Fix custom CA login behind TLS-inspecting proxies (#20676)

Refs:
https://linear.app/openai/issue/SE-6311/login-fails-for-experian-users-behind-tls-inspecting-proxy

## Summary
- When a custom CA bundle is configured, force the shared `codex-client`
reqwest builder onto rustls before registering custom roots.
- Add the `rustls-tls-native-roots` reqwest feature so the rustls client
preserves native roots plus the enterprise CA bundle.
- Add subprocess TLS coverage for both a direct local TLS 1.3 server and
a hermetic local CONNECT TLS-intercepting proxy that forwards a
token-exchange-shaped POST to a local origin.

## Plain-language explanation
Experian users are behind a TLS-inspecting proxy, so the login token
exchange needs to trust the enterprise CA bundle from
`CODEX_CA_CERTIFICATE` or `SSL_CERT_FILE`. Before this change, that
custom-CA branch still used reqwest default TLS selection, which could
fail in the proxy environment. Now, only when a custom CA is configured,
Codex selects rustls first and then adds the custom CA roots, matching
the validated behavior from the Experian test build while leaving normal
system-root clients unchanged.

The new regression test recreates the enterprise-proxy shape locally:
the probe client sends an HTTPS `POST /oauth/token` through an explicit
HTTP CONNECT proxy, the proxy presents a leaf certificate signed by a
runtime-generated test CA, decrypts the request, forwards it to a local
origin, and relays the `ok` response back.

## Scope note
- The actual production fix is the first commit: `8368119282 Fix custom
CA reqwest clients to use rustls`.
- The second commit is integration-test coverage only. It generates all
test CA and localhost certificate material at runtime.

## Validation
- `cd codex-rs && cargo test -p codex-client --test ca_env
posts_to_token_origin_through_tls_intercepting_proxy_with_custom_ca_bundle
-- --nocapture`
- `cd codex-rs && cargo test -p codex-client`
- `cd codex-rs && cargo test -p codex-login`
- `cd codex-rs && just fmt`
- `cd codex-rs && just bazel-lock-update`
- `cd codex-rs && just bazel-lock-check`
- `cd codex-rs && just fix -p codex-client`

codex-rs/Cargo.lock

@@ -2220,6 +2220,7 @@ dependencies = [
  "opentelemetry_sdk",
  "pretty_assertions",
  "rand 0.9.3",
+ "rcgen",
  "reqwest",
  "rustls",
  "rustls-native-certs",

codex-rs/Cargo.toml

@@ -322,6 +322,10 @@ quick-xml = "0.38.4"
 rand = "0.9"
 ratatui = "0.29.0"
 ratatui-macros = "0.6.0"
+rcgen = { version = "0.14.7", default-features = false, features = [
+    "aws_lc_rs",
+    "pem",
+] }
 regex = "1.12.3"
 regex-lite = "0.1.8"
 reqwest = { version = "0.12", features = ["cookies"] }

codex-rs/codex-client/Cargo.toml

@@ -12,7 +12,7 @@ futures = { workspace = true }
 http = { workspace = true }
 opentelemetry = { workspace = true }
 rand = { workspace = true }
-reqwest = { workspace = true, features = ["json", "stream"] }
+reqwest = { workspace = true, features = ["json", "rustls-tls-native-roots", "stream"] }
 rustls = { workspace = true }
 rustls-native-certs = { workspace = true }
 rustls-pki-types = { workspace = true }
@@ -32,5 +32,6 @@ workspace = true
 codex-utils-cargo-bin = { workspace = true }
 opentelemetry_sdk = { workspace = true }
 pretty_assertions = { workspace = true }
+rcgen = { workspace = true }
 tempfile = { workspace = true }
 tracing-subscriber = { workspace = true }

codex-rs/codex-client/src/bin/custom_ca_probe.rs

@@ -8,22 +8,93 @@
 //! - env precedence is respected,
 //! - multi-cert PEM bundles load,
 //! - error messages guide users when CA files are invalid.
+//! - optional HTTPS probes can complete a request through the constructed client.
 //!
 //! The detailed explanation of what "hermetic" means here lives in `codex_client::custom_ca`.
 //! This binary exists so the tests can exercise
 //! [`codex_client::build_reqwest_client_for_subprocess_tests`] in a separate process without
 //! duplicating client-construction logic.
 
