diff --git a/Cargo.lock b/Cargo.lock
index 69451f0..3e25afc 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -11,6 +11,15 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "ansi_term"
+version = "0.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d52a9bb7ec0cf484c551830a7ce27bd20d67eac647e1befb56b0be4ee39a55d2"
+dependencies = [
+ "winapi",
+]
+
 [[package]]
 name = "anyhow"
 version = "1.0.53"
@@ -26,6 +35,29 @@ dependencies = [
  "term",
 ]
 
+[[package]]
+name = "assay"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "238d82aacd5cfde8ccae5c981912be68ec3cfa2d92ff4ce34090be40584c96a6"
+dependencies = [
+ "assay-proc-macro",
+ "pretty_assertions",
+ "rusty-fork",
+ "tempdir",
+ "tokio",
+]
+
+[[package]]
+name = "assay-proc-macro"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0b5fd637c6a75fe224b372556511913f12d6ad481fbfef2fb7ecea2f7cb4965d"
+dependencies = [
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "assert-json-diff"
 version = "2.0.1"
@@ -674,6 +706,7 @@ name = "feroxbuster"
 version = "2.6.0"
 dependencies = [
  "anyhow",
+ "assay",
  "assert_cmd",
  "clap",
  "clap_complete",
@@ -752,6 +785,12 @@ dependencies = [
  "percent-encoding",
 ]
 
+[[package]]
+name = "fuchsia-cprng"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a06f77d526c1a601b7c4cdd98f54b5eaabffc14d5f2f0296febdc7f357c6d3ba"
+
 [[package]]
 name = "futf"
 version = "0.1.5"
@@ -1506,6 +1545,15 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "output_vt100"
+version = "0.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "628223faebab4e3e40667ee0b2336d34a5b960ff60ea743ddfdbcf7770bcfb66"
+dependencies = [
+ "winapi",
+]
+
 [[package]]
 name = "parking"
 version = "2.0.0"
@@ -1581,7 +1629,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "17367f0cc86f2d25802b2c26ee58a7b23faeccf78a396094c13dced0d0182526"
 dependencies = [
  "phf_shared 0.8.0",
- "rand",
+ "rand 0.7.3",
 ]
 
 [[package]]
@@ -1715,6 +1763,18 @@ dependencies = [
  "termtree",
 ]
 
+[[package]]
+name = "pretty_assertions"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "76d5b548b725018ab5496482b45cb8bef21e9fed1858a6d674e3a8a0f0bb5d50"
+dependencies = [
+ "ansi_term",
+ "ctor",
+ "diff",
+ "output_vt100",
+]
+
 [[package]]
 name = "proc-macro-hack"
 version = "0.5.19"
@@ -1730,6 +1790,12 @@ dependencies = [
  "unicode-xid",
 ]
 
+[[package]]
+name = "quick-error"
+version = "1.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a1d01941d82fa2ab50be1e79e6714289dd7cde78eba4c074bc5a4374f650dfe0"
+
 [[package]]
 name = "quote"
 version = "1.0.15"
@@ -1739,6 +1805,19 @@ dependencies = [
  "proc-macro2",
 ]
 
+[[package]]
+name = "rand"
+version = "0.4.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "552840b97013b1a26992c11eac34bdd778e464601a4c2054b5f0bff7c6761293"
+dependencies = [
+ "fuchsia-cprng",
+ "libc",
+ "rand_core 0.3.1",
+ "rdrand",
+ "winapi",
+]
+
 [[package]]
 name = "rand"
 version = "0.7.3"
@@ -1748,7 +1827,7 @@ dependencies = [
  "getrandom 0.1.16",
  "libc",
  "rand_chacha",
- "rand_core",
+ "rand_core 0.5.1",
  "rand_hc",
  "rand_pcg",
 ]
@@ -1760,9 +1839,24 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f4c8ed856279c9737206bf725bf36935d8666ead7aa69b52be55af369d193402"
 dependencies = [
  "ppv-lite86",
- "rand_core",
+ "rand_core 0.5.1",
 ]
 
+[[package]]
+name = "rand_core"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7a6fdeb83b075e8266dcc8762c22776f6877a63111121f5f8c7411e5be7eed4b"
+dependencies = [
+ "rand_core 0.4.2",
+]
+
+[[package]]
+name = "rand_core"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9c33a3c44ca05fa6f1807d8e6743f3824e8509beca625669633be0acbdf509dc"
+
 [[package]]
 name = "rand_core"
 version = "0.5.1"
@@ -1778,7 +1872,7 @@ version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ca3129af7b92a17112d59ad498c6f81eaf463253766b90396d39ea7a39d6613c"
 dependencies = [
- "rand_core",
+ "rand_core 0.5.1",
 ]
 
