add option to log payload

apps/framework-cli/src/cli.rs

@@ -589,10 +589,20 @@ pub async fn top_command_handler(
                 )))
             }
         }
-        Commands::Dev { no_infra, mcp } => {
+        Commands::Dev {
+            no_infra,
+            mcp,
+            log_payloads,
+        } => {
             info!("Running dev command");
             info!("Moose Version: {}", CLI_VERSION);
 
+            // Set environment variable for payload logging if flag is enabled
+            if *log_payloads {
+                std::env::set_var("MOOSE_LOG_PAYLOADS", "true");
+                info!("Payload logging enabled");
+            }
+
             let mut project = load_project(commands)?;
             project.set_is_production_env(false);
             let project_arc = Arc::new(project);

apps/framework-cli/src/cli/commands.rs

@@ -116,6 +116,10 @@ pub enum Commands {
         /// Enable or disable the MCP (Model Context Protocol) server
         #[arg(long, default_value = "true")]
         mcp: bool,
+
+        /// Log payloads at ingest API and streaming functions for debugging
+        #[arg(long)]
+        log_payloads: bool,
     },
     /// Start a remote environment for use in cloud deployments
     Prod {

apps/framework-cli/src/cli/local_webserver.rs

@@ -1318,6 +1318,28 @@ async fn handle_json_array_body(
 
     debug!("parsed json array for {}", topic_name);
 
+    // Log payload if enabled (compact JSON on one line)
+    if std::env::var("MOOSE_LOG_PAYLOADS")
+        .map(|v| v == "true")
+        .unwrap_or(false)
+    {
+        match serde_json::from_slice::<serde_json::Value>(&body) {
+            Ok(json_value) => {
+                if let Ok(compact_json) = serde_json::to_string(&json_value) {
+                    info!("[PAYLOAD:INGEST] {}: {}", topic_name, compact_json);
+                }
+            }
+            Err(_) => {
+                // If we can't parse it, log the raw body (shouldn't happen since we already parsed it above)
+                info!(
+                    "[PAYLOAD:INGEST] {}: {}",
+                    topic_name,
+                    String::from_utf8_lossy(&body)
+                );
+            }
+        }
+    }
+
     let mut records = match parsed {
         Err(e) => {
             if let Some(dlq) = dead_letter_queue {

apps/framework-docs/src/pages/moose/local-dev.mdx

@@ -46,6 +46,21 @@ workflows = false        # Controls Temporal startup
 apis = true              # Enables Analytics APIs server
 ```
 
+### Debugging Data Flow
+
+To debug data as it flows through your pipeline, use the `--log-payloads` flag:
+
+```bash
+moose dev --log-payloads
+```
+
+This logs payloads at three key points with searchable prefixes:
+- `PAYLOAD:INGEST` - Data received at ingest API
+- `PAYLOAD:STREAM_IN` - Data before streaming function
+- `PAYLOAD:STREAM_OUT` - Data after streaming function
+
+All logs appear in Moose logs and can be viewed using `moose logs --tail`.
+
 ### Extending Docker Infrastructure
 
 You can extend Moose's Docker Compose configuration with custom services by creating a `docker-compose.dev.override.yaml` file in your project root. This allows you to add additional infrastructure (databases, monitoring tools, etc.) that runs alongside your Moose development environment.

apps/framework-docs/src/pages/moose/moose-cli.mdx

@@ -41,10 +41,11 @@ moose build [--docker] [--amd64] [--arm64]
 ### Dev
 Starts a local development environment with hot reload and automatic infrastructure management.
 ```bash
-moose dev [--mcp] [--docker]
+moose dev [--mcp] [--no-infra] [--log-payloads]
 ```
 - `--mcp`: Enable or disable the MCP (Model Context Protocol) server (default: true). The MCP server provides AI-assisted development tools at `http://localhost:4000/mcp`. See [MCP Server documentation](/moose/mcp-server) for details.
-- `--docker`: Use Docker for infrastructure (default behavior in dev mode)
+- `--no-infra`: Skip starting docker containers for infrastructure
+- `--log-payloads`: Log payloads for debugging data flow
 
 The development server includes:
 - Hot reload for code changes

packages/py-moose-lib/moose_lib/streaming/streaming_function_runner.py

@@ -15,6 +15,7 @@
 
 import argparse
 import dataclasses
+import os
 import traceback
 from datetime import datetime, timezone
 from importlib import import_module
@@ -432,6 +433,10 @@ def process_messages():
                             # Parse the message into the input type
                             input_data = parse_input(streaming_function_input_type, message.value)
 
+                            # Log payload before transformation if enabled
+                            if os.getenv('MOOSE_LOG_PAYLOADS') == 'true':
+                                log(f"[PAYLOAD:STREAM_IN] {json.dumps(input_data, cls=EnhancedJSONEncoder)}")
+
                             # Run the flow
                             all_outputs = []
                             for (streaming_function_callable, dlq) in streaming_function_callables:
@@ -477,6 +482,15 @@ def process_messages():
                                 cli_log(CliLogData(action="Received",
                                                    message=f'{log_prefix} {len(output_data_list)} message(s)'))
 
+                            # Log payload after transformation if enabled (what we're actually sending to Kafka)
+                            if os.getenv('MOOSE_LOG_PAYLOADS') == 'true':
+                                # Filter out None values to match what actually gets sent
+                                outgoing_data = [item for item in all_outputs if item is not None]
+                                if len(outgoing_data) > 0:
+                                    log(f"[PAYLOAD:STREAM_OUT] {json.dumps(outgoing_data, cls=EnhancedJSONEncoder)}")
+                                else:
+                                    log("[PAYLOAD:STREAM_OUT] (no output from streaming function)")
+
                             if producer is not None:
                                 for item in all_outputs:
                                     # Ignore flow function returning null

packages/ts-moose-lib/src/streaming-functions/runner.ts

@@ -492,6 +492,12 @@ const handleMessage = async (
       payloadBuffer = payloadBuffer.subarray(5);
     }
     const parsedData = JSON.parse(payloadBuffer.toString(), jsonDateReviver);
+
+    // Log payload before transformation if enabled
+    if (process.env.MOOSE_LOG_PAYLOADS === "true") {
+      logger.log(`[PAYLOAD:STREAM_IN] ${JSON.stringify(parsedData)}`);
+    }
+
     const transformedData = await Promise.all(
       streamingFunctionWithConfigList.map(async ([fn, config]) => {
         try {
@@ -545,7 +551,7 @@ const handleMessage = async (
       }),
     );
 
-    return transformedData
+    const processedMessages = transformedData
       .map((userFunctionOutput, i) => {
         const [_, config] = streamingFunctionWithConfigList[i];
         if (userFunctionOutput) {
@@ -578,6 +584,19 @@ const handleMessage = async (
       })
       .flat()
       .filter((item) => item !== undefined && item !== null);
+
+    // Log payload after transformation if enabled (what we're actually sending to Kafka)
+    if (process.env.MOOSE_LOG_PAYLOADS === "true") {
+      if (processedMessages.length > 0) {
+        // msg.value is already JSON stringified, just construct array format
+        const outgoingJsonStrings = processedMessages.map((msg) => msg.value);
+        logger.log(`[PAYLOAD:STREAM_OUT] [${outgoingJsonStrings.join(",")}]`);
+      } else {
+        logger.log(`[PAYLOAD:STREAM_OUT] (no output from streaming function)`);
+      }
+    }
+
+    return processedMessages;
   } catch (e) {
     // TODO: Track failure rate
     logger.error(`Failed to transform data`);