diff --git a/internal/models/spec.go b/internal/models/spec.go
index 59de8e31..da718536 100644
--- a/internal/models/spec.go
+++ b/internal/models/spec.go
@@ -70,8 +70,16 @@ func (g *GraderConfig) UnmarshalYAML(node *yaml.Node) error {
 		Parameters yaml.Node  `yaml:"config,omitempty"`
 	}
 
+	// Marshal the node back to bytes so we can use a strict decoder that rejects unknown fields.
+	nodeBytes, err := yaml.Marshal(node)
+	if err != nil {
+		return fmt.Errorf("failed to marshal grader node: %w", err)
+	}
+	decoder := yaml.NewDecoder(bytes.NewReader(nodeBytes))
+	decoder.KnownFields(true)
+
 	var raw rawGraderConfig
-	if err := node.Decode(&raw); err != nil {
+	if err := decoder.Decode(&raw); err != nil {
 		return err
 	}
 
diff --git a/internal/models/spec_test.go b/internal/models/spec_test.go
index 73781d29..f2b27200 100644
--- a/internal/models/spec_test.go
+++ b/internal/models/spec_test.go
@@ -43,6 +43,40 @@ config:
   trials_per_task: 0
   timeout_seconds: 60
   executor: mock
+`,
+			expectError: true,
+		},
+		{
+			name: "unknown field inside grader entry",
+			specYAML: `name: valid
+skill: test-skill
+config:
+  trials_per_task: 1
+  timeout_seconds: 60
+  executor: mock
+graders:
+  - name: my-grader
+    type: text
+    unknown_grader_field: should cause error
+    config:
+      contains: ["hello"]
+`,
+			expectError: true,
+		},
+		{
+			name: "unknown field inside grader config",
+			specYAML: `name: valid
+skill: test-skill
+config:
+  trials_per_task: 1
+  timeout_seconds: 60
+  executor: mock
+graders:
+  - name: my-grader
+    type: text
+    config:
+      contains: ["hello"]
+      unknown_config_field: should cause error
 `,
 			expectError: true,
 		},