feat(vllm-cpp): wire the full engine config surface through engine_args

The vllm-cpp backend could configure four of the engine's knobs - block size, KV block count, max sequence length and max concurrent sequences - out of a config surface that is considerably larger. Speculative decoding, prefix caching, the chunked-prefill token budget, the scheduling policy and the external KV connector were all reachable from vllm.cpp's own HTTP server and from nothing LocalAI could write in a model config. Part of that gap was the C ABI itself, which carried strictly less than EngineParams does; that is fixed upstream in vllm.cpp ABI v9 (this bumps the pin to it). The rest was here: the backend parsed a flat `options:` list with five recognised keys and had no way to express a nested JSON document at all. Configuration now goes through `engine_args:`, the same map the vLLM and SGLang backends already take, with keys spelled as vLLM's own CLI flags - so a `speculative_config` or `kv_transfer_config` block written for vLLM works verbatim: engine_args: max_num_batched_tokens: 8192 enable_prefix_caching: true scheduling_policy: lpm speculative_config: method: dflash model: z-lab/Qwen3.6-27B-DFlash num_speculative_tokens: 4 kv_transfer_config: kv_connector: LMCacheConnector kv_role: kv_both kv_connector_extra_config: {host: 127.0.0.1, port: 65432} The `options:` list keeps working, and now reads every key too, so no existing config breaks; engine_args wins where both set the same key. Two details worth calling out. `enable_prefix_caching: false` maps to the ABI's force-OFF state (2), not the 0 that means "let the model capability decide" - collapsing them would silently turn the cache ON for the dense architectures that default it on. And cSamplingParams grows the ABI v8 logits-processor tail: LocalAI installs no processor, but the C side reads those fields off the pointer we hand it, so a Go struct that stopped short would have had the engine read 16 bytes past our allocation and call whatever sat there. The importer gets the safetensors counterpart of the llama-cpp MTP hook: a `vllm-cpp` import of a HuggingFace repo probes config.json and, on a checkpoint that declares an MTP head, writes speculative_config {method: mtp} into the generated engine_args. DFlash draft repositories are detected and refused with a warning rather than configured as standalone models, since a drafter cannot serve alone. The llama-cpp importer stops applying its own `spec_type:draft-mtp` options when the chosen backend is vllm-cpp: those are llama.cpp option keys vllm-cpp does not read, and vllm.cpp rejects MTP over a GGUF source anyway because the `mtp.*` draft tensors only exist in the safetensors checkpoint. docs/content/features/text-generation.md gains a vllm.cpp section - the backend had no documentation page at all - covering the engine_args table, all three speculative methods, LMCache, and the legacy options list. Signed-off-by: Ettore Di Giacinto <mudler@localai.io> Assisted-by: Claude Code:claude-opus-5 [ClaudeCode]
2026-07-30 09:57:57 -04:00 · 2026-07-28 08:11:43 +00:00
parent bc21f832aa
commit 5945b0eb11
11 changed files with 983 additions and 28 deletions
--- a/backend/go/vllm-cpp/Makefile
+++ b/backend/go/vllm-cpp/Makefile
@@ -11,7 +11,7 @@ JOBS?=$(shell nproc --ignore=1 2>/dev/null || sysctl -n hw.ncpu 2>/dev/null || e

 # vllm.cpp version
 VLLM_CPP_REPO?=https://github.com/mudler/vllm.cpp
-VLLM_CPP_VERSION?=9e1c9025ae61167a3335454d7cc0de6093c21845
+VLLM_CPP_VERSION?=974d9d72c2365d20dc76dbce4a98fddac32525c0

 # The backend consumes only the stable C ABI (libvllm + include/vllm.h), so the
 # server, examples and tests of the engine are never built here.
--- a/backend/go/vllm-cpp/backend.go
+++ b/backend/go/vllm-cpp/backend.go
@@ -116,34 +116,60 @@ func (v *VllmCpp) Load(opts *pb.ModelOptions) error {
 	if v.opts.numBlocks > 0 {
 		mp.NumBlocks = v.opts.numBlocks
 	}
+	// Sequence-length precedence, narrowest source last: context_size is the
+	// generic LocalAI knob every backend honours, max_model_len is the
+	// vLLM-specific one, and engine_args.max_model_len is the explicit
+	// vllm-cpp override.
 	if opts.ContextSize > 0 {
 		mp.MaxModelLen = opts.ContextSize
 	}
+	if opts.MaxModelLen > 0 {
+		mp.MaxModelLen = opts.MaxModelLen
+	}
+	if v.opts.maxModelLen > 0 {
+		mp.MaxModelLen = v.opts.maxModelLen
+	}
 	if v.opts.maxNumSeqs > 0 {
 		mp.MaxNumSeqs = v.opts.maxNumSeqs
 	}
+	if v.opts.maxNumBatchedTokens > 0 {
+		mp.MaxNumBatchedTokens = v.opts.maxNumBatchedTokens
+	}
+	mp.EnablePrefixCaching = v.opts.enablePrefixCaching

+	// Every string below is borrowed by C for the duration of the load call
+	// only (the library copies what it keeps), so the backing slices just have
+	// to outlive vllmEngineLoad - hence the single KeepAlive after it.
 	modelC := cString(model)
 	mp.ModelPath = uintptr(unsafe.Pointer(&modelC[0])) // #nosec G103 -- borrowed by C for the load call only
-	var toolParserC, reasoningParserC []byte
-	if v.opts.toolParser != "" {
-		toolParserC = cString(v.opts.toolParser)
-		mp.ToolParser = uintptr(unsafe.Pointer(&toolParserC[0])) // #nosec G103 -- borrowed by C for the load call only
-	}
-	if v.opts.reasoningParser != "" {
-		reasoningParserC = cString(v.opts.reasoningParser)
-		mp.ReasoningParser = uintptr(unsafe.Pointer(&reasoningParserC[0])) // #nosec G103 -- borrowed by C for the load call only
+	keep := [][]byte{modelC}
+	setStr := func(dst *uintptr, s string) {
+		if s == "" {
+			return
+		}
+		b := cString(s)
+		keep = append(keep, b)
+		*dst = uintptr(unsafe.Pointer(&b[0])) // #nosec G103 -- borrowed by C for the load call only
 	}
+	setStr(&mp.ToolParser, v.opts.toolParser)
+	setStr(&mp.ReasoningParser, v.opts.reasoningParser)
+	setStr(&mp.SpeculativeConfig, v.opts.speculativeConfig)
+	setStr(&mp.KVTransferConfig, v.opts.kvTransferConfig)
+	setStr(&mp.SchedulingPolicy, v.opts.schedulingPolicy)
+	setStr(&mp.TokenizerConfigPath, v.opts.tokenizerConfigPath)

 	xlog.Info("[vllm-cpp] Load", "model", model, "engine", vllmVersion(),
 		"blockSize", mp.BlockSize, "numBlocks", mp.NumBlocks,
-		"maxModelLen", mp.MaxModelLen, "maxNumSeqs", mp.MaxNumSeqs)
+		"maxModelLen", mp.MaxModelLen, "maxNumSeqs", mp.MaxNumSeqs,
+		"maxNumBatchedTokens", mp.MaxNumBatchedTokens,
+		"prefixCaching", prefixCachingName(mp.EnablePrefixCaching),
+		"schedulingPolicy", v.opts.schedulingPolicy,
+		"speculativeConfig", v.opts.speculativeConfig,
+		"kvTransferConfig", v.opts.kvTransferConfig)

 	var engine uintptr
 	rc := vllmEngineLoad(unsafe.Pointer(&mp), unsafe.Pointer(&engine)) // #nosec G103 -- POD out-params
-	runtime.KeepAlive(modelC)
-	runtime.KeepAlive(toolParserC)
-	runtime.KeepAlive(reasoningParserC)
+	runtime.KeepAlive(keep)
 	if rc != vllmOK {
 		return fmt.Errorf("vllm-cpp: engine load failed: %s", vllmLastError())
 	}
--- a/backend/go/vllm-cpp/govllmcpp.go
+++ b/backend/go/vllm-cpp/govllmcpp.go
@@ -1,6 +1,6 @@
 package main

