diff --git a/.changeset/odd-windows-brake.md b/.changeset/odd-windows-brake.md
new file mode 100644
index 00000000..6cbad64a
--- /dev/null
+++ b/.changeset/odd-windows-brake.md
@@ -0,0 +1,6 @@
+---
+"eppo_core": minor
+"ruby-sdk": minor
+---
+
+[ruby] support ruby-4.0
diff --git a/.github/workflows/ruby.yml b/.github/workflows/ruby.yml
index 0c64cbb8..44f2abae 100644
--- a/.github/workflows/ruby.yml
+++ b/.github/workflows/ruby.yml
@@ -41,6 +41,7 @@ jobs:
           - "3.2"
           - "3.3"
           - "3.4"
+          - "4.0.0-preview3"
 
     steps:
       - uses: actions/checkout@v5
diff --git a/eppo_core/Cargo.toml b/eppo_core/Cargo.toml
index 9dc9263e..f1772b9d 100644
--- a/eppo_core/Cargo.toml
+++ b/eppo_core/Cargo.toml
@@ -50,8 +50,8 @@ pyo3 = { version = "0.23.0", optional = true, default-features = false }
 serde-pyobject = { version = "0.6.0", optional = true }
 
 # magnus dependencies
-magnus = { version = "0.7.1", default-features = false, optional = true }
-serde_magnus = { version = "0.9.0", default-features = false, optional = true }
+magnus = { version = "0.8.0", default-features = false, optional = true }
+serde_magnus = { version = "0.10.0", default-features = false, optional = true }
 
 # elixir dependencies
 rustler = { version = "0.36.1", optional = true }
diff --git a/eppo_core/src/eval/eval_details.rs b/eppo_core/src/eval/eval_details.rs
index 6bd367ec..a794632e 100644
--- a/eppo_core/src/eval/eval_details.rs
+++ b/eppo_core/src/eval/eval_details.rs
@@ -270,8 +270,8 @@ mod magnus_impl {
     use super::{EvaluationDetails, EvaluationResultWithDetails};
 
     impl IntoValue for &EvaluationDetails {
-        fn into_value_with(self, _handle: &magnus::Ruby) -> magnus::Value {
-            serde_magnus::serialize(self)
+        fn into_value_with(self, handle: &magnus::Ruby) -> magnus::Value {
+            serde_magnus::serialize(handle, self)
                 .expect("EvaluationDetails should always be serializable to Ruby")
         }
     }
diff --git a/eppo_core/src/events.rs b/eppo_core/src/events.rs
index 4342bcc8..77784084 100644
--- a/eppo_core/src/events.rs
+++ b/eppo_core/src/events.rs
@@ -131,15 +131,15 @@ mod magnus_impl {
     use super::{AssignmentEvent, BanditEvent};
 
     impl IntoValue for AssignmentEvent {
-        fn into_value_with(self, _handle: &magnus::Ruby) -> magnus::Value {
-            serde_magnus::serialize(&self)
+        fn into_value_with(self, handle: &magnus::Ruby) -> magnus::Value {
+            serde_magnus::serialize(handle, &self)
                 .expect("AssignmentEvent should always be serializable to Ruby")
         }
     }
 
