diff --git a/codex-rs/Cargo.lock b/codex-rs/Cargo.lock
index bf68ff043f..c0cbcf1788 100644
--- a/codex-rs/Cargo.lock
+++ b/codex-rs/Cargo.lock
@@ -819,6 +819,8 @@ version = "1.2.30"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "deec109607ca693028562ed836a5f1c4b8bd77755c4e132fc5ce11b0b6211ae7"
 dependencies = [
+ "jobserver",
+ "libc",
  "shlex",
 ]
 
@@ -1197,6 +1199,7 @@ dependencies = [
  "tracing",
  "tracing-opentelemetry",
  "tracing-subscriber",
+ "zstd",
 ]
 
 [[package]]
@@ -1348,6 +1351,7 @@ dependencies = [
  "which",
  "wildmatch",
  "wiremock",
+ "zstd",
 ]
 
 [[package]]
@@ -3924,6 +3928,16 @@ version = "0.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8eaf4bc02d17cbdd7ff4c7438cafcdf7fb9a4613313ad11b4f8fefe7d3fa0130"
 
+[[package]]
+name = "jobserver"
+version = "0.1.34"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9afb3de4395d6b3e67a780b6de64b51c978ecf11cb9a462c66be7d4ca9039d33"
+dependencies = [
+ "getrandom 0.3.3",
+ "libc",
+]
+
 [[package]]
 name = "js-sys"
 version = "0.3.77"
@@ -8809,6 +8823,34 @@ dependencies = [
  "syn 2.0.104",
 ]
 
+[[package]]
+name = "zstd"
+version = "0.13.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e91ee311a569c327171651566e07972200e76fcfe2242a4fa446149a3881c08a"
+dependencies = [
+ "zstd-safe",
+]
+
+[[package]]
+name = "zstd-safe"
+version = "7.2.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8f49c4d5f0abb602a93fb8736af2a4f4dd9512e36f7f570d66e65ff867ed3b9d"
+dependencies = [
+ "zstd-sys",
+]
+
+[[package]]
+name = "zstd-sys"
+version = "2.0.16+zstd.1.5.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "91e19ebc2adc8f83e43039e79776e3fda8ca919132d68a1fed6a5faca2683748"
+dependencies = [
+ "cc",
+ "pkg-config",
+]
+
 [[package]]
 name = "zune-core"
 version = "0.4.12"
diff --git a/codex-rs/Cargo.toml b/codex-rs/Cargo.toml
index 2d0b5eec08..1dc043a4fc 100644
--- a/codex-rs/Cargo.toml
+++ b/codex-rs/Cargo.toml
@@ -218,6 +218,7 @@ tracing-subscriber = "0.3.22"
 tracing-test = "0.2.5"
 tree-sitter = "0.25.10"
 tree-sitter-bash = "0.25"
+zstd = "0.13"
 tree-sitter-highlight = "0.25.10"
 ts-rs = "11"
 tui-scrollbar = "0.2.1"
diff --git a/codex-rs/codex-api/src/endpoint/chat.rs b/codex-rs/codex-api/src/endpoint/chat.rs
index b7fa0572f0..f747c57411 100644
--- a/codex-rs/codex-api/src/endpoint/chat.rs
+++ b/codex-rs/codex-api/src/endpoint/chat.rs
@@ -10,6 +10,7 @@ use crate::provider::WireApi;
 use crate::sse::chat::spawn_chat_stream;
 use crate::telemetry::SseTelemetry;
 use codex_client::HttpTransport;
+use codex_client::RequestCompression;
 use codex_client::RequestTelemetry;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ReasoningItemContent;
@@ -80,7 +81,13 @@ impl<T: HttpTransport, A: AuthProvider> ChatClient<T, A> {
         extra_headers: HeaderMap,
     ) -> Result<ResponseStream, ApiError> {
         self.streaming
-            .stream(self.path(), body, extra_headers, spawn_chat_stream)
+            .stream(
+                self.path(),
+                body,
+                extra_headers,
+                RequestCompression::None,
+                spawn_chat_stream,
+            )
             .await
     }
 }
diff --git a/codex-rs/codex-api/src/endpoint/responses.rs b/codex-rs/codex-api/src/endpoint/responses.rs
index 476e8b8f13..57c7b0da03 100644
--- a/codex-rs/codex-api/src/endpoint/responses.rs
+++ b/codex-rs/codex-api/src/endpoint/responses.rs
@@ -9,9 +9,11 @@ use crate::provider::Provider;
 use crate::provider::WireApi;
 use crate::requests::ResponsesRequest;
 use crate::requests::ResponsesRequestBuilder;
+use crate::requests::responses::Compression;
 use crate::sse::spawn_response_stream;
 use crate::telemetry::SseTelemetry;
 use codex_client::HttpTransport;
+use codex_client::RequestCompression;
 use codex_client::RequestTelemetry;
 use codex_protocol::protocol::SessionSource;
 use http::HeaderMap;
@@ -33,6 +35,7 @@ pub struct ResponsesOptions {
     pub conversation_id: Option<String>,
     pub session_source: Option<SessionSource>,
     pub extra_headers: HeaderMap,
+    pub compression: Compression,
 }
 
