diff --git a/racket/src/io/port/bytes-input.rkt b/racket/src/io/port/bytes-input.rkt
index 3e76cb3519..b359646fca 100644
--- a/racket/src/io/port/bytes-input.rkt
+++ b/racket/src/io/port/bytes-input.rkt
@@ -23,7 +23,8 @@
          do-read-byte/core-port)
 
 (module+ internal
-  (provide do-read-bytes!))
+  (provide do-read-bytes!
+           peek-byte/core-port))
 
 ;; ----------------------------------------
 
@@ -132,11 +133,14 @@
   (check who input-port? orig-in)
   (check who exact-nonnegative-integer? skip-k)
   (let ([in (->core-input-port orig-in)])
-    (define peek-byte (and (zero? skip-k)
-                           (core-input-port-peek-byte in)))
-    (cond
-      [peek-byte (do-peek-byte who peek-byte in orig-in)]
-      [else (peek-byte-via-bytes in skip-k #:special-ok? #f)])))
+    (peek-byte/core-port in skip-k)))
+
+(define/who (peek-byte/core-port in skip-k)
+  (define peek-byte (and (zero? skip-k)
+                         (core-input-port-peek-byte in)))
+  (cond
+    [peek-byte (do-peek-byte who peek-byte in)]
+    [else (peek-byte-via-bytes in skip-k #:special-ok? #f)]))
 
 (define/who (peek-bytes amt skip-k [in (current-input-port)])
   (check who exact-nonnegative-integer? amt)
diff --git a/racket/src/io/port/read-and-peek.rkt b/racket/src/io/port/read-and-peek.rkt
index eb47c855bf..3d46922528 100644
--- a/racket/src/io/port/read-and-peek.rkt
+++ b/racket/src/io/port/read-and-peek.rkt
@@ -213,7 +213,7 @@
       v))
 
 ;; Use a `peek-byte` shortcut
-(define (do-peek-byte who peek-byte in orig-in)
+(define (do-peek-byte who peek-byte in)
   (let loop ()
     (start-atomic)
     (prepare-change in)
diff --git a/racket/src/io/port/special-input.rkt b/racket/src/io/port/special-input.rkt
index ceddc49bf7..87c6942647 100644
--- a/racket/src/io/port/special-input.rkt
+++ b/racket/src/io/port/special-input.rkt
@@ -41,7 +41,7 @@
   (let ([in (->core-input-port orig-in)])
     (define peek-byte (core-input-port-peek-byte in))
     (cond
-      [peek-byte (do-peek-byte who peek-byte in orig-in)]
+      [peek-byte (do-peek-byte who peek-byte in)]
       [else
        (extract-special-value (peek-byte-via-bytes in skip-k #:progress-evt progress-evt)
                               in source-name skip-k
diff --git a/racket/src/io/port/string-input.rkt b/racket/src/io/port/string-input.rkt
index 9a235bae9c..f7cb7fcf9b 100644
--- a/racket/src/io/port/string-input.rkt
+++ b/racket/src/io/port/string-input.rkt
@@ -197,24 +197,7 @@
 ;; A shortcut to implement `read-char` in terms of a port-specific
 ;; `read-byte`:
 (define (read-char-via-read-byte who in read-byte #:special-ok? [special-ok? #t])
-  (define b
-    (let loop ()
-      (start-atomic)
-      (prepare-change in)
-      (check-not-closed who in)
-      (define b (read-byte (core-port-self in)))
-      (cond
-        [(fixnum? b)
-         (port-count-byte! in b)
-         (end-atomic)
-         b]
-        [(eof-object? b)
-         (end-atomic)
-         eof]
-        [else
-         (end-atomic)
-         (sync b)
-         (loop)])))
+  (define b (do-read-byte who read-byte in))
   (cond
     [(eof-object? b) b]
     [else
@@ -222,48 +205,38 @@
        [(b . fx< . 128) (integer->char b)]
        [else
         ;; UTF-8 decoding... May need to peek bytes to discover
-        ;; whether the decoding will work (in which case this wasn't
-        ;; much of a shortcut)
-        (define bstr (bytes b))
-        (define str (make-string 1))
-        (define-values (used-bytes got-chars state)
-          (utf-8-decode! bstr 0 1
-                         #f 0 #f
-                         #:abort-mode 'state))
+        ;; whether the decoding will work
+        (define-values (accum remaining state)
+          (utf-8-decode-byte b 0 0))
         (cond
           [(eq? state 'error)
            ;; This happens if the byte is a UTF-8 continuation byte
            #\uFFFD]
           [else
-           ;; Need to peek ahead
-           (let loop ([skip-k 0] [state state])
-             (define v (peek-some-bytes! who in bstr 0 1 skip-k #:copy-bstr? #f #:special-ok? special-ok?))
+           ;; Need to peek ahead; don't consume any more bytes until
+           ;; complete, and consume only the already-consumed byte
+           ;; if there's a decoding error
+           (let loop ([skip-k 0] [accum accum] [remaining remaining])
+             (define b (peek-byte/core-port in skip-k))
              (cond
-               [(or (eof-object? v)
-                    (procedure? v))
-                ;; Already-consumed byte is an error byte
+               [(eof-object? b)
+                ;; Already-consumed byte is consume as an error byte
                 #\uFFFD]
                [else
-                (define-values (used-bytes got-chars new-state)
-                  (utf-8-decode! bstr 0 1
-                                 str 0 1
-                                 #:state state
-                                 #:error-char #\uFFFD
-                                 #:abort-mode 'state))
+                (define-values (next-accum next-remaining state)
+                  (utf-8-decode-byte b accum remaining))
                 (cond
-                  [(fx= got-chars 1)
-                   (define actually-used-bytes (fx+ skip-k used-bytes))
-                   (cond
-                     [(fx= actually-used-bytes 0) (void)]
-                     [(fx= actually-used-bytes 1) (do-read-byte who read-byte in)]
-                     [else
-                      (define finish-bstr (if (actually-used-bytes . fx<= . (bytes-length bstr))
-                                              bstr
-                                              (make-bytes actually-used-bytes)))
-                      (do-read-bytes! who in finish-bstr 0 actually-used-bytes)])
-                   (string-ref str 0)]
+                  [(eq? state 'complete)
+                   ;; Consume used bytes
+                   (let loop ([skip-k skip-k])
+                     (do-read-byte who read-byte in)
+                     (unless (fx= 0 skip-k)
+                       (loop (fx- skip-k 1))))
+                   (integer->char next-accum)]
+                  [(eq? state 'error)
+                   #\uFFFD]
                   [else
-                   (loop (fx+ skip-k 1) new-state)])]))])])]))
+                   (loop (fx+ 1 skip-k) next-accum next-remaining)])]))])])]))
 