     impl IntoValue for BanditEvent {
-        fn into_value_with(self, _handle: &magnus::Ruby) -> magnus::Value {
-            serde_magnus::serialize(&self)
+        fn into_value_with(self, handle: &magnus::Ruby) -> magnus::Value {
+            serde_magnus::serialize(handle, &self)
                 .expect("BanditEvent should always be serializable to Ruby")
         }
     }
diff --git a/eppo_core/src/ufc/assignment.rs b/eppo_core/src/ufc/assignment.rs
index 92d203c9..96d05888 100644
--- a/eppo_core/src/ufc/assignment.rs
+++ b/eppo_core/src/ufc/assignment.rs
@@ -380,8 +380,10 @@ mod magnus_impl {
                 AssignmentValue::Integer(i) => i.into_value_with(handle),
                 AssignmentValue::Numeric(n) => n.into_value_with(handle),
                 AssignmentValue::Boolean(b) => b.into_value_with(handle),
-                AssignmentValue::Json { raw: _, parsed } => serde_magnus::serialize(&parsed)
-                    .expect("JSON value should always be serializable to Ruby"),
+                AssignmentValue::Json { raw: _, parsed } => {
+                    serde_magnus::serialize(handle, &parsed)
+                        .expect("JSON value should always be serializable to Ruby")
+                }
             }
         }
     }
@@ -392,7 +394,7 @@ mod magnus_impl {
             let _ = hash.aset(handle.sym_new("value"), self.value);
             let _ = hash.aset(
                 handle.sym_new("event"),
-                serde_magnus::serialize::<_, magnus::Value>(&self.event)
+                serde_magnus::serialize::<_, magnus::Value>(handle, &self.event)
                     .expect("AssignmentEvent should always be serializable to Ruby"),
             );
             hash.as_value()
diff --git a/package-lock.json b/package-lock.json
index 74f28a5b..6be2a0e3 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -25,7 +25,7 @@
       "devDependencies": {}
     },
     "eppo_core": {
-      "version": "9.2.0",
+      "version": "9.3.0",
       "devDependencies": {}
     },
     "mock-server": {
@@ -3411,15 +3411,15 @@
       }
     },
     "python-sdk": {
-      "version": "4.3.1",
+      "version": "4.4.0",
       "devDependencies": {}
     },
     "ruby-sdk": {
-      "version": "3.7.2",
+      "version": "3.7.4",
       "devDependencies": {}
     },
     "rust-sdk": {
-      "version": "5.1.0",
+      "version": "5.2.0",
       "devDependencies": {}
     }
   }
diff --git a/ruby-sdk/Cargo.lock b/ruby-sdk/Cargo.lock
index c111ce19..eecd0db8 100644
--- a/ruby-sdk/Cargo.lock
+++ b/ruby-sdk/Cargo.lock
@@ -351,8 +351,6 @@ dependencies = [
 [[package]]
 name = "eppo_core"
 version = "9.3.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0f7a2aa615d3a5848b8cd60921fe67290122054e3e51519cce8e024bd6107c8d"
 dependencies = [
  "base64",
  "chrono",
@@ -932,9 +930,9 @@ dependencies = [
 
 [[package]]
 name = "magnus"
-version = "0.7.1"
+version = "0.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3d87ae53030f3a22e83879e666cb94e58a7bdf31706878a0ba48752994146dab"
+checksum = "3b36a5b126bbe97eb0d02d07acfeb327036c6319fd816139a49824a83b7f9012"
 dependencies = [
  "magnus-macros",
  "rb-sys",
@@ -944,9 +942,9 @@ dependencies = [
 
 [[package]]
 name = "magnus-macros"
-version = "0.6.0"
+version = "0.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5968c820e2960565f647819f5928a42d6e874551cab9d88d75e3e0660d7f71e3"
+checksum = "47607461fd8e1513cb4f2076c197d8092d921a1ea75bd08af97398f593751892"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -1226,18 +1224,18 @@ dependencies = [
 
 [[package]]
 name = "rb-sys"
-version = "0.9.111"
+version = "0.9.123"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "becea799ce051c16fb140be80f5e7cf781070f99ca099332383c2b17861249af"
+checksum = "45fb1a185af97ee456f1c9e56dbe6e2e662bec4fdeaf83c4c28e0e6adfb18816"
 dependencies = [
  "rb-sys-build",
 ]
 
 [[package]]
 name = "rb-sys-build"
-version = "0.9.111"
+version = "0.9.123"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "64691175abc704862f60a9ca8ef06174080cc50615f2bf1d4759f46db18b4d29"
+checksum = "a58ebd02d7a6033e6a5f6f8d150c1e9f16506039092b84a73e6bedce6d3adf41"
 dependencies = [
  "bindgen",
  "lazy_static",
@@ -1250,9 +1248,9 @@ dependencies = [
 
 [[package]]
 name = "rb-sys-env"
-version = "0.1.2"
+version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a35802679f07360454b418a5d1735c89716bde01d35b1560fc953c1415a0b3bb"
+checksum = "08f8d2924cf136a1315e2b4c7460a39f62ef11ee5d522df9b2750fab55b868b6"
 