 impl<T: HttpTransport, A: AuthProvider> ResponsesClient<T, A> {
@@ -56,7 +59,8 @@ impl<T: HttpTransport, A: AuthProvider> ResponsesClient<T, A> {
         &self,
         request: ResponsesRequest,
     ) -> Result<ResponseStream, ApiError> {
-        self.stream(request.body, request.headers).await
+        self.stream(request.body, request.headers, request.compression)
+            .await
     }
 
     #[instrument(level = "trace", skip_all, err)]
@@ -75,6 +79,7 @@ impl<T: HttpTransport, A: AuthProvider> ResponsesClient<T, A> {
             conversation_id,
             session_source,
             extra_headers,
+            compression,
         } = options;
 
         let request = ResponsesRequestBuilder::new(model, &prompt.instructions, &prompt.input)
@@ -88,6 +93,7 @@ impl<T: HttpTransport, A: AuthProvider> ResponsesClient<T, A> {
             .session_source(session_source)
             .store_override(store_override)
             .extra_headers(extra_headers)
+            .compression(compression)
             .build(self.streaming.provider())?;
 
         self.stream_request(request).await
@@ -104,9 +110,21 @@ impl<T: HttpTransport, A: AuthProvider> ResponsesClient<T, A> {
         &self,
         body: Value,
         extra_headers: HeaderMap,
+        compression: Compression,
     ) -> Result<ResponseStream, ApiError> {
+        let compression = match compression {
+            Compression::None => RequestCompression::None,
+            Compression::Zstd => RequestCompression::Zstd,
+        };
+
         self.streaming
-            .stream(self.path(), body, extra_headers, spawn_response_stream)
+            .stream(
+                self.path(),
+                body,
+                extra_headers,
+                compression,
+                spawn_response_stream,
+            )
             .await
     }
 }
diff --git a/codex-rs/codex-api/src/endpoint/streaming.rs b/codex-rs/codex-api/src/endpoint/streaming.rs
index 156d4084bc..de180845e8 100644
--- a/codex-rs/codex-api/src/endpoint/streaming.rs
+++ b/codex-rs/codex-api/src/endpoint/streaming.rs
@@ -6,6 +6,7 @@ use crate::provider::Provider;
 use crate::telemetry::SseTelemetry;
 use crate::telemetry::run_with_request_telemetry;
 use codex_client::HttpTransport;
+use codex_client::RequestCompression;
 use codex_client::RequestTelemetry;
 use codex_client::StreamResponse;
 use http::HeaderMap;
@@ -52,6 +53,7 @@ impl<T: HttpTransport, A: AuthProvider> StreamingClient<T, A> {
         path: &str,
         body: Value,
         extra_headers: HeaderMap,
+        compression: RequestCompression,
         spawner: fn(StreamResponse, Duration, Option<Arc<dyn SseTelemetry>>) -> ResponseStream,
     ) -> Result<ResponseStream, ApiError> {
         let builder = || {
@@ -62,6 +64,7 @@ impl<T: HttpTransport, A: AuthProvider> StreamingClient<T, A> {
                 http::HeaderValue::from_static("text/event-stream"),
             );
             req.body = Some(body.clone());
+            req.compression = compression;
             add_auth_headers(&self.auth, req)
         };
 
