diff --git a/.github/workflows/rust-ci-full.yml b/.github/workflows/rust-ci-full.yml
index 7cda682952..eb690a9bad 100644
--- a/.github/workflows/rust-ci-full.yml
+++ b/.github/workflows/rust-ci-full.yml
@@ -76,6 +76,8 @@ jobs:
       - name: Test argument comment lint package
         working-directory: tools/argument-comment-lint
         run: cargo test
+        env:
+          RUST_MIN_STACK: "8388608" # 8 MiB
 
   argument_comment_lint_prebuilt:
     name: Argument comment lint - ${{ matrix.name }}
@@ -671,6 +673,7 @@ jobs:
         run: cargo nextest run --no-fail-fast --target ${{ matrix.target }} --cargo-profile ci-test --timings
         env:
           RUST_BACKTRACE: 1
+          RUST_MIN_STACK: "8388608" # 8 MiB
           NEXTEST_STATUS_LEVEL: leak
 
       - name: Upload Cargo timings (nextest)
diff --git a/.github/workflows/rust-ci.yml b/.github/workflows/rust-ci.yml
index f62e5458a3..42a3ca8764 100644
--- a/.github/workflows/rust-ci.yml
+++ b/.github/workflows/rust-ci.yml
@@ -131,6 +131,8 @@ jobs:
       - name: Test argument comment lint package
         working-directory: tools/argument-comment-lint
         run: cargo test
+        env:
+          RUST_MIN_STACK: "8388608" # 8 MiB
 
   argument_comment_lint_prebuilt:
     name: Argument comment lint - ${{ matrix.name }}
diff --git a/defs.bzl b/defs.bzl
index d1e38ea91b..c5cc402658 100644
--- a/defs.bzl
+++ b/defs.bzl
@@ -18,11 +18,11 @@ PLATFORMS = [
 WINDOWS_RUSTC_LINK_FLAGS = select({
     "@rules_rs//rs/experimental/platforms/constraints:windows_gnullvm": [
         "-C",
-        "link-arg=-Wl,--stack,8388608",
+        "link-arg=-Wl,--stack,8388608",  # 8 MiB
     ],
     "@rules_rs//rs/experimental/platforms/constraints:windows_msvc": [
         "-C",
-        "link-arg=/STACK:8388608",
+        "link-arg=/STACK:8388608",  # 8 MiB
         "-C",
         "link-arg=/NODEFAULTLIB:libucrt.lib",
         "-C",
@@ -43,6 +43,13 @@ MACOS_WEBRTC_RUSTC_LINK_FLAGS = select({
     "//conditions:default": [],
 })
 
+RUST_LIBTEST_ENV = {
+    # Rust's libtest harness runs test bodies on std-spawned threads, whose
+    # default 2 MiB stack can be too small for large async test futures on
+    # Windows. Keep this aligned with the Windows test binary stack reserve.
+    "RUST_MIN_STACK": "8388608",  # 8 MiB
+}
+
 def multiplatform_binaries(name, platforms = PLATFORMS):
     for platform in platforms:
         platform_data(
@@ -187,7 +194,7 @@ def codex_rust_crate(
         extra_binaries: Additional binary labels to surface as test data and
             `CARGO_BIN_EXE_*` environment variables. These are only needed for binaries from a different crate.
     """
-    test_env = {
+    test_env = RUST_LIBTEST_ENV | {
         # The launcher resolves an absolute workspace root at runtime so
         # manifest-only platforms like macOS still point Insta at the real
         # `codex-rs` checkout.
@@ -359,7 +366,7 @@ def codex_rust_crate(
             # Important: do not merge `test_env` here. Its unit-test-only
             # `INSTA_WORKSPACE_ROOT="codex-rs"` is tuned for unit tests that
             # execute from the repo root and can misplace integration snapshots.
-            env = cargo_env,
+            env = RUST_LIBTEST_ENV | cargo_env,
             tags = test_tags,
             **test_kwargs
         )
diff --git a/justfile b/justfile
index d44bbda21d..ca74c2e5c6 100644
--- a/justfile
+++ b/justfile
@@ -1,6 +1,8 @@
 set working-directory := "codex-rs"
 set positional-arguments
 
+rust_min_stack := "8388608" # 8 MiB
+
 # Display help
 help:
     just -l
@@ -49,7 +51,7 @@ install:
 # Prefer this for routine local runs. Workspace crate features are banned, so
 # there should be no need to add `--all-features`.
 test:
-    cargo nextest run --no-fail-fast
+    RUST_MIN_STACK={{ rust_min_stack }} cargo nextest run --no-fail-fast
 
 # Build and run Codex from source using Bazel.
 # Note we have to use the combination of `[no-cd]` and `--run_under="cd $PWD &&"`