 [[package]]
 name = "redox_syscall"
@@ -1489,9 +1487,9 @@ dependencies = [
 
 [[package]]
 name = "serde_magnus"
-version = "0.9.0"
+version = "0.10.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "51b8b945a2dadb221f1c5490cfb411cab6c3821446b8eca50ee07e5a3893ec51"
+checksum = "c4a5121544138e6a14036e48ecbe589fb63577f35620caaa32642257b412c317"
 dependencies = [
  "magnus",
  "serde",
diff --git a/ruby-sdk/Gemfile.lock b/ruby-sdk/Gemfile.lock
index b8e6b6cd..7938ee1b 100644
--- a/ruby-sdk/Gemfile.lock
+++ b/ruby-sdk/Gemfile.lock
@@ -1,8 +1,9 @@
 PATH
   remote: .
   specs:
-    eppo-server-sdk (3.7.2)
-      rb_sys (~> 0.9.105)
+    eppo-server-sdk (3.7.4)
+      logger (~> 1.6)
+      rb_sys (~> 0.9.120)
 
 GEM
   remote: https://rubygems.org/
@@ -12,6 +13,7 @@ GEM
     json (2.15.2)
     language_server-protocol (3.17.0.5)
     lint_roller (1.1.0)
+    logger (1.7.0)
     parallel (1.27.0)
     parser (3.3.10.0)
       ast (~> 2.4.1)
diff --git a/ruby-sdk/eppo-server-sdk.gemspec b/ruby-sdk/eppo-server-sdk.gemspec
index 8f144989..4f0bc59a 100644
--- a/ruby-sdk/eppo-server-sdk.gemspec
+++ b/ruby-sdk/eppo-server-sdk.gemspec
@@ -32,8 +32,10 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
   spec.extensions = ["ext/eppo_client/extconf.rb"]
 
-  # 0.9.105 added Ruby 3.4 to the list of supported version
-  spec.add_dependency "rb_sys", "~> 0.9.105"
+  # 0.9.120 added Ruby 4.0 to the list of supported version
+  spec.add_dependency "rb_sys", "~> 0.9.120"
+
+  spec.add_dependency "logger", "~> 1.6"
 
   # For more information and examples about making a new gem, check out our
   # guide at: https://bundler.io/guides/creating_gem.html
diff --git a/ruby-sdk/ext/eppo_client/Cargo.toml b/ruby-sdk/ext/eppo_client/Cargo.toml
index 3da19db0..82537838 100644
--- a/ruby-sdk/ext/eppo_client/Cargo.toml
+++ b/ruby-sdk/ext/eppo_client/Cargo.toml
@@ -14,9 +14,9 @@ crate-type = ["cdylib"]
 env_logger = { version = "0.11.3", features = ["unstable-kv"] }
 eppo_core = { version = "=9.3.0", features = ["magnus", "event_ingestion"] }
 log = { version = "0.4.21", features = ["kv_serde"] }
-magnus = { version = "0.7.1" }
+magnus = { version = "0.8.2" }
 serde = { version = "1.0.203", features = ["derive"] }
-serde_magnus = "0.9.0"
-rb-sys = "0.9.105"
+serde_magnus = "0.10.0"
+rb-sys = "0.9.120"
 serde_json = "1.0.128"
 tokio = { version = "1.44.1", default-features = false, features = ["time"] }
diff --git a/ruby-sdk/ext/eppo_client/src/client.rs b/ruby-sdk/ext/eppo_client/src/client.rs
index a524b979..19156420 100644
--- a/ruby-sdk/ext/eppo_client/src/client.rs
+++ b/ruby-sdk/ext/eppo_client/src/client.rs
@@ -13,7 +13,7 @@ use eppo_core::{
     ufc::VariationType,
     Attributes, ContextAttributes, SdkKey,
 };
-use magnus::{error::Result, exception, prelude::*, Error, IntoValue, Ruby, TryConvert, Value};
+use magnus::{error::Result, prelude::*, Error, IntoValue, Ruby, TryConvert, Value};
 
