diff --git a/man/remind.1.in b/man/remind.1.in
index 6c0285de..88d4afab 100644
--- a/man/remind.1.in
+++ b/man/remind.1.in
@@ -553,7 +553,10 @@ In Agenda Mode, output JSON instead of the normal text-mode output.
 \fB\-\-json\fR also disables sorting (the \fB-g\fR option) and
 disables queueing (the \fB-q\fR option).  See the section
 "AGENDA MODE JSON OUTPUT" for more details.  The \fB\-\-json\fR option
-is ignored in Calendar Mode.
+is ignored in Calendar Mode.  Note that in JSON mode, the output from
+any RUN command that would normally appear on standard output is redirected
+to standard error instead; this is so that RUN commands don't mess up
+the output and cause invalid JSON to be produced on standard output.
 .TP
 .B \-\-print-errs
 The \fB\-\-print-errs\fR option causes \fBRemind\fR to print all
diff --git a/src/files.c b/src/files.c
index f0e504af..eb63c5cc 100644
--- a/src/files.c
+++ b/src/files.c
@@ -179,17 +179,14 @@ got_a_fresh_line(void)
     WarnedAboutImplicit = 0;
 }
 
-static void set_cloexec(FILE *fp)
+void set_cloexec(int fd)
 {
     int flags;
-    int fd;
-    if (fp) {
-        fd = fileno(fp);
-        flags = fcntl(fd, F_GETFD);
-        if (flags >= 0) {
-            flags |= FD_CLOEXEC;
-            fcntl(fd, F_SETFD, flags);
-        }
+
+    flags = fcntl(fd, F_GETFD);
+    if (flags >= 0) {
+        flags |= FD_CLOEXEC;
+        fcntl(fd, F_SETFD, flags);
     }
 }
 
@@ -218,7 +215,7 @@ static void OpenPurgeFile(char const *fname, char const *mode)
         fprintf(ErrFp, tr("Cannot open `%s' for writing: %s"), DBufValue(&fname_buf), strerror(errno));
         fprintf(ErrFp, "\n");
     }
-    set_cloexec(PurgeFP);
+    set_cloexec(fileno(PurgeFP));
     DBufFree(&fname_buf);
 }
 
@@ -427,7 +424,9 @@ static int OpenFile(char const *fname)
         }
     } else {
         fp = fopen(fname, "r");
-        set_cloexec(fp);
+        if (fp) {
+            set_cloexec(fileno(fp));
+        }
         if (DebugFlag & DB_TRACE_FILES) {
             fprintf(ErrFp, tr("Reading `%s': Opening file on disk"), fname);
             fprintf(ErrFp, "\n");
@@ -449,7 +448,7 @@ static int OpenFile(char const *fname)
             if (strcmp(fname, "-")) {
                 fp = fopen(fname, "r");
                 if (!fp || !CheckSafety()) return E_CANT_OPEN;
-                set_cloexec(fp);
+                set_cloexec(fileno(fp));
                 if (PurgeMode) OpenPurgeFile(fname, "w");
             } else {
                 fp = stdin;
@@ -650,7 +649,7 @@ static int PopFile(void)
         if (strcmp(i->filename, "-")) {
             fp = fopen(i->filename, "r");
             if (!fp || !CheckSafety()) return E_CANT_OPEN;
-            set_cloexec(fp);
+            set_cloexec(fileno(fp));
             if (PurgeMode) OpenPurgeFile(i->filename, "a");
         } else {
             fp = stdin;
diff --git a/src/init.c b/src/init.c
index ad847b08..3b388565 100644
--- a/src/init.c
+++ b/src/init.c
@@ -803,6 +803,8 @@ void InitRemind(int argc, char const *argv[])
         SortByTime = SORT_NONE;
         SortByDate = SORT_NONE;
         SortByPrio = SORT_NONE;
+        /* Make sure we don't blat errors to stdout! */
+        ErrFp = stderr;
     }
 
     /* Figure out the offset from UTC */
diff --git a/src/main.c b/src/main.c
index d08fe9e1..5fe7b61f 100644
--- a/src/main.c
+++ b/src/main.c
@@ -2146,7 +2146,12 @@ System(char const *cmd, int is_queued)
         }
     }
     /* This is the child process or original if we never forked */
-    (void) system(cmd);
+    if (JSONMode) {
+
+        (void) system_to_stderr(cmd);
+    } else {
+        (void) system(cmd);
+    }
     if (do_exit) {
         /* In the child process, so exit! */
         exit(0);
diff --git a/src/protos.h b/src/protos.h
index d4fe5c60..6d2566eb 100644
--- a/src/protos.h
+++ b/src/protos.h
@@ -292,3 +292,5 @@ void SetCurrentFilename(char const *fname);
 char const *GetCurrentFilename(void);
 int get_scanfrom(Trigger const *t);
 void remove_trailing_newlines(DynamicBuffer *buf);
+void set_cloexec(int fd);
+int system_to_stderr(char const *cmd);
diff --git a/src/utils.c b/src/utils.c
index 64b37f04..86a88b93 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -17,6 +17,7 @@ static char const DontEscapeMe[] =
 #include "err.h"
 
