diff --git a/crates/preloop-runner-server/src/actions.rs b/crates/preloop-runner-server/src/actions.rs
index ee14587dd..15dd6216e 100644
--- a/crates/preloop-runner-server/src/actions.rs
+++ b/crates/preloop-runner-server/src/actions.rs
@@ -351,8 +351,7 @@ pub async fn download_action_tarball(
         repo, git_ref, github_url, "Downloading action to server cache"
     );
 
-    let client = reqwest::Client::builder()
-        .user_agent("preloop-runner-server")
+    let client = crate::shared_http::github_client_builder()
         .build()
         .map_err(|e| ApiError::internal(format!("failed to build reqwest client: {e}")))?;
 
diff --git a/crates/preloop-runner-server/src/github_app.rs b/crates/preloop-runner-server/src/github_app.rs
index 7318386df..25030406e 100644
--- a/crates/preloop-runner-server/src/github_app.rs
+++ b/crates/preloop-runner-server/src/github_app.rs
@@ -1269,7 +1269,7 @@ async fn verify_repo_access_at(
     repo: &str,
 ) -> anyhow::Result<Vec<(String, String)>> {
     let (owner, name) = split_repository(repo)?;
-    let client = reqwest::Client::builder()
+    let client = crate::shared_http::github_client_builder()
         .user_agent("preloop-doctor")
         .build()
         .context("building GitHub API client")?;
diff --git a/crates/preloop-runner-server/src/remote_workflows.rs b/crates/preloop-runner-server/src/remote_workflows.rs
index e8cfa1336..45bbe029e 100644
--- a/crates/preloop-runner-server/src/remote_workflows.rs
+++ b/crates/preloop-runner-server/src/remote_workflows.rs
@@ -71,8 +71,7 @@ pub async fn resolve_remote_workflows(
                 continue;
             }
             let client = client.get_or_insert(
-                reqwest::Client::builder()
-                    .user_agent("preloop-runner-server")
+                crate::shared_http::github_client_builder()
                     .build()
                     .map_err(|error| ApiError::internal(format!("build GitHub client: {error}")))?,
             );
diff --git a/crates/preloop-runner-server/src/shared_http.rs b/crates/preloop-runner-server/src/shared_http.rs
index 4fa3ee421..3b50db9e2 100644
--- a/crates/preloop-runner-server/src/shared_http.rs
+++ b/crates/preloop-runner-server/src/shared_http.rs
@@ -1,12 +1,127 @@
 use std::time::Duration;
 
