diff --git a/runner/ollamarunner/runner.go b/runner/ollamarunner/runner.go
index e6b1a6df4..56d553eee 100644
--- a/runner/ollamarunner/runner.go
+++ b/runner/ollamarunner/runner.go
@@ -62,6 +62,11 @@ type Sequence struct {
 	// tokens that have been generated but not returned yet (e.g. for stop sequences)
 	pendingResponses []string
 
+	// startGate
+	startGate *sync.Mutex
+
+	grammarReady bool
+
 	// input cache being used by this sequence
 	cache *InputCacheSlot
 
@@ -164,6 +169,7 @@ func (s *Server) NewSequence(prompt string, images []llm.ImageData, params NewSe
 
 	// TODO(jessegross): Ingest cached history for grammar
 
+	startGate := &sync.Mutex{}
 	return &Sequence{
 		ctxs:                ctxs,
 		mmStore:             mmStore,
@@ -179,6 +185,8 @@ func (s *Server) NewSequence(prompt string, images []llm.ImageData, params NewSe
 		embeddingOnly:       params.embedding,
 		stop:                params.stop,
 		numKeep:             params.numKeep,
+		startGate:           startGate,
+		grammarReady:        false,
 	}, nil
 }
 
@@ -707,11 +715,18 @@ func (s *Server) computeBatch(activeBatch batchState) {
 		// sample a token
 		vocabSize := len(outputs) / len(activeBatch.batch.Outputs)
 		logutil.Trace("computeBatch: vocab details", "batchID", activeBatch.id, "seqIdx", i, "len(logits)", len(outputs), "len(activeBatch.batch.Outputs)", len(activeBatch.batch.Outputs), "vocabSize", vocabSize, "iBatches", iBatches)
+
+		if !seq.grammarReady {
+			seq.startGate.Lock()
+		}
 		token, err := seq.sampler.Sample(outputs[iBatches[i]*vocabSize : (iBatches[i]+1)*vocabSize])
 		if err != nil {
 			s.hardErrCh <- fmt.Errorf("failed to sample token: %w", err)
 			return
 		}
+		if !seq.grammarReady {
+			seq.startGate.Unlock()
+		}
 
 		nextBatchTokens[i].Token = token
 
@@ -830,11 +845,9 @@ func (s *Server) completion(w http.ResponseWriter, r *http.Request) {
 	}
 
 	tokenParser := parser.NewTokenParser(req.ParserType, req.PrefillString)
-	switch req.ParserType {
-	case parser.TokenParserTypeHarmony:
-		// Do not set grammar until model allows constraining
-	default:
-		seq.sampler.SetGrammar(grammar)
+	// this accounts for the default case and also the case where there is a prefill which moves the state of the parser to allow for constraints
+	if tokenParser.ConstraintsAllowed() {
+		seq.grammarReady = true
 	}
 
 	// Ensure there is a place to put the sequence, released when removed from s.seqs
@@ -873,7 +886,6 @@ func (s *Server) completion(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	grammarSet := false
 	for {
 		select {
 		case <-r.Context().Done():
@@ -881,6 +893,9 @@ func (s *Server) completion(w http.ResponseWriter, r *http.Request) {
 			return
 		case content, ok := <-seq.responses:
 			if ok {
+				if !seq.grammarReady {
+					seq.startGate.Lock()
+				}
 				var thinking string
 				var err error
 				content, thinking, err = tokenParser.AddContent(content)
@@ -890,9 +905,11 @@ func (s *Server) completion(w http.ResponseWriter, r *http.Request) {
 					return
 				}
 
-				if !grammarSet && grammar != nil && tokenParser.ConstraintsAllowed() {
-					seq.sampler.SetGrammar(grammar)
-					grammarSet = true
+				// only apply the grammar once
+				if tokenParser.ConstraintsAllowed() && !seq.grammarReady {
+					seq.sampler.SetGrammar(grammar, &s.mu)
+					seq.grammarReady = true
+					seq.startGate.Unlock()
 				}
 
 				if err := json.NewEncoder(w).Encode(&llm.CompletionResponse{
@@ -921,6 +938,9 @@ func (s *Server) completion(w http.ResponseWriter, r *http.Request) {
 
 				return
 			}
+			if !seq.grammarReady {
+				seq.startGate.Unlock()
+			}
 		}
 	}
 }
diff --git a/sample/samplers.go b/sample/samplers.go
index 1c913be9f..af58730dc 100644
--- a/sample/samplers.go
+++ b/sample/samplers.go
@@ -5,6 +5,7 @@ import (
 	"math"
 	"math/rand/v2"
 	"slices"
+	"sync"
 
 	"github.com/ollama/ollama/llama"
 	"github.com/ollama/ollama/model"
@@ -25,7 +26,9 @@ type Sampler struct {
 	grammar     *GrammarSampler
 }
 
-func (s *Sampler) SetGrammar(grammar *GrammarSampler) {
+func (s *Sampler) SetGrammar(grammar *GrammarSampler, mutex *sync.Mutex) {
+	mutex.Lock()
+	defer mutex.Unlock()
 	s.grammar = grammar
 }