diff --git a/tests/it_context_optimizer.rs b/tests/it_context_optimizer.rs
new file mode 100644
index 0000000..12f2707
--- /dev/null
+++ b/tests/it_context_optimizer.rs
@@ -0,0 +1,212 @@
+//! Integration tests for M3.8 Context Optimizer
+//!
+//! Tests the full 4-stage pipeline: Detection → Compression → CCR → Integration
+
+use mem_core::{ContextOptimizer, ContextOptimizerConfig, ContentType};
+
+#[test]
+fn test_optimizer_detects_json() {
+    let optimizer = ContextOptimizer::new().expect("failed to create optimizer");
+    let json = r#"{"status": "success", "count": 42, "items": [1, 2, 3]}"#;
+
+    let result = optimizer.optimize(json).expect("optimize failed");
+    assert_eq!(result.content_type, ContentType::Json);
+    assert!(result.original_tokens > 0);
+}
+
+#[test]
+fn test_optimizer_detects_log() {
+    let optimizer = ContextOptimizer::new().expect("failed to create optimizer");
+    let log = "ERROR: connection timeout at 2026-08-28T09:15:00Z\n  at main.rs:42";
+
+    let result = optimizer.optimize(log).expect("optimize failed");
+    assert_eq!(result.content_type, ContentType::Log);
+}
+
+#[test]
+fn test_optimizer_detects_code() {
+    let optimizer = ContextOptimizer::new().expect("failed to create optimizer");
+    let code = r#"
+fn main() {
+    println!("hello");
+}
+    "#;
+
+    let result = optimizer.optimize(code).expect("optimize failed");
+    assert_eq!(result.content_type, ContentType::Code);
+}
+
+#[test]
+fn test_optimizer_compresses_npm_logs() {
+    let optimizer = ContextOptimizer::new().expect("failed to create optimizer");
+
+    let npm_log = r#"
+npm WARN deprecated info@0.8.0: package no longer supported
+npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@2.3.2
+npm notice created a lockfile as package-lock.json.
+npm ERR! 404 Not Found - GET https://registry.npmjs.org/react@99.0.0
+npm ERR! 404
+npm ERR!  It is likely you are behind a proxy.
+npm ERR! A complete log of this error:
+npm ERR! /home/user/.npm/_logs/error.log
+    "#;
+
+    let result = optimizer.optimize(npm_log).expect("optimize failed");
+
+    // Should detect as log
+    assert_eq!(result.content_type, ContentType::Log);
+
+    // Should compress
+    assert!(result.compressed_tokens < result.original_tokens);
+
+    // Should keep error markers
+    assert!(result.compressed.to_lowercase().contains("npm err!"));
+    assert!(result.compressed.contains("404"));
+
+    // Should drop noise
+    assert!(!result.compressed.contains("WARN deprecated"));
+    assert!(!result.compressed.contains("notice created"));
+}
+
+#[test]
+fn test_optimizer_handles_json_array() {
+    let optimizer = ContextOptimizer::new().expect("failed to create optimizer");
+
+    let json_array = r#"
+[
+  {"id": 1, "name": "Alice", "status": "active"},
+  {"id": 2, "name": "Bob", "status": "inactive"},
+  {"id": 3, "name": "Charlie", "status": "active"},
+  {"id": 4, "name": "Diana", "status": "active"}
+]
+    "#;
+
+    let result = optimizer.optimize(json_array).expect("optimize failed");
+
+    // Should detect JSON
+    assert_eq!(result.content_type, ContentType::Json);
+    assert!(result.original_tokens > 0);
+}
+
+#[test]
+fn test_optimizer_detects_diff() {
+    let optimizer = ContextOptimizer::new().expect("failed to create optimizer");
+
+    let diff = r#"--- a/config.yaml
++++ b/config.yaml
+@@ -1,3 +1,3 @@
+ server:
+   host: localhost
+-  port: 8080
++  port: 9000
+    "#;
+
+    let result = optimizer.optimize(diff).expect("optimize failed");
+    assert_eq!(result.content_type, ContentType::Diff);
+}
+
+#[test]
+fn test_optimizer_passthrough_when_disabled() {
