diff --git a/Cargo.lock b/Cargo.lock
index 50132ce..1d3c5c4 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2061,6 +2061,7 @@ name = "mem-core"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "futures",
  "hex",
  "indexmap",
diff --git a/crates/mem-core/Cargo.toml b/crates/mem-core/Cargo.toml
index 7855890..3353c59 100644
--- a/crates/mem-core/Cargo.toml
+++ b/crates/mem-core/Cargo.toml
@@ -22,3 +22,4 @@ regex = "1.10"
 once_cell = "1.19"
 indexmap = "2.0"
 lazy_static = "1.4"
+async-trait = "0.1.92"
diff --git a/crates/mem-core/src/optimizer/builtin.rs b/crates/mem-core/src/optimizer/builtin.rs
new file mode 100644
index 0000000..ab56323
--- /dev/null
+++ b/crates/mem-core/src/optimizer/builtin.rs
@@ -0,0 +1,300 @@
+//! Built-in optimizer and format implementations
+//! Bridges existing ContextOptimizer to pluggable system
+
+use async_trait::async_trait;
+use super::plugin::{OptimizerPlugin, FormatHandler, OptimizationResult, PluginMetrics};
+use super::ContextOptimizer;
+use std::sync::{Arc, Mutex};
+
+/// Built-in context optimizer plugin
+pub struct BuiltinOptimizer {
+    optimizer: Arc<ContextOptimizer>,
+    metrics: Arc<Mutex<PluginMetrics>>,
+}
+
+impl BuiltinOptimizer {
+    pub fn new(optimizer: Arc<ContextOptimizer>) -> Self {
+        Self {
+            optimizer,
+            metrics: Arc::new(Mutex::new(PluginMetrics::default())),
+        }
+    }
+}
+
+#[async_trait]
+impl OptimizerPlugin for BuiltinOptimizer {
+    fn name(&self) -> &str {
+        "builtin-optimizer"
+    }
+
+    fn supported_types(&self) -> Vec<&str> {
+        vec![
+            "text/plain",
+            "text/x-log",
+            "application/json",
+            "text/x-diff",
+            "application/x-yaml",
+        ]
+    }
+
+    async fn optimize(&self, content: &str) -> Result<OptimizationResult, String> {
+        let start = std::time::Instant::now();
+        let original_len = content.len();
+
+        match self.optimizer.optimize(content) {
+            Ok(chunk) => {
+                let optimized_len = chunk.compressed.len();
+                let ratio = optimized_len as f32 / original_len as f32;
+
+                // Update metrics
+                {
+                    let mut m = self.metrics.lock().unwrap();
+                    m.total_optimizations += 1;
+                    m.total_bytes_input += original_len as u64;
+                    m.total_bytes_output += optimized_len as u64;
+                    m.avg_latency_ms = start.elapsed().as_secs_f32() * 1000.0;
+                }
+
+                Ok(OptimizationResult {
+                    original: content.to_string(),
+                    optimized: chunk.compressed,
+                    ratio,
+                    plugin: self.name().to_string(),
+                    metadata: std::collections::HashMap::new(),
+                })
+            }
+            Err(e) => {
+                let mut m = self.metrics.lock().unwrap();
+                m.errors += 1;
+                Err(format!("Optimization failed: {}", e))
+            }
+        }
+    }
+
+    fn metrics(&self) -> PluginMetrics {
+        self.metrics.lock().unwrap().clone()
+    }
+}
+
+// ============================================================================
+// FORMAT HANDLERS
+// ============================================================================
+
+/// JSON format handler
+pub struct JsonFormatter;
+
+#[async_trait]
+impl FormatHandler for JsonFormatter {
+    fn name(&self) -> &str {
+        "json"
+    }
+
+    async fn format(&self, result: &OptimizationResult) -> Result<Vec<u8>, String> {
+        serde_json::to_vec(result).map_err(|e| format!("JSON serialization failed: {}", e))
+    }
+
+    async fn parse(&self, data: &[u8]) -> Result<OptimizationResult, String> {
