diff --git a/README.md b/README.md
index baea3d3..1c7cf23 100644
--- a/README.md
+++ b/README.md
@@ -777,6 +777,10 @@ request for a model that is not available gets a `503` saying so.
 Add this repo as a `:git/url` dep, run `jolt kernels` and `jolt prepare`
 for the native library and `data/`, then:
 
+jolt runs no prep step for a dependency, so `jolt kernels` runs in lev's
+own checkout, the gitlib that `jolt path` names. A Windows build of the
+program links no kernels, so the encoders are unavailable there.
+
 ```clojure
 (require '[lev.agent :as ag] '[lev.server :as server] '[lev.config :as cfg])
 (def agent (ag/load-agent (cfg/setting (cfg/context {}) "--data" "LEV_DATA" :data "data")))
@@ -1008,7 +1012,10 @@ entries, and the build task branches on OS.
 - **HTTP**: `jolt-lang/ring-chez-adapter` serves the API, and
   `org.clojars.askonomm/ruuter` from Clojars dispatches its routes.
 - **BLAS**: `cblas_sgemm` from the Accelerate framework on mac, OpenBLAS
-  on linux.
+  on linux, else the reference `libblas.so.3` (about 10x slower). It is an
+  optional native, so a program that uses lev as a library still starts
+  without one; the encoders then refuse to load, and
+  `lev.tensors/available?` says whether they can.
 
 On linux, install the OpenBLAS runtime package.
 
diff --git a/deps.edn b/deps.edn
index 1fcd916..2239213 100644
--- a/deps.edn
+++ b/deps.edn
@@ -23,11 +23,13 @@
  ;; kernels: built from native/lev_kernels.c by the `kernels` task.
  ;; blas: Accelerate framework on mac, OpenBLAS elsewhere.
  ;; `jolt build` links the kernels statically from the archive so the binary
- ;; is self-contained; run/repl/test load the shared object.
+ ;; is self-contained; run/repl/test load the shared object. By platform, so
+ ;; a Windows build of a program that uses lev as a library links none.
  :jolt/native [{:name "lev_kernels"
                 :darwin ["native/liblev_kernels.dylib"]
                 :linux  ["native/liblev_kernels.so"]
-                :static {:archive "native/liblev_kernels.a"}}
+                :static {:darwin {:archive "native/liblev_kernels.a"}
+                         :linux {:archive "native/liblev_kernels.a"}}}
                ;; llama.cpp behind native/lev_llm.c, built by `jolt llama`
                ;; (optional at runtime: without it only the thinker is missing)
                {:name "lev_llm"
@@ -75,10 +77,16 @@
                 :linux []
                 :static {:darwin {:lib "MetalKit" :libdir "native/frameworks"}}}
                ;; on mac the static llm archive (ggml-cpu) also needs Accelerate
-               ;; at link time, through its stub like the frameworks above
+               ;; at link time, through its stub like the frameworks above.
+               ;; Optional, so a program that uses lev as a library starts on a
+               ;; host without it; the encoders then refuse to load
+               ;; (lev.tensors/available?). On linux libblas.so.3 is the
+               ;; fallback: the reference CBLAS, ~10x slower than OpenBLAS, or
+               ;; OpenBLAS itself where the alternatives point it there.
                {:name "blas"
+                :optional true
                 :darwin ["/System/Library/Frameworks/Accelerate.framework/Accelerate"]
-                :linux  ["libopenblas.so.0" "libopenblas.so"]
+                :linux  ["libopenblas.so.0" "libopenblas.so" "libblas.so.3"]
                 :static {:darwin {:lib "Accelerate" :libdir "native/frameworks"}}}]
 
  :tasks
diff --git a/src/lev/tensors.clj b/src/lev/tensors.clj
index c48fb34..d10f9dc 100644
--- a/src/lev/tensors.clj
+++ b/src/lev/tensors.clj
@@ -75,6 +75,12 @@
   (set-threads* (int (max 1 n)))
   n)
 
+(defn available?
+  "Are the kernels and a BLAS loaded? Both are declared natives, BLAS an
+  optional one: without them nothing here runs."
+  []
+  (boolean (and (ffi/find-symbol "lev_set_threads") (ffi/find-symbol "cblas_sgemm"))))
+
 (def ^:private sgemm-address
   "cblas_sgemm as the kernel library reaches it: the same BLAS entry the
   binding above calls, found once through the declared natives."
diff --git a/src/lev/tinydecide.clj b/src/lev/tinydecide.clj
index e8f7409..c609806 100644
--- a/src/lev/tinydecide.clj
+++ b/src/lev/tinydecide.clj
@@ -75,6 +75,8 @@
   [dir]
   (when-not (release? dir)
     (unavailable dir (str dir " has no meta.json and model.bin (hf download TheREZOR/TinyDecide)")))
+  (when-not (t/available?)
+    (unavailable dir "no kernels or BLAS loaded (jolt kernels; OpenBLAS on linux)"))
   (let [meta (json/read-str (slurp (str dir "/meta.json")))
         cfg (get meta "cfg")
         tok (get meta "tokenizer")]