {
   }
 }
 
-
-
 // User Context is used to refer to one particular user session that is executing
 // queries in the backend.
 message UserContext {
@@ -406,6 +404,10 @@ message ExecutePlanResponse {
     // Pipeline command response
     PipelineCommandResult pipeline_command_result = 22;
 
+    // A signal from the server to the client to execute the query function for a flow, and to
+    // register its result with the server.
+    PipelineQueryFunctionExecutionSignal pipeline_query_function_execution_signal = 23;
+
     // Support arbitrary result objects.
     google.protobuf.Any extension = 999;
   }
diff --git a/sql/connect/common/src/main/protobuf/spark/connect/pipelines.proto b/sql/connect/common/src/main/protobuf/spark/connect/pipelines.proto
index c5a6312645902..1159b6a2e4991 100644
--- a/sql/connect/common/src/main/protobuf/spark/connect/pipelines.proto
+++ b/sql/connect/common/src/main/protobuf/spark/connect/pipelines.proto
@@ -90,6 +90,24 @@ message PipelineCommand {
     optional string format = 8;
   }
 
+  // Metadata about why a query function failed to be executed successfully.
+  message QueryFunctionFailure {
+    // Identifier for a dataset within the graph that the query function needed to know the schema
+    // of but which had not yet been analyzed itself.
+    optional string missing_dependency = 1;
+  }
+
+  // The result of executing a user-defined query function.
+  message QueryFunctionResult {
+    oneof flow_function_evaluation_result {
+      // If the query function executed successfully, the unresolved logical plan produced by it.
+      spark.connect.Relation plan = 1;
+
+      // If the query function failed, metadata about the failure.
+      QueryFunctionFailure failure = 2;
+    }
+  }
+
   // Request to define a flow targeting a dataset.
   message DefineFlow {
     // The graph to attach this flow to.
@@ -101,14 +119,18 @@ message PipelineCommand {
     // Name of the dataset this flow writes to. Can be partially or fully qualified.
     optional string target_dataset_name = 3;
 
-    // An unresolved relation that defines the dataset's flow.
-    optional spark.connect.Relation relation = 4;
+    // The result of executing the flow's query function
+    optional QueryFunctionResult result = 4;
 
     // SQL configurations set when running this flow.
     map<string, string> sql_conf = 5;
 
     // If true, this flow will only be run once per full refresh.
     optional bool once = 6;
+
+    // Identifier for the client making the request. The server uses this to determine what flow
+    // evaluation request stream to dispatch evaluation requests to for this flow.
+    optional string client_id = 7;
   }
 
   // Resolves all datasets and flows and start a pipeline update. Should be called after all
@@ -129,8 +151,30 @@ message PipelineCommand {
     // The contents of the SQL file.
     optional string sql_text = 3;
   }
-}
 
+  // Request to get the stream of query function execution signals for a graph. Responses should
+  // be a stream of PipelineQueryFunctionExecutionSignal messages.
+  message GetQueryFunctionExecutionSignalStream {
+    // The graph to get the query function execution signal stream for.
+    optional string dataflow_graph_id = 1;
+
+    // Identifier for the client that is requesting the stream.
+    optional string client_id = 2;
+  }
+
+  // Request from the client to update the flow function evaluation result
+  // for a previously unanalyzed flow.
+  message DefineFlowQueryFunctionResult {
+    // The fully qualified name of the flow being updated.
+    optional string flow_name = 1;
+
+    // The ID of the graph this flow belongs to.
+    optional string dataflow_graph_id = 2;
+
+    // The result of executing the flow's query function
+    optional QueryFunctionResult result = 3;
+  }
+}
 
 // Dispatch object for pipelines command results.
 message PipelineCommandResult {
@@ -166,3 +210,9 @@ message PipelineEvent {
   // The message that should be displayed to users.
   optional string message = 2;
 }
+
+// A signal from the server to the client to execute the query function for one or more flows, and
+// to register their results with the server.
+message PipelineQueryFunctionExecutionSignal {
+  repeated string flow_names = 1;
+}