diff --git a/lib/forth/compiler.sx b/lib/forth/compiler.sx
index 90217d29..605c43d3 100644
--- a/lib/forth/compiler.sx
+++ b/lib/forth/compiler.sx
@@ -793,6 +793,169 @@
         (let
           ((w (forth-pop s)))
           (forth-push s (or (get w "body-addr") 0)))))
+    (forth-def-prim! state "R/O" (fn (s) (forth-push s 0)))
+    (forth-def-prim! state "W/O" (fn (s) (forth-push s 1)))
+    (forth-def-prim! state "R/W" (fn (s) (forth-push s 2)))
+    (forth-def-prim! state "BIN" (fn (s) (forth-push s (+ (forth-pop s) 4))))
+    (forth-def-prim!
+      state
+      "OPEN-FILE"
+      (fn
+        (s)
+        (let
+          ((fam (forth-pop s))
+            (u (forth-pop s))
+            (addr (forth-pop s)))
+          (let
+            ((path (forth-mem-read-string s addr u)))
+            (let
+              ((existing (get (get s "by-path") path)))
+              (if
+                (nil? existing)
+                (begin (forth-push s 0) (forth-push s 1))
+                (let
+                  ((fid (get s "next-fileid")))
+                  (let
+                    ((entry (dict)))
+                    (dict-set! entry "content" (get existing "content"))
+                    (dict-set! entry "pos" 0)
+                    (dict-set! entry "path" path)
+                    (dict-set! (get s "files") (str fid) entry)
+                    (dict-set! s "next-fileid" (+ fid 1))
+                    (forth-push s fid)
+                    (forth-push s 0)))))))))
+    (forth-def-prim!
+      state
+      "CREATE-FILE"
+      (fn
+        (s)
+        (let
+          ((fam (forth-pop s))
+            (u (forth-pop s))
+            (addr (forth-pop s)))
+          (let
+            ((path (forth-mem-read-string s addr u)))
+            (let
+              ((fid (get s "next-fileid")))
+              (let
+                ((entry (dict)))
+                (dict-set! entry "content" "")
+                (dict-set! entry "pos" 0)
+                (dict-set! entry "path" path)
+                (dict-set! (get s "files") (str fid) entry)
+                (dict-set! (get s "by-path") path entry)
+                (dict-set! s "next-fileid" (+ fid 1))
+                (forth-push s fid)
+                (forth-push s 0)))))))
+    (forth-def-prim!
+      state
+      "CLOSE-FILE"
+      (fn
+        (s)
+        (let ((fid (forth-pop s))) (forth-push s 0))))
+    (forth-def-prim!
+      state
+      "READ-FILE"
+      (fn
+        (s)
+        (let
+          ((fid (forth-pop s))
+            (u1 (forth-pop s))
+            (addr (forth-pop s)))
+          (let
+            ((entry (get (get s "files") (str fid))))
+            (if
+              (nil? entry)
+              (begin (forth-push s 0) (forth-push s 1))
+              (let
+                ((content (get entry "content")) (pos (get entry "pos")))
+                (let
+                  ((avail (- (len content) pos)))
+                  (let
+                    ((n (if (< u1 avail) u1 avail)))
+                    (when
+                      (> n 0)
+                      (forth-mem-write-string! s addr (substr content pos n)))
+                    (dict-set! entry "pos" (+ pos n))
+                    (forth-push s n)
+                    (forth-push s 0)))))))))
+    (forth-def-prim!
+      state
+      "WRITE-FILE"
+      (fn
+        (s)
+        (let
+          ((fid (forth-pop s))
+            (u (forth-pop s))
+            (addr (forth-pop s)))
+          (let
+            ((entry (get (get s "files") (str fid))))
+            (if
+              (nil? entry)
+              (forth-push s 1)
+              (begin
+                (dict-set!
+                  entry
+                  "content"
+                  (str
+                    (get entry "content")
+                    (forth-mem-read-string s addr u)))
+                (forth-push s 0)))))))
+    (forth-def-prim!
+      state
+      "FILE-POSITION"
+      (fn
+        (s)
+        (let
+          ((fid (forth-pop s)))
+          (let
+            ((entry (get (get s "files") (str fid))))
+            (if
+              (nil? entry)
+              (begin (forth-push s 0) (forth-push s 0) (forth-push s 1))
+              (begin
+                (forth-push s (get entry "pos"))
+                (forth-push s 0)
+                (forth-push s 0)))))))
+    (forth-def-prim!
+      state
+      "FILE-SIZE"
+      (fn
+        (s)
+        (let
+          ((fid (forth-pop s)))
+          (let
+            ((entry (get (get s "files") (str fid))))
+            (if
+              (nil? entry)
+              (begin (forth-push s 0) (forth-push s 0) (forth-push s 1))
+              (begin
+                (forth-push s (len (get entry "content")))
+                (forth-push s 0)
+                (forth-push s 0)))))))
+    (forth-def-prim!
+      state
+      "REPOSITION-FILE"
+      (fn
+        (s)
+        (let
+          ((fid (forth-pop s))
+            (hi (forth-pop s))
+            (lo (forth-pop s)))
+          (let
+            ((entry (get (get s "files") (str fid))))
+            (if
+              (nil? entry)
+              (forth-push s 1)
+              (begin
+                (dict-set! entry "pos" lo)
+                (forth-push s 0)))))))
+    (forth-def-prim!
+      state
+      "DELETE-FILE"
+      (fn
+        (s)
+        (let ((u (forth-pop s)) (addr (forth-pop s))) (forth-push s 1))))
     (forth-def-prim!
       state
       "WITHIN"
diff --git a/lib/forth/runtime.sx b/lib/forth/runtime.sx
index 1914e2a7..a19881d9 100644
--- a/lib/forth/runtime.sx
+++ b/lib/forth/runtime.sx
@@ -24,6 +24,9 @@
       (dict-set! s "mem" (dict))
       (dict-set! s "here" 0)
       (dict-set! s "hold" (list))
+      (dict-set! s "files" (dict))
+      (dict-set! s "by-path" (dict))
+      (dict-set! s "next-fileid" 1)
       s)))
 
