diff --git a/Cargo.lock b/Cargo.lock
index 63f7dc5..6cba7df 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -181,6 +181,7 @@ dependencies = [
  "schemars 1.2.0",
  "serde",
  "serde_json",
+ "serde_with",
  "serde_yaml",
  "similar",
  "sysinfo",
diff --git a/Cargo.toml b/Cargo.toml
index 37f8522..2cf6c9a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -156,6 +156,7 @@ opentelemetry-proto.workspace = true
 prost.workspace = true
 similar.workspace = true
 tempfile.workspace = true
+serde_with = { version = "3.16.1", default-features = false, features = ["macros"] }
 
 [dev-dependencies]
 tempfile.workspace = true
diff --git a/src/dev/process.rs b/src/dev/process.rs
index 3b9cd1f..83c5557 100644
--- a/src/dev/process.rs
+++ b/src/dev/process.rs
@@ -24,7 +24,10 @@ use crate::common::{ApxCommand, BunCommand, UvCommand, handle_spawn_error, read_
 use crate::dev::common::CLIENT_HOST;
 use crate::dev::otel::forward_log_to_flux;
 use crate::dotenv::DotenvFile;
-use crate::python_logging::{DevConfig, resolve_log_config};
+use crate::python_logging::{
+    DevConfig, LogConfigResult, default_logging_config, resolve_log_config,
+    write_logging_config_json,
+};
 
 #[derive(Debug, Clone, Copy)]
 enum LogSource {
@@ -794,7 +797,53 @@ impl ProcessManager {
 
         // Resolve uvicorn logging config (inline TOML, external Python file, or default)
         let log_config_result = resolve_log_config(dev_config, app_slug, app_dir).await?;
-        let log_config_str = log_config_result.to_string_path();
+
+        // Validate JSON configs before use (skip validation for Python file configs)
+        let log_config_str = match &log_config_result {
+            LogConfigResult::PythonFile(path) => path.display().to_string(),
+            LogConfigResult::JsonConfig(config_path) => {
+                // Validate the JSON config can be loaded by Python's logging.config.dictConfig
+                let validation_script = format!(
+                    "import json, logging.config; logging.config.dictConfig(json.load(open('{}')))",
+                    config_path.display()
+                );
+
+                let mut validation_cmd = UvCommand::new("python").tokio_command();
+                validation_cmd
+                    .args(["-c", &validation_script])
+                    .current_dir(app_dir)
+                    .stdout(Stdio::null())
+                    .stderr(Stdio::piped());
+
+                let output = validation_cmd
+                    .output()
+                    .await
+                    .map_err(|e| format!("Failed to validate logging config: {e}"))?;
+
+                if output.status.success() {
+                    config_path.display().to_string()
+                } else {
+                    // Validation failed - log error and fall back to default config
+                    let stderr = String::from_utf8_lossy(&output.stderr);
+                    warn!(
+                        "Logging config validation failed, falling back to default:\n{}",
+                        stderr
+                    );
+                    eprintln!(
+                        "⚠️  Custom logging config is invalid, using default config:\n{}",
+                        stderr
+                    );
+
+                    // Generate and write default config
+                    let default_config = default_logging_config(app_slug);
+                    let fallback_path =
+                        write_logging_config_json(&default_config, app_dir)
+                            .await
+                            .map_err(|e| format!("Failed to write fallback logging config: {e}"))?;
+                    fallback_path.display().to_string()
+                }
+            }
+        };
 
         // Run uvicorn via uv to ensure correct Python environment
         let mut cmd = UvCommand::new("uvicorn").tokio_command();
diff --git a/src/python_logging.rs b/src/python_logging.rs
index a1e789a..5915eed 100644
--- a/src/python_logging.rs
+++ b/src/python_logging.rs
@@ -7,6 +7,7 @@
 //! When neither is specified, generates a default logging configuration.
 
 use serde::{Deserialize, Serialize};
+use serde_with::skip_serializing_none;
 use std::collections::HashMap;
 use std::path::{Path, PathBuf};
 
@@ -20,6 +21,7 @@ pub struct DevConfig {
 }
 
 /// Python logging.dictConfig format
+#[skip_serializing_none]
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct LoggingConfig {
     pub version: i32,
@@ -36,6 +38,7 @@ pub struct LoggingConfig {
 }
 
 /// Formatter configuration
+#[skip_serializing_none]
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct FormatterConfig {
     #[serde(default)]
@@ -47,6 +50,7 @@ pub struct FormatterConfig {
 }
 
 /// Handler configuration
+#[skip_serializing_none]
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct HandlerConfig {
     #[serde(rename = "class")]
@@ -64,6 +68,7 @@ pub struct HandlerConfig {
 }
 
 /// Logger configuration
+#[skip_serializing_none]
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct LoggerConfig {
     #[serde(default)]
@@ -75,6 +80,7 @@ pub struct LoggerConfig {
 }
 
 /// Root logger configuration
+#[skip_serializing_none]
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct RootLoggerConfig {
     #[serde(default)]
@@ -789,4 +795,45 @@ console = { class = "logging.StreamHandler", formatter = "custom", stream = "ext
         // User's myapp logger should also be present
         assert!(merged.loggers.contains_key("myapp"));
     }
+
+    /// Test that the default logging config can be loaded by Python's logging.config.dictConfig.
+    /// This ensures the generated JSON is valid and doesn't contain unexpected fields like
+    /// `filename: null` that would cause StreamHandler to fail.
+    #[test]
+    fn test_default_config_python_validation() {
+        use std::io::Write;
+
+        let config = default_logging_config("testapp");
+        let json = serde_json::to_string_pretty(&config).expect("Failed to serialize config");
+
+        // Write config to temp file
+        let mut temp_file = tempfile::NamedTempFile::new().expect("Failed to create temp file");
+        temp_file
+            .write_all(json.as_bytes())
+            .expect("Failed to write config");
+        let config_path = temp_file.path().to_string_lossy().to_string();
+
+        // Validate using Python's logging.config.dictConfig
+        let output = std::process::Command::new("uv")
+            .args([
+                "run",
+                "--no-sync",
+                "python",
+                "-c",
+                &format!(
+                    "import json, logging.config; logging.config.dictConfig(json.load(open('{}')))",
+                    config_path
+                ),
+            ])
+            .output()
+            .expect("Failed to run Python validation");
+
+        if !output.status.success() {
+            let stderr = String::from_utf8_lossy(&output.stderr);
+            panic!(
+                "Default logging config failed Python validation:\n{}\n\nConfig JSON:\n{}",
+                stderr, json
+            );
+        }
+    }
 }