+    let config = ContextOptimizerConfig {
+        enabled: false,
+        ..Default::default()
+    };
+    let optimizer = ContextOptimizer::with_config(config).expect("failed to create optimizer");
+
+    let content = "ERROR: something failed";
+    let result = optimizer.optimize(content).expect("optimize failed");
+
+    // Should be unchanged
+    assert_eq!(result.compressed, content);
+    assert_eq!(result.original_tokens, result.compressed_tokens);
+}
+
+#[test]
+fn test_optimizer_with_compression_disabled() {
+    let config = ContextOptimizerConfig {
+        enabled: true,
+        compress_logs: false,
+        ..Default::default()
+    };
+    let optimizer = ContextOptimizer::with_config(config).expect("failed to create optimizer");
+
+    let log = "ERROR: connection timeout\nWARN: slow response\nINFO: connected";
+    let result = optimizer.optimize(log).expect("optimize failed");
+
+    // Should be unchanged since compression is disabled
+    assert_eq!(result.compressed, log);
+}
+
+#[test]
+fn test_optimizer_token_counting() {
+    let optimizer = ContextOptimizer::new().expect("failed to create optimizer");
+
+    let short = "hello world";
+    let short_result = optimizer.optimize(short).expect("optimize failed");
+
+    let long = "This is a much longer piece of text with many words to test the token counting. ".repeat(10);
+    let long_result = optimizer.optimize(&long).expect("optimize failed");
+
+    // Longer text should have more tokens
+    assert!(long_result.original_tokens > short_result.original_tokens);
+}
+
+#[test]
+fn test_optimizer_with_cargo_error() {
+    let optimizer = ContextOptimizer::new().expect("failed to create optimizer");
+
+    let cargo_error = r#"
+   Compiling myapp v0.1.0
+error[E0425]: cannot find value `unknown_var` in this scope
+   --> src/main.rs:42:5
+    |
+ 42 |     unknown_var = 42;
+    |     ^^^^^^^^^^^ not found in this scope
+
+error: aborting due to 1 previous error
+    "#;
+
+    let result = optimizer.optimize(cargo_error).expect("optimize failed");
+
+    // Should detect as log
+    assert_eq!(result.content_type, ContentType::Log);
+
+    // Should keep error lines
+    assert!(result.compressed.contains("error[E0425]"));
+    assert!(result.compressed.contains("src/main.rs:42"));
+
+    // Should drop compilation noise
+    assert!(!result.compressed.contains("Compiling myapp"));
+}
+
+#[test]
+fn test_optimizer_real_world_scenario() {
+    let optimizer = ContextOptimizer::new().expect("failed to create optimizer");
+
+    // Simulate a chunk from a real failure log with mixed content
+    let mixed_content = r#"
+2026-08-28T09:15:00Z DEBUG: Starting npm install
+npm WARN deprecated info@0.8.0: package no longer supported
+npm notice created a lockfile
+npm WARN optional SKIPPING OPTIONAL DEPENDENCY
+2026-08-28T09:15:05Z INFO: dependency resolution
+npm ERR! 404 Not Found - GET https://registry.npmjs.org/react@99.0.0
+npm ERR! 404
+npm ERR!  It is likely you are behind a proxy
+npm ERR! A complete log: /home/user/.npm/_logs/error.log
+2026-08-28T09:15:10Z WARN: slow network
+    "#;
+
+    let result = optimizer.optimize(mixed_content).expect("optimize failed");
+
+    // Should detect as log (has timestamps + errors)
+    assert_eq!(result.content_type, ContentType::Log);
+
+    // Should reduce size
+    assert!(result.compressed.len() < mixed_content.len());
+
+    // Should preserve critical information
+    assert!(result.compressed.to_lowercase().contains("npm err!"));
+    assert!(result.compressed.contains("404"));
+    assert!(result.compressed.contains("2026-08-28")); // Timestamps may be kept
+}