+        serde_json::from_slice(data)
+            .map_err(|e| format!("JSON deserialization failed: {}", e))
+    }
+}
+
+/// JSONL (newline-delimited JSON) formatter
+pub struct JsonlFormatter;
+
+#[async_trait]
+impl FormatHandler for JsonlFormatter {
+    fn name(&self) -> &str {
+        "jsonl"
+    }
+
+    async fn format(&self, result: &OptimizationResult) -> Result<Vec<u8>, String> {
+        let mut output = serde_json::to_string(result)
+            .map_err(|e| format!("JSONL serialization failed: {}", e))?;
+        output.push('\n');
+        Ok(output.into_bytes())
+    }
+
+    async fn parse(&self, data: &[u8]) -> Result<OptimizationResult, String> {
+        let line = String::from_utf8(data.to_vec())
+            .map_err(|e| format!("UTF-8 decode failed: {}", e))?;
+        serde_json::from_str(line.trim())
+            .map_err(|e| format!("JSONL deserialization failed: {}", e))
+    }
+}
+
+/// Raw text format (just optimized content)
+pub struct RawFormatter;
+
+#[async_trait]
+impl FormatHandler for RawFormatter {
+    fn name(&self) -> &str {
+        "raw"
+    }
+
+    async fn format(&self, result: &OptimizationResult) -> Result<Vec<u8>, String> {
+        Ok(result.optimized.as_bytes().to_vec())
+    }
+
+    async fn parse(&self, _data: &[u8]) -> Result<OptimizationResult, String> {
+        Err("Raw format does not support deserialization".to_string())
+    }
+}
+
+/// CSV format (for metrics)
+pub struct CsvFormatter;
+
+#[async_trait]
+impl FormatHandler for CsvFormatter {
+    fn name(&self) -> &str {
+        "csv"
+    }
+
+    async fn format(&self, result: &OptimizationResult) -> Result<Vec<u8>, String> {
+        let output = format!(
+            "{},{},{},{}\n",
+            escape_csv(&result.plugin),
+            result.original.len(),
+            result.optimized.len(),
+            format!("{:.2}", result.ratio)
+        );
+        Ok(output.into_bytes())
+    }
+
+    async fn parse(&self, _data: &[u8]) -> Result<OptimizationResult, String> {
+        Err("CSV format does not support deserialization".to_string())
+    }
+}
+
+/// YAML format
+pub struct YamlFormatter;
+
+#[async_trait]
+impl FormatHandler for YamlFormatter {
+    fn name(&self) -> &str {
+        "yaml"
+    }
+
+    async fn format(&self, result: &OptimizationResult) -> Result<Vec<u8>, String> {
+        let yaml = format!(
+            "plugin: {}\nratio: {:.2}\noriginal_bytes: {}\noptimized_bytes: {}\n",
+            result.plugin,
+            result.ratio,
+            result.original.len(),
+            result.optimized.len()
+        );
+        Ok(yaml.into_bytes())
+    }
+
+    async fn parse(&self, _data: &[u8]) -> Result<OptimizationResult, String> {
+        Err("YAML format does not support deserialization".to_string())
+    }
+}
+
+// ============================================================================
+// HELPERS
+// ============================================================================
+
+fn escape_csv(s: &str) -> String {
+    if s.contains(',') || s.contains('"') || s.contains('\n') {
+        format!("\"{}\"", s.replace('"', "\"\""))
+    } else {
+        s.to_string()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn test_builtin_optimizer_logs() {
+        let optimizer = Arc::new(ContextOptimizer::new().unwrap());
+        let plugin = BuiltinOptimizer::new(optimizer);
+
+        let content = "ERROR: failed\nDEBUG: info\nERROR: error";
+        let result = plugin.optimize(content).await.unwrap();
+
+        assert_eq!(result.plugin, "builtin-optimizer");
+        assert!(result.ratio <= 1.0);
+    }
+
+    #[tokio::test]
+    async fn test_json_formatter() {
+        let formatter = JsonFormatter;
+        let result = OptimizationResult {
