fix: allow having no rust tokenizer when using dyn-chat-processor vllm (#7697)

lib/llm/src/discovery/watcher.rs

@@ -474,8 +474,19 @@ impl ModelWatcher {
                 None
             };
 
-            // This is expensive, we are loading ~10MiB JSON, so only do it once
-            let tokenizer = card.tokenizer().context("tokenizer")?;
+            // Loading the tokenizer is expensive (~10 MiB JSON), so only do it
+            // once and only when a local pipeline actually needs it.  Models
+            // without tokenizer.json (e.g. Qwen3-Omni) set tokenizer = None;
+            // they rely on a Python chat_engine_factory for tokenization.
+            // When a chat_engine_factory handles chat and no completions are
+            // needed, skip tokenizer loading entirely — even if the file exists.
+            let needs_rust_tokenizer =
+                needs_local_chat_pipeline || needs_local_completions_pipeline;
+            let tokenizer = if needs_rust_tokenizer && card.has_tokenizer() {
+                Some(card.tokenizer().context("tokenizer")?)
+            } else {
+                None
+            };
 
             // Create prefill chooser once if we're building pipelines
             // Both chat and completions will share the same prefill chooser instance
@@ -538,6 +549,13 @@ impl ModelWatcher {
                 let chat_engine = if let Some(engine) = factory_engine {
                     engine
                 } else {
+                    let tk = tokenizer.clone().ok_or_else(|| {
+                        anyhow::anyhow!(
+                            "Model has no supported Rust tokenizer and no chat_engine_factory. \
+                             Use --dyn-chat-processor vllm/sglang or provide a supported \
+                             tokenizer file (tokenizer.json, tiktoken.model, or *.tiktoken)."
+                        )
+                    })?;
                     entrypoint::build_routed_pipeline::<
                         NvCreateChatCompletionRequest,
                         NvCreateChatCompletionStreamResponse,
@@ -548,7 +566,7 @@ impl ModelWatcher {
                         self.router_config.router_mode,
                         worker_monitor.clone(),
                         kv_chooser.clone(),
-                        tokenizer.clone(),
+                        tk,
                         prefill_chooser.clone(),
                         self.router_config.enforce_disagg,
                         self.migration_limit,
@@ -561,12 +579,14 @@ impl ModelWatcher {
                 tracing::info!("Chat completions is ready");
             }
 
-            // Add completions engine only if the model supports completions.
+            // Add completions engine only if the model supports completions
+            // and we have a tokenizer (completions always uses the Rust preprocessor).
             if card.model_type.supports_completions() {
+                if let Some(tk) = tokenizer {
                     let formatter = PromptFormatter::no_op();
                     let PromptFormatter::OAI(formatter) = formatter;
                     let preprocessor =
-                    OpenAIPreprocessor::new_with_parts(card.clone(), formatter, tokenizer.clone())
+                        OpenAIPreprocessor::new_with_parts(card.clone(), formatter, tk.clone())
                             .context("OpenAIPreprocessor::new_with_parts")?;
                     let completions_engine = entrypoint::build_routed_pipeline_with_preprocessor::<
                         NvCreateCompletionRequest,
@@ -579,7 +599,7 @@ impl ModelWatcher {
                         worker_monitor,
                         kv_chooser,
                         preprocessor,
-                    tokenizer,
+                        tk,
                         prefill_chooser,
                         self.router_config.enforce_disagg,
                         self.migration_limit,
@@ -589,6 +609,24 @@ impl ModelWatcher {
                     .context("build_routed_pipeline_with_preprocessor")?;
                     worker_set.completions_engine = Some(completions_engine);
                     tracing::info!("Completions is ready");
+                } else {
+                    tracing::warn!(
+                        "Skipping completions engine: no Rust tokenizer available for this model"
+                    );
+                }
+            }
+
+            // Verify we built at least one serving engine. A Tokens model that
+            // ends up with no chat AND no completions engine (e.g. completions-only
+            // model with no tokenizer) should fail fast rather than register an
+            // empty WorkerSet that can't serve any requests.
+            if !worker_set.has_decode_engine() {
+                anyhow::bail!(
+                    "Model '{}' requires frontend tokenization/preprocessing (ModelInput::Tokens) \
+                     but no serving engine could be built. Provide a working tokenizer config or \
+                     perform tokenization in the backend (ModelInput::Text).",
+                    card.name()
+                );
             }
         } else if card.model_input == ModelInput::Text && card.model_type.supports_embedding() {
             // Case: Text + Embeddings

lib/llm/src/model_card.rs

@@ -452,7 +452,12 @@ impl ModelDeploymentCard {
             }
             None => {
                 anyhow::bail!(
-                    "Blank ModelDeploymentCard does not have a tokenizer. Is this a mistral model? If so, the `--use-<framework>-tokenizer` flag in the engine command is required."
+                    "ModelDeploymentCard for '{}' does not have a tokenizer. \
+                     Provide a supported tokenizer file (tokenizer.json, tiktoken.model, \
+                     or *.tiktoken), use --use-<framework>-tokenizer to delegate \
+                     tokenization to the backend, or use a non-Rust chat processor \
+                     (e.g. --dyn-chat-processor vllm).",
+                    self.display_name
                 );
             }
         }
@@ -676,7 +681,7 @@ impl ModelDeploymentCard {
         let (model_info, tokenizer, gen_config, prompt_formatter) = if !is_mistral_model {
             (
                 Some(ModelInfoType::from_disk(local_path)?),
-                Some(TokenizerKind::from_disk(local_path)?),
+                TokenizerKind::from_disk(local_path)?,
                 GenerationConfig::from_disk(local_path).ok(),
                 PromptFormatterArtifact::from_disk(local_path)?,
             )
@@ -1007,29 +1012,45 @@ impl PromptFormatterArtifact {
 }
 
