diff --git a/cube.c b/cube.c
index 2ce7a4f..afd2ebd 100644
--- a/cube.c
+++ b/cube.c
@@ -6,6 +6,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <assert.h>
+#include <ctype.h>
 #include <math.h>
 
 #include "puzzles.h"
@@ -276,6 +277,37 @@ game_params *dup_params(game_params *params)
     return ret;
 }
 
+game_params *decode_params(char const *string)
+{
+    game_params *ret = default_params();
+
+    switch (*string) {
+      case 't': ret->solid = TETRAHEDRON; string++; break;
+      case 'c': ret->solid = CUBE;        string++; break;
+      case 'o': ret->solid = OCTAHEDRON;  string++; break;
+      case 'i': ret->solid = ICOSAHEDRON; string++; break;
+      default: break;
+    }
+    ret->d1 = ret->d2 = atoi(string);
+    while (*string && isdigit(*string)) string++;
+    if (*string == 'x') {
+        string++;
+        ret->d2 = atoi(string);
+    }
+
+    return ret;
+}
+
+char *encode_params(game_params *params)
+{
+    char data[256];
+
+    assert(params->solid >= 0 && params->solid < 4);
+    sprintf(data, "%c%dx%d", "tcoi"[params->solid], params->d1, params->d2);
+
+    return dupstr(data);
+}
+
 static void enum_grid_squares(game_params *params,
                               void (*callback)(void *, struct grid_square *),
                               void *ctx)
@@ -653,7 +685,7 @@ char *new_game_seed(game_params *params, random_state *rs)
     /*
      * Choose a non-blue square for the polyhedron.
      */
-    sprintf(p, ":%d", data.gridptrs[0][random_upto(rs, m)]);
+    sprintf(p, ",%d", data.gridptrs[0][random_upto(rs, m)]);
 
     sfree(data.gridptrs[0]);
     sfree(flags);
@@ -824,13 +856,13 @@ char *validate_seed(game_params *params, char *seed)
 	/* NB if seed[j]=='\0' that will also be caught here, so we're safe */
     }
 
-    if (seed[i] != ':')
-	return "Expected ':' after hex digits";
+    if (seed[i] != ',')
+	return "Expected ',' after hex digits";
 
     i++;
     do {
 	if (seed[i] < '0' || seed[i] > '9')
-	    return "Expected decimal integer after ':'";
+	    return "Expected decimal integer after ','";
 	i++;
     } while (seed[i]);
 
@@ -883,7 +915,7 @@ game_state *new_game(game_params *params, char *seed)
 		j = 8;
 	}
 
-	if (*p == ':')
+	if (*p == ',')
 	    p++;
 
 	state->current = atoi(p);
diff --git a/fifteen.c b/fifteen.c
index 78060f5..2788604 100644
--- a/fifteen.c
+++ b/fifteen.c
@@ -6,6 +6,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <assert.h>
+#include <ctype.h>
 #include <math.h>
 
 #include "puzzles.h"
@@ -72,6 +73,29 @@ game_params *dup_params(game_params *params)
     return ret;
 }
 
+game_params *decode_params(char const *string)
+{
+    game_params *ret = default_params();
+
+    ret->w = ret->h = atoi(string);
+    while (*string && isdigit(*string)) string++;
+    if (*string == 'x') {
+        string++;
+        ret->h = atoi(string);
+    }
+
+    return ret;
+}
+
+char *encode_params(game_params *params)
+{
+    char data[256];
+
+    sprintf(data, "%dx%d", params->w, params->h);
+
+    return dupstr(data);
+}
+
 config_item *game_configure(game_params *params)
 {
     config_item *ret;
diff --git a/midend.c b/midend.c
index 63ba193..6a93856 100644
--- a/midend.c
+++ b/midend.c
@@ -333,7 +333,7 @@ int midend_wants_statusbar(midend_data *me)
 
 config_item *midend_get_config(midend_data *me, int which, char **wintitle)
 {
-    char *titlebuf;
+    char *titlebuf, *parstr;
     config_item *ret;
 
     titlebuf = snewn(40 + strlen(game_name), char);
@@ -352,7 +352,15 @@ config_item *midend_get_config(midend_data *me, int which, char **wintitle)
 	ret[0].type = C_STRING;
 	ret[0].name = "Game ID";
 	ret[0].ival = 0;
-	ret[0].sval = dupstr(me->seed);
+        /*
+         * The text going in here will be a string encoding of the
+         * parameters, plus a colon, plus the game seed. This is a
+         * full game ID.
+         */
+        parstr = encode_params(me->params);
+        ret[0].sval = snewn(strlen(parstr) + strlen(me->seed) + 2, char);
+        sprintf(ret[0].sval, "%s:%s", parstr, me->seed);
+        sfree(parstr);
 
 	ret[1].type = C_END;
 	ret[1].name = ret[1].sval = NULL;
@@ -367,7 +375,7 @@ config_item *midend_get_config(midend_data *me, int which, char **wintitle)
 
 char *midend_set_config(midend_data *me, int which, config_item *cfg)
 {
-    char *error;
+    char *error, *p;
     game_params *params;
 