+use std::env;
 use std::process;
+use std::time::Duration;
+
+const PROBE_TLS13_ENV: &str = "CODEX_CUSTOM_CA_PROBE_TLS13";
+const PROBE_PROXY_ENV: &str = "CODEX_CUSTOM_CA_PROBE_PROXY";
+const PROBE_URL_ENV: &str = "CODEX_CUSTOM_CA_PROBE_URL";
 
 fn main() {
-    match codex_client::build_reqwest_client_for_subprocess_tests(reqwest::Client::builder()) {
-        Ok(_) => {
-            println!("ok");
+    let runtime = match tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()
+    {
+        Ok(runtime) => runtime,
+        Err(error) => {
+            eprintln!("failed to create probe runtime: {error}");
+            process::exit(1);
         }
+    };
+
+    match runtime.block_on(run_probe()) {
+        Ok(()) => println!("ok"),
         Err(error) => {
             eprintln!("{error}");
             process::exit(1);
         }
     }
 }
+
+async fn run_probe() -> Result<(), String> {
+    let proxy_url = env::var(PROBE_PROXY_ENV).ok();
+    let target_url = env::var(PROBE_URL_ENV).ok();
+    let mut builder = reqwest::Client::builder();
+    if target_url.is_some() {
+        builder = builder.timeout(Duration::from_secs(5));
+    }
+    if env::var_os(PROBE_TLS13_ENV).is_some() {
+        builder = builder.min_tls_version(reqwest::tls::Version::TLS_1_3);
+    }
+
+    let client = build_probe_client(builder, proxy_url.as_deref())?;
+    if let Some(url) = target_url {
+        post_probe_request(&client, &url).await?;
+    }
+    Ok(())
+}
+
+fn build_probe_client(
+    builder: reqwest::ClientBuilder,
+    proxy_url: Option<&str>,
+) -> Result<reqwest::Client, String> {
+    if let Some(proxy_url) = proxy_url {
+        let proxy = reqwest::Proxy::https(proxy_url)
+            .map_err(|error| format!("failed to configure probe proxy {proxy_url}: {error}"))?;
+        return codex_client::build_reqwest_client_with_custom_ca(builder.proxy(proxy))
+            .map_err(|error| error.to_string());
+    }
+
+    codex_client::build_reqwest_client_for_subprocess_tests(builder)
+        .map_err(|error| error.to_string())
+}
+
+async fn post_probe_request(client: &reqwest::Client, url: &str) -> Result<(), String> {
+    let response = client
+        .post(url)
+        .header("Content-Type", "application/x-www-form-urlencoded")
+        .body("grant_type=authorization_code&code=test")
+        .send()
+        .await
+        .map_err(|error| format!("probe request failed: {error:?}"))?;
+    let status = response.status();
+    let body = response
+        .text()
+        .await
+        .map_err(|error| format!("failed to read probe response body: {error}"))?;
+    if !status.is_success() {
+        return Err(format!("probe request returned {status}: {body}"));
+    }
+    if body != "ok" {
+        return Err(format!("probe response body mismatch: {body}"));
+    }
+    Ok(())
+}

