diff --git a/vllm/attention/backends/abstract.py b/vllm/attention/backends/abstract.py
index 84cca8e68607..b60cfd5c50b4 100644
--- a/vllm/attention/backends/abstract.py
+++ b/vllm/attention/backends/abstract.py
@@ -357,8 +357,8 @@ def forward(
         self,
         layer: AttentionLayer,
         query: torch.Tensor,
-        key: torch.Tensor,
-        value: torch.Tensor,
+        key: torch.Tensor | None,
+        value: torch.Tensor | None,
         kv_cache: torch.Tensor,
         attn_metadata: T,
         output: torch.Tensor | None = None,
diff --git a/vllm/v1/attention/backends/flash_attn.py b/vllm/v1/attention/backends/flash_attn.py
index f5ad98cf2125..6fa554743f0c 100755
--- a/vllm/v1/attention/backends/flash_attn.py
+++ b/vllm/v1/attention/backends/flash_attn.py
@@ -562,8 +562,8 @@ def forward(
         self,
         layer: torch.nn.Module,
         query: torch.Tensor,
-        key: torch.Tensor,
-        value: torch.Tensor,
+        key: torch.Tensor | None,
+        value: torch.Tensor | None,
         kv_cache: torch.Tensor,
         attn_metadata: FlashAttentionMetadata,
         output: torch.Tensor | None = None,
@@ -611,6 +611,8 @@ def forward(
 
         # Handle encoder attention differently - no KV cache needed
         if attn_type in (AttentionType.ENCODER_ONLY, AttentionType.ENCODER):
+            # key/value are only None with ENCODER_DECODER attention
+            assert key is not None and value is not None
             # For encoder attention,
             # we use direct Q, K, V tensors without caching
             return self._forward_encoder_attention(
@@ -670,6 +672,7 @@ def forward(
             descale_shape = (cu_seqlens_q.shape[0] - 1, self.num_kv_heads)
 
             if self.dcp_world_size > 1:
+                assert key is not None and value is not None
                 self._forward_with_dcp(
                     query[:num_actual_tokens],
                     key[:num_actual_tokens],
diff --git a/vllm/v1/attention/backends/flashinfer.py b/vllm/v1/attention/backends/flashinfer.py
index 8e9d764e4a12..d67e3d068e00 100755
--- a/vllm/v1/attention/backends/flashinfer.py
+++ b/vllm/v1/attention/backends/flashinfer.py
@@ -294,6 +294,13 @@ def get_supported_kernel_block_sizes() -> list[int | MultipleOf]:
     def get_name() -> str:
         return "FLASHINFER"
 
+    @classmethod
+    def supports_attn_type(cls, attn_type: str) -> bool:
+        return attn_type in (
+            AttentionType.DECODER,
+            AttentionType.ENCODER_DECODER,
+        )
+
     @staticmethod
     def get_impl_cls() -> type["FlashInferImpl"]:
         return FlashInferImpl
@@ -1019,12 +1026,9 @@ def __init__(
 
         self.num_queries_per_kv = self.num_heads // self.num_kv_heads
 
-        if attn_type != AttentionType.DECODER:
+        if attn_type not in (AttentionType.DECODER, AttentionType.ENCODER_DECODER):
             raise NotImplementedError(
-                "Encoder self-attention and "
-                "encoder/decoder cross-attention "
-                "are not implemented for "
-                "FlashInferImpl"
+                "Encoder self-attention is not implemented for FlashInferImpl"
             )
 
         self.sinks: torch.Tensor | None = None
@@ -1063,8 +1067,8 @@ def forward(
         self,
         layer: torch.nn.Module,
         query: torch.Tensor,
-        key: torch.Tensor,
-        value: torch.Tensor,
+        key: torch.Tensor | None,
+        value: torch.Tensor | None,
         kv_cache: torch.Tensor,
         attn_metadata: FlashInferMetadata,
         output: torch.Tensor | None = None,
@@ -1155,16 +1159,20 @@ def forward(
             # and value[:num_actual_tokens] because the reshape_and_cache_flash
             # op uses the slot_mapping's shape to determine the number of
             # actual tokens.
-            torch.ops._C_cache_ops.reshape_and_cache_flash(
-                key,
-                value,
-                kv_cache[:, 0],
-                kv_cache[:, 1],
-                attn_metadata.slot_mapping,
-                self.kv_cache_dtype,
-                layer._k_scale,
-                layer._v_scale,
-            )
+            if key is not None and value is not None:
+                # ENCODER_DECODER attention is called with key and value
+                # set to None after the first decode step. They are cached
+                # on the first pass and do not change.
+                torch.ops._C_cache_ops.reshape_and_cache_flash(
+                    key,
+                    value,
+                    kv_cache[:, 0],
+                    kv_cache[:, 1],
+                    attn_metadata.slot_mapping,
+                    self.kv_cache_dtype,
+                    layer._k_scale,
+                    layer._v_scale,
+                )
 
             # The FlashInfer api requires data to be in fp8_e4m3 or fp8_e5m2
             # to process the cache when the kv_cache_dtype is fp8
@@ -1176,8 +1184,9 @@ def forward(
 
         # Inputs and outputs may be padded for CUDA graphs
         query = query[:num_actual_tokens]
-        key = key[:num_actual_tokens]
-        value = value[:num_actual_tokens]
+        if key is not None and value is not None:
+            key = key[:num_actual_tokens]
+            value = value[:num_actual_tokens]
         output_padded = output
         output = output[:num_actual_tokens]
 
@@ -1218,6 +1227,8 @@ def forward(
                     )
                     assert prefill_wrapper._new_tokens._sm_scale == self.scale
                     assert prefill_wrapper._new_tokens._causal
+                    assert key is not None
+                    assert value is not None
 
                     prefill_wrapper.run(
                         layer,