diff --git a/src/api/api_seq.cpp b/src/api/api_seq.cpp
index 6d3524544d1..311e238cbee 100644
--- a/src/api/api_seq.cpp
+++ b/src/api/api_seq.cpp
@@ -163,12 +163,32 @@ extern "C" {
             SET_ERROR_CODE(Z3_INVALID_ARG, "expression is not a string literal");
             return "";
         }
-        mk_c(c)->m_char_buffer.reset();
+        auto& buffer = mk_c(c)->m_char_buffer;
+        buffer.reset();
+        svector<char> buff;
         for (unsigned i = 0; i < str.length(); ++i) {
-            mk_c(c)->m_char_buffer.push_back((char)str[i]);
+            unsigned ch = str[i];
+            if (ch >= 256) {
+                buff.reset();
+                buffer.push_back('\\');
+                buffer.push_back('\\');  // possibly replace by native non-escaped version?
+                buffer.push_back('u');
+                buffer.push_back('{');
+                while (ch > 0) {
+                    buff.push_back('0' + (ch & 0xF));
+                    ch /= 16;
+                }
+                for (unsigned j = buff.size(); j-- > 0; ) {
+                    buffer.push_back(buff[j]);
+                }
+                buffer.push_back('}');
+            }
+            else {
+                buffer.push_back((char)ch);
+            }
         }
-        *length = str.length();
-        return mk_c(c)->m_char_buffer.c_ptr();
+        *length = buffer.size();
+        return buffer.c_ptr();
         Z3_CATCH_RETURN("");
     }
 
diff --git a/src/ast/rewriter/seq_rewriter.cpp b/src/ast/rewriter/seq_rewriter.cpp
index 4eed120b5a8..0cb6f7394ee 100644
--- a/src/ast/rewriter/seq_rewriter.cpp
+++ b/src/ast/rewriter/seq_rewriter.cpp
@@ -1739,7 +1739,7 @@ br_status seq_rewriter::mk_str_lt(expr* a, expr* b, expr_ref& result) {
 br_status seq_rewriter::mk_str_from_code(expr* a, expr_ref& result) {
     rational r;
     if (m_autil.is_numeral(a, r)) {
-        if (r.is_neg() || r > m_util.str.max_char_value()) {
+        if (r.is_neg() || r > zstring::max_char()) {
             result = m_util.str.mk_string(symbol(""));
         }
         else {
diff --git a/src/ast/seq_decl_plugin.cpp b/src/ast/seq_decl_plugin.cpp
index 4ffe77d1abe..3360e300e41 100644
--- a/src/ast/seq_decl_plugin.cpp
+++ b/src/ast/seq_decl_plugin.cpp
@@ -158,28 +158,18 @@ static bool is_escape_char(char const *& s, unsigned& result) {
     return false;
 }
 
-zstring::zstring() {}
-
 zstring::zstring(char const* s) {
-    unsigned mask = 0xFF; // TBD for Unicode
     while (*s) {
-        unsigned ch;
+        unsigned ch = 0;
         if (is_escape_char(s, ch)) {
-            m_buffer.push_back(ch & mask);
+            m_buffer.push_back(ch);
         }
         else {
-            m_buffer.push_back(*s & mask);
+            m_buffer.push_back(*s);
             ++s;
         }
     }
-}
-
-zstring::zstring(zstring const& other) {
-    m_buffer = other.m_buffer;
-}
-
-zstring::zstring(unsigned sz, unsigned const* s) {
-    m_buffer.append(sz, s);
+    SASSERT(well_formed());
 }
 
 zstring::zstring(unsigned num_bits, bool const* ch) {
@@ -189,10 +179,18 @@ zstring::zstring(unsigned num_bits, bool const* ch) {
         n |= (((unsigned)ch[i]) << i);
     }
     m_buffer.push_back(n);
+    SASSERT(well_formed());
+}
+
+bool zstring::well_formed() const {
+    for (unsigned ch : m_buffer) {
+        if (ch > max_char())
+            return false;
+    }
+    return true;
 }
 
 zstring::zstring(unsigned ch) {
-    SASSERT(ch <= 196607);
     m_buffer.push_back(ch);
 }
 
@@ -267,20 +265,6 @@ std::string zstring::encode() const {
     return strm.str();
 }
 
-std::string zstring::as_string() const {
-    std::ostringstream strm;
-    char buffer[100];
-    unsigned offset = 0;
-    for (unsigned i = 0; i < m_buffer.size(); ++i) {
-        if (offset == 99) { _flush(); }
-        unsigned char ch = m_buffer[i];
-        buffer[offset++] = (char)(ch);        
-    }
-    _flush();
-    return strm.str();
-}
-
-
 bool zstring::suffixof(zstring const& other) const {
     if (length() > other.length()) return false;
     bool suffix = true;
@@ -1079,13 +1063,12 @@ app* seq_util::str::mk_string(zstring const& s) const {
     return u.seq.mk_string(s); 
 }
 