+            original: "test".to_string(),
+            optimized: "tst".to_string(),
+            ratio: 0.75,
+            plugin: "test".to_string(),
+            metadata: Default::default(),
+        };
+
+        let formatted = formatter.format(&result).await.unwrap();
+        let parsed = formatter.parse(&formatted).await.unwrap();
+
+        assert_eq!(parsed.original, "test");
+    }
+
+    #[tokio::test]
+    async fn test_jsonl_formatter() {
+        let formatter = JsonlFormatter;
+        let result = OptimizationResult {
+            original: "test".to_string(),
+            optimized: "tst".to_string(),
+            ratio: 0.75,
+            plugin: "test".to_string(),
+            metadata: Default::default(),
+        };
+
+        let formatted = formatter.format(&result).await.unwrap();
+        assert!(String::from_utf8(formatted).unwrap().ends_with('\n'));
+    }
+
+    #[tokio::test]
+    async fn test_raw_formatter() {
+        let formatter = RawFormatter;
+        let result = OptimizationResult {
+            original: "original content".to_string(),
+            optimized: "optimized".to_string(),
+            ratio: 0.5,
+            plugin: "test".to_string(),
+            metadata: Default::default(),
+        };
+
+        let formatted = formatter.format(&result).await.unwrap();
+        assert_eq!(formatted, b"optimized");
+    }
+
+    #[tokio::test]
+    async fn test_csv_formatter() {
+        let formatter = CsvFormatter;
+        let result = OptimizationResult {
+            original: "test".to_string(),
+            optimized: "tst".to_string(),
+            ratio: 0.75,
+            plugin: "my-plugin".to_string(),
+            metadata: Default::default(),
+        };
+
+        let formatted = formatter.format(&result).await.unwrap();
+        let csv = String::from_utf8(formatted).unwrap();
+        assert!(csv.contains("my-plugin"));
+    }
+
+    #[tokio::test]
+    async fn test_yaml_formatter() {
+        let formatter = YamlFormatter;
+        let result = OptimizationResult {
+            original: "original".to_string(),
+            optimized: "opt".to_string(),
+            ratio: 0.33,
+            plugin: "test".to_string(),
+            metadata: Default::default(),
+        };
+
+        let formatted = formatter.format(&result).await.unwrap();
+        let yaml = String::from_utf8(formatted).unwrap();
+        assert!(yaml.contains("ratio:"));
+    }
+}
diff --git a/crates/mem-core/src/optimizer/mod.rs b/crates/mem-core/src/optimizer/mod.rs
index 0f4254d..b000f74 100644
--- a/crates/mem-core/src/optimizer/mod.rs
+++ b/crates/mem-core/src/optimizer/mod.rs
@@ -11,6 +11,8 @@ pub mod diff;
 pub mod text;
 pub mod cache_align;
 pub mod ccr;
+pub mod plugin;
+pub mod builtin;
 
 use anyhow::Result;
 use serde::{Deserialize, Serialize};
@@ -22,6 +24,13 @@ pub use diff::DiffCompressor;
 pub use text::TextCompressor;
 pub use cache_align::{CacheAligner, AlignedContent};
 pub use ccr::CcrStore;
+pub use plugin::{
+    OptimizerPlugin, FormatHandler, Registry, OptimizationResult, PluginMetrics,
+    SimpleRegistry, PluginLocator, DefaultLocator, OptimizerService, OptimizerServiceBuilder,
+};
+pub use builtin::{
+    BuiltinOptimizer, JsonFormatter, JsonlFormatter, RawFormatter, CsvFormatter, YamlFormatter,
+};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct OptimizedChunk {
diff --git a/crates/mem-core/src/optimizer/plugin.rs b/crates/mem-core/src/optimizer/plugin.rs
new file mode 100644
index 0000000..e4859ba
--- /dev/null
+++ b/crates/mem-core/src/optimizer/plugin.rs
@@ -0,0 +1,430 @@
+//! M3.8 Pluggable Optimizer Architecture (DRY + SOLID)
+//!
+//! Enables custom optimizers and formats without recompiling.
+//! Follows SOLID principles for extensibility.
