diff --git a/harmony/harmonyparser.go b/harmony/harmonyparser.go
index 3ec2c21f1..62bc8a47a 100644
--- a/harmony/harmonyparser.go
+++ b/harmony/harmonyparser.go
@@ -47,12 +47,13 @@ func (s harmonyParserState) String() string {
 }
 
 type HarmonyParser struct {
-	state           harmonyParserState
-	MessageStartTag string
-	MessageEndTag   string
-	HeaderEndTag    string
-	acc             strings.Builder
-	lifetimeAcc     strings.Builder
+	state              harmonyParserState
+	MessageStartTag    string
+	MessageEndTag      string
+	HeaderEndTag       string
+	constraintsAllowed bool
+	acc                strings.Builder
+	lifetimeAcc        strings.Builder
 }
 
 type HarmonyEvent interface {
@@ -89,6 +90,10 @@ func (s *HarmonyParser) AddImplicitStart() {
 	s.acc.WriteString("<|start|>assistant")
 }
 
+func (s *HarmonyParser) ConstraintsAllowed() bool {
+	return s.constraintsAllowed
+}
+
 func Prefill(lastMessage api.Message) string {
 	if lastMessage.Role != "assistant" {
 		return ""
@@ -341,6 +346,7 @@ func (h *HarmonyMessageHandler) AddContent(content string) (string, string, stri
 				}
 			case "final":
 				h.state = harmonyMessageState_Normal
+				h.HarmonyParser.constraintsAllowed = true
 			}
 		case HarmonyEventContentEmitted:
 			logutil.Trace("harmony event content", "content", event.Content, "state", h.state)
diff --git a/parser/token_parser.go b/parser/token_parser.go
index 812458299..a889a19fd 100644
--- a/parser/token_parser.go
+++ b/parser/token_parser.go
@@ -33,6 +33,7 @@ type MessageHandler interface {
 
 type ParserInternals interface {
 	AddImplicitStartOrPrefill(prefillString string)
+	ConstraintsAllowed() bool
 }
 
 type ToolParser interface {
@@ -51,6 +52,10 @@ type defaultEngine struct{}
 
 func (defaultEngine) AddImplicitStartOrPrefill(prefillString string) {}
 
+func (defaultEngine) ConstraintsAllowed() bool {
+	return true
+}
+
 type defaultToolParser struct{}
 
 func (defaultToolParser) Add(token string) {}
@@ -104,6 +109,10 @@ func (p *TokenParser) repeatLimitReached(token string) bool {
 	return p.tokenRepeat >= p.repeatLimit
 }
 
+func (p *TokenParser) ConstraintsAllowed() bool {
+	return p.parserEngine.ConstraintsAllowed()
+}
+
 // TODO: update to work with multiple toolcalls - unmarshalling should also happen on parser level
 func (p *TokenParser) Drain() []api.ToolCall {
 	toolName, toolContent := p.toolParser.Drain()
diff --git a/runner/ollamarunner/runner.go b/runner/ollamarunner/runner.go
index 201d55a16..e6b1a6df4 100644
--- a/runner/ollamarunner/runner.go
+++ b/runner/ollamarunner/runner.go
@@ -782,8 +782,6 @@ func (s *Server) completion(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	tokenParser := parser.NewTokenParser(req.ParserType, req.PrefillString)
-
 	if req.Options == nil {
 		opts := api.DefaultOptions()
 		req.Options = &opts
@@ -816,7 +814,7 @@ func (s *Server) completion(w http.ResponseWriter, r *http.Request) {
 		req.Options.TopP,
 		req.Options.MinP,
 		req.Options.Seed,
-		grammar,
+		nil,
 	)
 
 	seq, err := s.NewSequence(req.Prompt, req.Images, NewSequenceParams{
@@ -831,6 +829,14 @@ func (s *Server) completion(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	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)
+	}
+
 	// Ensure there is a place to put the sequence, released when removed from s.seqs
 	if err := s.seqsSem.Acquire(r.Context(), 1); err != nil {
 		if errors.Is(err, context.Canceled) {
@@ -867,6 +873,7 @@ func (s *Server) completion(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	grammarSet := false
 	for {
 		select {
 		case <-r.Context().Done():
@@ -883,6 +890,11 @@ func (s *Server) completion(w http.ResponseWriter, r *http.Request) {
 					return
 				}
 
+				if !grammarSet && grammar != nil && tokenParser.ConstraintsAllowed() {
+					seq.sampler.SetGrammar(grammar)
+					grammarSet = true
+				}
+
 				if err := json.NewEncoder(w).Encode(&llm.CompletionResponse{
 					Content:  content,
 					Thinking: thinking,
diff --git a/sample/samplers.go b/sample/samplers.go
index d395650d9..1c913be9f 100644
--- a/sample/samplers.go
+++ b/sample/samplers.go
@@ -25,6 +25,10 @@ type Sampler struct {
 	grammar     *GrammarSampler
 }
 
+func (s *Sampler) SetGrammar(grammar *GrammarSampler) {
+	s.grammar = grammar
+}
+
 func (s *Sampler) Sample(logits []float32) (int32, error) {
 	if len(logits) == 0 {
 		return -1, errors.New("sample: no logits provided to sample")