 #[derive(Debug)]
 #[magnus::wrap(class = "EppoClient::Core::Config", size, free_immediately)]
@@ -29,6 +29,7 @@ pub struct Config {
 impl TryConvert for Config {
     // `val` is expected to be of type EppoClient::Config.
     fn try_convert(val: Value) -> Result<Self> {
+        let ruby = Ruby::get_with(val);
         let sdk_key = String::try_convert(val.funcall("api_key", ())?)?;
         let base_url = String::try_convert(val.funcall("base_url", ())?)?;
         let poll_interval_seconds =
@@ -38,7 +39,7 @@ impl TryConvert for Config {
             let s = Option::<String>::try_convert(val.funcall("log_level", ())?)?;
             s.map(|s| {
                 log::LevelFilter::from_str(&s)
-                    .map_err(|err| Error::new(exception::runtime_error(), err.to_string()))
+                    .map_err(|err| Error::new(ruby.exception_runtime_error(), err.to_string()))
             })
             .transpose()?
         };
@@ -139,8 +140,8 @@ impl Client {
         subject_attributes: Value,
         expected_type: Value,
     ) -> Result<Value> {
-        let expected_type: VariationType = serde_magnus::deserialize(expected_type)?;
-        let subject_attributes: Attributes = serde_magnus::deserialize(subject_attributes)?;
+        let expected_type: VariationType = serde_magnus::deserialize(ruby, expected_type)?;
+        let subject_attributes: Attributes = serde_magnus::deserialize(ruby, subject_attributes)?;
 
         let result = rb_self
             .evaluator
@@ -151,24 +152,23 @@ impl Client {
                 Some(expected_type),
             )
             // TODO: maybe expose possible errors individually.
-            .map_err(|err| Error::new(exception::runtime_error(), err.to_string()))?;
+            .map_err(|err| Error::new(ruby.exception_runtime_error(), err.to_string()))?;
 
         Ok(result.into_value_with(&ruby))
     }
 
     pub fn get_assignment_details(
-        &self,
+        ruby: &Ruby,
+        rb_self: &Self,
         flag_key: String,
         subject_key: String,
         subject_attributes: Value,
         expected_type: Value,
     ) -> Result<Value> {
-        let ruby = Ruby::get_with(subject_attributes);
+        let expected_type: VariationType = serde_magnus::deserialize(ruby, expected_type)?;
+        let subject_attributes: Attributes = serde_magnus::deserialize(ruby, subject_attributes)?;
 
-        let expected_type: VariationType = serde_magnus::deserialize(expected_type)?;
-        let subject_attributes: Attributes = serde_magnus::deserialize(subject_attributes)?;
-
-        let result = self.evaluator.get_assignment_details(
+        let result = rb_self.evaluator.get_assignment_details(
             &flag_key,
             &subject_key.into(),
             &Arc::new(subject_attributes),
@@ -179,25 +179,26 @@ impl Client {
     }
 
     pub fn get_bandit_action(
-        &self,
+        ruby: &Ruby,
+        rb_self: &Self,
         flag_key: String,
         subject_key: String,
         subject_attributes: Value,
         actions: Value,
         default_variation: String,
     ) -> Result<Value> {
-        let subject_attributes = serde_magnus::deserialize::<_, ContextAttributes>(
-            subject_attributes,
-        )
-        .map_err(|err| {
-            Error::new(
-                exception::runtime_error(),
-                format!("Unexpected value for subject_attributes: {err}"),
-            )
-        })?;
-        let actions = serde_magnus::deserialize(actions)?;
+        let subject_attributes =
+            serde_magnus::deserialize::<_, ContextAttributes>(ruby, subject_attributes).map_err(
+                |err| {
+                    Error::new(
+                        ruby.exception_runtime_error(),
+                        format!("Unexpected value for subject_attributes: {err}"),
+                    )
+                },
+            )?;
+        let actions = serde_magnus::deserialize(ruby, actions)?;
 
-        let result = self.evaluator.get_bandit_action(
+        let result = rb_self.evaluator.get_bandit_action(
             &flag_key,
             &subject_key.into(),
             &subject_attributes,
@@ -205,29 +206,30 @@ impl Client {
             &default_variation.into(),
         );
 