codex-rs/codex-client/src/custom_ca.rs

@@ -14,10 +14,9 @@
 //!   `TRUSTED CERTIFICATE` labels and bundles that also contain CRLs
 //! - return user-facing errors that explain how to fix misconfigured CA files
 //!
-//! It does not validate certificate chains or perform a handshake in tests. Its contract is
-//! narrower: produce a transport configuration whose root store contains every parseable
-//! certificate block from the configured PEM bundle, or fail early with a precise error before
-//! the caller starts network traffic.
+//! Its production contract is narrow: produce a transport configuration whose root store contains
+//! every parseable certificate block from the configured PEM bundle, or fail early with a precise
+//! error before the caller starts network traffic.
 //!
 //! In this module's test setup, a hermetic test is one whose result depends only on the CA file
 //! and environment variables that the test chose for itself. That matters here because the normal
@@ -36,7 +35,8 @@
 //! - unit tests in this module cover env-selection logic without constructing a real client
 //! - subprocess integration tests under `tests/` cover real client construction through
 //!   [`build_reqwest_client_for_subprocess_tests`], which disables reqwest proxy autodetection so
-//!   the tests can observe custom-CA success and failure directly
+//!   the tests can observe custom-CA success and failure directly, including one TLS handshake
+//!   through a local HTTPS server
 //! - those subprocess tests also scrub inherited CA environment variables before launch so their
 //!   result depends only on the test fixtures and env vars set by the test itself
 
@@ -266,12 +266,21 @@ fn maybe_build_rustls_client_config_with_env(
 /// This exists so tests can exercise precedence behavior deterministically without mutating the
 /// real process environment. It selects the CA bundle, delegates file parsing to
 /// [`ConfiguredCaBundle::load_certificates`], preserves the caller's chosen `reqwest` builder
-/// configuration, and finally registers each parsed certificate with that builder.
+/// configuration, forces rustls when a custom CA is configured, and finally registers each parsed
+/// certificate with that builder.
 fn build_reqwest_client_with_env(
     env_source: &dyn EnvSource,
     mut builder: reqwest::ClientBuilder,
 ) -> Result<reqwest::Client, BuildCustomCaTransportError> {
     if let Some(bundle) = env_source.configured_ca_bundle() {
+        ensure_rustls_crypto_provider();
+        info!(
+            source_env = bundle.source_env,
+            ca_path = %bundle.path.display(),
+            "building HTTP client with rustls backend for custom CA bundle"
+        );
+        builder = builder.use_rustls_tls();
+
         let certificates = bundle.load_certificates()?;
 
         for (idx, cert) in certificates.iter().enumerate() {

codex-rs/codex-client/tests/ca_env.rs

@@ -4,24 +4,83 @@
 //! `build_reqwest_client_for_subprocess_tests` instead of calling the helper in-process. The
 //! detailed explanation of what "hermetic" means here lives in `codex_client::custom_ca`; these
 //! tests add the process-level half of that contract by scrubbing inherited CA environment
-//! variables before each subprocess launch. They still stop at client construction: the
-//! assertions here cover CA file selection, PEM parsing, and user-facing errors, not a full TLS
-//! handshake.
+//! variables before each subprocess launch. Most assertions here cover CA file selection, PEM
+//! parsing, and user-facing errors. The HTTPS probes go further and perform real POSTs against
+//! locally generated certificates, including through a TLS-intercepting CONNECT proxy.
 
 use codex_utils_cargo_bin::cargo_bin;
+use rcgen::BasicConstraints;
+use rcgen::CertificateParams;
+use rcgen::CertifiedIssuer;
+use rcgen::DistinguishedName;
+use rcgen::DnType;
+use rcgen::ExtendedKeyUsagePurpose;
+use rcgen::IsCa;
+use rcgen::KeyPair;
+use rcgen::KeyUsagePurpose;
+use rcgen::PKCS_ECDSA_P256_SHA256;
+use rustls_pki_types::CertificateDer;
+use rustls_pki_types::PrivateKeyDer;
 use std::fs;
+use std::io;
+use std::io::Read;
+use std::io::Write;
+use std::net::TcpListener;
+use std::net::TcpStream;
 use std::path::Path;
+use std::path::PathBuf;
 use std::process::Command;
+use std::sync::Arc;
+use std::sync::mpsc;
+use std::thread;
+use std::time::Duration;
+use std::time::Instant;
 use tempfile::TempDir;
 