diff --git a/codex-rs/codex-api/src/provider.rs b/codex-rs/codex-api/src/provider.rs
index 8bd5fc9093..846a25bf5e 100644
--- a/codex-rs/codex-api/src/provider.rs
+++ b/codex-rs/codex-api/src/provider.rs
@@ -1,4 +1,5 @@
 use codex_client::Request;
+use codex_client::RequestCompression;
 use codex_client::RetryOn;
 use codex_client::RetryPolicy;
 use http::Method;
@@ -87,6 +88,7 @@ impl Provider {
             url: self.url_for_path(path),
             headers: self.headers.clone(),
             body: None,
+            compression: RequestCompression::None,
             timeout: None,
         }
     }
diff --git a/codex-rs/codex-api/src/requests/responses.rs b/codex-rs/codex-api/src/requests/responses.rs
index 543b79bbe9..a18a147aba 100644
--- a/codex-rs/codex-api/src/requests/responses.rs
+++ b/codex-rs/codex-api/src/requests/responses.rs
@@ -11,10 +11,18 @@ use codex_protocol::protocol::SessionSource;
 use http::HeaderMap;
 use serde_json::Value;
 
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
+pub enum Compression {
+    #[default]
+    None,
+    Zstd,
+}
+
 /// Assembled request body plus headers for a Responses stream request.
 pub struct ResponsesRequest {
     pub body: Value,
     pub headers: HeaderMap,
+    pub compression: Compression,
 }
 
 #[derive(Default)]
@@ -32,6 +40,7 @@ pub struct ResponsesRequestBuilder<'a> {
     session_source: Option<SessionSource>,
     store_override: Option<bool>,
     headers: HeaderMap,
+    compression: Compression,
 }
 
 impl<'a> ResponsesRequestBuilder<'a> {
@@ -94,6 +103,11 @@ impl<'a> ResponsesRequestBuilder<'a> {
         self
     }
 
+    pub fn compression(mut self, compression: Compression) -> Self {
+        self.compression = compression;
+        self
+    }
+
     pub fn build(self, provider: &Provider) -> Result<ResponsesRequest, ApiError> {
         let model = self
             .model
@@ -138,7 +152,11 @@ impl<'a> ResponsesRequestBuilder<'a> {
             insert_header(&mut headers, "x-openai-subagent", &subagent);
         }
 
-        Ok(ResponsesRequest { body, headers })
+        Ok(ResponsesRequest {
+            body,
+            headers,
+            compression: self.compression,
+        })
     }
 }
 
diff --git a/codex-rs/codex-api/tests/clients.rs b/codex-rs/codex-api/tests/clients.rs
index 3dafaf74fa..70af9fe829 100644
--- a/codex-rs/codex-api/tests/clients.rs
+++ b/codex-rs/codex-api/tests/clients.rs
@@ -11,6 +11,7 @@ use codex_api::Provider;
 use codex_api::ResponsesClient;
 use codex_api::ResponsesOptions;
 use codex_api::WireApi;
+use codex_api::requests::responses::Compression;
 use codex_client::HttpTransport;
 use codex_client::Request;
 use codex_client::Response;
@@ -229,7 +230,9 @@ async fn responses_client_uses_responses_path_for_responses_wire() -> Result<()>
     let client = ResponsesClient::new(transport, provider("openai", WireApi::Responses), NoAuth);
 
     let body = serde_json::json!({ "echo": true });
-    let _stream = client.stream(body, HeaderMap::new()).await?;
+    let _stream = client
+        .stream(body, HeaderMap::new(), Compression::None)
+        .await?;
 
     let requests = state.take_stream_requests();
     assert_path_ends_with(&requests, "/responses");
@@ -243,7 +246,9 @@ async fn responses_client_uses_chat_path_for_chat_wire() -> Result<()> {
     let client = ResponsesClient::new(transport, provider("openai", WireApi::Chat), NoAuth);
 