-/// Shared HTTP client for server-side outbound requests (GitHub API, scheduler).
-/// Configured with standard timeouts and user-agent.
-pub static CLIENT: std::sync::LazyLock<reqwest::Client> = std::sync::LazyLock::new(|| {
-    reqwest::Client::builder()
+/// Root certificates loaded for server-side outbound HTTPS.
+///
+/// Sources (in order):
+///   1. `PRELOOP_GITHUB_CA_FILE` — a PEM bundle that should additionally be
+///      trusted when talking to a GHES / GitHub-emulator forge (gh-simulate
+///      serves a self-signed CA). Forge-scoped, additive to the system roots.
+///   2. `SSL_CERT_FILE` — the OpenSSL/native convention the runner already
+///      honors in `client::http::with_control`; kept here so the engine's
+///      GitHub clients follow the same environment override on platforms
+///      whose reqwest backend does not read it.
+///
+/// Both are optional and additive to the bundled webpki/native roots. A
+/// missing or unreadable file fails closed (the client simply has the default
+/// roots) — we log rather than abort startup, since TLS verification itself
+/// stays on.
+fn ca_certificates() -> Vec<reqwest::Certificate> {
+    let mut out = Vec::new();
+    for var in ["PRELOOP_GITHUB_CA_FILE", "SSL_CERT_FILE"] {
+        let Ok(path) = std::env::var(var) else {
+            continue;
+        };
+        if path.trim().is_empty() {
+            continue;
+        }
+        out.extend(load_ca_bundle(std::path::Path::new(&path), var));
+    }
+    out
+}
+
+/// Read one PEM bundle from `path` and parse its certificates. Missing or
+/// invalid files log and return empty — verification stays on, we just add no
+/// roots. Split out so tests exercise it without mutating process env.
+fn load_ca_bundle(path: &std::path::Path, env: &str) -> Vec<reqwest::Certificate> {
+    match std::fs::read(path) {
+        Ok(pem) => match reqwest::Certificate::from_pem_bundle(&pem) {
+            Ok(certs) => {
+                tracing::info!(path = %path.display(), env, count = certs.len(),
+                    "loaded additional TLS root certificates for outbound GitHub calls");
+                certs
+            }
+            Err(e) => {
+                tracing::warn!(path = %path.display(), env, error = %e,
+                    "ignoring CA bundle with invalid PEM");
+                Vec::new()
+            }
+        },
+        Err(e) => {
+            tracing::warn!(path = %path.display(), env, error = %e,
+                "ignoring unreadable CA bundle");
+            Vec::new()
+        }
+    }
+}
+
+/// Build a reqwest client with the shared timeouts and the extra CA roots
+/// from [`ca_certificates`]. Use this for every server-side outbound HTTPS
+/// call that can target a GHES/emulator forge instead of ad-hoc
+/// `reqwest::Client::builder()` so custom roots apply uniformly.
+pub fn github_client_builder() -> reqwest::ClientBuilder {
+    let mut b = reqwest::Client::builder()
         .timeout(Duration::from_secs(30))
         .connect_timeout(Duration::from_secs(10))
-        .user_agent("preloop-runner-server")
+        .user_agent("preloop-runner-server");
+    for cert in ca_certificates() {
+        b = b.add_root_certificate(cert);
+    }
+    b
+}
+
+/// Shared HTTP client for server-side outbound requests (GitHub API, scheduler).
+/// Configured with standard timeouts, user-agent, and the CA roots from
+/// [`ca_certificates`] so `PRELOOP_GITHUB_CA_FILE`/`SSL_CERT_FILE` reach every
+/// caller.
+pub static CLIENT: std::sync::LazyLock<reqwest::Client> = std::sync::LazyLock::new(|| {
+    github_client_builder()
         .build()
         .expect("HTTP client builder")
 });
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn write_self_signed_pem(path: &std::path::Path) {
+        let key = std::env::temp_dir().join("ghsim_ca_test_key.pem");
+        let _ = std::process::Command::new("openssl")
+            .args([
+                "req",
+                "-x509",
+                "-newkey",
+                "rsa:2048",
+                "-keyout",
+                key.to_str().unwrap(),
+                "-out",
+                path.to_str().unwrap(),
+                "-days",
+                "1",
+                "-nodes",
+                "-subj",
+                "/CN=gh-simulate-test",
+            ])
+            .output();
+    }
+
+    #[test]
+    fn parses_pem_bundle() {
+        let p = std::env::temp_dir().join("ghsim_ca_file.pem");
+        write_self_signed_pem(&p);
+        let certs = load_ca_bundle(&p, "TEST");
+        assert!(!certs.is_empty());
+    }
+
+    #[test]
+    fn missing_ca_file_returns_empty_not_panic() {
+        let certs = load_ca_bundle(std::path::Path::new("/nonexistent/ca.pem"), "TEST");
+        assert!(certs.is_empty());
+    }
+
+    #[test]
+    fn invalid_pem_returns_empty_not_panic() {
+        let p = std::env::temp_dir().join("ghsim_bad.pem");
+        std::fs::write(&p, b"not a pem").unwrap();
+        assert!(load_ca_bundle(&p, "TEST").is_empty());
+    }
+}
diff --git a/crates/preloop-runner-server/src/snapshots.rs b/crates/preloop-runner-server/src/snapshots.rs
index 7af88c8f4..edd8c56e9 100644
--- a/crates/preloop-runner-server/src/snapshots.rs
+++ b/crates/preloop-runner-server/src/snapshots.rs
@@ -1906,10 +1906,8 @@ async fn lfs_batch_response(
 /// purpose: blobs stream straight to disk and can be large; the per-object
 /// byte cap, not the clock, is the backstop.
 static LFS_FORGE_CLIENT: std::sync::LazyLock<reqwest::Client> = std::sync::LazyLock::new(|| {
-    reqwest::Client::builder()
+    crate::shared_http::github_client_builder()
         .timeout(std::time::Duration::from_secs(300))
-        .connect_timeout(std::time::Duration::from_secs(10))
-        .user_agent("preloop-runner-server")
         .build()
         .expect("LFS forge client builds")
 });