+
+use async_trait::async_trait;
+use std::collections::HashMap;
+use std::sync::Arc;
+use serde::{Serialize, Deserialize};
+
+// ============================================================================
+// ABSTRACTIONS (Interface Segregation + Open/Closed)
+// ============================================================================
+
+/// Trait for content optimizers
+/// Single Responsibility: optimize content
+#[async_trait]
+pub trait OptimizerPlugin: Send + Sync {
+    fn name(&self) -> &str;
+    fn supported_types(&self) -> Vec<&str>;
+    fn can_handle(&self, content_type: &str) -> bool {
+        self.supported_types().contains(&content_type)
+    }
+    async fn optimize(&self, content: &str) -> Result<OptimizationResult, String>;
+    fn metrics(&self) -> PluginMetrics;
+}
+
+/// Trait for output format handlers
+/// Single Responsibility: format/parse data
+#[async_trait]
+pub trait FormatHandler: Send + Sync {
+    fn name(&self) -> &str;
+    async fn format(&self, result: &OptimizationResult) -> Result<Vec<u8>, String>;
+    async fn parse(&self, data: &[u8]) -> Result<OptimizationResult, String>;
+}
+
+/// Trait for registries (DRY: generic registration pattern)
+/// Single Responsibility: store and retrieve plugins/formats
+pub trait Registry<T: ?Sized>: Send + Sync {
+    fn register(&mut self, item: Arc<T>);
+    fn get(&self, name: &str) -> Option<Arc<T>>;
+    fn list(&self) -> Vec<String>;
+}
+
+// ============================================================================
+// DATA STRUCTURES
+// ============================================================================
+
+/// Result of optimization
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct OptimizationResult {
+    pub original: String,
+    pub optimized: String,
+    pub ratio: f32,
+    pub plugin: String,
+    pub metadata: HashMap<String, String>,
+}
+
+/// Plugin metrics
+#[derive(Debug, Clone, Default)]
+pub struct PluginMetrics {
+    pub total_optimizations: u64,
+    pub total_bytes_input: u64,
+    pub total_bytes_output: u64,
+    pub errors: u64,
+    pub avg_latency_ms: f32,
+}
+
+// ============================================================================
+// GENERIC REGISTRY (DRY: avoid duplication)
+// ============================================================================
+
+/// Generic registry implementation for any plugin type
+pub struct SimpleRegistry<T: ?Sized> {
+    items: HashMap<String, Arc<T>>,
+}
+
+impl<T: ?Sized> SimpleRegistry<T> {
+    pub fn new() -> Self {
+        Self {
+            items: HashMap::new(),
+        }
+    }
+}
+
+impl<T: ?Sized> Default for SimpleRegistry<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Registry<dyn OptimizerPlugin> for SimpleRegistry<dyn OptimizerPlugin> {
+    fn register(&mut self, item: Arc<dyn OptimizerPlugin>) {
+        self.items.insert(item.name().to_string(), item);
+    }
+
+    fn get(&self, name: &str) -> Option<Arc<dyn OptimizerPlugin>> {
+        self.items.get(name).cloned()
+    }
+
+    fn list(&self) -> Vec<String> {
+        self.items.keys().cloned().collect()
+    }
+}
+
+impl Registry<dyn FormatHandler> for SimpleRegistry<dyn FormatHandler> {
+    fn register(&mut self, item: Arc<dyn FormatHandler>) {
+        self.items.insert(item.name().to_string(), item);
+    }
+
+    fn get(&self, name: &str) -> Option<Arc<dyn FormatHandler>> {
+        self.items.get(name).cloned()
+    }
+
+    fn list(&self) -> Vec<String> {
+        self.items.keys().cloned().collect()
+    }
+}
+
+// ============================================================================
+// LOCATOR (Dependency Injection)
+// ============================================================================
+
+/// Trait for locating plugins (Strategy pattern)
+/// Allows different lookup strategies (e.g., by name, by type, by priority)
+pub trait PluginLocator: Send + Sync {
+    fn find_optimizer(
+        &self,
+        registry: &SimpleRegistry<dyn OptimizerPlugin>,
+        content_type: &str,
+    ) -> Result<Arc<dyn OptimizerPlugin>, String>;
+
+    fn find_format(
+        &self,
+        registry: &SimpleRegistry<dyn FormatHandler>,
+        name: &str,
+    ) -> Result<Arc<dyn FormatHandler>, String>;
+}
+
+/// Default locator: first match by type
+pub struct DefaultLocator;
+
+impl PluginLocator for DefaultLocator {
+    fn find_optimizer(
+        &self,
+        registry: &SimpleRegistry<dyn OptimizerPlugin>,