 const CODEX_CA_CERT_ENV: &str = "CODEX_CA_CERTIFICATE";
+const PROBE_PROXY_ENV: &str = "CODEX_CUSTOM_CA_PROBE_PROXY";
+const PROBE_TLS13_ENV: &str = "CODEX_CUSTOM_CA_PROBE_TLS13";
+const PROBE_URL_ENV: &str = "CODEX_CUSTOM_CA_PROBE_URL";
 const SSL_CERT_FILE_ENV: &str = "SSL_CERT_FILE";
+const PROXY_ENV_VARS: &[&str] = &[
+    "HTTP_PROXY",
+    "http_proxy",
+    "HTTPS_PROXY",
+    "https_proxy",
+    "ALL_PROXY",
+    "all_proxy",
+    "NO_PROXY",
+    "no_proxy",
+];
 
 const TEST_CERT_1: &str = include_str!("fixtures/test-ca.pem");
 const TEST_CERT_2: &str = include_str!("fixtures/test-intermediate.pem");
 const TRUSTED_TEST_CERT: &str = include_str!("fixtures/test-ca-trusted.pem");
 
-fn write_cert_file(temp_dir: &TempDir, name: &str, contents: &str) -> std::path::PathBuf {
+struct Tls13Material {
+    ca_cert_pem: String,
+    server_cert: CertificateDer<'static>,
+    server_key: PrivateKeyDer<'static>,
+}
+
+struct Tls13TestServer {
+    ca_cert_pem: String,
+    request_rx: mpsc::Receiver<Result<String, String>>,
+    url: String,
+}
+
+struct PlainHttpOrigin {
+    request_rx: mpsc::Receiver<Result<String, String>>,
+    url: String,
+}
+
+struct TlsInterceptingProxy {
+    ca_cert_pem: String,
+    request_rx: mpsc::Receiver<Result<String, String>>,
+    url: String,
+}
+
+fn write_cert_file(temp_dir: &TempDir, name: &str, contents: &str) -> PathBuf {
     let path = temp_dir.path().join(name);
     fs::write(&path, contents).unwrap_or_else(|error| {
         panic!("write cert fixture failed for {}: {error}", path.display())
@@ -29,7 +88,7 @@ fn write_cert_file(temp_dir: &TempDir, name: &str, contents: &str) -> std::path:
     path
 }
 
-fn run_probe(envs: &[(&str, &Path)]) -> std::process::Output {
+fn probe_command() -> Command {
     let mut cmd = Command::new(
         cargo_bin("custom_ca_probe")
             .unwrap_or_else(|error| panic!("failed to locate custom_ca_probe: {error}")),
@@ -37,7 +96,18 @@ fn run_probe(envs: &[(&str, &Path)]) -> std::process::Output {
     // `Command` inherits the parent environment by default, so scrub CA-related variables first or
     // these tests can accidentally pass/fail based on the developer shell or CI runner.
     cmd.env_remove(CODEX_CA_CERT_ENV);
+    cmd.env_remove(PROBE_PROXY_ENV);
+    cmd.env_remove(PROBE_TLS13_ENV);
+    cmd.env_remove(PROBE_URL_ENV);
     cmd.env_remove(SSL_CERT_FILE_ENV);
+    for env_var in PROXY_ENV_VARS {
+        cmd.env_remove(env_var);
+    }
+    cmd
+}
+
+fn run_probe(envs: &[(&str, &Path)]) -> std::process::Output {
+    let mut cmd = probe_command();
     for (key, value) in envs {
         cmd.env(key, value);
     }
@@ -45,6 +115,286 @@ fn run_probe(envs: &[(&str, &Path)]) -> std::process::Output {
         .unwrap_or_else(|error| panic!("failed to run custom_ca_probe: {error}"))
 }
 