-app*  seq_util::str::mk_char(zstring const& s, unsigned idx) const {
-    // TBD: change to unicode
+app* seq_util::str::mk_char(zstring const& s, unsigned idx) const {
+    // TBD for unicode
     return u.bv().mk_numeral(s[idx], 8);
 }
 
-app*  seq_util::str::mk_char(char ch) const {
-    // TBD: change to unicode
+app* seq_util::str::mk_char(char ch) const {
     zstring s(ch);
     return mk_char(s, 0);
 }
diff --git a/src/ast/seq_decl_plugin.h b/src/ast/seq_decl_plugin.h
index 81e9009dcea..578e76a4d4b 100644
--- a/src/ast/seq_decl_plugin.h
+++ b/src/ast/seq_decl_plugin.h
@@ -17,6 +17,8 @@ Revision History:
 
     Updated to string sequences 2015-12-5
 
+    Add SMTLIB 2.6 support 2020-5-17
+
 --*/
 #ifndef SEQ_DECL_PLUGIN_H_
 #define SEQ_DECL_PLUGIN_H_
@@ -103,17 +105,18 @@ enum seq_op_kind {
 class zstring {
 private:
     buffer<unsigned> m_buffer;
+    bool well_formed() const;
 public:
-    zstring();
+    static unsigned max_char() { return 196607; }
+    zstring() {}
     zstring(char const* s);
-    zstring(unsigned sz, unsigned const* s);
-    zstring(zstring const& other);
+    zstring(unsigned sz, unsigned const* s) { m_buffer.append(sz, s); SASSERT(well_formed()); }
+    zstring(zstring const& other): m_buffer(other.m_buffer) {}
     zstring(unsigned num_bits, bool const* ch);
     zstring(unsigned ch);
     zstring& operator=(zstring const& other);
     zstring replace(zstring const& src, zstring const& dst) const;
     std::string encode() const;
-    std::string as_string() const;
     unsigned length() const { return m_buffer.size(); }
     unsigned operator[](unsigned i) const { return m_buffer[i]; }
     bool empty() const { return m_buffer.empty(); }
@@ -257,9 +260,6 @@ class seq_util {
     public:
         str(seq_util& u): u(u), m(u.m), m_fid(u.m_fid) {}
 
-        unsigned min_char_value() const { return 0; }
-        unsigned max_char_value() const { return 196607; }
-
         sort* mk_seq(sort* s) const { parameter param(s); return m.mk_sort(m_fid, SEQ_SORT, 1, &param); }
         sort* mk_string_sort() const { return m.mk_sort(m_fid, _STRING_SORT, 0, nullptr); }
         app* mk_empty(sort* s) const { return m.mk_const(m.mk_func_decl(m_fid, OP_SEQ_EMPTY, 0, nullptr, 0, (expr*const*)nullptr, s)); }
diff --git a/src/smt/seq_axioms.cpp b/src/smt/seq_axioms.cpp
index 620653c8c25..dccf8c81fe2 100644
--- a/src/smt/seq_axioms.cpp
+++ b/src/smt/seq_axioms.cpp
@@ -802,7 +802,7 @@ void seq_axioms::add_str_to_code_axiom(expr* n) {
     VERIFY(seq.str.is_to_code(n, e)); 
     literal len_is1 = mk_eq(mk_len(e), a.mk_int(1));
     add_axiom(~len_is1, mk_ge(n, 0)); 
-    add_axiom(~len_is1, mk_le(n, seq.str.max_char_value())); 
+    add_axiom(~len_is1, mk_le(n, zstring::max_char()));
     add_axiom(len_is1, mk_eq(n, a.mk_int(-1)));
 }
 
@@ -815,7 +815,7 @@ void seq_axioms::add_str_from_code_axiom(expr* n) {
     expr* e = nullptr;
     VERIFY(seq.str.is_from_code(n, e)); 
     literal ge = mk_ge(e, 0);
-    literal le = mk_le(e, seq.str.max_char_value());
+    literal le = mk_le(e, zstring::max_char());
     literal emp = mk_literal(seq.str.mk_is_empty(n));
     add_axiom(~ge, ~le, mk_eq(mk_len(n), a.mk_int(1)));
     add_axiom(~ge, ~le, mk_eq(seq.str.mk_to_code(n), e));