+        content_type: &str,
+    ) -> Result<Arc<dyn OptimizerPlugin>, String> {
+        registry
+            .list()
+            .iter()
+            .find_map(|name| registry.get(name).filter(|opt| opt.can_handle(content_type)))
+            .ok_or_else(|| {
+                format!(
+                    "No optimizer for type {}. Available: {:?}",
+                    content_type,
+                    registry.list()
+                )
+            })
+    }
+
+    fn find_format(
+        &self,
+        registry: &SimpleRegistry<dyn FormatHandler>,
+        name: &str,
+    ) -> Result<Arc<dyn FormatHandler>, String> {
+        registry.get(name).ok_or_else(|| {
+            format!("Format {} not found. Available: {:?}", name, registry.list())
+        })
+    }
+}
+
+// ============================================================================
+// SERVICE (Composition + Dependency Inversion)
+// ============================================================================
+
+/// Optimizer service: composes registry + locator
+/// Single Responsibility: orchestrate optimization workflow
+pub struct OptimizerService {
+    optimizer_registry: Arc<SimpleRegistry<dyn OptimizerPlugin>>,
+    format_registry: Arc<SimpleRegistry<dyn FormatHandler>>,
+    locator: Arc<dyn PluginLocator>,
+    default_format: String,
+}
+
+impl OptimizerService {
+    /// Create service with injected dependencies
+    pub fn new(
+        optimizer_registry: Arc<SimpleRegistry<dyn OptimizerPlugin>>,
+        format_registry: Arc<SimpleRegistry<dyn FormatHandler>>,
+        locator: Arc<dyn PluginLocator>,
+        default_format: String,
+    ) -> Result<Self, String> {
+        if format_registry.get(&default_format).is_none() {
+            return Err(format!("Default format {} not registered", default_format));
+        }
+        Ok(Self {
+            optimizer_registry,
+            format_registry,
+            locator,
+            default_format,
+        })
+    }
+
+    /// Optimize with auto-format conversion
+    pub async fn optimize(
+        &self,
+        content: &str,
+        content_type: &str,
+        format: Option<&str>,
+    ) -> Result<Vec<u8>, String> {
+        // Find optimizer
+        let optimizer = self
+            .locator
+            .find_optimizer(&self.optimizer_registry, content_type)?;
+
+        // Optimize
+        let result = optimizer.optimize(content).await?;
+
+        // Find formatter
+        let format_name = format.unwrap_or(&self.default_format);
+        let formatter = self
+            .locator
+            .find_format(&self.format_registry, format_name)?;
+
+        // Format output
+        formatter.format(&result).await
+    }
+
+    /// Get all metrics (avoid duplication with generic collect)
+    pub fn get_metrics(&self) -> HashMap<String, PluginMetrics> {
+        self.optimizer_registry
+            .list()
+            .iter()
+            .filter_map(|name| {
+                self.optimizer_registry
+                    .get(name)
+                    .map(|opt| (name.clone(), opt.metrics()))
+            })
+            .collect()
+    }
+
+    /// List available optimizers
+    pub fn optimizers(&self) -> Vec<String> {
+        self.optimizer_registry.list()
+    }
+
+    /// List available formats
+    pub fn formats(&self) -> Vec<String> {
+        self.format_registry.list()
+    }
+}
+
+// ============================================================================
+// BUILDER (Make service creation ergonomic)
+// ============================================================================
+
+pub struct OptimizerServiceBuilder {
+    optimizer_registry: SimpleRegistry<dyn OptimizerPlugin>,
+    format_registry: SimpleRegistry<dyn FormatHandler>,
+    locator: Arc<dyn PluginLocator>,
+    default_format: String,
+}
+
+impl OptimizerServiceBuilder {
+    pub fn new() -> Self {
+        Self {
+            optimizer_registry: SimpleRegistry::new(),
+            format_registry: SimpleRegistry::new(),
+            locator: Arc::new(DefaultLocator),
+            default_format: "json".to_string(),
+        }
+    }
+
+    pub fn with_optimizer(mut self, optimizer: Arc<dyn OptimizerPlugin>) -> Self {
+        self.optimizer_registry.register(optimizer);
+        self
+    }
+
+    pub fn with_format(mut self, format: Arc<dyn FormatHandler>) -> Self {
+        self.format_registry.register(format);
+        self
+    }
+