     let body = serde_json::json!({ "echo": true });
-    let _stream = client.stream(body, HeaderMap::new()).await?;
+    let _stream = client
+        .stream(body, HeaderMap::new(), Compression::None)
+        .await?;
 
     let requests = state.take_stream_requests();
     assert_path_ends_with(&requests, "/chat/completions");
@@ -258,7 +263,9 @@ async fn streaming_client_adds_auth_headers() -> Result<()> {
     let client = ResponsesClient::new(transport, provider("openai", WireApi::Responses), auth);
 
     let body = serde_json::json!({ "model": "gpt-test" });
-    let _stream = client.stream(body, HeaderMap::new()).await?;
+    let _stream = client
+        .stream(body, HeaderMap::new(), Compression::None)
+        .await?;
 
     let requests = state.take_stream_requests();
     assert_eq!(requests.len(), 1);
diff --git a/codex-rs/codex-api/tests/sse_end_to_end.rs b/codex-rs/codex-api/tests/sse_end_to_end.rs
index b91cf3a5d8..f324cc7480 100644
--- a/codex-rs/codex-api/tests/sse_end_to_end.rs
+++ b/codex-rs/codex-api/tests/sse_end_to_end.rs
@@ -9,6 +9,7 @@ use codex_api::Provider;
 use codex_api::ResponseEvent;
 use codex_api::ResponsesClient;
 use codex_api::WireApi;
+use codex_api::requests::responses::Compression;
 use codex_client::HttpTransport;
 use codex_client::Request;
 use codex_client::Response;
@@ -124,7 +125,11 @@ async fn responses_stream_parses_items_and_completed_end_to_end() -> Result<()>
     let client = ResponsesClient::new(transport, provider("openai", WireApi::Responses), NoAuth);
 
     let mut stream = client
-        .stream(serde_json::json!({"echo": true}), HeaderMap::new())
+        .stream(
+            serde_json::json!({"echo": true}),
+            HeaderMap::new(),
+            Compression::None,
+        )
         .await?;
 
     let mut events = Vec::new();
@@ -189,7 +194,11 @@ async fn responses_stream_aggregates_output_text_deltas() -> Result<()> {
     let client = ResponsesClient::new(transport, provider("openai", WireApi::Responses), NoAuth);
 
     let stream = client
-        .stream(serde_json::json!({"echo": true}), HeaderMap::new())
+        .stream(
+            serde_json::json!({"echo": true}),
+            HeaderMap::new(),
+            Compression::None,
+        )
         .await?;
 
     let mut stream = stream.aggregate();
diff --git a/codex-rs/codex-client/Cargo.toml b/codex-rs/codex-client/Cargo.toml
index 2eeb456937..233bea4088 100644
--- a/codex-rs/codex-client/Cargo.toml
+++ b/codex-rs/codex-client/Cargo.toml
@@ -19,6 +19,7 @@ thiserror = { workspace = true }
 tokio = { workspace = true, features = ["macros", "rt", "time", "sync"] }
 tracing = { workspace = true }
 tracing-opentelemetry = { workspace = true }
+zstd = { workspace = true }
 
 [lints]
 workspace = true
diff --git a/codex-rs/codex-client/src/default_client.rs b/codex-rs/codex-client/src/default_client.rs
index efb4d5aec4..781ded3614 100644
--- a/codex-rs/codex-client/src/default_client.rs
+++ b/codex-rs/codex-client/src/default_client.rs
@@ -104,6 +104,13 @@ impl CodexRequestBuilder {
         self.map(|builder| builder.json(value))
     }
 