-// purego bindings for the vllm.cpp stable C ABI (include/vllm.h, ABI v2).
+// purego bindings for the vllm.cpp stable C ABI (include/vllm.h, ABI v9).
 //
 // The structs below are hand-mirrored PODs of the C declarations, with
 // explicit padding so the Go layout matches the C layout on linux/darwin
@@ -18,23 +18,52 @@ import (
 )

 // abiVersion is the VLLM_ABI_VERSION this file mirrors (vllm.h).
-const abiVersion = 5
+const abiVersion = 9
+
+// Prefix-caching tri-state (vllm_model_params.enable_prefix_caching, ABI v7).
+// 0 is the model-capability default, which is what a config that says nothing
+// about prefix caching must produce.
+const (
+	prefixCachingModelDefault int32 = 0
+	prefixCachingOn           int32 = 1
+	prefixCachingOff          int32 = 2
+)
+
+// prefixCachingName renders the tri-state for the load log line, where "0"
+// would otherwise read as "off" rather than "whatever the model defaults to".
+func prefixCachingName(state int32) string {
+	switch state {
+	case prefixCachingOn:
+		return "on"
+	case prefixCachingOff:
+		return "off"
+	default:
+		return "model-default"
+	}
+}

 // vllm_status (vllm.h).
 const (
 	vllmOK = 0
 )

-// cModelParams mirrors vllm_model_params.
+// cModelParams mirrors vllm_model_params. Every field is naturally aligned on
+// LP64 (the int32 pairs sit together), so the Go layout needs no explicit
+// padding; the offsets are asserted in vllmcpp_test.go.
 type cModelParams struct {
 	ModelPath           uintptr // const char*
-	TokenizerConfigPath uintptr // const char*
+	TokenizerConfigPath uintptr // const char*; NULL = <model_dir>/... (ABI v9)
 	BlockSize           int32
 	NumBlocks           int32
 	MaxModelLen         int32
 	MaxNumSeqs          int32
 	ToolParser          uintptr // const char*; NULL = auto-detect (ABI v4)
 	ReasoningParser     uintptr // const char*; NULL = auto-detect (ABI v5)
+	SpeculativeConfig   uintptr // const char* JSON; NULL = no speculation (ABI v6)
+	EnablePrefixCaching int32   // tri-state 0/1/2 (ABI v7)
+	MaxNumBatchedTokens int32   // <= 0 = per-arch default (ABI v9)
+	SchedulingPolicy    uintptr // const char*; NULL = "fcfs" (ABI v9)
+	KVTransferConfig    uintptr // const char* JSON; NULL = no connector (ABI v9)
 }

 // cSamplingParams mirrors vllm_sampling_params (ABI v2, structured fields
@@ -65,6 +94,12 @@ type cSamplingParams struct {
 	StructuredGrammar    uintptr // const char*
 	StructuredJSONObject int32
 	_                    [4]byte
+	// ABI v8 tail. LocalAI installs no custom logits processor, but the fields
+	// MUST be mirrored: the C side reads them off the pointer we hand it, so a
+	// Go struct that stopped at StructuredJSONObject would have the engine read
+	// 16 bytes past our allocation and call whatever garbage sat there.
+	LogitsProcessor         uintptr // vllm_logits_processor; NULL = none
+	LogitsProcessorUserData uintptr // void*
 }

 // cCompletion mirrors vllm_completion.
--- a/backend/go/vllm-cpp/options.go
+++ b/backend/go/vllm-cpp/options.go
@@ -1,30 +1,72 @@
 package main

-// Engine-sizing knobs carried through the model config's free-form
-// `options:` list ("key:value" entries), mirroring how the other in-house
-// backends pass engine-specific settings that have no proto field.
+// Load-time engine configuration, from two config surfaces:
+//
+//   - `engine_args:` (ModelOptions.EngineArgs, a JSON object) is the canonical
+//     one. Keys are spelled exactly as vLLM's own CLI flags, so a config written
+//     against vLLM works verbatim here - `speculative_config` and
+//     `kv_transfer_config` in particular take the same JSON documents vLLM's
+//     --speculative-config / --kv-transfer-config accept, and are handed to the
+//     engine unparsed.
+//   - `options:` (the free-form "key:value" list) is the older surface this
+//     backend shipped with. It is still honoured so existing configs keep
+//     working; engine_args wins on any key set in both.
+//
+// Anything unrecognised is ignored rather than fatal: the engine validates the
+// documents it is given and reports a precise error at load, and a config that
+// also carries knobs for a different backend must not fail the load here.

 import (
+	"encoding/json"
 	"strconv"
 	"strings"

 	pb "github.com/mudler/LocalAI/pkg/grpc/proto"
+	"github.com/mudler/xlog"
 )

 type loadOptions struct {
 	blockSize  int32 // KV block size (tokens/block); engine default 32.
 	numBlocks  int32 // KV blocks to allocate; engine default 256.
 	maxNumSeqs int32 // max concurrent sequences; engine default 8.
+	// Max sequence length. Also settable through the model config's
+	// context_size / max_model_len; see Load for the precedence.
+	maxModelLen int32
+	// Per-step chunked-prefill token budget (ABI v9). 0 = the engine's
+	// bounded per-arch default.
+	maxNumBatchedTokens int32
+	// Automatic prefix caching tri-state (ABI v7): 0 = the model-capability
+	// default, 1 = force on, 2 = force off.
+	enablePrefixCaching int32
+	// Scheduler admission policy (ABI v9): "" = fcfs, else fcfs|priority|lpm.
+	schedulingPolicy string
 	// Engine-side parser selection (ABI v4/v5). Empty = the engine
 	// auto-detects from the chat template; "none" disables the reasoning
 	// split; unknown names fail the first chat call.
 	toolParser      string
 	reasoningParser string
+	// Speculative decoding (ABI v6), as vLLM's --speculative-config JSON:
+	// {"method":"mtp"|"dflash"|"ngram", ...}. Empty = no speculation.
+	speculativeConfig string
+	// External KV connector / LMCache (ABI v9), as vLLM's --kv-transfer-config
+	// JSON. Empty = no connector.
+	kvTransferConfig string
+	// Override for the tokenizer_config.json the chat template is read from
+	// (ABI v9). Empty = <model_dir>/tokenizer_config.json.
+	tokenizerConfigPath string
 }