+fn run_probe_posting_to_tls13_server(envs: &[(&str, &Path)], url: &str) -> std::process::Output {
+    let mut cmd = probe_command();
+    for (key, value) in envs {
+        cmd.env(key, value);
+    }
+    cmd.env(PROBE_TLS13_ENV, "1");
+    cmd.env(PROBE_URL_ENV, url);
+    cmd.output()
+        .unwrap_or_else(|error| panic!("failed to run custom_ca_probe: {error}"))
+}
+
+fn run_probe_posting_through_tls_intercepting_proxy(
+    envs: &[(&str, &Path)],
+    url: &str,
+    proxy_url: &str,
+) -> std::process::Output {
+    let mut cmd = probe_command();
+    for (key, value) in envs {
+        cmd.env(key, value);
+    }
+    cmd.env(PROBE_PROXY_ENV, proxy_url);
+    cmd.env(PROBE_TLS13_ENV, "1");
+    cmd.env(PROBE_URL_ENV, url);
+    cmd.output()
+        .unwrap_or_else(|error| panic!("failed to run custom_ca_probe: {error}"))
+}
+
+fn spawn_tls13_test_server() -> Tls13TestServer {
+    codex_utils_rustls_provider::ensure_rustls_crypto_provider();
+    let material = generate_tls13_material();
+    let listener = TcpListener::bind(("127.0.0.1", 0))
+        .unwrap_or_else(|error| panic!("bind TLS test server: {error}"));
+    listener
+        .set_nonblocking(true)
+        .unwrap_or_else(|error| panic!("set TLS test server nonblocking: {error}"));
+    let port = listener
+        .local_addr()
+        .unwrap_or_else(|error| panic!("TLS test server addr: {error}"))
+        .port();
+    let config = Arc::new(
+        rustls::ServerConfig::builder_with_protocol_versions(&[&rustls::version::TLS13])
+            .with_no_client_auth()
+            .with_single_cert(vec![material.server_cert], material.server_key)
+            .unwrap_or_else(|error| panic!("TLS 1.3 server config: {error}")),
+    );
+    let (request_tx, request_rx) = mpsc::channel();
+
+    thread::spawn(move || {
+        let result = accept_tls13_request(listener, config);
+        let _ = request_tx.send(result.map_err(|error| error.to_string()));
+    });
+
+    Tls13TestServer {
+        ca_cert_pem: material.ca_cert_pem,
+        request_rx,
+        url: format!("https://127.0.0.1:{port}/oauth/token"),
+    }
+}
+
+fn spawn_plain_http_origin() -> PlainHttpOrigin {
+    let listener = TcpListener::bind(("127.0.0.1", 0))
+        .unwrap_or_else(|error| panic!("bind plain HTTP origin: {error}"));
+    listener
+        .set_nonblocking(true)
+        .unwrap_or_else(|error| panic!("set plain HTTP origin nonblocking: {error}"));
+    let port = listener
+        .local_addr()
+        .unwrap_or_else(|error| panic!("plain HTTP origin addr: {error}"))
+        .port();
+    let (request_tx, request_rx) = mpsc::channel();
+
+    thread::spawn(move || {
+        let result = accept_plain_http_origin_request(listener);
+        let _ = request_tx.send(result.map_err(|error| error.to_string()));
+    });
+
+    PlainHttpOrigin {
+        request_rx,
+        url: format!("https://127.0.0.1:{port}/oauth/token"),
+    }
+}
+
+fn spawn_tls_intercepting_proxy() -> TlsInterceptingProxy {
+    codex_utils_rustls_provider::ensure_rustls_crypto_provider();
+    let material = generate_tls13_material();
+    let listener = TcpListener::bind(("127.0.0.1", 0))
+        .unwrap_or_else(|error| panic!("bind TLS intercepting proxy: {error}"));
+    listener
+        .set_nonblocking(true)
+        .unwrap_or_else(|error| panic!("set TLS intercepting proxy nonblocking: {error}"));
+    let port = listener
+        .local_addr()
+        .unwrap_or_else(|error| panic!("TLS intercepting proxy addr: {error}"))
+        .port();
+    let config = Arc::new(
+        rustls::ServerConfig::builder_with_protocol_versions(&[&rustls::version::TLS13])
+            .with_no_client_auth()
+            .with_single_cert(vec![material.server_cert], material.server_key)