+    pub fn body<B>(self, body: B) -> Self
+    where
+        B: Into<reqwest::Body>,
+    {
+        self.map(|builder| builder.body(body))
+    }
+
     pub async fn send(self) -> Result<Response, reqwest::Error> {
         let headers = trace_headers();
 
diff --git a/codex-rs/codex-client/src/lib.rs b/codex-rs/codex-client/src/lib.rs
index 66d1083c07..089d777c3a 100644
--- a/codex-rs/codex-client/src/lib.rs
+++ b/codex-rs/codex-client/src/lib.rs
@@ -11,6 +11,7 @@ pub use crate::default_client::CodexRequestBuilder;
 pub use crate::error::StreamError;
 pub use crate::error::TransportError;
 pub use crate::request::Request;
+pub use crate::request::RequestCompression;
 pub use crate::request::Response;
 pub use crate::retry::RetryOn;
 pub use crate::retry::RetryPolicy;
diff --git a/codex-rs/codex-client/src/request.rs b/codex-rs/codex-client/src/request.rs
index f3d205de99..c2c9cf2b33 100644
--- a/codex-rs/codex-client/src/request.rs
+++ b/codex-rs/codex-client/src/request.rs
@@ -5,12 +5,20 @@ use serde::Serialize;
 use serde_json::Value;
 use std::time::Duration;
 
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
+pub enum RequestCompression {
+    #[default]
+    None,
+    Zstd,
+}
+
 #[derive(Debug, Clone)]
 pub struct Request {
     pub method: Method,
     pub url: String,
     pub headers: HeaderMap,
     pub body: Option<Value>,
+    pub compression: RequestCompression,
     pub timeout: Option<Duration>,
 }
 
@@ -21,6 +29,7 @@ impl Request {
             url,
             headers: HeaderMap::new(),
             body: None,
+            compression: RequestCompression::None,
             timeout: None,
         }
     }
@@ -29,6 +38,11 @@ impl Request {
         self.body = serde_json::to_value(body).ok();
         self
     }
+
+    pub fn with_compression(mut self, compression: RequestCompression) -> Self {
+        self.compression = compression;
+        self
+    }
 }
 
 #[derive(Debug, Clone)]
diff --git a/codex-rs/codex-client/src/transport.rs b/codex-rs/codex-client/src/transport.rs
index abe6e29ee5..50e9f8fab7 100644
--- a/codex-rs/codex-client/src/transport.rs
+++ b/codex-rs/codex-client/src/transport.rs
@@ -2,6 +2,7 @@ use crate::default_client::CodexHttpClient;
 use crate::default_client::CodexRequestBuilder;
 use crate::error::TransportError;
 use crate::request::Request;
+use crate::request::RequestCompression;
 use crate::request::Response;
 use async_trait::async_trait;
 use bytes::Bytes;
@@ -41,18 +42,70 @@ impl ReqwestTransport {
     }
 
     fn build(&self, req: Request) -> Result<CodexRequestBuilder, TransportError> {
-        let mut builder = self
-            .client
-            .request(
-                Method::from_bytes(req.method.as_str().as_bytes()).unwrap_or(Method::GET),
-                &req.url,
-            )
-            .headers(req.headers);
-        if let Some(timeout) = req.timeout {
+        let Request {
+            method,
+            url,
+            mut headers,
+            body,
+            compression,
+            timeout,
+        } = req;
+
+        let mut builder = self.client.request(
+            Method::from_bytes(method.as_str().as_bytes()).unwrap_or(Method::GET),
+            &url,
+        );
+
+        if let Some(timeout) = timeout {
             builder = builder.timeout(timeout);
         }
-        if let Some(body) = req.body {
-            builder = builder.json(&body);
+
+        if let Some(body) = body {
+            if compression != RequestCompression::None {
+                if headers.contains_key(http::header::CONTENT_ENCODING) {
+                    return Err(TransportError::Build(
+                        "request compression was requested but content-encoding is already set"
+                            .to_string(),
+                    ));
+                }
+
+                let json = serde_json::to_vec(&body)
+                    .map_err(|err| TransportError::Build(err.to_string()))?;
+                let pre_compression_bytes = json.len();
+                let compression_start = std::time::Instant::now();
+                let (compressed, content_encoding) = match compression {
+                    RequestCompression::None => unreachable!("guarded by compression != None"),
+                    RequestCompression::Zstd => (
+                        zstd::stream::encode_all(std::io::Cursor::new(json), 3)
+                            .map_err(|err| TransportError::Build(err.to_string()))?,
+                        http::HeaderValue::from_static("zstd"),
+                    ),
+                };
+                let post_compression_bytes = compressed.len();
+                let compression_duration = compression_start.elapsed();
+
+                // Ensure the server knows to unpack the request body.
+                headers.insert(http::header::CONTENT_ENCODING, content_encoding);
+                if !headers.contains_key(http::header::CONTENT_TYPE) {
+                    headers.insert(
+                        http::header::CONTENT_TYPE,
+                        http::HeaderValue::from_static("application/json"),
+                    );
+                }
+
+                tracing::info!(
+                    pre_compression_bytes,
+                    post_compression_bytes,
+                    compression_duration_ms = compression_duration.as_millis(),
+                    "Compressed request body with zstd"
+                );
+
+                builder = builder.headers(headers).body(compressed);
+            } else {
+                builder = builder.headers(headers).json(&body);
+            }
+        } else {
+            builder = builder.headers(headers);
         }
         Ok(builder)
     }
diff --git a/codex-rs/core/Cargo.toml b/codex-rs/core/Cargo.toml
index 51fab19dec..5060092290 100644
--- a/codex-rs/core/Cargo.toml
+++ b/codex-rs/core/Cargo.toml
@@ -137,6 +137,7 @@ tracing-subscriber = { workspace = true }
 tracing-test = { workspace = true, features = ["no-env-filter"] }
 walkdir = { workspace = true }
 wiremock = { workspace = true }
+zstd = { workspace = true }
 