 func parseOptions(opts *pb.ModelOptions) loadOptions {
 	lo := loadOptions{}
-	for _, o := range opts.GetOptions() {
+	applyOptionsList(&lo, opts.GetOptions())
+	applyEngineArgs(&lo, opts.GetEngineArgs())
+	return lo
+}
+
+// applyOptionsList reads the legacy free-form "key:value" list. strings.Cut
+// splits on the FIRST colon only, so a JSON object value survives intact.
+func applyOptionsList(lo *loadOptions, options []string) {
+	for _, o := range options {
 		k, v, found := strings.Cut(o, ":")
 		if !found {
 			continue
@@ -36,13 +78,132 @@ func parseOptions(opts *pb.ModelOptions) loadOptions {
 			lo.numBlocks = parseInt32(v, lo.numBlocks)
 		case "max_num_seqs":
 			lo.maxNumSeqs = parseInt32(v, lo.maxNumSeqs)
-		case "tool_parser":
+		case "max_num_batched_tokens":
+			lo.maxNumBatchedTokens = parseInt32(v, lo.maxNumBatchedTokens)
+		case "max_model_len":
+			lo.maxModelLen = parseInt32(v, lo.maxModelLen)
+		case "scheduling_policy", "schedule_policy":
+			lo.schedulingPolicy = strings.TrimSpace(v)
+		case "tool_parser", "tool_call_parser":
 			lo.toolParser = strings.TrimSpace(v)
 		case "reasoning_parser":
 			lo.reasoningParser = strings.TrimSpace(v)
+		case "speculative_config":
+			lo.speculativeConfig = strings.TrimSpace(v)
+		case "kv_transfer_config":
+			lo.kvTransferConfig = strings.TrimSpace(v)
+		case "tokenizer_config", "tokenizer_config_path":
+			lo.tokenizerConfigPath = strings.TrimSpace(v)
+		case "enable_prefix_caching", "enable_radix_attention":
+			if b, err := strconv.ParseBool(strings.TrimSpace(v)); err == nil {
+				lo.enablePrefixCaching = prefixCachingTriState(b)
+			}
 		}
 	}
-	return lo
+}
+
+// applyEngineArgs overlays the `engine_args:` JSON object. A document that does
+// not parse is logged and skipped: engine_args is shared with the other engines
+// (the vLLM and SGLang backends read the same field), so a stray key must not
+// take the model down.
+func applyEngineArgs(lo *loadOptions, engineArgs string) {
+	if strings.TrimSpace(engineArgs) == "" {
+		return
+	}
+	var args map[string]any
+	if err := json.Unmarshal([]byte(engineArgs), &args); err != nil {
+		xlog.Warn("[vllm-cpp] ignoring unparseable engine_args", "error", err)
+		return
+	}
+	for k, v := range args {
+		switch k {
+		case "block_size":
+			lo.blockSize = jsonInt32(v, lo.blockSize)
+		case "num_blocks":
+			lo.numBlocks = jsonInt32(v, lo.numBlocks)
+		case "max_num_seqs":
+			lo.maxNumSeqs = jsonInt32(v, lo.maxNumSeqs)
+		case "max_num_batched_tokens":
+			lo.maxNumBatchedTokens = jsonInt32(v, lo.maxNumBatchedTokens)
+		case "max_model_len":
+			lo.maxModelLen = jsonInt32(v, lo.maxModelLen)
+		case "scheduling_policy", "schedule_policy":
+			lo.schedulingPolicy = jsonString(v, lo.schedulingPolicy)
+		case "tool_parser", "tool_call_parser":
+			lo.toolParser = jsonString(v, lo.toolParser)
+		case "reasoning_parser":
+			lo.reasoningParser = jsonString(v, lo.reasoningParser)
+		case "tokenizer_config", "tokenizer_config_path":
+			lo.tokenizerConfigPath = jsonString(v, lo.tokenizerConfigPath)
+		case "speculative_config":
+			lo.speculativeConfig = jsonDocument(v, lo.speculativeConfig, k)
+		case "kv_transfer_config":
+			lo.kvTransferConfig = jsonDocument(v, lo.kvTransferConfig, k)
+		case "enable_prefix_caching", "enable_radix_attention":
+			if b, ok := v.(bool); ok {
+				lo.enablePrefixCaching = prefixCachingTriState(b)
+			}
+		default:
+			xlog.Debug("[vllm-cpp] ignoring unknown engine_args key", "key", k)
+		}
+	}
+}
+
+// prefixCachingTriState maps a YAML/JSON boolean onto the C ABI's tri-state. An
+// explicit `false` must reach the engine as force-OFF (2), NOT as the 0 that
+// means "let the model capability decide" - those differ for the hybrid archs,
+// which default the cache off, and for the dense ones, which default it on.
+func prefixCachingTriState(on bool) int32 {
+	if on {
+		return prefixCachingOn
+	}
+	return prefixCachingOff
+}
+
+// jsonDocument normalises an object-valued engine_args entry to a JSON string
+// for the C ABI. YAML nesting arrives as a map (the natural spelling); a
+// pre-encoded JSON string is accepted too, since a config round-tripped through
+// a flat store may carry it that way.
+func jsonDocument(v any, fallback string, key string) string {
+	switch t := v.(type) {
+	case string:
+		if strings.TrimSpace(t) == "" {
+			return fallback
+		}
+		return t
+	default:
+		buf, err := json.Marshal(t)
+		if err != nil {
+			xlog.Warn("[vllm-cpp] ignoring unencodable engine_args value", "key", key, "error", err)
+			return fallback
+		}
+		return string(buf)
+	}
+}
+
+func jsonString(v any, fallback string) string {
+	s, ok := v.(string)
+	if !ok {
+		return fallback
+	}
+	return strings.TrimSpace(s)
+}
+
+// jsonInt32 accepts the float64 a JSON number decodes to, plus the string
+// spelling a YAML config may produce. Non-positive values keep the fallback:
+// every knob this covers uses "<= 0 means the engine default".
+func jsonInt32(v any, fallback int32) int32 {
+	switch t := v.(type) {
+	case float64:
+		if t <= 0 || t > 1<<31-1 {
+			return fallback
+		}
+		return int32(t)
+	case string:
+		return parseInt32(t, fallback)
+	default:
+		return fallback
+	}
 }

 func parseInt32(s string, fallback int32) int32 {
--- a/backend/go/vllm-cpp/vllmcpp_test.go
+++ b/backend/go/vllm-cpp/vllmcpp_test.go
@@ -16,7 +16,7 @@ func TestVllmCpp(t *testing.T) {
 	RunSpecs(t, "vllm-cpp suite")
 }

-// The Go POD mirrors must match the C struct layout of vllm.h (ABI v2)
+// The Go POD mirrors must match the C struct layout of vllm.h (ABI v9)
 // byte-for-byte: these offsets are the C offsets on LP64 (linux/darwin
 // amd64+arm64). A failure here means govllmcpp.go drifted from vllm.h.
 var _ = Describe("C ABI struct mirrors", func() {
@@ -30,10 +30,15 @@ var _ = Describe("C ABI struct mirrors", func() {
 		Expect(unsafe.Offsetof(p.MaxNumSeqs)).To(Equal(uintptr(28)))
 		Expect(unsafe.Offsetof(p.ToolParser)).To(Equal(uintptr(32)))
 		Expect(unsafe.Offsetof(p.ReasoningParser)).To(Equal(uintptr(40)))
-		Expect(unsafe.Sizeof(p)).To(Equal(uintptr(48)))
+		Expect(unsafe.Offsetof(p.SpeculativeConfig)).To(Equal(uintptr(48)))
+		Expect(unsafe.Offsetof(p.EnablePrefixCaching)).To(Equal(uintptr(56)))
+		Expect(unsafe.Offsetof(p.MaxNumBatchedTokens)).To(Equal(uintptr(60)))
+		Expect(unsafe.Offsetof(p.SchedulingPolicy)).To(Equal(uintptr(64)))
+		Expect(unsafe.Offsetof(p.KVTransferConfig)).To(Equal(uintptr(72)))
+		Expect(unsafe.Sizeof(p)).To(Equal(uintptr(80)))
 	})