+            .unwrap_or_else(|error| panic!("TLS intercepting proxy config: {error}")),
+    );
+    let (request_tx, request_rx) = mpsc::channel();
+
+    thread::spawn(move || {
+        let result = accept_tls_intercepting_proxy_request(listener, config);
+        let _ = request_tx.send(result.map_err(|error| error.to_string()));
+    });
+
+    TlsInterceptingProxy {
+        ca_cert_pem: material.ca_cert_pem,
+        request_rx,
+        url: format!("http://127.0.0.1:{port}"),
+    }
+}
+
+fn generate_tls13_material() -> Tls13Material {
+    let mut ca_params = CertificateParams::default();
+    ca_params.is_ca = IsCa::Ca(BasicConstraints::Unconstrained);
+    ca_params.key_usages = vec![KeyUsagePurpose::KeyCertSign, KeyUsagePurpose::CrlSign];
+    let mut ca_distinguished_name = DistinguishedName::new();
+    ca_distinguished_name.push(DnType::CommonName, "codex test CA");
+    ca_params.distinguished_name = ca_distinguished_name;
+    let ca_key_pair = KeyPair::generate_for(&PKCS_ECDSA_P256_SHA256)
+        .unwrap_or_else(|error| panic!("generate test CA key pair: {error}"));
+    let ca = CertifiedIssuer::self_signed(ca_params, ca_key_pair)
+        .unwrap_or_else(|error| panic!("generate test CA certificate: {error}"));
+
+    let mut server_params =
+        CertificateParams::new(vec!["localhost".to_string(), "127.0.0.1".to_string()])
+            .unwrap_or_else(|error| panic!("create test server certificate params: {error}"));
+    server_params.extended_key_usages = vec![ExtendedKeyUsagePurpose::ServerAuth];
+    server_params.key_usages = vec![
+        KeyUsagePurpose::DigitalSignature,
+        KeyUsagePurpose::KeyEncipherment,
+    ];
+    let server_key_pair = KeyPair::generate_for(&PKCS_ECDSA_P256_SHA256)
+        .unwrap_or_else(|error| panic!("generate test server key pair: {error}"));
+    let server_cert = server_params
+        .signed_by(&server_key_pair, &ca)
+        .unwrap_or_else(|error| panic!("generate test server certificate: {error}"));
+
+    Tls13Material {
+        ca_cert_pem: ca.pem(),
+        server_cert: server_cert.der().clone(),
+        server_key: PrivateKeyDer::from(server_key_pair),
+    }
+}
+
+fn accept_plain_http_origin_request(listener: TcpListener) -> io::Result<String> {
+    let mut stream = accept_with_timeout(listener, Duration::from_secs(5))?;
+    stream.set_nonblocking(false)?;
+    stream.set_read_timeout(Some(Duration::from_secs(5)))?;
+    stream.set_write_timeout(Some(Duration::from_secs(5)))?;
+
+    let request = read_http_message(&mut stream)?;
+    stream.write_all(b"HTTP/1.1 200 OK\r\nContent-Length: 2\r\nConnection: close\r\n\r\nok")?;
+    stream.flush()?;
+    Ok(request)
+}
+
+fn accept_tls13_request(
+    listener: TcpListener,
+    config: Arc<rustls::ServerConfig>,
+) -> io::Result<String> {
+    let stream = accept_with_timeout(listener, Duration::from_secs(5))?;
+    stream.set_nonblocking(false)?;
+    stream.set_read_timeout(Some(Duration::from_secs(5)))?;
+    stream.set_write_timeout(Some(Duration::from_secs(5)))?;
+
+    let connection = rustls::ServerConnection::new(config).map_err(io::Error::other)?;
+    let mut tls = rustls::StreamOwned::new(connection, stream);
+    let request = read_http_message(&mut tls)?;
+    tls.write_all(b"HTTP/1.1 200 OK\r\nContent-Length: 2\r\nConnection: close\r\n\r\nok")?;
+    tls.flush()?;
+    Ok(request)
+}
+
+fn accept_tls_intercepting_proxy_request(
+    listener: TcpListener,
+    config: Arc<rustls::ServerConfig>,
+) -> io::Result<String> {
+    let mut stream = accept_with_timeout(listener, Duration::from_secs(5))?;
+    stream.set_nonblocking(false)?;
+    stream.set_read_timeout(Some(Duration::from_secs(5)))?;
+    stream.set_write_timeout(Some(Duration::from_secs(5)))?;