 ;; ----------------------------------------
 
diff --git a/racket/src/io/string/utf-8-decode.rkt b/racket/src/io/string/utf-8-decode.rkt
index 26c1bdf9d2..5034e5f507 100644
--- a/racket/src/io/string/utf-8-decode.rkt
+++ b/racket/src/io/string/utf-8-decode.rkt
@@ -3,7 +3,9 @@
 
 (provide utf-8-decode!
          utf-8-max-aborts-amt
-         
+
+         utf-8-decode-byte
+
          utf-8-state?
          utf-8-state-pending-amt)
 
@@ -57,6 +59,21 @@
   ;; Iterate through the given byte string
   (let loop ([i in-start] [j out-start] [base-i base-i] [accum accum] [remaining remaining])
 
+    ;; Shared handling for success:
+    (define (complete accum)
+      (when out-str (string-set! out-str j (integer->char accum)))
+      (define next-j (fx+ j 1))
+      (define next-i (fx+ i 1))
+      (cond
+        [(and out-end (fx= next-j out-end))
+         (values (fx- next-i in-start)
+                 (fx- next-j out-start)
+                 (if (fx= next-i in-end)
+                     'complete
+                     'continues))]
+        [else
+         (loop next-i next-j next-i 0 0)]))
+
     ;; Shared handling for encoding failures:
     (define (encoding-failure)
       (cond
@@ -76,20 +93,6 @@
                 (fx- j out-start)
                 'error)]))
     