-	It("cSamplingParams matches vllm_sampling_params (ABI v2)", func() {
+	It("cSamplingParams matches vllm_sampling_params (ABI v8)", func() {
 		var p cSamplingParams
 		Expect(unsafe.Offsetof(p.Temperature)).To(Equal(uintptr(0)))
 		Expect(unsafe.Offsetof(p.TopP)).To(Equal(uintptr(4)))
@@ -55,7 +60,9 @@ var _ = Describe("C ABI struct mirrors", func() {
 		Expect(unsafe.Offsetof(p.NStructuredChoice)).To(Equal(uintptr(96)))
 		Expect(unsafe.Offsetof(p.StructuredGrammar)).To(Equal(uintptr(104)))
 		Expect(unsafe.Offsetof(p.StructuredJSONObject)).To(Equal(uintptr(112)))
-		Expect(unsafe.Sizeof(p)).To(Equal(uintptr(120)))
+		Expect(unsafe.Offsetof(p.LogitsProcessor)).To(Equal(uintptr(120)))
+		Expect(unsafe.Offsetof(p.LogitsProcessorUserData)).To(Equal(uintptr(128)))
+		Expect(unsafe.Sizeof(p)).To(Equal(uintptr(136)))
 	})

 	It("cCompletion matches vllm_completion", func() {
@@ -83,6 +90,112 @@ var _ = Describe("parseOptions", func() {
 		}})
 		Expect(lo).To(Equal(loadOptions{}))
 	})
+
+	It("carries a speculative_config JSON value through the legacy options list", func() {
+		// strings.Cut splits on the FIRST colon only, so a JSON object value
+		// survives the "key:value" spelling intact.
+		lo := parseOptions(&pb.ModelOptions{Options: []string{
+			`speculative_config:{"method":"mtp","num_speculative_tokens":1}`,
+		}})
+		Expect(lo.speculativeConfig).To(Equal(`{"method":"mtp","num_speculative_tokens":1}`))
+	})
+})
+
+var _ = Describe("engine_args", func() {
+	It("maps every load knob onto the C model params", func() {
+		lo := parseOptions(&pb.ModelOptions{EngineArgs: `{
+			"block_size": 64,
+			"num_blocks": 1024,
+			"max_model_len": 16384,
+			"max_num_seqs": 32,
+			"max_num_batched_tokens": 8192,
+			"enable_prefix_caching": true,
+			"scheduling_policy": "lpm",
+			"tool_parser": "qwen3",
+			"reasoning_parser": "deepseek_r1",
+			"tokenizer_config": "/models/tok/tokenizer_config.json"
+		}`})
+		Expect(lo.blockSize).To(Equal(int32(64)))
+		Expect(lo.numBlocks).To(Equal(int32(1024)))
+		Expect(lo.maxModelLen).To(Equal(int32(16384)))
+		Expect(lo.maxNumSeqs).To(Equal(int32(32)))
+		Expect(lo.maxNumBatchedTokens).To(Equal(int32(8192)))
+		Expect(lo.enablePrefixCaching).To(Equal(int32(1)))
+		Expect(lo.schedulingPolicy).To(Equal("lpm"))
+		Expect(lo.toolParser).To(Equal("qwen3"))
+		Expect(lo.reasoningParser).To(Equal("deepseek_r1"))
+		Expect(lo.tokenizerConfigPath).To(Equal("/models/tok/tokenizer_config.json"))
+	})
+
+	It("re-marshals a nested speculative_config object to JSON for the engine", func() {
+		lo := parseOptions(&pb.ModelOptions{EngineArgs: `{
+			"speculative_config": {"method": "mtp", "num_speculative_tokens": 1}
+		}`})
+		Expect(lo.speculativeConfig).To(MatchJSON(`{"method":"mtp","num_speculative_tokens":1}`))
+	})
+
+	It("re-marshals a nested kv_transfer_config object (LMCache) to JSON", func() {
+		lo := parseOptions(&pb.ModelOptions{EngineArgs: `{
+			"kv_transfer_config": {
+				"kv_connector": "LMCacheConnector",
+				"kv_role": "kv_both",
+				"kv_connector_extra_config": {"host": "127.0.0.1", "port": 65432}
+			}
+		}`})
+		Expect(lo.kvTransferConfig).To(MatchJSON(`{
+			"kv_connector":"LMCacheConnector",
+			"kv_role":"kv_both",
+			"kv_connector_extra_config":{"host":"127.0.0.1","port":65432}
+		}`))
+	})
+
+	It("accepts a pre-encoded JSON string for the object-valued knobs", func() {
+		// A config written by hand (or round-tripped through a flat store) may
+		// carry the object as a string; both spellings reach the engine the same.
+		lo := parseOptions(&pb.ModelOptions{EngineArgs: `{
+			"speculative_config": "{\"method\":\"ngram\",\"num_speculative_tokens\":4}"
+		}`})
+		Expect(lo.speculativeConfig).To(MatchJSON(`{"method":"ngram","num_speculative_tokens":4}`))
+	})
+
+	It("maps enable_prefix_caching false onto the force-OFF tri-state", func() {
+		// The C ABI tri-state is 0=model default, 1=on, 2=off, so an explicit
+		// `false` must NOT collapse to the 0 that means "let the model decide".
+		lo := parseOptions(&pb.ModelOptions{EngineArgs: `{"enable_prefix_caching": false}`})
+		Expect(lo.enablePrefixCaching).To(Equal(int32(2)))
+	})
+
+	It("leaves the prefix-caching tri-state at the model default when unset", func() {
+		lo := parseOptions(&pb.ModelOptions{EngineArgs: `{"max_num_seqs": 4}`})
+		Expect(lo.enablePrefixCaching).To(Equal(int32(0)))
+	})
+
+	It("accepts the radix-attention alias upstream documents for prefix caching", func() {
+		lo := parseOptions(&pb.ModelOptions{EngineArgs: `{"enable_radix_attention": true}`})
+		Expect(lo.enablePrefixCaching).To(Equal(int32(1)))
+	})
+
+	It("lets engine_args override the legacy options list", func() {
+		lo := parseOptions(&pb.ModelOptions{
+			Options:    []string{"max_num_seqs:8", "block_size:16"},
+			EngineArgs: `{"max_num_seqs": 64}`,
+		})
+		Expect(lo.maxNumSeqs).To(Equal(int32(64))) // engine_args wins
+		Expect(lo.blockSize).To(Equal(int32(16)))  // untouched keys survive
+	})
+
+	It("ignores malformed engine_args rather than failing the load", func() {
+		lo := parseOptions(&pb.ModelOptions{
+			Options:    []string{"max_num_seqs:8"},
+			EngineArgs: `{not json`,
+		})
+		Expect(lo.maxNumSeqs).To(Equal(int32(8)))
+	})
+
+	It("ignores unknown keys", func() {
+		lo := parseOptions(&pb.ModelOptions{EngineArgs: `{"gpu_memory_utilization": 0.9}`})
+		Expect(lo).To(Equal(loadOptions{}))
+	})
 })

