diff --git a/Cargo.toml b/Cargo.toml
index aa8e63d..38e73ed 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -58,6 +58,7 @@ tree-sitter-inference = "0.0.38"
 anyhow = "1.0.100"
 thiserror = "2.0.18"
 serde = { version = "1.0.228", features = ["derive", "rc"] }
+cov-mark = "2.2.0"
 leb128 = "0.2.5"
 rustc-hash = "2.1.1"
 inkwell = { version = "0.8.0", features = ["llvm21-1"] }
diff --git a/core/type-checker/Cargo.toml b/core/type-checker/Cargo.toml
index dc41e17..9321f74 100644
--- a/core/type-checker/Cargo.toml
+++ b/core/type-checker/Cargo.toml
@@ -11,3 +11,4 @@ inference-ast.workspace = true
 anyhow.workspace = true
 thiserror.workspace = true
 rustc-hash.workspace = true
+cov-mark.workspace = true
diff --git a/core/type-checker/src/type_checker.rs b/core/type-checker/src/type_checker.rs
index 82de1b2..9f7bee7 100644
--- a/core/type-checker/src/type_checker.rs
+++ b/core/type-checker/src/type_checker.rs
@@ -2092,8 +2092,10 @@ impl TypeChecker {
         };
         if let Some(key) = key {
             if self.reported_error_keys.contains(&key) {
+                cov_mark::hit!(type_checker_error_dedup_skips_duplicate);
                 return;
             }
+            cov_mark::hit!(type_checker_error_dedup_first_occurrence);
             self.reported_error_keys.insert(key);
         }
         self.errors.push(error);
diff --git a/tests/Cargo.toml b/tests/Cargo.toml
index 47e3b0a..02fb9bd 100644
--- a/tests/Cargo.toml
+++ b/tests/Cargo.toml
@@ -10,6 +10,7 @@ repository = { workspace = true }
 anyhow.workspace = true
 serde_json = "1.0.99"
 wasmtime="40.0.0"
+cov-mark.workspace = true
 
 inference-ast.workspace = true
 inference-wasm-codegen.workspace = true
diff --git a/tests/src/type_checker/error_recovery.rs b/tests/src/type_checker/error_recovery.rs
index 512cf47..cb17765 100644
--- a/tests/src/type_checker/error_recovery.rs
+++ b/tests/src/type_checker/error_recovery.rs
@@ -101,6 +101,7 @@ mod error_recovery_tests {
                 return helper(10);
             }
         "#;
+        cov_mark::check!(type_checker_error_dedup_first_occurrence);
         let result = try_type_check(source);
         assert!(
             result.is_err(),
@@ -125,14 +126,12 @@ mod error_recovery_tests {
     #[test]
     fn test_error_deduplication() {
         let source = r#"
-            struct Container {
-                value: UnknownType;
-            }
-            fn test(c: Container) -> UnknownType {
-                let x: UnknownType = c.value;
-                return x;
+            fn test(a: UnknownType, b: UnknownType) -> UnknownType {
+                let x: UnknownType = a;
+                return b;
             }
         "#;
+        cov_mark::check!(type_checker_error_dedup_skips_duplicate);
         let result = try_type_check(source);
         assert!(
             result.is_err(),