 [package.metadata.cargo-shear]
 ignored = ["openssl-sys"]
diff --git a/codex-rs/core/src/client.rs b/codex-rs/core/src/client.rs
index c43c074c44..b1b9480e75 100644
--- a/codex-rs/core/src/client.rs
+++ b/codex-rs/core/src/client.rs
@@ -17,6 +17,7 @@ use codex_api::TransportError;
 use codex_api::common::Reasoning;
 use codex_api::create_text_param_for_request;
 use codex_api::error::ApiError;
+use codex_api::requests::responses::Compression;
 use codex_app_server_protocol::AuthMode;
 use codex_otel::otel_manager::OtelManager;
 use codex_protocol::ThreadId;
@@ -47,6 +48,7 @@ use crate::default_client::build_reqwest_client;
 use crate::error::CodexErr;
 use crate::error::Result;
 use crate::features::FEATURES;
+use crate::features::Feature;
 use crate::flags::CODEX_RS_SSE_FIXTURE;
 use crate::model_provider_info::ModelProviderInfo;
 use crate::model_provider_info::WireApi;
@@ -250,6 +252,20 @@ impl ModelClient {
             let api_auth = auth_provider_from_auth(auth.clone(), &self.provider).await?;
             let transport = ReqwestTransport::new(build_reqwest_client());
             let (request_telemetry, sse_telemetry) = self.build_streaming_telemetry();
+            let compression = if self
+                .config
+                .features
+                .enabled(Feature::EnableRequestCompression)
+                && auth
+                    .as_ref()
+                    .is_some_and(|auth| auth.mode == AuthMode::ChatGPT)
+                && self.provider.is_openai()
+            {
+                Compression::Zstd
+            } else {
+                Compression::None
+            };
+
             let client = ApiResponsesClient::new(transport, api_provider, api_auth)
                 .with_telemetry(Some(request_telemetry), Some(sse_telemetry));
 
@@ -262,6 +278,7 @@ impl ModelClient {
                 conversation_id: Some(conversation_id.clone()),
                 session_source: Some(session_source.clone()),
                 extra_headers: beta_feature_headers(&self.config),
+                compression,
             };
 
             let stream_result = client
diff --git a/codex-rs/core/src/features.rs b/codex-rs/core/src/features.rs
index 7ef74cfb7a..e1af096af5 100644
--- a/codex-rs/core/src/features.rs
+++ b/codex-rs/core/src/features.rs
@@ -89,6 +89,8 @@ pub enum Feature {
     Tui2,
     /// Enforce UTF8 output in Powershell.
     PowershellUtf8,
+    /// Compress request bodies (zstd) when sending streaming requests to codex-backend.
+    EnableRequestCompression,
 }
 
