diff --git a/crates/socket-patch-cli/tests/common/yarn_classic_vex.rs b/crates/socket-patch-cli/tests/common/yarn_classic_vex.rs
index fc6d912fe..83010cbc2 100644
--- a/crates/socket-patch-cli/tests/common/yarn_classic_vex.rs
+++ b/crates/socket-patch-cli/tests/common/yarn_classic_vex.rs
@@ -160,6 +160,29 @@ pub fn require_yarn_classic(suite: &str, isolate: impl Fn(&mut Command)) -> bool
     false
 }
 
+/// Run `cmd` (a suite's `corepack <pm> ...` invocation), re-running it after 2 s
+/// and then 5 s while it dies on a transient DNS failure (`EAI_AGAIN`).
+///
+/// yarn 1 re-queues a registry request on `ENOTFOUND`, `ECONNRESET` and
+/// `ETIMEDOUT` (`RequestManager.isPossibleOfflineError` in 1.22.22) but not
+/// on `EAI_AGAIN`, so one resolver blip on a CI host fails the install
+/// outright, and under [`REQUIRED_ENV`]`=1` the whole leg with it (#1734's
+/// CI, 2026-10-26: `getaddrinfo EAI_AGAIN registry.yarnpkg.com`). A failed
+/// lookup writes nothing, so the re-run starts from the same tree. Any
+/// other failure returns on the first attempt.
+pub fn output_retrying_dns(cmd: &mut Command) -> std::process::Output {
+    let mut out = cmd.output().expect("failed to run corepack");
+    for backoff in [2, 5] {
+        if out.status.success() || !String::from_utf8_lossy(&out.stderr).contains("EAI_AGAIN") {
+            break;
+        }
+        eprintln!("yarn hit a transient DNS failure (EAI_AGAIN); retrying in {backoff} s");
+        std::thread::sleep(std::time::Duration::from_secs(backoff));
+        out = cmd.output().expect("failed to run corepack");
+    }
+    out
+}
+
 // ── manifest-less VEX ─────────────────────────────────────────────────
 
 /// Which wiring the flow produced.
diff --git a/crates/socket-patch-cli/tests/e2e_redirect_yarn_classic_build.rs b/crates/socket-patch-cli/tests/e2e_redirect_yarn_classic_build.rs
index ce38670ef..fc7b1b5eb 100644
--- a/crates/socket-patch-cli/tests/e2e_redirect_yarn_classic_build.rs
+++ b/crates/socket-patch-cli/tests/e2e_redirect_yarn_classic_build.rs
@@ -111,7 +111,7 @@ fn corepack(cwd: &Path, pm: &str, args: &[&str], extra_env: &[(&str, &str)]) ->
     for (k, v) in extra_env {
         cmd.env(k, v);
     }
-    cmd.output().expect("failed to run corepack")
+    yarn_classic_vex::output_retrying_dns(&mut cmd)
 }
 
 fn run_socket(cwd: &Path, args: &[&str]) -> (i32, String, String) {
diff --git a/crates/socket-patch-cli/tests/e2e_vendor_yarn_classic_build.rs b/crates/socket-patch-cli/tests/e2e_vendor_yarn_classic_build.rs
index b388f5b9f..5293c8ebe 100644
--- a/crates/socket-patch-cli/tests/e2e_vendor_yarn_classic_build.rs
+++ b/crates/socket-patch-cli/tests/e2e_vendor_yarn_classic_build.rs
@@ -112,7 +112,7 @@ fn corepack(cwd: &Path, pm: &str, args: &[&str], extra_env: &[(&str, &str)]) ->
     for (k, v) in extra_env {
         cmd.env(k, v);
     }
-    cmd.output().expect("failed to run corepack")
+    yarn_classic_vex::output_retrying_dns(&mut cmd)
 }
 
 /// Run the socket-patch binary with a scrubbed environment.
diff --git a/crates/socket-patch-cli/tests/e2e_vendor_yarn_classic_dev_flow.rs b/crates/socket-patch-cli/tests/e2e_vendor_yarn_classic_dev_flow.rs
index 0f51f0748..01bb85ff0 100644
--- a/crates/socket-patch-cli/tests/e2e_vendor_yarn_classic_dev_flow.rs
+++ b/crates/socket-patch-cli/tests/e2e_vendor_yarn_classic_dev_flow.rs
@@ -132,7 +132,7 @@ fn corepack(cwd: &Path, pm: &str, args: &[&str], extra_env: &[(&str, &str)]) ->
     for (k, v) in extra_env {
         cmd.env(k, v);
     }
-    cmd.output().expect("failed to run corepack")
+    yarn_classic_vex::output_retrying_dns(&mut cmd)
 }
 
 /// Run the socket-patch binary with a scrubbed environment.
diff --git a/crates/socket-patch-cli/tests/e2e_vex_build/pdm.rs b/crates/socket-patch-cli/tests/e2e_vex_build/pdm.rs
index 7331301bd..16efed366 100644
--- a/crates/socket-patch-cli/tests/e2e_vex_build/pdm.rs
+++ b/crates/socket-patch-cli/tests/e2e_vex_build/pdm.rs
@@ -137,6 +137,12 @@ fn pins_for(v: &str) -> Vec<String> {
         if ((2, 21, 0)..(2, 26, 1)).contains(&t) {
             pins.push("hishel<1");
         }
+        // 2.x declares an unbounded `unearth`, and unearth 0.19 (2026-10-27)
+        // moved to `httpx2`: PDM's PyPI client is no longer the `httpx`
+        // client unearth expects, so `pdm lock` dies with "PDMPyPIClient is
+        // not an instance or subtype of type (Client)" (seen on 2.29.2).
+        // `<0.19` resolves the 0.18 line every 2.x ran with before.
+        pins.push("unearth<0.19");
     }
     let mut out = vec![format!("pdm=={v}")];
     out.extend(pins.into_iter().map(String::from));
diff --git a/crates/socket-patch-cli/tests/mode_migration_npm.rs b/crates/socket-patch-cli/tests/mode_migration_npm.rs
index c8091f68e..78fac9495 100644
--- a/crates/socket-patch-cli/tests/mode_migration_npm.rs
+++ b/crates/socket-patch-cli/tests/mode_migration_npm.rs
@@ -93,7 +93,7 @@ fn corepack(cwd: &Path, pm: &str, args: &[&str], extra_env: &[(&str, &str)]) ->
     for (k, v) in extra_env {
         cmd.env(k, v);
     }
-    cmd.output().expect("failed to run corepack")
+    yarn_classic_vex::output_retrying_dns(&mut cmd)
 }
 
 fn run_socket(cwd: &Path, args: &[&str]) -> (i32, String, String) {
diff --git a/scripts/backtest-pdm.py b/scripts/backtest-pdm.py
index e7f4db789..d403cb545 100644
--- a/scripts/backtest-pdm.py
+++ b/scripts/backtest-pdm.py
@@ -204,6 +204,11 @@ def pins_for(version):
         # ModuleNotFoundError before locking.
         if (2, 21) <= v < (2, 26, 1):
             pins.append("hishel<1")
+        # 2.x declares an unbounded `unearth`; 0.19 (2026-10-27) moved to
+        # httpx2 and breaks PDM's PyPI client ("PDMPyPIClient is not an
+        # instance or subtype of type (Client)" on `pdm lock`, seen on
+        # 2.29.2). `<0.19` resolves the 0.18 line 2.x ran with before.
+        pins.append("unearth<0.19")
     return pins