 var _ = Describe("samplingFromPredict", func() {
--- a/core/config/vllm_spec.go
+++ b/core/config/vllm_spec.go
@@ -0,0 +1,117 @@
+package config
+
+// Speculative-decoding auto-defaults for the vllm-cpp backend, the safetensors
+// counterpart of the GGUF/llama.cpp hook in mtp.go.
+//
+// The two engines detect and spell the same feature differently. llama.cpp
+// reads `<arch>.nextn_predict_layers` out of the GGUF header and takes
+// `spec_type:draft-mtp` in `options:`; vllm.cpp reads `mtp_num_hidden_layers`
+// out of the checkpoint's config.json and takes vLLM's own
+// `--speculative-config` JSON, which LocalAI carries in `engine_args`. The
+// engine resolves the draft depth and the default k itself, so the config only
+// has to name the method.
+
+import (
+	"encoding/json"
+
+	"github.com/mudler/xlog"
+)
+
+// hfSpecConfig is the subset of a HuggingFace config.json that decides whether
+// speculative decoding can be auto-enabled.
+type hfSpecConfig struct {
+	ModelType string `json:"model_type"`
+	// MtpNumHiddenLayers is the MTP head depth (upstream speculative.py reads
+	// it as n_predict for the qwen3_5 / qwen3_5_moe families).
+	MtpNumHiddenLayers uint32 `json:"mtp_num_hidden_layers"`
+	// DFlashConfig marks a z-lab DFlash DRAFT checkpoint (mask_token_id +
+	// target_layer_ids). Its presence means this repo is a draft, not a
+	// servable target.
+	DFlashConfig json.RawMessage `json:"dflash_config"`
+	// TextConfig is where multimodal checkpoints nest the language-model
+	// config, and therefore the MTP depth.
+	TextConfig *hfSpecConfig `json:"text_config"`
+}
+
+// parseHFSpecConfig decodes the speculative-relevant subset of a config.json.
+// A document that does not parse yields nothing rather than an error: detection
+// is best-effort and must never break an import.
+func parseHFSpecConfig(configJSON []byte) (hfSpecConfig, bool) {
+	if len(configJSON) == 0 {
+		return hfSpecConfig{}, false
+	}
+	var c hfSpecConfig
+	if err := json.Unmarshal(configJSON, &c); err != nil {
+		xlog.Debug("[vllm-spec] config.json did not parse; skipping detection", "error", err)
+		return hfSpecConfig{}, false
+	}
+	return c, true
+}
+
+// IsDFlashDraftConfig reports whether a HuggingFace config.json describes a
+// DFlash DRAFT checkpoint. Unlike MTP - whose head ships inside the target
+// checkpoint's `mtp.*` tensors - a DFlash draft is its own repo that can only
+// run paired with a target it verifies against, so it must never be configured
+// as a standalone model.
+func IsDFlashDraftConfig(configJSON []byte) bool {
+	c, ok := parseHFSpecConfig(configJSON)
+	if !ok {
+		return false
+	}
+	return len(c.DFlashConfig) > 0 ||
+		(c.TextConfig != nil && len(c.TextConfig.DFlashConfig) > 0)
+}
+
+// HasSafetensorsMTPHead reports whether a HuggingFace config.json declares a
+// self-speculating Multi-Token Prediction head, returning its depth. The depth
+// is informational: vllm.cpp resolves n_predict and the default
+// num_speculative_tokens from the checkpoint itself.
+//
+// DFlash drafts are excluded for the same reason `gemma4-assistant` GGUFs are
+// excluded from the llama.cpp hook: they carry head metadata but cannot
+// self-speculate.
+//
+// NOTE this is a safetensors-only signal. vllm.cpp rejects an MTP config over a
+// GGUF source, because the `mtp.*` draft tensors only exist in the safetensors
+// checkpoint - so the GGUF import path must not use this.
+func HasSafetensorsMTPHead(configJSON []byte) (uint32, bool) {
+	c, ok := parseHFSpecConfig(configJSON)
+	if !ok {
+		return 0, false
+	}
+	if IsDFlashDraftConfig(configJSON) {
+		return 0, false
+	}
+	n := c.MtpNumHiddenLayers
+	if n == 0 && c.TextConfig != nil {
+		n = c.TextConfig.MtpNumHiddenLayers
+	}
+	return n, n > 0
+}
+
+// ApplyVLLMSpeculativeDefaults enables MTP speculative decoding in cfg's
+// engine_args when nothing is configured there yet. It is a no-op when the user
+// already set a speculative_config, so an explicit choice (a different method,
+// an explicit k, a DFlash draft) is never clobbered.
+//
+// `layers` is the detected head depth and is only used for the diagnostic log
+// line - the engine derives the real k from the checkpoint.
+func ApplyVLLMSpeculativeDefaults(cfg *ModelConfig, layers uint32) {
+	if cfg == nil {
+		return
+	}
+	if _, set := cfg.EngineArgs["speculative_config"]; set {
+		xlog.Debug("[vllm-spec] MTP head detected but speculative_config already configured; leaving user choice intact",
+			"name", cfg.Name, "mtp_num_hidden_layers", layers)
+		return
+	}
+	if cfg.EngineArgs == nil {
+		cfg.EngineArgs = map[string]any{}
+	}
+	// Only the method: vllm.cpp defaults num_speculative_tokens to the
+	// checkpoint's own n_predict (speculative.py:865-875), which is the right
+	// value far more reliably than anything guessable here.
+	cfg.EngineArgs["speculative_config"] = map[string]any{"method": "mtp"}