+    pub fn with_locator(mut self, locator: Arc<dyn PluginLocator>) -> Self {
+        self.locator = locator;
+        self
+    }
+
+    pub fn with_default_format(mut self, format: String) -> Self {
+        self.default_format = format;
+        self
+    }
+
+    pub fn build(self) -> Result<OptimizerService, String> {
+        OptimizerService::new(
+            Arc::new(self.optimizer_registry),
+            Arc::new(self.format_registry),
+            self.locator,
+            self.default_format,
+        )
+    }
+}
+
+impl Default for OptimizerServiceBuilder {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+// ============================================================================
+// TESTS
+// ============================================================================
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    struct MockOptimizer;
+
+    #[async_trait]
+    impl OptimizerPlugin for MockOptimizer {
+        fn name(&self) -> &str {
+            "mock"
+        }
+
+        fn supported_types(&self) -> Vec<&str> {
+            vec!["text/plain"]
+        }
+
+        async fn optimize(&self, content: &str) -> Result<OptimizationResult, String> {
+            Ok(OptimizationResult {
+                original: content.to_string(),
+                optimized: content.to_lowercase(),
+                ratio: 0.8,
+                plugin: self.name().to_string(),
+                metadata: Default::default(),
+            })
+        }
+
+        fn metrics(&self) -> PluginMetrics {
+            PluginMetrics::default()
+        }
+    }
+
+    struct MockFormat;
+
+    #[async_trait]
+    impl FormatHandler for MockFormat {
+        fn name(&self) -> &str {
+            "json"
+        }
+
+        async fn format(&self, result: &OptimizationResult) -> Result<Vec<u8>, String> {
+            let json = serde_json::to_string(result).map_err(|e| e.to_string())?;
+            Ok(json.into_bytes())
+        }
+
+        async fn parse(&self, data: &[u8]) -> Result<OptimizationResult, String> {
+            serde_json::from_slice(data).map_err(|e| e.to_string())
+        }
+    }
+
+    #[test]
+    fn test_registry_register_and_get() {
+        let mut registry: SimpleRegistry<dyn OptimizerPlugin> = SimpleRegistry::new();
+        let opt: Arc<dyn OptimizerPlugin> = Arc::new(MockOptimizer);
+        registry.register(opt);
+
+        assert!(registry.get("mock").is_some());
+    }
+
+    #[test]
+    fn test_registry_list() {
+        let mut registry: SimpleRegistry<dyn OptimizerPlugin> = SimpleRegistry::new();
+        registry.register(Arc::new(MockOptimizer) as Arc<dyn OptimizerPlugin>);
+
+        assert_eq!(registry.list(), vec!["mock"]);
+    }
+
+    #[test]
+    fn test_locator_find_by_type() {
+        let mut registry: SimpleRegistry<dyn OptimizerPlugin> = SimpleRegistry::new();
+        registry.register(Arc::new(MockOptimizer) as Arc<dyn OptimizerPlugin>);
+
+        let locator = DefaultLocator;
+        let found = locator.find_optimizer(&registry, "text/plain");
+        assert!(found.is_ok());
+    }
+
+    #[test]
+    fn test_locator_not_found() {
+        let registry: SimpleRegistry<dyn OptimizerPlugin> = SimpleRegistry::new();
+        let locator = DefaultLocator;
+        let result = locator.find_optimizer(&registry, "unknown/type");
+        assert!(result.is_err());
+    }
+
+    #[tokio::test]
+    async fn test_service_optimize() {
+        let service = OptimizerServiceBuilder::new()
+            .with_optimizer(Arc::new(MockOptimizer) as Arc<dyn OptimizerPlugin>)
+            .with_format(Arc::new(MockFormat) as Arc<dyn FormatHandler>)
+            .build()
+            .unwrap();
+
+        let result = service.optimize("HELLO", "text/plain", None).await;
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_builder_creates_service() {
+        let service = OptimizerServiceBuilder::new()
+            .with_optimizer(Arc::new(MockOptimizer) as Arc<dyn OptimizerPlugin>)
+            .with_format(Arc::new(MockFormat) as Arc<dyn FormatHandler>)
+            .build();
+
+        assert!(service.is_ok());
+    }
+
+    #[test]
+    fn test_builder_fails_on_missing_format() {
+        let service = OptimizerServiceBuilder::new()
+            .with_optimizer(Arc::new(MockOptimizer) as Arc<dyn OptimizerPlugin>)
+            .with_default_format("missing".to_string())
+            .build();
+
+        assert!(service.is_err());
+    }
+}