diff --git a/compression/python/compression_clif_aux.cc b/compression/python/compression_clif_aux.cc
index 3568ad34..f4502a0c 100644
--- a/compression/python/compression_clif_aux.cc
+++ b/compression/python/compression_clif_aux.cc
@@ -148,7 +148,9 @@ SbsWriter::SbsWriter(const std::string& path)
     : impl_(HWY_DYNAMIC_DISPATCH(NewSbsWriter)(path)) {}
 
 SbsReader::SbsReader(const std::string& path)
-    : reader_(Path(path)), model_(reader_) {}
+    : reader_(Path(path)),
+      model_(reader_, Path(), /*wrapping=*/Tristate::kDefault,
+             /*allow_unknown_config=*/Tristate::kTrue) {}
 
 }  // namespace gcpp
 #endif  // HWY_ONCE
diff --git a/gemma/model_store.cc b/gemma/model_store.cc
index 76f0c754..0adee85e 100644
--- a/gemma/model_store.cc
+++ b/gemma/model_store.cc
@@ -23,6 +23,7 @@
 #include <charconv>
 #include <cstdlib>
 #include <cstring>  // strcmp
+#include <optional>
 #include <string>
 #include <system_error>  // std::errc  // NOLINT
 
@@ -250,7 +251,8 @@ static int DeduceLayerTypes(const BlobReader& reader) {
 // `wrapping_override` is forwarded from the command line. For pre-2025 files
 // without `ModelConfig`, it is the only way to force PT.
 static ModelConfig ReadOrDeduceConfig(BlobReader& reader,
-                                      Tristate wrapping_override) {
+                                      Tristate wrapping_override,
+                                      Tristate allow_unknown_config) {
   const TypePrefix type_prefix(reader.Keys(), reader);
   Type deduced_weight = Type::kUnknown;
   if (type_prefix.HasPrefixes()) {
@@ -258,15 +260,18 @@ static ModelConfig ReadOrDeduceConfig(BlobReader& reader,
     type_prefix.PrintTypeBytes();
   }
 
-  // Always deduce so we can verify it against the config we read.
-  const size_t layers = DeduceNumLayers(reader.Keys());
-  const int layer_types = DeduceLayerTypes(reader);
-  const Model deduced_model =
-      DeduceModel(reader.blob_path(), layers, layer_types);
+  // Optionally deduce so we can verify it against the config we read.
+  std::optional<Model> deduced_model;
+  const bool has_config = reader.Find(kConfigName);
+  if (!has_config || allow_unknown_config != Tristate::kTrue) {
+    const size_t layers = DeduceNumLayers(reader.Keys());
+    const int layer_types = DeduceLayerTypes(reader);
+    deduced_model = DeduceModel(reader.blob_path(), layers, layer_types);
+  }
 
   ModelConfig config;
   // Check first to prevent `CallWithSpan` from printing a warning.
-  if (reader.Find(kConfigName)) {
+  if (has_config) {
     HWY_ASSERT(reader.CallWithSpan<uint32_t>(
         kConfigName, [&config](const SerializedSpan serialized) {
           const IFields::ReadResult result = config.Read(serialized, 0);
@@ -274,7 +279,6 @@ static ModelConfig ReadOrDeduceConfig(BlobReader& reader,
           HWY_ASSERT_M(result.pos != 0, "Error deserializing config");
         }));
 
-    HWY_ASSERT(config.model != Model::UNKNOWN);
     HWY_ASSERT(config.wrapping != PromptWrapping::kSentinel);
     HWY_ASSERT(config.weight != Type::kUnknown);
     for (const LayerConfig& layer_config : config.layer_configs) {
@@ -285,18 +289,19 @@ static ModelConfig ReadOrDeduceConfig(BlobReader& reader,
 
     // We trust the deserialized config, but checking helps to validate the
     // deduction, which we rely on below for pre-2025 files.
-    if (config.model != deduced_model) {
+    if (deduced_model.has_value() && config.model != *deduced_model) {
       const std::string suffix = WrappingSuffix(config.wrapping);
       HWY_WARN("Detected model %s does not match config %s.",
-               (std::string(ModelPrefix(deduced_model)) + suffix).c_str(),
+               (std::string(ModelPrefix(*deduced_model)) + suffix).c_str(),
                (std::string(ModelPrefix(config.model)) + suffix).c_str());
     }
     return config;
   }
 
   // Pre-2025 format: no config, rely on deduction plus `wrapping_override`.
-  return ModelConfig(deduced_model, deduced_weight,
-                     ChooseWrapping(deduced_model, wrapping_override));
+  HWY_ASSERT(deduced_model.has_value());
+  return ModelConfig(*deduced_model, deduced_weight,
+                     ChooseWrapping(*deduced_model, wrapping_override));
 }
 
 static std::vector<float> ReadScales(BlobReader& reader,
@@ -387,8 +392,8 @@ void ModelStore::CreateMatPtrs(BlobReader& reader) {
 }
 
 ModelStore::ModelStore(BlobReader& reader, const Path& tokenizer_path,
-                       Tristate wrapping)
-    : config_(ReadOrDeduceConfig(reader, wrapping)),
+                       Tristate wrapping, Tristate allow_unknown_config)
+    : config_(ReadOrDeduceConfig(reader, wrapping, allow_unknown_config)),
       tokenizer_(ReadTokenizer(reader, tokenizer_path)) {
   if (!ReadMatPtrs(reader)) {  // Pre-2025 format.
     CreateMatPtrs(reader);
@@ -460,7 +465,6 @@ static void AddBlob(const char* name, const std::vector<uint32_t>& data,
 void WriteSingleFile(const ModelConfig& config, const GemmaTokenizer& tokenizer,
                      const std::vector<uint32_t>& serialized_mat_ptrs,
                      BlobWriter& writer) {
-  HWY_ASSERT(config.model != Model::UNKNOWN);
   HWY_ASSERT(config.weight != Type::kUnknown);
   HWY_ASSERT(config.wrapping != PromptWrapping::kSentinel);
   const std::vector<uint32_t> serialized_config = config.Write();
diff --git a/gemma/model_store.h b/gemma/model_store.h
index 506fb77c..76be557b 100644
--- a/gemma/model_store.h
+++ b/gemma/model_store.h
@@ -51,19 +51,19 @@ class Gemma;
 // both, but only write single-file format.
 class ModelStore {
  public:
-  // Reads from file(s) or aborts on error. The latter two arguments are only
-  // used for pre-2025 files.
+  // Reads from file(s) or aborts on error. `tokenizer_path` and `wrapping` are
+  // only used for pre-2025 files. `allow_unknown_config` allows loading models
+  // with unknown config, if the config is present in the file.
   ModelStore(BlobReader& reader, const Path& tokenizer_path = Path(),
-             Tristate wrapping = Tristate::kDefault);
+             Tristate wrapping = Tristate::kDefault,
+             Tristate allow_unknown_config = Tristate::kDefault);
   ~ModelStore();
 
   const ModelConfig& Config() const {
-    HWY_ASSERT(config_.model != Model::UNKNOWN);
     return config_;
   }
 
   ModelConfig& MutableConfig() {
-    HWY_ASSERT(config_.model != Model::UNKNOWN);
     return config_;
   }