diff --git a/src/tools/miri/ci/ci.sh b/src/tools/miri/ci/ci.sh
index 9d2c3f362e6d1..0fbbd6e506f2d 100755
--- a/src/tools/miri/ci/ci.sh
+++ b/src/tools/miri/ci/ci.sh
@@ -58,8 +58,9 @@ function run_tests {
     MIRIFLAGS="${MIRIFLAGS:-} -O -Zmir-opt-level=4 -Cdebug-assertions=yes" MIRI_SKIP_UI_CHECKS=1 ./miri test -- tests/{pass,panic}
 
     # Also run some many-seeds tests. 64 seeds means this takes around a minute per test.
+    # (Need to invoke via explicit `bash -c` for Windows.)
     for FILE in tests/many-seeds/*.rs; do
-      MIRI_SEEDS=64 ./miri many-seeds ./miri run "$FILE"
+      MIRI_SEEDS=64 ./miri many-seeds "$BASH" -c "./miri run '$FILE'"
     done
 
     # Check that the benchmarks build and run, but without actually benchmarking.
diff --git a/src/tools/miri/miri-script/src/commands.rs b/src/tools/miri/miri-script/src/commands.rs
index 7fff6ae80b429..66f323290b2b0 100644
--- a/src/tools/miri/miri-script/src/commands.rs
+++ b/src/tools/miri/miri-script/src/commands.rs
@@ -178,7 +178,7 @@ impl Command {
             .context("Please install rustup-toolchain-install-master by running 'cargo install rustup-toolchain-install-master'")?;
         let sh = Shell::new()?;
         sh.change_dir(miri_dir()?);
-        let new_commit = Some(sh.read_file("rust-version")?.trim().to_owned());
+        let new_commit = sh.read_file("rust-version")?.trim().to_owned();
         let current_commit = {
             let rustc_info = cmd!(sh, "rustc +miri --version -v").read();
             if rustc_info.is_err() {
@@ -193,7 +193,7 @@ impl Command {
             }
         };
         // Check if we already are at that commit.
-        if current_commit == new_commit {
+        if current_commit.as_ref() == Some(&new_commit) {
             if active_toolchain()? != "miri" {
                 cmd!(sh, "rustup override set miri").run()?;
             }
@@ -202,7 +202,7 @@ impl Command {
         // Install and setup new toolchain.
         cmd!(sh, "rustup toolchain uninstall miri").run()?;
 
-        cmd!(sh, "rustup-toolchain-install-master -n miri -c cargo -c rust-src -c rustc-dev -c llvm-tools -c rustfmt -c clippy {flags...} -- {new_commit...}").run()?;
+        cmd!(sh, "rustup-toolchain-install-master -n miri -c cargo -c rust-src -c rustc-dev -c llvm-tools -c rustfmt -c clippy {flags...} -- {new_commit}").run()?;
         cmd!(sh, "rustup override set miri").run()?;
         // Cleanup.
         cmd!(sh, "cargo clean").run()?;
@@ -380,9 +380,9 @@ impl Command {
                 .env("MIRIFLAGS", miriflags)
                 .quiet()
                 .run();
-            if status.is_err() {
+            if let Err(err) = status {
                 println!("Failing seed: {seed}");
-                break;
+                return Err(err.into());
             }
         }
         Ok(())