+
+    let connect_request = read_http_message(&mut stream)?;
+    let origin_authority = connect_authority_from_request(&connect_request)?;
+    stream.write_all(b"HTTP/1.1 200 Connection Established\r\n\r\n")?;
+    stream.flush()?;
+
+    let connection = rustls::ServerConnection::new(config).map_err(io::Error::other)?;
+    let mut tls = rustls::StreamOwned::new(connection, stream);
+    let request = read_http_message(&mut tls)?;
+
+    let mut origin = TcpStream::connect(origin_authority.as_str())?;
+    origin.set_read_timeout(Some(Duration::from_secs(5)))?;
+    origin.set_write_timeout(Some(Duration::from_secs(5)))?;
+    origin.write_all(request.as_bytes())?;
+    origin.flush()?;
+    let response = read_http_message(&mut origin)?;
+
+    tls.write_all(response.as_bytes())?;
+    tls.flush()?;
+    Ok(request)
+}
+
+fn connect_authority_from_request(request: &str) -> io::Result<String> {
+    let request_line = request
+        .lines()
+        .next()
+        .ok_or_else(|| io::Error::new(io::ErrorKind::InvalidData, "empty CONNECT request"))?;
+    let mut parts = request_line.split_whitespace();
+    match (parts.next(), parts.next(), parts.next()) {
+        (Some("CONNECT"), Some(authority), Some(_version)) => Ok(authority.to_string()),
+        _ => Err(io::Error::new(
+            io::ErrorKind::InvalidData,
+            format!("invalid CONNECT request line: {request_line}"),
+        )),
+    }
+}
+
+fn accept_with_timeout(listener: TcpListener, timeout: Duration) -> io::Result<TcpStream> {
+    let deadline = Instant::now() + timeout;
+    loop {
+        match listener.accept() {
+            Ok((stream, _)) => return Ok(stream),
+            Err(error) if error.kind() == io::ErrorKind::WouldBlock => {
+                if Instant::now() >= deadline {
+                    return Err(io::Error::new(
+                        io::ErrorKind::TimedOut,
+                        "timed out waiting for TLS test client",
+                    ));
+                }
+                thread::sleep(Duration::from_millis(10));
+            }
+            Err(error) => return Err(error),
+        }
+    }
+}
+
+fn read_http_message(stream: &mut impl Read) -> io::Result<String> {
+    let mut buffer = Vec::new();
+    let mut chunk = [0; 1024];
+    loop {
+        let bytes_read = stream.read(&mut chunk)?;
+        if bytes_read == 0 {
+            break;
+        }
+        buffer.extend_from_slice(&chunk[..bytes_read]);
+        if let Some(header_end) = buffer.windows(4).position(|window| window == b"\r\n\r\n") {
+            let body_start = header_end + 4;
+            let headers = String::from_utf8_lossy(&buffer[..body_start]);
+            let content_length = headers
+                .lines()
+                .filter_map(|line| line.split_once(':'))
+                .find_map(|(name, value)| {
+                    name.eq_ignore_ascii_case("content-length")
+                        .then(|| value.trim().parse::<usize>().ok())
+                        .flatten()
+                })
+                .unwrap_or(0);
+            if buffer.len() >= body_start + content_length {
+                break;
+            }
+        }
+    }
+    Ok(String::from_utf8_lossy(&buffer).into_owned())
+}
+
+fn assert_token_exchange_request(request: &str) {
+    assert!(
+        request.starts_with("POST /oauth/token HTTP/1.1"),
+        "unexpected request:\n{request}"
+    );
+    assert!(
+        request.contains("grant_type=authorization_code&code=test"),
+        "unexpected request body:\n{request}"
+    );
+}
+
 #[test]
 fn uses_codex_ca_cert_env() {
     let temp_dir = TempDir::new().expect("tempdir");
@@ -90,6 +440,59 @@ fn handles_multi_certificate_bundle() {
     assert!(output.status.success());
 }
 