 #include <string.h>
+#include <unistd.h>
 
 #ifdef HAVE_STRINGS_H
 #include <strings.h>
@@ -30,6 +31,43 @@ static char const DontEscapeMe[] =
 #include "globals.h"
 #include "protos.h"
 
+/***************************************************************/
+/*                                                             */
+/*  system_to_stderr                                           */
+/*                                                             */
+/*  Run system(...) but with stdout redirected to stderr       */
+/*                                                             */
+/***************************************************************/
+int system_to_stderr(char const *cmd)
+{
+    int stdout_dup = dup(STDOUT_FILENO);
+    int r;
+
+    if (stdout_dup < 0) {
+        perror("dup");
+        return -1;
+    }
+
+    /* Duplicate STDERR onto STDOUT */
+    if (dup2(STDERR_FILENO, STDOUT_FILENO) < 0) {
+        (void) close(stdout_dup);
+        return -1;
+    }
+
+    /* Set close-on-exec flag on stdout_dup */
+    set_cloexec(stdout_dup);
+
+    r = system(cmd);
+
+    /* Restore original stdout */
+    /* If this dup2 fails... there's not a whole lot we can do. */
+    (void) dup2(stdout_dup, STDOUT_FILENO);
+    if (STDOUT_FILENO != stdout_dup) {
+        (void) close(stdout_dup);
+    }
+    return r;
+}
+
 /***************************************************************/
 /*                                                             */
 /*  StrnCpy                                                    */
diff --git a/tests/json-redirect.rem b/tests/json-redirect.rem
new file mode 100644
index 00000000..40afc075
--- /dev/null
+++ b/tests/json-redirect.rem
@@ -0,0 +1,5 @@
+BANNER %
+SET $AddBlankLines 0
+REM MSG Hello
+REM RUN echo This is executed by the shell.
+REM MSG Goodbye
\ No newline at end of file
diff --git a/tests/test-rem b/tests/test-rem
index f5362e02..54dee474 100644
--- a/tests/test-rem
+++ b/tests/test-rem
@@ -771,6 +771,13 @@ echo "Testing TODOS in calendar mode with completed todos hidden" >> ../tests/te
 ../src/remind -s --hide-completed-todos ../tests/todos.rem 2025-08-13 >> ../tests/test.out 2>&1
 echo "Testing TODOS and JSON mode" >> ../tests/test.out 2>&1
 ../src/remind --json ../tests/todos.rem 2025-08-13 >> ../tests/test.out 2>&1
+
+echo "Testing proper redirection of RUN stdout in JSON mode... here's stdout" >> ../tests/test.out 2>&1
+../src/remind --json ../tests/json-redirect.rem 1 Jan 2025 >> ../tests/test.out 2>/dev/null
+
+echo "... and here is stderr" >> ../tests/test.out 2>&1
+../src/remind --json ../tests/json-redirect.rem 1 Jan 2025 > /dev/null 2>> ../tests/test.out
+
 cmp -s ../tests/test.out ../tests/test.cmp
 if [ "$?" = "0" ]; then
    echo "Remind:  Acceptance test PASSED"
diff --git a/tests/test.cmp b/tests/test.cmp
index 07b9e978..86af0a7a 100644
--- a/tests/test.cmp
+++ b/tests/test.cmp
@@ -39615,3 +39615,10 @@ Testing TODOS and JSON mode
 {"date":"2025-08-11","filename":"../tests/todos.rem","lineno":17,"d":11,"m":8,"y":2025,"is_todo":1,"trigbase":"2025-08-11","max_overdue":3,"priority":5000,"body":"Yup3 on 2025-08-11"},
 {"date":"2025-08-10","filename":"../tests/todos.rem","lineno":18,"d":10,"m":8,"y":2025,"is_todo":1,"trigbase":"2025-08-10","max_overdue":3,"priority":5000,"body":"Yup4 on 2025-08-10"}
 ]
+Testing proper redirection of RUN stdout in JSON mode... here's stdout
+[
+{"date":"2025-01-01","filename":"../tests/json-redirect.rem","lineno":3,"is_todo":0,"priority":5000,"body":"Hello"},
+{"date":"2025-01-01","filename":"../tests/json-redirect.rem","lineno":5,"is_todo":0,"priority":5000,"body":"Goodbye"}
+]
+... and here is stderr
+This is executed by the shell.