diff --git a/.clusterfuzzlite/build.sh b/.clusterfuzzlite/build.sh
index 8f7cb8a1..14342132 100755
--- a/.clusterfuzzlite/build.sh
+++ b/.clusterfuzzlite/build.sh
@@ -6,7 +6,10 @@
 # `fuzz/` and stage each binary into $OUT. Run by `compile` inside the
 # base-builder-rust image (see .clusterfuzzlite/Dockerfile).
 cd "$SRC/my-lang"
-cargo +nightly fuzz build -O
+# Build for the sanitizer OSS-Fuzz requests (defaults to address). NOTE: only
+# `address` is configured — Rust/cargo-fuzz cannot emit a valid UBSan binary
+# (it links ASan regardless), which `bad_build_check` rejects; see project.yaml.
+cargo +nightly fuzz build -O --sanitizer "${SANITIZER:-address}"
 
 release="$SRC/my-lang/fuzz/target/x86_64-unknown-linux-gnu/release"
 for target in fuzz/fuzz_targets/*.rs; do
diff --git a/.clusterfuzzlite/project.yaml b/.clusterfuzzlite/project.yaml
index 565a66e0..78f17ce6 100644
--- a/.clusterfuzzlite/project.yaml
+++ b/.clusterfuzzlite/project.yaml
@@ -1,6 +1,9 @@
 homepage: "https://github.com/hyperpolymath/my-lang"
 language: rust
 main_repo: "https://github.com/hyperpolymath/my-lang"
+# Only ASan: Rust/cargo-fuzz does not produce a valid UndefinedBehaviorSanitizer
+# build (it links ASan regardless), which OSS-Fuzz's `bad_build_check` rejects
+# ("UBSan build ... seems to be compiled with ASan"). ASan is the supported,
+# meaningful sanitizer for Rust fuzzing here.
 sanitizers:
   - address
-  - undefined
diff --git a/.github/workflows/cflite_batch.yml b/.github/workflows/cflite_batch.yml
index 9cd88072..f902698f 100644
--- a/.github/workflows/cflite_batch.yml
+++ b/.github/workflows/cflite_batch.yml
@@ -11,7 +11,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        sanitizer: [address, undefined]
+        # ASan only: Rust/cargo-fuzz can't emit a valid UBSan binary (links ASan
+        # regardless), so `undefined` failed bad_build_check. See project.yaml.
+        sanitizer: [address]
     steps:
       - uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5
       - uses: google/clusterfuzzlite/actions/build_fuzzers@884713a6c30a92e5e8544c39945cd7cb630abcd1 # v1
diff --git a/.github/workflows/cflite_pr.yml b/.github/workflows/cflite_pr.yml
index f6f4f607..e84d5b0a 100644
--- a/.github/workflows/cflite_pr.yml
+++ b/.github/workflows/cflite_pr.yml
@@ -14,7 +14,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        sanitizer: [address, undefined]
+        # ASan only: Rust/cargo-fuzz can't emit a valid UBSan binary (links ASan
+        # regardless), so `undefined` failed bad_build_check. See project.yaml.
+        sanitizer: [address]
     steps:
       - uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5
       - uses: google/clusterfuzzlite/actions/build_fuzzers@884713a6c30a92e5e8544c39945cd7cb630abcd1 # v1