 impl TokenizerKind {
-    pub fn from_disk(directory: &Path) -> Result<Self> {
+    /// Try to discover a tokenizer in the given directory.
+    ///
+    /// Returns `Ok(Some(..))` when a supported tokenizer is found,
+    /// `Ok(None)` when no tokenizer files are present (e.g. models that
+    /// ship only `vocab.json` + `merges.txt`), and `Err` for ambiguous
+    /// layouts or filesystem failures that should be treated as hard errors.
+    pub fn from_disk(directory: &Path) -> Result<Option<Self>> {
+        // Helper: probe a single well-known file.  Returns Ok(None) when the
+        // file simply does not exist, Ok(Some(..)) on success, and Err for
+        // anything else (unreadable file, checksum failure, etc.).
+        fn probe(path: std::path::PathBuf) -> Result<Option<CheckedFile>> {
+            if !path.exists() {
+                return Ok(None);
+            }
+            Ok(Some(CheckedFile::from_disk(path)?))
+        }
+
         // 1. Try tokenizer.json (HuggingFace)
-        if let Ok(f) = CheckedFile::from_disk(directory.join("tokenizer.json")) {
-            return Ok(Self::HfTokenizerJson(f));
+        if let Some(f) = probe(directory.join("tokenizer.json"))? {
+            return Ok(Some(Self::HfTokenizerJson(f)));
         }
 
         // 2. Try tiktoken.model
-        if let Ok(f) = CheckedFile::from_disk(directory.join("tiktoken.model")) {
-            return Ok(Self::TikTokenModel(f));
+        if let Some(f) = probe(directory.join("tiktoken.model"))? {
+            return Ok(Some(Self::TikTokenModel(f)));
         }
 
         // 3. Search for any *.tiktoken file
         let tiktoken_files: Vec<_> = std::fs::read_dir(directory)
+            .with_context(|| format!("Failed to read directory {}", directory.display()))?
+            .collect::<std::io::Result<Vec<_>>>()
+            .with_context(|| format!("Failed to iterate directory {}", directory.display()))?
             .into_iter()
-            .flatten()
-            .flatten()
             .filter(|entry| entry.path().extension().is_some_and(|e| e == "tiktoken"))
             .collect();
 
         if tiktoken_files.len() == 1 {
-            if let Ok(f) = CheckedFile::from_disk(tiktoken_files[0].path()) {
-                return Ok(Self::TikTokenModel(f));
-            }
+            let f = CheckedFile::from_disk(tiktoken_files[0].path())?;
+            return Ok(Some(Self::TikTokenModel(f)));
         } else if tiktoken_files.len() > 1 {
             let names: Vec<_> = tiktoken_files
                 .iter()
@@ -1042,10 +1063,13 @@ impl TokenizerKind {
             );
         }
 
-        anyhow::bail!(
-            "No tokenizer.json or tiktoken model file found in {}",
+        tracing::warn!(
+            "No supported tokenizer found in {} \
+             (expected tokenizer.json or a tiktoken file). \
+             Features that depend on the Rust tokenizer will not be available.",
             directory.display()
-        )
+        );
+        Ok(None)
     }
 }
 

lib/llm/tests/data/sample-models/mock-no-tokenizer-json/config.json

+{
+  "architectures": ["Qwen3OmniForConditionalGeneration"],
+  "bos_token_id": 151643,
+  "eos_token_id": 151645,
+  "max_position_embeddings": 32768,
+  "model_type": "qwen3_omni",
+  "vocab_size": 151936
+}

lib/llm/tests/data/sample-models/mock-no-tokenizer-json/tokenizer_config.json

+{
+  "bos_token": "<|endoftext|>",
+  "eos_token": "<|im_end|>",
+  "model_max_length": 32768,
+  "tokenizer_class": "Qwen2Tokenizer",
+  "chat_template": "{% for message in messages %}{{ message.content }}{% endfor %}"
+}

lib/llm/tests/model_card.rs

@@ -54,3 +54,19 @@ async fn test_missing_required_files() {
     // Should fail because config.json is missing
     assert!(err.contains("unable to extract"));
 }
+
+/// Models without tokenizer.json (e.g. Qwen3-Omni which ships vocab.json + merges.txt)
+/// should load successfully with tokenizer set to None. The frontend must use a
+/// non-Rust chat processor for these models (e.g. --dyn-chat-processor vllm).
+#[tokio::test]
+async fn test_model_loads_without_tokenizer_json() {
+    let path = "tests/data/sample-models/mock-no-tokenizer-json";
+    let mdc = ModelDeploymentCard::load_from_disk(path, None).unwrap();
+    assert!(
+        mdc.tokenizer.is_none(),
+        "Expected tokenizer to be None for model without tokenizer.json"
+    );
+    assert!(!mdc.has_tokenizer(), "has_tokenizer() should be false");
+    // Model info should still be loaded
+    assert!(mdc.model_info.is_some());
+}