+	xlog.Info("[vllm-spec] MTP head detected; enabling mtp speculative decoding",
+		"name", cfg.Name, "mtp_num_hidden_layers", layers)
+}
--- a/core/config/vllm_spec_test.go
+++ b/core/config/vllm_spec_test.go
@@ -0,0 +1,117 @@
+package config_test
+
+import (
+	. "github.com/mudler/LocalAI/core/config"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("vllm-cpp speculative-decoding auto-defaults", func() {
+	Context("HasSafetensorsMTPHead", func() {
+		It("detects a top-level mtp_num_hidden_layers", func() {
+			n, ok := HasSafetensorsMTPHead([]byte(`{
+				"model_type": "qwen3_5_moe",
+				"mtp_num_hidden_layers": 1
+			}`))
+			Expect(ok).To(BeTrue())
+			Expect(n).To(Equal(uint32(1)))
+		})
+
+		It("detects the head nested under text_config", func() {
+			// Multimodal checkpoints nest the language-model config, which is
+			// where the MTP depth lives (mirrors the engine's own resolution
+			// off config.raw text_config).
+			n, ok := HasSafetensorsMTPHead([]byte(`{
+				"model_type": "qwen3_5_moe",
+				"text_config": {"mtp_num_hidden_layers": 2}
+			}`))
+			Expect(ok).To(BeTrue())
+			Expect(n).To(Equal(uint32(2)))
+		})
+
+		It("reports no head when the key is absent", func() {
+			n, ok := HasSafetensorsMTPHead([]byte(`{"model_type": "llama"}`))
+			Expect(ok).To(BeFalse())
+			Expect(n).To(BeZero())
+		})
+
+		It("reports no head for a zero depth", func() {
+			_, ok := HasSafetensorsMTPHead([]byte(`{"mtp_num_hidden_layers": 0}`))
+			Expect(ok).To(BeFalse())
+		})
+
+		It("ignores a DFlash draft checkpoint", func() {
+			// A DFlash draft is a SEPARATE checkpoint that cannot serve alone:
+			// it needs a target to verify against. Same exclusion the GGUF path
+			// makes for gemma4-assistant drafts.
+			_, ok := HasSafetensorsMTPHead([]byte(`{
+				"model_type": "qwen3_dflash",
+				"mtp_num_hidden_layers": 1,
+				"dflash_config": {"mask_token_id": 151666, "target_layer_ids": [0, 1]}
+			}`))
+			Expect(ok).To(BeFalse())
+		})
+
+		It("reports no head on unparseable JSON", func() {
+			_, ok := HasSafetensorsMTPHead([]byte(`{not json`))
+			Expect(ok).To(BeFalse())
+		})
+
+		It("reports no head on empty input", func() {
+			_, ok := HasSafetensorsMTPHead(nil)
+			Expect(ok).To(BeFalse())
+		})
+	})
+
+	Context("IsDFlashDraftConfig", func() {
+		It("recognises a draft by its dflash_config block", func() {
+			Expect(IsDFlashDraftConfig([]byte(`{
+				"dflash_config": {"mask_token_id": 151666, "target_layer_ids": [0]}
+			}`))).To(BeTrue())
+		})
+
+		It("does not flag an ordinary checkpoint", func() {
+			Expect(IsDFlashDraftConfig([]byte(`{"model_type": "qwen3_5_moe"}`))).To(BeFalse())
+		})
+	})
+
+	Context("ApplyVLLMSpeculativeDefaults", func() {
+		It("writes the mtp method into engine_args", func() {
+			cfg := &ModelConfig{Name: "qwen"}
+			ApplyVLLMSpeculativeDefaults(cfg, 1)
+			Expect(cfg.EngineArgs).To(HaveKey("speculative_config"))
+			spec, ok := cfg.EngineArgs["speculative_config"].(map[string]any)
+			Expect(ok).To(BeTrue())
+			Expect(spec["method"]).To(Equal("mtp"))
+		})
+
+		It("leaves an existing speculative_config alone", func() {
+			cfg := &ModelConfig{
+				Name: "qwen",
+				LLMConfig: LLMConfig{
+					EngineArgs: map[string]any{
+						"speculative_config": map[string]any{"method": "ngram", "num_speculative_tokens": 4},
+					},
+				},
+			}
+			ApplyVLLMSpeculativeDefaults(cfg, 1)
+			spec := cfg.EngineArgs["speculative_config"].(map[string]any)
+			Expect(spec["method"]).To(Equal("ngram"))
+		})
+
+		It("preserves unrelated engine_args keys", func() {
+			cfg := &ModelConfig{
+				Name:      "qwen",
+				LLMConfig: LLMConfig{EngineArgs: map[string]any{"max_num_seqs": 32}},
+			}
+			ApplyVLLMSpeculativeDefaults(cfg, 1)
+			Expect(cfg.EngineArgs).To(HaveKeyWithValue("max_num_seqs", 32))
+			Expect(cfg.EngineArgs).To(HaveKey("speculative_config"))
+		})
+
+		It("tolerates a nil config", func() {
+			Expect(func() { ApplyVLLMSpeculativeDefaults(nil, 1) }).ToNot(Panic())
+		})
+	})
+})
--- a/core/gallery/importers/llama-cpp.go
+++ b/core/gallery/importers/llama-cpp.go
@@ -298,7 +298,15 @@ func (i *LlamaCPPImporter) Import(details Details) (gallery.ModelConfig, error)
 	// imported configs already carry spec_type:draft-mtp before the model is
 	// ever loaded - users see it in the YAML preview rather than discovering
 	// it after the first start.
-	maybeApplyMTPDefaults(&modelConfig, details, &cfg)
+	//
+	// vllm-cpp is excluded on both counts: `spec_type:*` are llama.cpp option
+	// keys it does not read, and vllm.cpp rejects an MTP config over a GGUF
+	// source outright (the `mtp.*` draft tensors exist only in the safetensors
+	// checkpoint). Its MTP auto-config runs in the vllm importer instead, over
+	// the safetensors config.json.
+	if backend != "vllm-cpp" {
+		maybeApplyMTPDefaults(&modelConfig, details, &cfg)
+	}