 [[package]]
@@ -1787,7 +1881,16 @@ version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "16abd0c1b639e9eb4d7c50c0b8100b0d0f849be2349829c740fe8e6eb4816429"
 dependencies = [
- "rand_core",
+ "rand_core 0.5.1",
+]
+
+[[package]]
+name = "rdrand"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "678054eb77286b51581ba43620cc911abf02758c91f93f479767aed0f90458b2"
+dependencies = [
+ "rand_core 0.3.1",
 ]
 
 [[package]]
@@ -1902,6 +2005,18 @@ version = "1.0.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f2cc38e8fa666e2de3c4aba7edeb5ffc5246c1c2ed0e3d17e560aeeba736b23f"
 
+[[package]]
+name = "rusty-fork"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cb3dcc6e454c328bb824492db107ab7c0ae8fcffe4ad210136ef014458c1bc4f"
+dependencies = [
+ "fnv",
+ "quick-error",
+ "tempfile",
+ "wait-timeout",
+]
+
 [[package]]
 name = "ryu"
 version = "1.0.9"
@@ -2176,6 +2291,16 @@ dependencies = [
  "unicode-xid",
 ]
 
+[[package]]
+name = "tempdir"
+version = "0.3.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "15f2b5fb00ccdf689e0149d1b1b3c03fead81c2b37735d812fa8bddbbf41b6d8"
+dependencies = [
+ "rand 0.4.6",
+ "remove_dir_all",
+]
+
 [[package]]
 name = "tempfile"
 version = "3.3.0"
diff --git a/Cargo.toml b/Cargo.toml
index abf7551..3e44961 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -55,6 +55,7 @@ tempfile = "3.3"
 httpmock = "0.6"
 assert_cmd = "2.0"
 predicates = "2.1"
+assay = "0.1.0"
 
 [profile.release]
 lto = true
diff --git a/src/heuristics.rs b/src/heuristics.rs
index e999133..1a5fb25 100644
--- a/src/heuristics.rs
+++ b/src/heuristics.rs
@@ -407,6 +407,7 @@ impl HeuristicTests {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use assay::assay;
 
     #[test]
     /// request a unique string of 32bytes * a value returns correct result
@@ -417,4 +418,51 @@ mod tests {
             assert_eq!(tester.unique_string(i).len(), i * 32);
         }
     }
+
+    #[assay]
+    /// `detect_directory_listing` correctly identifies tomcat/python instances
+    fn detect_directory_listing_finds_tomcat_python() {
+        let html = "<title>directory listing for /</title>";
+        let parsed = Html::parse_document(html);
+        let handles = Handles::for_testing(None, None);
+        let heuristics = HeuristicTests::new(Arc::new(handles.0));
+        let dirlist_type = heuristics.detect_directory_listing(&parsed);
+        assert!(matches!(
+            dirlist_type.unwrap(),
+            DirListingType::TomCatOrPython
+        ));
+    }
+
+    #[assay]
+    /// `detect_directory_listing` correctly identifies apache instances
+    fn detect_directory_listing_finds_apache() {
+        let html = "<title>index of /</title>";
+        let parsed = Html::parse_document(html);
+        let handles = Handles::for_testing(None, None);
+        let heuristics = HeuristicTests::new(Arc::new(handles.0));
+        let dirlist_type = heuristics.detect_directory_listing(&parsed);
+        assert!(matches!(dirlist_type.unwrap(), DirListingType::Apache));
+    }
+
+    #[assay]
+    /// `detect_directory_listing` correctly identifies ASP.NET instances
+    fn detect_directory_listing_finds_asp_dot_net() {
+        let html = "<title>directory listing -- /</title>";
+        let parsed = Html::parse_document(html);
+        let handles = Handles::for_testing(None, None);
+        let heuristics = HeuristicTests::new(Arc::new(handles.0));
+        let dirlist_type = heuristics.detect_directory_listing(&parsed);
+        assert!(matches!(dirlist_type.unwrap(), DirListingType::AspDotNet));
+    }
+
+    #[assay]
+    /// `detect_directory_listing` returns None when heuristic doesn't match
+    fn detect_directory_listing_returns_none_as_default() {
+        let html = "<title>derp listing -- /</title>";
+        let parsed = Html::parse_document(html);
+        let handles = Handles::for_testing(None, None);
+        let heuristics = HeuristicTests::new(Arc::new(handles.0));
+        let dirlist_type = heuristics.detect_directory_listing(&parsed);
+        assert!(dirlist_type.is_none());
+    }
 }
diff --git a/src/response.rs b/src/response.rs
index 27938ac..e4084ef 100644
--- a/src/response.rs
+++ b/src/response.rs
@@ -294,6 +294,11 @@ impl FeroxResponse {
                 // only add extensions to those responses that pass our checks; filtered out
                 // status codes are handled by should_filter, but we need to still check against
                 // the allow list for what we want to keep
+                #[cfg(test)]
+                handles
+                    .send_scan_command(Command::AddDiscoveredExtension(extension.to_owned()))
+                    .unwrap_or_default();
+                #[cfg(not(test))]
                 handles.send_scan_command(Command::AddDiscoveredExtension(extension.to_owned()))?;
             }
         }
@@ -652,6 +657,7 @@ impl<'de> Deserialize<'de> for FeroxResponse {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::config::Configuration;
     use std::default::Default;
 