+#[test]
+fn posts_to_tls13_server_using_custom_ca_bundle() {
+    let temp_dir = TempDir::new().expect("tempdir");
+    let server = spawn_tls13_test_server();
+    let cert_path = write_cert_file(&temp_dir, "tls-ca.pem", &server.ca_cert_pem);
+
+    let output =
+        run_probe_posting_to_tls13_server(&[(CODEX_CA_CERT_ENV, cert_path.as_path())], &server.url);
+    let server_result = server.request_rx.recv_timeout(Duration::from_secs(5));
+
+    assert!(
+        output.status.success(),
+        "custom_ca_probe failed\nstdout:\n{}\nstderr:\n{}\nserver:\n{server_result:?}",
+        String::from_utf8_lossy(&output.stdout),
+        String::from_utf8_lossy(&output.stderr)
+    );
+    let request = server_result
+        .expect("TLS test server should report a request")
+        .expect("TLS test server should accept the probe request");
+    assert_token_exchange_request(&request);
+}
+
+#[test]
+fn posts_to_token_origin_through_tls_intercepting_proxy_with_custom_ca_bundle() {
+    let temp_dir = TempDir::new().expect("tempdir");
+    let origin = spawn_plain_http_origin();
+    let proxy = spawn_tls_intercepting_proxy();
+    let cert_path = write_cert_file(&temp_dir, "proxy-ca.pem", &proxy.ca_cert_pem);
+
+    let output = run_probe_posting_through_tls_intercepting_proxy(
+        &[(CODEX_CA_CERT_ENV, cert_path.as_path())],
+        &origin.url,
+        &proxy.url,
+    );
+    let proxy_result = proxy.request_rx.recv_timeout(Duration::from_secs(5));
+    let origin_result = origin.request_rx.recv_timeout(Duration::from_secs(5));
+
+    assert!(
+        output.status.success(),
+        "custom_ca_probe failed\nstdout:\n{}\nstderr:\n{}\nproxy:\n{proxy_result:?}\norigin:\n{origin_result:?}",
+        String::from_utf8_lossy(&output.stdout),
+        String::from_utf8_lossy(&output.stderr)
+    );
+    let proxy_request = proxy_result
+        .expect("TLS intercepting proxy should report a request")
+        .expect("TLS intercepting proxy should accept the probe request");
+    let origin_request = origin_result
+        .expect("plain HTTP origin should report a request")
+        .expect("plain HTTP origin should accept the forwarded request");
+    assert_token_exchange_request(&proxy_request);
+    assert_token_exchange_request(&origin_request);
+}
+
 #[test]
 fn rejects_empty_pem_file_with_hint() {
     let temp_dir = TempDir::new().expect("tempdir");