-    ;; Shared handling for decoding success:
-    (define (continue)
-      (define next-j (fx+ j 1))
-      (define next-i (fx+ i 1))
-      (cond
-       [(and out-end (fx= next-j out-end))
-        (values (fx- next-i in-start)
-                (fx- next-j out-start)
-                (if (fx= next-i in-end)
-                    'complete
-                    'continues))]
-       [else
-        (loop next-i next-j next-i 0 0)]))
-    
     ;; Dispatch on byte:
     (cond
      [(fx= i in-end)
@@ -116,74 +119,101 @@
       (encoding-failure)]
      [else
       (define b (bytes-ref in-bstr i))
-      (cond
-       [(b . fx< . 128)
-        (cond
-         [(fx= remaining 0)
-          ;; Found ASCII
-          (when out-str (string-set! out-str j (integer->char b)))
-          (continue)]
-         [else
-          ;; We were accumulating bytes for an encoding, and
-          ;; the encoding didn't complete
-          (encoding-failure)])]
+      (utf-8-decode-byte/inline b accum remaining
+                                complete
+                                (lambda (accum remaining)
+                                  (loop (fx+ i 1) j i accum remaining))
+                                (lambda (accum remaining)
+                                  (loop (fx+ i 1) j base-i accum remaining))
+                                encoding-failure)])))
+
+(define-syntax-rule (utf-8-decode-byte/inline b accum remaining
+                                              complete-k
+                                              init-continue-k
+                                              next-continue-k
+                                              error-k)
+  (cond
+    [(b . fx< . 128)
+     (cond
+       [(fx= remaining 0)
+        ;; Found ASCII
+        (complete-k b)]
        [else
-        ;; Encoding...
+        ;; We were accumulating bytes for an encoding, and
+        ;; the encoding didn't complete
+        (error-k)])]
+    [else
+     ;; Encoding...
+     (cond
+       [(fx= #b10000000 (fxand b #b11000000))
+        ;; A continuation byte
         (cond
-          [(fx= #b10000000 (fxand b #b11000000))
-           ;; A continuation byte
-          (cond
-           [(fx= remaining 0)
-            ;; We weren't continuing
-            (encoding-failure)]
-           [else
-            (define next (fxand b #b00111111))
-            (define next-accum (fxior (fxlshift accum 6) next))
-            (cond
-              [(fx= 1 remaining)
-               (cond
-                 [(and (next-accum . fx> . 127)
-                       (next-accum . fx<= . #x10FFFF)
-                       (not (and (next-accum . fx>= . #xD800)
-                                 (next-accum . fx<= . #xDFFF))))
-                  (when out-str (string-set! out-str j (integer->char next-accum)))
-                  (continue)]
-                 [else
-                  ;; Not a valid character
-                  (encoding-failure)])]
-              [(and (fx= 2 remaining)
-                    (next-accum . fx<= . #b11111))
-               ;; A shorter byte sequence would work, so this is an
-               ;; encoding mistae.
-               (encoding-failure)]
-              [(and (fx= 3 remaining)
-                    (next-accum . fx<= . #b1111))
-               ;; A shorter byte sequence would work
-               (encoding-failure)]
-              [else
-               ;; Continue an encoding.
-               (loop (fx+ i 1) j base-i next-accum (fx- remaining 1))])])]
-          [(not (fx= remaining 0))
-           ;; Trying to start a new encoding while one is in
-           ;; progress
-           (encoding-failure)]
-          [(fx= #b11000000 (fxand b #b11100000))
-           ;; Start a two-byte encoding
-           (define accum (fxand b #b11111))
-           ;; If `accum` is zero, that's an encoding mistake,
-           ;; because a shorted byte sequence would work.
+          [(fx= remaining 0)
+           ;; We weren't continuing
+           (error-k)]
+          [else
+           (define next (fxand b #b00111111))
+           (define next-accum (fxior (fxlshift accum 6) next))
            (cond
-             [(fx= accum 0) (encoding-failure)]
-             [else (loop (fx+ i 1) j i accum 1)])]
-         [(fx= #b11100000 (fxand b #b11110000))
-          ;; Start a three-byte encoding
-          (define accum (fxand b #b1111))
-          (loop (fx+ i 1) j i accum 2)]
-         [(fx= #b11110000 (fxand b #b11111000))
-          ;; Start a four-byte encoding
-          (define accum (fxand b #b111))
-          (loop (fx+ i 1) j i  accum 3)]
-         [else
-          ;; Five- or six-byte encodings don't produce valid
-          ;; characters
-          (encoding-failure)])])])))
+             [(fx= 1 remaining)
+              (cond
+                [(and (next-accum . fx> . 127)
+                      (next-accum . fx<= . #x10FFFF)
+                      (not (and (next-accum . fx>= . #xD800)
+                                (next-accum . fx<= . #xDFFF))))
+                 (complete-k next-accum)]
+                [else
+                 ;; Not a valid character
+                 (error-k)])]
+             [(and (fx= 2 remaining)
+                   (next-accum . fx<= . #b11111))
+              ;; A shorter byte sequence would work, so this is an
+              ;; encoding mistae.
+              (error-k)]
+             [(and (fx= 3 remaining)
+                   (next-accum . fx<= . #b1111))
+              ;; A shorter byte sequence would work
+              (error-k)]
+             [else
+              ;; Continue an encoding
+              (next-continue-k next-accum (fx- remaining 1))])])]
+       [(not (fx= remaining 0))
+        ;; Trying to start a new encoding while one is in
+        ;; progress
+        (error-k)]
+       [(fx= #b11000000 (fxand b #b11100000))
+        ;; Start a two-byte encoding
+        (define accum (fxand b #b11111))
+        ;; If `accum` is zero, that's an encoding mistake,
+        ;; because a shorter byte sequence would work.
+        (cond
+          [(fx= accum 0) (error-k)]
+          [else (init-continue-k accum 1)])]
+       [(fx= #b11100000 (fxand b #b11110000))
+        ;; Start a three-byte encoding
+        (define accum (fxand b #b1111))
+        (init-continue-k accum 2)]
+       [(fx= #b11110000 (fxand b #b11111000))
+        ;; Start a four-byte encoding
+        (define accum (fxand b #b111))
+        (init-continue-k accum 3)]
+       [else
+        ;; Five- or six-byte encodings don't produce valid
+        ;; characters
+        (error-k)])]))
+
+;; Takes a byte and a decoding state and returns
+;; one of
+;;   - (values code-point 0 'complete)
+;;   - (values #f 0 'error)
+;;   - (values new-accum new-remaining 'continues)
+(define (utf-8-decode-byte b accum remaining)
+  (utf-8-decode-byte/inline b accum remaining
+                            (lambda (accum)
+                              (values accum 0 'complete))
+                            (lambda (accum remaining)
+                              (values accum remaining 'continues))
+                            (lambda (accum remaining)
+                              (values accum remaining 'continues))
+                            (lambda ()
+                              (values #f 0 'error))))