diff --git a/libafl/src/executors/command.rs b/libafl/src/executors/command.rs
index ea0e3885a4..fca9f6e6cf 100644
--- a/libafl/src/executors/command.rs
+++ b/libafl/src/executors/command.rs
@@ -212,7 +212,7 @@ where
     pub fn from_cmd_with_file<P>(
         cmd: &Command,
         debug_child: bool,
-        mut observers: OT,
+        observers: OT,
         path: P,
     ) -> Result<Self, Error>
     where
@@ -529,7 +529,7 @@ impl CommandExecutorBuilder {
     /// Builds the `ComandExecutor`
     pub fn build<EM, OT, S, Z>(
         &self,
-        mut observers: OT,
+        observers: OT,
     ) -> Result<CommandExecutor<EM, OT, S, StdCommandConfigurator, Z>, Error>
     where
         OT: Debug + MatchName + ObserversTuple<S>,
diff --git a/libafl/src/executors/differential.rs b/libafl/src/executors/differential.rs
index a46bd9b9fa..4e24b4d114 100644
--- a/libafl/src/executors/differential.rs
+++ b/libafl/src/executors/differential.rs
@@ -141,13 +141,13 @@ where
 
     /// Returns true if a `stdout` observer was added to the list
     #[inline]
-    fn observes_stdout(&mut self) -> bool {
-        self.primary.as_mut().observes_stdout() || self.secondary.as_mut().observes_stdout()
+    fn observes_stdout(&self) -> bool {
+        self.primary.as_ref().observes_stdout() || self.secondary.as_ref().observes_stdout()
     }
     /// Returns true if a `stderr` observer was added to the list
     #[inline]
-    fn observes_stderr(&mut self) -> bool {
-        self.primary.as_mut().observes_stderr() || self.secondary.as_mut().observes_stderr()
+    fn observes_stderr(&self) -> bool {
+        self.primary.as_ref().observes_stderr() || self.secondary.as_ref().observes_stderr()
     }
 
     /// Runs `observe_stdout` for all stdout observers in the list
diff --git a/libafl/src/observers/mod.rs b/libafl/src/observers/mod.rs
index 3eaedc1bdb..8702187c64 100644
--- a/libafl/src/observers/mod.rs
+++ b/libafl/src/observers/mod.rs
@@ -93,12 +93,12 @@ where
 
     /// If this observer observes `stdout`
     #[inline]
-    fn observes_stdout(&mut self) -> bool {
+    fn observes_stdout(&self) -> bool {
         false
     }
     /// If this observer observes `stderr`
     #[inline]
-    fn observes_stderr(&mut self) -> bool {
+    fn observes_stderr(&self) -> bool {
         false
     }
 
@@ -107,6 +107,7 @@ where
     #[inline]
     #[allow(unused_variables)]
     fn observe_stdout(&mut self, stdout: &str) {}
+
     /// React to new `stderr`
     /// To use this, always return `true` from `observes_stderr`
     #[inline]
@@ -149,9 +150,9 @@ where
     ) -> Result<(), Error>;
 
     /// Returns true if a `stdout` observer was added to the list
-    fn observes_stdout(&mut self) -> bool;
+    fn observes_stdout(&self) -> bool;
     /// Returns true if a `stderr` observer was added to the list
-    fn observes_stderr(&mut self) -> bool;
+    fn observes_stderr(&self) -> bool;
 
     /// Runs `observe_stdout` for all stdout observers in the list
     fn observe_stdout(&mut self, stdout: &str);
@@ -191,12 +192,13 @@ where
 
     /// Returns true if a `stdout` observer was added to the list
     #[inline]
-    fn observes_stdout(&mut self) -> bool {
+    fn observes_stdout(&self) -> bool {
         false
     }
+
     /// Returns true if a `stderr` observer was added to the list
     #[inline]
-    fn observes_stderr(&mut self) -> bool {
+    fn observes_stderr(&self) -> bool {
         false
     }
 
@@ -204,6 +206,7 @@ where
     #[inline]
     #[allow(unused_variables)]
     fn observe_stdout(&mut self, stdout: &str) {}
+
     /// Runs `observe_stderr` for all stderr observers in the list
     #[inline]
     #[allow(unused_variables)]
@@ -248,12 +251,13 @@ where
 
     /// Returns true if a `stdout` observer was added to the list
     #[inline]
-    fn observes_stdout(&mut self) -> bool {
+    fn observes_stdout(&self) -> bool {
         self.0.observes_stdout() || self.1.observes_stdout()
     }
+
     /// Returns true if a `stderr` observer was added to the list
     #[inline]
-    fn observes_stderr(&mut self) -> bool {
+    fn observes_stderr(&self) -> bool {
         self.0.observes_stderr() || self.1.observes_stderr()
     }
 
@@ -261,10 +265,13 @@ where
     #[inline]
     fn observe_stdout(&mut self, stdout: &str) {
         self.0.observe_stdout(stdout);
+        self.1.observe_stdout(stdout);
     }
+
     /// Runs `observe_stderr` for all stderr observers in the list
     #[inline]
     fn observe_stderr(&mut self, stderr: &str) {
+        self.0.observe_stderr(stderr);
         self.1.observe_stderr(stderr);
     }
 }
@@ -974,12 +981,12 @@ pub mod pybind {
 
         // TODO: expose stdout/stderr to python
         #[inline]
-        fn observes_stdout(&mut self) -> bool {
+        fn observes_stdout(&self) -> bool {
             false
         }
 
         #[inline]
-        fn observes_stderr(&mut self) -> bool {
+        fn observes_stderr(&self) -> bool {
             false
         }
 
diff --git a/libafl/src/observers/stacktrace.rs b/libafl/src/observers/stacktrace.rs
index e3585648ff..e228b38880 100644
--- a/libafl/src/observers/stacktrace.rs
+++ b/libafl/src/observers/stacktrace.rs
@@ -264,7 +264,7 @@ where
 
     /// Do nothing on new `stderr`
     #[inline]
-    fn observes_stderr(&mut self) -> bool {
+    fn observes_stderr(&self) -> bool {
         true
     }
 
diff --git a/libafl/src/observers/stdio.rs b/libafl/src/observers/stdio.rs
index 7ef35003a7..ca046781aa 100644
--- a/libafl/src/observers/stdio.rs
+++ b/libafl/src/observers/stdio.rs
@@ -32,9 +32,10 @@ where
     S: UsesInput,
 {
     #[inline]
-    fn observes_stdout(&mut self) -> bool {
+    fn observes_stdout(&self) -> bool {
         true
     }
+
     /// React to new `stdout`
     fn observe_stdout(&mut self, stdout: &str) {
         self.stdout = Some(stdout.into());
@@ -71,11 +72,11 @@ where
     S: UsesInput,
 {
     #[inline]
-    fn observes_stderr(&mut self) -> bool {
+    fn observes_stderr(&self) -> bool {
         true
     }
 
-    /// Do nothing on new `stderr`
+    /// React to new `stderr`
     fn observe_stderr(&mut self, stderr: &str) {
         self.stderr = Some(stderr.into());
     }