[Bugfix][Spec Decode] Disable dynamic speculative decoding for fixed-K proposers instead of crashing (#46399)

Signed-off-by: Ting Sun <suntcrick@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: Wentao Ye <44945378+yewentao256@users.noreply.github.com>

vllm/config/vllm.py

@@ -1056,6 +1056,28 @@ class VllmConfig:
         )
         self.compilation_config.cudagraph_mode = CUDAGraphMode.PIECEWISE
 
+    def _maybe_disable_dynamic_sd_for_v1(self) -> None:
+        speculative_config = self.speculative_config
+        # V2 truncates drafts centrally. V1 requires proposer support.
+        if (
+            speculative_config is None
+            or not speculative_config.uses_dynamic_speculative_decoding()
+            or self.use_v2_model_runner
+            or speculative_config.method
+            in ("ngram", "eagle", "eagle3", "mtp", "draft_model", "dflash")
+        ):
+            return
+
+        logger.warning_once(
+            "Dynamic speculative decoding is not supported with the '%s' "
+            "speculative method on Model Runner V1. "
+            "Disabling num_speculative_tokens_per_batch_size "
+            "and falling back to static num_speculative_tokens=%d.",
+            speculative_config.method,
+            speculative_config.num_speculative_tokens,
+        )
+        speculative_config.num_speculative_tokens_per_batch_size = None
+
     def _maybe_disable_dynamic_sd_for_data_parallel(self) -> None:
         speculative_config = self.speculative_config
         if (
@@ -1829,6 +1851,7 @@ class VllmConfig:
             )
 
         self._maybe_disable_dynamic_sd_for_data_parallel()
+        self._maybe_disable_dynamic_sd_for_v1()
         self._maybe_override_dynamic_sd_cudagraph_mode()
 
         if (