     #[test]
@@ -723,4 +729,65 @@ mod tests {
         let result = response.reached_max_depth(0, 2, handles);
         assert!(result);
     }
+
+    #[test]
+    /// simple case of a single extension gets parsed correctly and stored on the `FeroxResponse`
+    fn parse_extension_finds_simple_extension() {
+        let config = Configuration {
+            collect_extensions: true,
+            ..Default::default()
+        };
+
+        let (handles, _) = Handles::for_testing(None, Some(Arc::new(config)));
+
+        let url = Url::parse("http://localhost/derp.js").unwrap();
+
+        let mut response = FeroxResponse {
+            url,
+            ..Default::default()
+        };
+
+        response.parse_extension(Arc::new(handles)).unwrap();
+
+        assert_eq!(response.extension, Some(String::from("js")));
+    }
+
+    #[test]
+    /// hidden files shouldn't be parsed as extensions, i.e. `/.bash_history`
+    fn parse_extension_ignores_hidden_files() {
+        let config = Configuration {
+            collect_extensions: true,
+            ..Default::default()
+        };
+
+        let (handles, _) = Handles::for_testing(None, Some(Arc::new(config)));
+
+        let url = Url::parse("http://localhost/.bash_history").unwrap();
+
+        let mut response = FeroxResponse {
+            url,
+            ..Default::default()
+        };
+
+        response.parse_extension(Arc::new(handles)).unwrap();
+
+        assert_eq!(response.extension, None);
+    }
+
+    #[test]
+    /// `parse_extension` should return immediately if `--collect-extensions` isn't used
+    fn parse_extension_early_returns_based_on_config() {
+        let (handles, _) = Handles::for_testing(None, None);
+
+        let url = Url::parse("http://localhost/derp.js").unwrap();
+
+        let mut response = FeroxResponse {
+            url,
+            ..Default::default()
+        };
+
+        response.parse_extension(Arc::new(handles)).unwrap();
+
+        assert_eq!(response.extension, None);
+    }
 }
diff --git a/tests/test_scanner.rs b/tests/test_scanner.rs
index 76a1d3b..12a1ce7 100644
--- a/tests/test_scanner.rs
+++ b/tests/test_scanner.rs
@@ -1,8 +1,12 @@
 mod utils;
+use assay::assay;
 use assert_cmd::prelude::*;
 use httpmock::Method::GET;
 use httpmock::MockServer;
 use predicates::prelude::*;
+use std::env::temp_dir;
+use std::thread::sleep;
+use std::time::Duration;
 use std::{process::Command, time};
 use utils::{setup_tmp_directory, teardown_tmp_directory};
 
@@ -638,3 +642,43 @@ fn rate_limit_enforced_when_specified() {
 
     teardown_tmp_directory(tmp_dir);
 }
+
+#[assay]
+/// ensure that auto-discovered extensions are tracked in statistics and bar lengths are updated
+fn add_discovered_extension_updates_bars_and_stats() {
+    let srv = MockServer::start();
+    let (tmp_dir, file) = setup_tmp_directory(
+        &["LICENSE".to_string(), "stuff.php".to_string()],
+        "wordlist",
+    )
+    .unwrap();
+
+    let mock = srv.mock(|when, then| {
+        when.method(GET).path("/stuff.php");
+        then.status(200).body("cool... coolcoolcool");
+    });
+
+    let file_path = tmp_dir.path().join("debug-file.txt");
+
+    assert!(!file_path.exists());
+
+    Command::cargo_bin("feroxbuster")?
+        .arg("--url")
+        .arg(srv.url("/"))
+        .arg("--wordlist")
+        .arg(file.as_os_str())
+        .arg("--extract-links")
+        .arg("--collect-extensions")
+        .arg("-vvvv")
+        .arg("--debug-log")
+        .arg(file_path.as_os_str())
+        .unwrap()
+        .assert()
+        .success();
+
+    let contents = std::fs::read_to_string(file_path).unwrap();
+    println!("{}", contents);
+    assert!(contents.contains("discovered new extension: php"));
+    assert!(contents.contains("extensions_collected: 1"));
+    assert!(contents.contains("expected_per_scan: 6"));
+}