 	data, err := yaml.Marshal(modelConfig)
 	if err != nil {
--- a/core/gallery/importers/vllm.go
+++ b/core/gallery/importers/vllm.go
@@ -1,13 +1,21 @@
 package importers

 import (
+	"context"
 	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
 	"path/filepath"
 	"strings"
+	"time"

 	"github.com/mudler/LocalAI/core/config"
 	"github.com/mudler/LocalAI/core/gallery"
 	"github.com/mudler/LocalAI/core/schema"
+	"github.com/mudler/LocalAI/pkg/downloader"
+	"github.com/mudler/LocalAI/pkg/httpclient"
+	"github.com/mudler/xlog"
 	"go.yaml.in/yaml/v2"
 )

@@ -107,6 +115,12 @@ func (i *VLLMImporter) Import(details Details) (gallery.ModelConfig, error) {
 		// vllm python backend, so use_tokenizer_template carries over), but
 		// tool/reasoning parsing is the engine's own autoparser pipeline -
 		// the vllm-python tool_parser/reasoning_parser options don't apply.
+		//
+		// Auto-detect a Multi-Token Prediction head, the safetensors analogue
+		// of the llama-cpp importer's GGUF hook, so a freshly imported
+		// Qwen3.5 / Qwen3.6 config already carries speculative decoding in its
+		// engine_args instead of leaving the throughput on the table.
+		maybeApplyVLLMSpeculativeDefaults(&modelConfig, details)
 	} else {
 		// Auto-detect tool_parser and reasoning_parser for known model families.
 		// Surfacing them in the generated YAML lets users see and edit the choices.
@@ -132,3 +146,89 @@ func (i *VLLMImporter) Import(details Details) (gallery.ModelConfig, error) {
 		ConfigFile:  string(data),
 	}, nil
 }
+
+// maxSpecConfigProbeBytes caps the config.json body we read. Real ones are a
+// few KB; the cap keeps a hostile or mislabelled URL from streaming into the
+// importer.
+const maxSpecConfigProbeBytes = 1 << 20 // 1 MiB
+
+// specConfigProbeTimeout bounds the config.json fetch. Detection is an
+// optimisation, so it must never hold an import open for long.
+const specConfigProbeTimeout = 30 * time.Second
+
+// specConfigFetcher is the seam the config.json probe goes through, so tests can
+// drive the whole import path without a network round trip.
+var specConfigFetcher = fetchProbeBody
+
+// maybeApplyVLLMSpeculativeDefaults fetches the repository's config.json and,
+// when it declares a Multi-Token Prediction head, enables MTP speculative
+// decoding in the emitted engine_args. This is the safetensors counterpart of
+// the llama-cpp importer's GGUF header probe.
+//
+// Every failure is non-fatal and logged at debug: a network blip, a private
+// repo, or a config.json this doesn't understand must leave the import working
+// exactly as it did before, just without the speculative default.
+func maybeApplyVLLMSpeculativeDefaults(modelConfig *config.ModelConfig, details Details) {
+	probeURL := vllmSpecProbeURL(details)
+	if probeURL == "" {
+		return
+	}
+
+	body, err := specConfigFetcher(probeURL)
+	if err != nil {
+		xlog.Debug("[vllm-spec-importer] could not read config.json for MTP detection", "uri", probeURL, "error", err)
+		return
+	}
+
+	applySpecFromConfigJSON(modelConfig, body, details.URI)
+}
+
+// applySpecFromConfigJSON is the decision half of the probe, split out so it can
+// be exercised without a network round trip.
+func applySpecFromConfigJSON(modelConfig *config.ModelConfig, body []byte, uri string) {
+	if config.IsDFlashDraftConfig(body) {
+		// A DFlash draft cannot serve on its own - it only proposes tokens for
+		// a target model to verify. Say so rather than emitting a config that
+		// would fail at load.
+		xlog.Warn("[vllm-spec-importer] this repository is a DFlash DRAFT checkpoint, not a servable model; "+
+			"import the TARGET model and point engine_args.speculative_config at this repo "+
+			`({"method":"dflash","model":"<this repo>"})`, "uri", uri)
+		return
+	}
+
+	n, ok := config.HasSafetensorsMTPHead(body)
+	if !ok {
+		return
+	}
+	config.ApplyVLLMSpeculativeDefaults(modelConfig, n)
+}
+
+// vllmSpecProbeURL returns the HTTP(S) URL of the repository's config.json, or
+// "" when the import isn't backed by a HuggingFace repo we can fetch from (a
+// local directory import, an OCI artifact, ...).
+func vllmSpecProbeURL(details Details) string {
+	if details.HuggingFace == nil || details.HuggingFace.ModelID == "" {
+		return ""
+	}
+	return resolveHTTPProbe(downloader.HuggingFacePrefix + details.HuggingFace.ModelID + "/config.json")
+}
+
+// fetchProbeBody GETs a small remote JSON document under a short timeout.
+func fetchProbeBody(url string) ([]byte, error) {
+	ctx, cancel := context.WithTimeout(context.Background(), specConfigProbeTimeout)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+	resp, err := httpclient.NewWithTimeout(specConfigProbeTimeout).Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer func() { _ = resp.Body.Close() }()
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("unexpected status %d", resp.StatusCode)
+	}
+	return io.ReadAll(io.LimitReader(resp.Body, maxSpecConfigProbeBytes))
+}
--- a/core/gallery/importers/vllm_spec_internal_test.go
+++ b/core/gallery/importers/vllm_spec_internal_test.go
@@ -0,0 +1,118 @@
+package importers
+
+import (
+	"encoding/json"
+	"errors"
+
+	"github.com/mudler/LocalAI/core/config"
+	hfapi "github.com/mudler/LocalAI/pkg/huggingface-api"
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("vllm-cpp speculative auto-config (importer)", func() {
+	Context("applySpecFromConfigJSON", func() {
+		It("enables mtp when the checkpoint declares an MTP head", func() {
+			cfg := &config.ModelConfig{Name: "qwen3.5"}
+			applySpecFromConfigJSON(cfg, []byte(`{
+				"model_type": "qwen3_5_moe",
+				"mtp_num_hidden_layers": 1
+			}`), "huggingface://Qwen/Qwen3.5-A3B")
+			Expect(cfg.EngineArgs).To(HaveKeyWithValue("speculative_config",
+				map[string]any{"method": "mtp"}))
+		})
+
+		It("leaves a plain checkpoint untouched", func() {
+			cfg := &config.ModelConfig{Name: "llama"}
+			applySpecFromConfigJSON(cfg, []byte(`{"model_type": "llama"}`), "huggingface://meta/llama")
+			Expect(cfg.EngineArgs).To(BeEmpty())
+		})
+
+		It("refuses to configure a DFlash draft as a servable model", func() {
+			// The draft only proposes tokens; configuring it standalone would
+			// produce a model that cannot load.
+			cfg := &config.ModelConfig{Name: "dflash-draft"}
+			applySpecFromConfigJSON(cfg, []byte(`{
+				"model_type": "qwen3_dflash",
+				"dflash_config": {"mask_token_id": 151666, "target_layer_ids": [0, 1]}
+			}`), "huggingface://z-lab/Qwen3.6-27B-DFlash")
+			Expect(cfg.EngineArgs).To(BeEmpty())
+		})
+
+		It("survives a config.json it cannot parse", func() {
+			cfg := &config.ModelConfig{Name: "weird"}
+			Expect(func() {
+				applySpecFromConfigJSON(cfg, []byte(`<html>404</html>`), "huggingface://a/b")
+			}).ToNot(Panic())
+			Expect(cfg.EngineArgs).To(BeEmpty())
+		})
+	})
+
+	Context("Import over a repository with an MTP head", func() {
+		var restore func()
+
+		BeforeEach(func() {
+			original := specConfigFetcher
+			restore = func() { specConfigFetcher = original }
+		})
+		AfterEach(func() { restore() })
+
+		importWith := func(backend, configJSON string) string {
+			specConfigFetcher = func(string) ([]byte, error) {
+				return []byte(configJSON), nil
+			}
+			importer := &VLLMImporter{}
+			out, err := importer.Import(Details{
+				URI:         "huggingface://Qwen/Qwen3.5-A3B",
+				Preferences: json.RawMessage(`{"backend": "` + backend + `"}`),
+				HuggingFace: &hfapi.ModelDetails{ModelID: "Qwen/Qwen3.5-A3B"},
+			})
+			Expect(err).ToNot(HaveOccurred())
+			return out.ConfigFile
+		}
+
+		It("emits engine_args.speculative_config for vllm-cpp", func() {
+			yaml := importWith("vllm-cpp", `{"model_type":"qwen3_5_moe","mtp_num_hidden_layers":1}`)
+			Expect(yaml).To(ContainSubstring("engine_args:"))
+			Expect(yaml).To(ContainSubstring("speculative_config:"))
+			Expect(yaml).To(ContainSubstring("method: mtp"))
+		})
+
+		It("emits nothing speculative for the python vllm backend", func() {
+			// The python backend has its own speculative surface and its own
+			// version-dependent MTP support; this hook is vllm-cpp only.
+			yaml := importWith("vllm", `{"model_type":"qwen3_5_moe","mtp_num_hidden_layers":1}`)
+			Expect(yaml).NotTo(ContainSubstring("speculative_config"))
+		})
+
+		It("emits nothing speculative when the probe fails", func() {
+			specConfigFetcher = func(string) ([]byte, error) {
+				return nil, errors.New("network down")
+			}
+			importer := &VLLMImporter{}
+			out, err := importer.Import(Details{
+				URI:         "huggingface://Qwen/Qwen3.5-A3B",
+				Preferences: json.RawMessage(`{"backend": "vllm-cpp"}`),
+				HuggingFace: &hfapi.ModelDetails{ModelID: "Qwen/Qwen3.5-A3B"},
+			})
+			Expect(err).ToNot(HaveOccurred())
+			Expect(out.ConfigFile).NotTo(ContainSubstring("speculative_config"))
+		})
+	})
+
+	Context("vllmSpecProbeURL", func() {
+		It("resolves the repository's config.json to an HTTPS URL", func() {
+			url := vllmSpecProbeURL(Details{
+				URI:         "huggingface://Qwen/Qwen3.5-A3B",
+				HuggingFace: &hfapi.ModelDetails{ModelID: "Qwen/Qwen3.5-A3B"},
+			})
+			Expect(url).To(ContainSubstring("Qwen/Qwen3.5-A3B"))
+			Expect(url).To(HaveSuffix("config.json"))
+			Expect(url).To(HavePrefix("https://"))
+		})
+
+		It("skips the probe when there is no HuggingFace repo behind the import", func() {
+			Expect(vllmSpecProbeURL(Details{URI: "/models/local-dir"})).To(BeEmpty())
+		})
+	})
+})
--- a/docs/content/features/text-generation.md
+++ b/docs/content/features/text-generation.md
@@ -889,6 +889,166 @@ options:
 The full list of registered parsers lives in `sglang.srt.function_call`
 and `sglang.srt.parser.reasoning_parser`.

