diff --git a/cmd/loopvec-toolexec/compile.go b/cmd/loopvec-toolexec/compile.go
index 13f0d91..da8e8eb 100644
--- a/cmd/loopvec-toolexec/compile.go
+++ b/cmd/loopvec-toolexec/compile.go
@@ -113,7 +113,7 @@ func tryRewrite(tool string, args []string) (out []string, ok bool) {
 		debugf(c.pkg, "skipped %s: simd unavailable or unsupported build mode", c.pkg)
 		return nil, false
 	}
-	if want := toolchainRelease(); want != builtRelease() {
+	if want := toolchainRelease(); want == "" || want != builtRelease() {
 		// go/types can only read export data written by the Go release it was
 		// built with.
 		debugf(c.pkg, "skipped %s: loopvec-toolexec was built with %s but the toolchain is %s", c.pkg, builtRelease(), want)
diff --git a/cmd/loopvec-toolexec/simd.go b/cmd/loopvec-toolexec/simd.go
index 4b5d8fc..cd3c847 100644
--- a/cmd/loopvec-toolexec/simd.go
+++ b/cmd/loopvec-toolexec/simd.go
@@ -39,14 +39,31 @@ func simdOn(tool string) bool {
 	return simdEnabled
 }
 
-var releaseRE = regexp.MustCompile(`go1\.[0-9]+`)
+// releaseRE matches a Go version as a whole word: a release and its patch
+// (go1.27.3), a prerelease (go1.28rc1), a development build (go1.28-6ae0b1f8a9)
+// or a vendor's build (go1.27.3-acme).
+var releaseRE = regexp.MustCompile(`go1\.[^\s]+`)
+
+// release returns the Go version in s, a compiler's -V=full output or a
+// runtime.Version, without the experiments: the compiler separates them with a
+// space (go1.27.3 X:simd), but a program built with an experiment reports its
+// version with a dash when the version has none (go1.27.3-X:simd).
+func release(s string) string {
+	v, _, _ := strings.Cut(releaseRE.FindString(s), "-X:")
+	return v
+}
 
-// toolchainRelease returns the Go release (such as go1.28) of the compiler
+// toolchainRelease returns the Go version (such as go1.27.3) of the compiler
 // probed by simdOn.
-func toolchainRelease() string { return releaseRE.FindString(toolVersion) }
-
-// builtRelease returns the Go release this program was built with.
-func builtRelease() string { return releaseRE.FindString(runtime.Version()) }
+func toolchainRelease() string { return release(toolVersion) }
+
+// builtRelease returns the Go version this program was built with. It must be
+// the toolchain's exact version, patch included: the simd package can change
+// in a patch release without go/types seeing it (go1.27.0 declares
+// ShiftAllRight(uint64) but compiles it as taking a uint8; go1.27.1 takes a
+// uint64), and then the rewritten package type-checks here but does not
+// compile.
+func builtRelease() string { return release(runtime.Version()) }
 
 // build holds the per-build state shared by the wrapper processes of one go
 // command invocation. It lives in the go command's work directory, so it is
diff --git a/cmd/loopvec-toolexec/simd_test.go b/cmd/loopvec-toolexec/simd_test.go
new file mode 100644
index 0000000..12e7deb
--- /dev/null
+++ b/cmd/loopvec-toolexec/simd_test.go
@@ -0,0 +1,25 @@
+package main
+
+import "testing"
+
+func TestRelease(t *testing.T) {
+	for _, tt := range []struct{ in, want string }{
+		{"go1.27.3", "go1.27.3"},
+		{"go1.27.3-X:simd", "go1.27.3"},
+		{"go1.27.3-X:simd,fieldtrack", "go1.27.3"},
+		{"compile version go1.27.3", "go1.27.3"},
+		{"compile version go1.27.3 X:simd\n", "go1.27.3"},
+		{"go1.28rc1-X:simd", "go1.28rc1"},
+		{"compile version go1.28rc1 X:simd", "go1.28rc1"},
+		{"devel go1.28-6ae0b1f8a9 Tue Oct 27 10:00:00 2026 -0700 X:simd", "go1.28-6ae0b1f8a9"},
+		{"compile version devel go1.28-6ae0b1f8a9 Tue Oct 27 10:00:00 2026 -0700 X:simd buildID=x/y", "go1.28-6ae0b1f8a9"},
+		{"go1.27.3-acme X:simd", "go1.27.3-acme"},
+		{"compile version go1.27.3-acme X:simd", "go1.27.3-acme"},
+		{"", ""},
+		{"devel +6ae0b1f8a9", ""},
+	} {
+		if got := release(tt.in); got != tt.want {
+			t.Errorf("release(%q) = %q, want %q", tt.in, got, tt.want)
+		}
+	}
+}
diff --git a/testdata/toolexec_toolchain_mismatch.txt b/testdata/toolexec_toolchain_mismatch.txt
index 4c9c9a7..6145616 100644
--- a/testdata/toolexec_toolchain_mismatch.txt
+++ b/testdata/toolexec_toolchain_mismatch.txt
@@ -1,16 +1,24 @@
 # go/types can only read export data written by the Go release it was built
-# with. When loopvec-toolexec was built with a different release than the
-# toolchain running the build, packages are compiled unchanged, and the log
-# (with LOOPVEC_TOOLEXEC_DEBUG) says why.
+# with, and the simd package can change in a patch release without go/types
+# seeing it (go1.27.0 compiles ShiftAllRight as taking a uint8). When
+# loopvec-toolexec was built with a different Go version than the toolchain
+# running the build, patch included, packages are compiled unchanged, and the
+# log (with LOOPVEC_TOOLEXEC_DEBUG) says why.
 
-chmod 0755 compile
+chmod 0755 compile patch/compile
 env LOOPVEC_TOOLEXEC_DEBUG=1
 env LOOPVEC_TOOLEXEC_LOG=$WORK/log
 exec $LOOPVEC_TOOLEXEC $WORK/compile -p example.com/x -importcfg $WORK/b001/importcfg x.go
 cmpenv stdout stdout.want
 ! stderr .
 stdout -count=1 '^fake compile: '
-grep -count=1 '^skipped example\.com/x: loopvec-toolexec was built with go1\.[0-9]+ but the toolchain is go1\.1$' log
+grep -count=1 '^skipped example\.com/x: loopvec-toolexec was built with go1\.[^ ]+ but the toolchain is go1\.1$' log
+
+# Same release, another patch.
+exec $LOOPVEC_TOOLEXEC $WORK/patch/compile -p example.com/y -importcfg $WORK/b001/importcfg x.go
+stdout -count=1 '^fake compile: -p example\.com/y '
+! stderr .
+grep -count=1 '^skipped example\.com/y: loopvec-toolexec was built with go1\.[^ ]+ but the toolchain is go1\.27\.0$' log
 
 -- compile --
 #!/bin/sh
@@ -19,6 +27,13 @@ if [ "$1" = "-V=full" ]; then
 	exit 0
 fi
 echo "fake compile: $@"
+-- patch/compile --
+#!/bin/sh
+if [ "$1" = "-V=full" ]; then
+	echo "compile version go1.27.0 X:simd"
+	exit 0
+fi
+echo "fake compile: $@"
 -- x.go --
 package x
 -- stdout.want --