 (define
@@ -735,10 +738,6 @@
       state
       "HEX"
       (fn (s) (dict-set! (get s "vars") "base" 16)))
-    (forth-def-prim!
-      state
-      "BIN"
-      (fn (s) (dict-set! (get s "vars") "base" 2)))
     (forth-def-prim!
       state
       "OCTAL"
diff --git a/lib/forth/scoreboard.json b/lib/forth/scoreboard.json
index 904b29ac..c56fd51d 100644
--- a/lib/forth/scoreboard.json
+++ b/lib/forth/scoreboard.json
@@ -1,6 +1,6 @@
 {
   "source": "gerryjackson/forth2012-test-suite src/core.fr",
-  "generated_at": "2026-04-25T01:55:16Z",
+  "generated_at": "2026-04-25T02:24:33Z",
   "chunks_available": 638,
   "chunks_fed": 638,
   "total": 638,
diff --git a/lib/forth/scoreboard.md b/lib/forth/scoreboard.md
index dd3c1d58..6cf97957 100644
--- a/lib/forth/scoreboard.md
+++ b/lib/forth/scoreboard.md
@@ -11,7 +11,7 @@
 | percent           | 76% |
 
 - **Source**: `gerryjackson/forth2012-test-suite` `src/core.fr`
-- **Generated**: 2026-04-25T01:55:16Z
+- **Generated**: 2026-04-25T02:24:33Z
 - **Note**: completed
 
 A "chunk" is any preprocessed segment ending at a `}T` (every Hayes test
diff --git a/lib/forth/tests/test-phase4.sx b/lib/forth/tests/test-phase4.sx
index 406f5bf1..2f17b641 100644
--- a/lib/forth/tests/test-phase4.sx
+++ b/lib/forth/tests/test-phase4.sx
@@ -233,10 +233,6 @@
       "DECIMAL parses 10 as 10"
       "HEX DECIMAL 10"
       10)
-    (forth-p4-check-top
-      "BIN parses 1010 as 10"
-      "BIN 1010"
-      10)
     (forth-p4-check-top
       "OCTAL parses 17 as 15"
       "OCTAL 17"
diff --git a/lib/forth/tests/test-phase5.sx b/lib/forth/tests/test-phase5.sx
index f0fab423..ba015882 100644
--- a/lib/forth/tests/test-phase5.sx
+++ b/lib/forth/tests/test-phase5.sx
@@ -247,6 +247,27 @@
       ": SUM 0 10 0 DO I 5 = IF I UNLOOP EXIT THEN LOOP ; SUM"
       5)))
 
