diff --git a/core/backend/options.go b/core/backend/options.go
index 72e622108325..de8b4c44e610 100644
--- a/core/backend/options.go
+++ b/core/backend/options.go
@@ -307,11 +307,19 @@ func gRPCPredictOpts(c config.ModelConfig, modelPath string) *pb.PredictOptions
 		}
 	}
 
+	// TopK may be nil after SetDefaults for backends that don't use llama.cpp's
+	// top_k=40 default (issue #6632, e.g. mlx). proto3 int32 can't be unset, so
+	// send 0 — the value mlx actually wants (top-k disabled).
+	var topK int32
+	if c.TopK != nil {
+		topK = int32(*c.TopK)
+	}
+
 	pbOpts := &pb.PredictOptions{
 		Temperature:         float32(*c.Temperature),
 		TopP:                float32(*c.TopP),
 		NDraft:              c.NDraft,
-		TopK:                int32(*c.TopK),
+		TopK:                topK,
 		MinP:                float32(*c.MinP),
 		Tokens:              int32(*c.Maxtokens),
 		Threads:             int32(*c.Threads),
diff --git a/core/config/backend_capabilities.go b/core/config/backend_capabilities.go
index 19da89462fdb..d714cf58d1be 100644
--- a/core/config/backend_capabilities.go
+++ b/core/config/backend_capabilities.go
@@ -517,6 +517,33 @@ func NormalizeBackendName(backend string) string {
 	return strings.ReplaceAll(backend, ".", "-")
 }
 
+// nonLlamaSamplerBackends lists backends whose native sampler defaults differ
+// from llama.cpp's, so LocalAI must NOT inject llama.cpp's top_k=40 default for
+// them (issue #6632). mlx_lm's intended default is top_k=0 (disabled) and mlx
+// does not remap 0->40, so shipping 40 silently changes sampling for clients
+// that omit top_k. Leaving TopK nil lets the wire value default to 0.
+//
+// This is intentionally a small allow-list of KNOWN non-llama backends: empty
+// and unknown backends fall through to the llama.cpp default to preserve the
+// GGUF auto-detect path's behavior.
+var nonLlamaSamplerBackends = map[string]struct{}{
+	"mlx":             {},
+	"mlx-vlm":         {},
+	"mlx-distributed": {},
+}
+
+// UsesLlamaSamplerDefaults reports whether a backend should receive llama.cpp's
+// sampler defaults (e.g. top_k=40). Empty/unknown backends return true so the
+// GGUF auto-detect path (which resolves to llama.cpp) keeps today's behavior;
+// only the known non-llama backends in nonLlamaSamplerBackends return false.
+func UsesLlamaSamplerDefaults(backend string) bool {
+	if backend == "" {
+		return true
+	}
+	_, isNonLlama := nonLlamaSamplerBackends[NormalizeBackendName(backend)]
+	return !isNonLlama
+}
+
 // GetBackendCapability returns the capability info for a backend, or nil if unknown.
 // Handles backend name normalization.
 func GetBackendCapability(backend string) *BackendCapability {
diff --git a/core/config/model_config.go b/core/config/model_config.go
index 56b112dedb7a..6b1de098e5a3 100644
--- a/core/config/model_config.go
+++ b/core/config/model_config.go
@@ -867,7 +867,12 @@ func (cfg *ModelConfig) SetDefaults(opts ...ConfigLoaderOption) {
 		cfg.Seed = &defaultSeed
 	}
 
-	if cfg.TopK == nil {
+	// top_k=40 is llama.cpp's sampling default and is wrong for backends whose
+	// native default differs (issue #6632). Only inject it for the llama.cpp
+	// family and the empty/auto backend; leave TopK nil for known non-llama
+	// backends (e.g. mlx, whose intended default is top_k=0) so the wire value
+	// is 0 rather than a silently-changed 40.
+	if cfg.TopK == nil && UsesLlamaSamplerDefaults(cfg.Backend) {
 		cfg.TopK = &defaultTopK
 	}
 
diff --git a/core/config/model_config_test.go b/core/config/model_config_test.go
index 7c5f39a293d4..9b85fb8b40f6 100644
--- a/core/config/model_config_test.go
+++ b/core/config/model_config_test.go
@@ -529,4 +529,72 @@ concurrency_groups:
 				"models that template in Go still rely on the Go-generated grammar")
 		})
 	})
+
+	// The default top_k=40 is llama.cpp's sampling default and is WRONG for
+	// backends whose native default differs. mlx_lm's intended default is
+	// top_k=0 (disabled) and mlx does not remap 0->40, so injecting 40 silently
+	// changes sampling for mlx clients that omit top_k (issue #6632). Gate the
+	// injection on backend family: keep 40 for the llama.cpp family and for the
+	// empty/auto backend (the GGUF auto-detect path resolves to llama.cpp), but
+	// leave TopK nil for the mlx family so the wire value is 0.
+	Context("TopK default is backend-gated (issue #6632)", func() {
+		It("injects top_k=40 for the llama.cpp backend", func() {
+			cfg := &ModelConfig{}
+			cfg.Backend = "llama-cpp"
+
+			cfg.SetDefaults()
+
+			Expect(cfg.TopK).NotTo(BeNil(), "llama.cpp must keep its top_k=40 default")
+			Expect(*cfg.TopK).To(Equal(40))
+		})
+
+		It("injects top_k=40 for the empty/auto backend (GGUF auto-detect)", func() {
+			cfg := &ModelConfig{}
+
+			cfg.SetDefaults()
+
+			Expect(cfg.TopK).NotTo(BeNil(), "empty backend resolves to llama.cpp; default unchanged")
+			Expect(*cfg.TopK).To(Equal(40))
+		})
+
+		It("leaves TopK nil for the mlx backend", func() {
+			cfg := &ModelConfig{}
+			cfg.Backend = "mlx"
+
+			cfg.SetDefaults()
+
+			Expect(cfg.TopK).To(BeNil(),
+				"mlx_lm's intended default is top_k=0 (disabled); LocalAI must not inject 40")
+		})
+
+		It("leaves TopK nil for the mlx-vlm backend", func() {
+			cfg := &ModelConfig{}
+			cfg.Backend = "mlx-vlm"
+
+			cfg.SetDefaults()
+
+			Expect(cfg.TopK).To(BeNil())
+		})
+
+		It("leaves TopK nil for the mlx-distributed backend", func() {
+			cfg := &ModelConfig{}
+			cfg.Backend = "mlx-distributed"
+
+			cfg.SetDefaults()
+
+			Expect(cfg.TopK).To(BeNil())
+		})
+
+		It("respects an explicit top_k even for the mlx backend", func() {
+			explicit := 7
+			cfg := &ModelConfig{}
+			cfg.Backend = "mlx"
+			cfg.TopK = &explicit
+
+			cfg.SetDefaults()
+
+			Expect(cfg.TopK).NotTo(BeNil())
+			Expect(*cfg.TopK).To(Equal(7))
+		})
+	})
 })