 impl Feature {
@@ -374,6 +376,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Experimental,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::EnableRequestCompression,
+        key: "enable_request_compression",
+        stage: Stage::Experimental,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::Tui2,
         key: "tui2",
diff --git a/codex-rs/core/tests/suite/mod.rs b/codex-rs/core/tests/suite/mod.rs
index 7242641072..effbc8a931 100644
--- a/codex-rs/core/tests/suite/mod.rs
+++ b/codex-rs/core/tests/suite/mod.rs
@@ -44,6 +44,7 @@ mod prompt_caching;
 mod quota_exceeded;
 mod read_file;
 mod remote_models;
+mod request_compression;
 mod resume;
 mod resume_warning;
 mod review;
diff --git a/codex-rs/core/tests/suite/request_compression.rs b/codex-rs/core/tests/suite/request_compression.rs
new file mode 100644
index 0000000000..271f67e1af
--- /dev/null
+++ b/codex-rs/core/tests/suite/request_compression.rs
@@ -0,0 +1,116 @@
+#![cfg(not(target_os = "windows"))]
+
+use codex_core::CodexAuth;
+use codex_core::features::Feature;
+use codex_core::protocol::EventMsg;
+use codex_core::protocol::Op;
+use codex_protocol::user_input::UserInput;
+use core_test_support::responses::ev_completed;
+use core_test_support::responses::ev_response_created;
+use core_test_support::responses::get_responses_requests;
+use core_test_support::responses::mount_sse_once;
+use core_test_support::responses::sse;
+use core_test_support::responses::start_mock_server;
+use core_test_support::skip_if_no_network;
+use core_test_support::test_codex::test_codex;
+use core_test_support::wait_for_event;
+use pretty_assertions::assert_eq;
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn request_body_is_zstd_compressed_for_codex_backend_when_enabled() -> anyhow::Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    mount_sse_once(
+        &server,
+        sse(vec![ev_response_created("resp-1"), ev_completed("resp-1")]),
+    )
+    .await;
+
+    let base_url = format!("{}/backend-api/codex/v1", server.uri());
+    let mut builder = test_codex()
+        .with_auth(CodexAuth::create_dummy_chatgpt_auth_for_testing())
+        .with_config(move |config| {
+            config.features.enable(Feature::EnableRequestCompression);
+            config.model_provider.base_url = Some(base_url);
+        });
+    let codex = builder.build(&server).await?.codex;
+
+    codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "compress me".into(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await?;
+
+    // Wait until the task completes so the request definitely hit the server.
+    wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
+
+    let requests = get_responses_requests(&server).await;
+    assert_eq!(requests.len(), 1);
+
+    let request = &requests[0];
+    let content_encoding = request
+        .headers
+        .get("content-encoding")
+        .and_then(|v| v.to_str().ok());
+    assert_eq!(content_encoding, Some("zstd"));
+
+    let decompressed = zstd::stream::decode_all(std::io::Cursor::new(request.body.clone()))?;
+    let json: serde_json::Value = serde_json::from_slice(&decompressed)?;
+    assert!(
+        json.get("input").is_some(),
+        "expected request body to decode as Responses API JSON"
+    );
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn request_body_is_not_compressed_for_api_key_auth_even_when_enabled() -> anyhow::Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    mount_sse_once(
+        &server,
+        sse(vec![ev_response_created("resp-1"), ev_completed("resp-1")]),
+    )
+    .await;
+
+    let base_url = format!("{}/backend-api/codex/v1", server.uri());
+    let mut builder = test_codex().with_config(move |config| {
+        config.features.enable(Feature::EnableRequestCompression);
+        config.model_provider.base_url = Some(base_url);
+    });
+    let codex = builder.build(&server).await?.codex;
+
+    codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "do not compress".into(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await?;
+
+    wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
+
+    let requests = get_responses_requests(&server).await;
+    assert_eq!(requests.len(), 1);
+
+    let request = &requests[0];
+    assert!(
+        request.headers.get("content-encoding").is_none(),
+        "did not expect request compression for API-key auth"
+    );
+
+    let json: serde_json::Value = serde_json::from_slice(&request.body)?;
+    assert!(
+        json.get("input").is_some(),
+        "expected request body to be plain Responses API JSON"
+    );
+
+    Ok(())
+}