+### vllm.cpp
+
+[vllm.cpp](https://github.com/mudler/vllm.cpp) is the LocalAI team's C++ port of
+vLLM: the same continuous-batching scheduler, paged KV cache and prefix caching,
+with no Python at inference time. It consumes either a HuggingFace safetensors
+model directory or a `.gguf` file, and applies the model's chat template,
+tool-call parsing and reasoning split engine-side.
+
+#### Setup
+
+```yaml
+name: vllm-cpp
+backend: vllm-cpp
+parameters:
+  model: "Qwen/Qwen3-4B"
+context_size: 8192
+template:
+  use_tokenizer_template: true
+```
+
+#### Configuring the engine with `engine_args`
+
+The same `engine_args:` map the vLLM and SGLang backends accept is honoured
+here, with keys spelled exactly as vLLM's own CLI flags - so a `speculative_config`
+or `kv_transfer_config` block written for vLLM works verbatim. Unknown keys are
+ignored rather than fatal; the engine validates the documents it is handed and
+reports a precise error at load.
+
+```yaml
+name: qwen35-a3b
+backend: vllm-cpp
+parameters:
+  model: "Qwen/Qwen3.5-A3B"
+context_size: 16384
+template:
+  use_tokenizer_template: true
+engine_args:
+  # KV cache sizing: num_blocks * block_size tokens of cache.
+  block_size: 32
+  num_blocks: 1024
+  # Concurrency and the per-step chunked-prefill token budget.
+  max_num_seqs: 32
+  max_num_batched_tokens: 8192
+  # Automatic prefix caching. Omit to keep the model's own default
+  # (on for dense models, off for hybrid / attention-free ones).
+  enable_prefix_caching: true
+  # Scheduler admission order: fcfs (default), priority, or lpm
+  # (cache-aware longest-prefix-match; needs prefix caching to have any effect).
+  scheduling_policy: lpm
+```
+
+| Key | Meaning | Default |
+|-----|---------|---------|
+| `block_size` | KV-cache block size, in tokens per block | 32 |
+| `num_blocks` | KV-cache blocks to allocate | 256 |
+| `max_model_len` | Max sequence length; also settable as `context_size` / `max_model_len` | model config |
+| `max_num_seqs` | Max concurrent sequences the scheduler admits | 8 |
+| `max_num_batched_tokens` | Per-step chunked-prefill token budget | per-arch (2048 dense, 4096/8192 MoE) |
+| `enable_prefix_caching` | Automatic prefix caching; `enable_radix_attention` is an accepted alias | model default |
+| `scheduling_policy` | `fcfs`, `priority`, or `lpm` | `fcfs` |
+| `tool_parser` / `reasoning_parser` | Force a parser instead of chat-template auto-detection | auto |
+| `tokenizer_config` | Override the `tokenizer_config.json` the chat template is read from | `<model_dir>/tokenizer_config.json` |
+| `speculative_config` | Speculative decoding (see below) | disabled |
+| `kv_transfer_config` | External KV connector / LMCache (see below) | none |
+
+Raising `max_num_batched_tokens` lets more prefill land in a single step, at the
+cost of decode latency for requests queued behind it. The default deliberately
+does not scale with `max_num_seqs`, which is what keeps a large concurrent
+prefill from blowing up the per-step activation on the hybrid architectures.
+
+#### Speculative decoding
+
+`speculative_config:` takes the same JSON object as vLLM's
+`--speculative-config`. Three methods are supported:
+
+**MTP** (Multi-Token Prediction) uses a draft head shipped inside the target
+checkpoint's own `mtp.*` tensors, so there is no second model to download. It
+requires a **safetensors** checkpoint - the `mtp.*` tensors do not survive GGUF
+conversion, and an MTP config over a `.gguf` model is rejected at load.
+
+```yaml
+engine_args:
+  speculative_config:
+    method: mtp
+    # Optional; defaults to the checkpoint's own head depth, which is
+    # usually the right value. Must be a multiple of that depth.
+    num_speculative_tokens: 1
+```
+
+**DFlash** uses a separate block-diffusion drafter that proposes a whole block
+of tokens in one non-autoregressive forward pass. Unlike MTP, the draft is its
+own checkpoint, so `model:` is **required**:
+
+```yaml
+engine_args:
+  speculative_config:
+    method: dflash
+    model: z-lab/Qwen3.6-27B-DFlash
+    num_speculative_tokens: 4
+```
+
+**N-gram** needs no draft model at all - it proposes from the prompt's own
+suffix history. `num_speculative_tokens` is required:
+
+```yaml
+engine_args:
+  speculative_config:
+    method: ngram
+    num_speculative_tokens: 4
+    prompt_lookup_min: 5
+    prompt_lookup_max: 5
+```
+
+> **Auto-configuration on import.** When you import a safetensors repository
+> with `backend: vllm-cpp`, LocalAI reads the checkpoint's `config.json` and, if
+> it declares an MTP head (`mtp_num_hidden_layers`), writes
+> `speculative_config: {method: mtp}` into the generated `engine_args` for you.
+> An explicit `speculative_config` in your own config is never overwritten.
+> Importing a DFlash *draft* repository is refused with a warning: a drafter
+> cannot serve on its own, so import the target model and point
+> `speculative_config.model` at the draft.
+
+#### External KV cache with LMCache
+
+`kv_transfer_config:` takes vLLM's `--kv-transfer-config` JSON and selects an
+external KV-cache connector. The `lm://` LMCache client lets prefill KV be
+stored to and reloaded from a shared `lmcache.v1.server`, so a prefix computed
+by one replica does not have to be recomputed by the next:
+
+```yaml
+engine_args:
+  kv_transfer_config:
+    kv_connector: LMCacheConnector
+    kv_role: kv_both          # required whenever kv_connector is set
+    kv_connector_extra_config:
+      host: 127.0.0.1
+      port: 65432
+```
+
+`kv_role` is one of `kv_producer` (store only), `kv_consumer` (load only), or
+`kv_both`. An unregistered connector name, a missing role, or a malformed
+document fails the load with an explicit error rather than silently running
+without the cache.
+
+#### Legacy `options:` list
+
+Earlier versions configured this backend through the flat `options:` list, and
+those configs keep working. Every key in the table above is still read from
+there in `key:value` form, and `engine_args` wins on any key set in both:
+
+```yaml
+options:
+  - max_num_seqs:32
+  - enable_prefix_caching:true
+```
+
+New configs should prefer `engine_args:`, which is the only place the nested
+`speculative_config` / `kv_transfer_config` documents can be written naturally
+rather than as a single-line JSON string.
+
 ### Transformers

 [Transformers](https://huggingface.co/docs/transformers/index) is a State-of-the-art Machine Learning library for PyTorch, TensorFlow, and JAX.