-        serde_magnus::serialize(&result)
+        serde_magnus::serialize(ruby, &result)
     }
 
     pub fn get_bandit_action_details(
-        &self,
+        ruby: &Ruby,
+        rb_self: &Self,
         flag_key: String,
         subject_key: String,
         subject_attributes: Value,
         actions: Value,
         default_variation: String,
     ) -> Result<Value> {
-        let subject_attributes = serde_magnus::deserialize::<_, ContextAttributes>(
-            subject_attributes,
-        )
-        .map_err(|err| {
-            Error::new(
-                exception::runtime_error(),
-                format!("Unexpected value for subject_attributes: {err}"),
-            )
-        })?;
-        let actions = serde_magnus::deserialize(actions)?;
+        let subject_attributes =
+            serde_magnus::deserialize::<_, ContextAttributes>(ruby, subject_attributes).map_err(
+                |err| {
+                    Error::new(
+                        ruby.exception_runtime_error(),
+                        format!("Unexpected value for subject_attributes: {err}"),
+                    )
+                },
+            )?;
+        let actions = serde_magnus::deserialize(ruby, actions)?;
 
-        let result = self.evaluator.get_bandit_action_details(
+        let result = rb_self.evaluator.get_bandit_action_details(
             &flag_key,
             &subject_key.into(),
             &subject_attributes,
@@ -235,7 +237,7 @@ impl Client {
             &default_variation.into(),
         );
 
-        serde_magnus::serialize(&result)
+        serde_magnus::serialize(ruby, &result)
     }
 
     pub fn wait_for_initialization(&self, timeout_secs: f64) {
@@ -278,22 +280,23 @@ impl Client {
 
     pub fn shutdown(&self) {
         if let Some(thread) = self.background_thread.take() {
-            thread.graceful_shutdown();
+            thread.shutdown();
         }
     }
 
-    pub fn track(&self, event_type: String, payload: Value) -> Result<()> {
-        let Some(event_ingestion) = &self.event_ingestion else {
+    pub fn track(ruby: &Ruby, rb_self: &Self, event_type: String, payload: Value) -> Result<()> {
+        let Some(event_ingestion) = &rb_self.event_ingestion else {
             // Event ingestion is disabled, do nothing.
             return Ok(());
         };
 
-        let payload: serde_json::Value = serde_magnus::deserialize(payload).map_err(|err| {
-            Error::new(
-                exception::runtime_error(),
-                format!("Unexpected value for payload: {err}"),
-            )
-        })?;
+        let payload: serde_json::Value =
+            serde_magnus::deserialize(ruby, payload).map_err(|err| {
+                Error::new(
+                    ruby.exception_runtime_error(),
+                    format!("Unexpected value for payload: {err}"),
+                )
+            })?;
 
         event_ingestion.track(event_type, payload);
 
diff --git a/ruby-sdk/ext/eppo_client/src/configuration.rs b/ruby-sdk/ext/eppo_client/src/configuration.rs
index ae281061..6a4a8891 100644
--- a/ruby-sdk/ext/eppo_client/src/configuration.rs
+++ b/ruby-sdk/ext/eppo_client/src/configuration.rs
@@ -9,7 +9,7 @@ use crate::{gc_lock::GcLock, SDK_METADATA};
 pub(crate) fn init(ruby: &Ruby) -> Result<(), Error> {
     let eppo_client = ruby.define_module("EppoClient")?;
 
-    let configuration = eppo_client.define_class("Configuration", magnus::class::object())?;
+    let configuration = eppo_client.define_class("Configuration", ruby.class_object())?;
     configuration.define_singleton_method("new", function!(Configuration::new, 1))?;
     configuration.define_method(
         "flags_configuration",
diff --git a/ruby-sdk/ext/eppo_client/src/lib.rs b/ruby-sdk/ext/eppo_client/src/lib.rs
index 97957b6f..84ce2e97 100644
--- a/ruby-sdk/ext/eppo_client/src/lib.rs
+++ b/ruby-sdk/ext/eppo_client/src/lib.rs
@@ -16,7 +16,7 @@ pub(crate) const SDK_METADATA: SdkMetadata = SdkMetadata {
 fn init(ruby: &Ruby) -> Result<(), Error> {
     let eppo_client = ruby.define_module("EppoClient")?;
     let core = eppo_client.define_module("Core")?;
-    let core_client = core.define_class("Client", magnus::class::object())?;
+    let core_client = core.define_class("Client", ruby.class_object())?;
     core_client.define_singleton_method("new", function!(Client::new, 1))?;
     core_client.define_method("get_assignment", method!(Client::get_assignment, 4))?;
     core_client.define_method(