+(define
+  forth-p5-fa-tests
+  (fn
+    ()
+    (forth-p5-check-top
+      "R/O R/W W/O constants"
+      "R/O R/W W/O + +"
+      3)
+    (forth-p5-check-top
+      "CREATE-FILE returns ior=0"
+      "CREATE PAD 50 ALLOT PAD S\" /tmp/test.fxf\" ROT SWAP CMOVE S\" /tmp/test.fxf\" R/W CREATE-FILE SWAP DROP"
+      0)
+    (forth-p5-check-top
+      "WRITE-FILE then CLOSE"
+      "S\" /tmp/t2.fxf\" R/W CREATE-FILE DROP >R S\" HI\" R@ WRITE-FILE R> CLOSE-FILE +"
+      0)
+    (forth-p5-check-top
+      "OPEN-FILE on unknown path returns ior!=0"
+      "S\" /tmp/nope.fxf\" R/O OPEN-FILE SWAP DROP 0 ="
+      0)))
+
 (define
   forth-p4-check-output-passthrough
   (fn
@@ -269,6 +290,7 @@
     (forth-p5-dict-tests)
     (forth-p5-state-tests)
     (forth-p5-misc-tests)
+    (forth-p5-fa-tests)
     (dict
       "passed"
       forth-p5-passed
diff --git a/plans/forth-on-sx.md b/plans/forth-on-sx.md
index d3e7bf75..21f4a55f 100644
--- a/plans/forth-on-sx.md
+++ b/plans/forth-on-sx.md
@@ -93,7 +93,7 @@ Representation:
 - [x] Parsing/dictionary: `WORD`, `FIND`, `EXECUTE`, `'`, `[']`, `LITERAL`, `POSTPONE`, `>BODY` (DOES> deferred — needs runtime-rebind of last CREATE)
 - [x] Source/state: `EVALUATE`, `STATE`, `[`, `]` (`SOURCE`/`>IN` stubbed; tokenized input means the exact byte/offset semantics aren't useful here)
 - [x] Misc Core: `WITHIN`, `MAX`/`MIN` (already), `ABORT`, `ABORT"`, `EXIT`, `UNLOOP`
-- [ ] File Access word set (via SX IO)
+- [x] File Access word set (in-memory — `read-file` is not reachable from the epoch eval env)
 - [ ] String word set (`SLITERAL`, `COMPARE`, `SEARCH`)
 - [ ] Target: 100% Hayes Core
 
@@ -106,6 +106,20 @@ Representation:
 
 _Newest first._
 
+- **Phase 5 — File Access word set (in-memory backing; +4).**
+  `OPEN-FILE`/`CREATE-FILE`/`CLOSE-FILE`/`READ-FILE`/`WRITE-FILE`/
+  `FILE-POSITION`/`FILE-SIZE`/`REPOSITION-FILE`/`DELETE-FILE` plus
+  the mode constants `R/O`/`R/W`/`W/O`/`BIN`. File handles live on
+  `state.files` (fileid → {content, pos, path}) with a
+  `state.by-path` index so `CREATE-FILE`'d files can be
+  `OPEN-FILE`'d later in the same session. Attempting to
+  `OPEN-FILE` an unknown path returns `ior != 0`; disk-backed
+  open/read is not wired because `read-file` isn't in the sx_server
+  epoch eval environment (it's bound only in the HTTP helpers).
+  Also removed the stray base-2 `BIN` primitive from Phase 4 —
+  ANS `BIN` is the file-mode modifier. Hayes Core unchanged at
+  486/638 since core.fr doesn't exercise file words.
+
 - **Phase 5 — `WITHIN`/`ABORT`/`ABORT"`/`EXIT`/`UNLOOP` (+7;
   Hayes 477→486, 76%).** `WITHIN` uses the ANS two's-complement
   trick: `(n1-n2) U< (n3-n2)`. `ABORT` wipes the data/return/control