     switch (which) {
@@ -385,12 +393,34 @@ char *midend_set_config(midend_data *me, int which, config_item *cfg)
 	break;
 
       case CFG_SEED:
-	error = validate_seed(me->params, cfg[0].sval);
+
+        /*
+         * The game ID will often (in fact, mostly) have a prefix
+         * containing a string-encoded parameter specification
+         * followed by a colon. So first find the colon, and then
+         * split the string up.
+         */
+        p = strchr(cfg[0].sval, ':');
+
+        if (p) {
+            *p++ = '\0';               /* p now points to game seed */
+            params = decode_params(cfg[0].sval);
+            error = validate_params(params);
+            if (error) {
+                free_params(params);
+                return error;
+            }
+            free_params(me->params);
+            me->params = params;
+        } else
+            p = cfg[0].sval;
+
+	error = validate_seed(me->params, p);
 	if (error)
 	    return error;
 
 	sfree(me->seed);
-	me->seed = dupstr(cfg[0].sval);
+	me->seed = dupstr(p);
 	me->fresh_seed = TRUE;
 
 	break;
diff --git a/net.c b/net.c
index d616d03..3c3f971 100644
--- a/net.c
+++ b/net.c
@@ -6,6 +6,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <assert.h>
+#include <ctype.h>
 #include <math.h>
 
 #include "puzzles.h"
@@ -183,6 +184,44 @@ game_params *dup_params(game_params *params)
     return ret;
 }
 
+game_params *decode_params(char const *string)
+{
+    game_params *ret = default_params();
+    char const *p = string;
+
+    ret->width = atoi(p);
+    while (*p && isdigit(*p)) p++;
+    if (*p == 'x') {
+        p++;
+        ret->height = atoi(p);
+        while (*p && isdigit(*p)) p++;
+        if ( (ret->wrapping = (*p == 'w')) != 0 )
+            p++;
+        if (*p == 'b')
+            ret->barrier_probability = atof(p+1);
+    } else {
+        ret->height = ret->width;
+    }
+
+    return ret;
+}
+
+char *encode_params(game_params *params)
+{
+    char ret[400];
+    int len;
+
+    len = sprintf(ret, "%dx%d", params->width, params->height);
+    if (params->wrapping)
+        ret[len++] = 'w';
+    if (params->barrier_probability)
+        len += sprintf(ret+len, "b%g", params->barrier_probability);
+    assert(len < lenof(ret));
+    ret[len] = '\0';
+
+    return dupstr(ret);
+}
+
 config_item *game_configure(game_params *params)
 {
     config_item *ret;
diff --git a/nullgame.c b/nullgame.c
index 32e6e2c..67982b9 100644
--- a/nullgame.c
+++ b/nullgame.c
@@ -15,6 +15,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <assert.h>
+#include <ctype.h>
 #include <math.h>
 
 #include "puzzles.h"
@@ -61,6 +62,20 @@ game_params *dup_params(game_params *params)
     return ret;
 }
 
+game_params *decode_params(char const *string)
+{
+    game_params *ret = snew(game_params);
+
+    ret->FIXME = 0;
+
+    return ret;
+}
+
+char *encode_params(game_params *params)
+{
+    return dupstr("FIXME");
+}
+
 config_item *game_configure(game_params *params)
 {
     return NULL;
diff --git a/puzzles.h b/puzzles.h
index 46c0f15..37c190a 100644
--- a/puzzles.h
+++ b/puzzles.h
@@ -162,6 +162,8 @@ extern const char *const game_name;
 const int game_can_configure;
 game_params *default_params(void);
 int game_fetch_preset(int i, char **name, game_params **params);
+game_params *decode_params(char const *string);
+char *encode_params(game_params *);
 void free_params(game_params *params);
 game_params *dup_params(game_params *params);
 config_item *game_configure(game_params *params);
diff --git a/rect.c b/rect.c
index dcd8ef7..9b7816c 100644
--- a/rect.c
+++ b/rect.c
@@ -34,6 +34,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <assert.h>
+#include <ctype.h>
 #include <math.h>
 
 #include "puzzles.h"
@@ -129,6 +130,29 @@ game_params *dup_params(game_params *params)
     return ret;
 }
 
+game_params *decode_params(char const *string)
+{
+    game_params *ret = default_params();
+
+    ret->w = ret->h = atoi(string);
+    while (*string && isdigit(*string)) string++;
+    if (*string == 'x') {
+        string++;
+        ret->h = atoi(string);
+    }
+
+    return ret;
+}
+
+char *encode_params(game_params *params)
+{
+    char data[256];
+
+    sprintf(data, "%dx%d", params->w, params->h);
+
+    return dupstr(data);
+}
+
 config_item *game_configure(game_params *params)
 {
     config_item *ret;
diff --git a/sixteen.c b/sixteen.c
index cce11d9..5b7b98e 100644
--- a/sixteen.c
+++ b/sixteen.c
@@ -8,6 +8,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <assert.h>
+#include <ctype.h>
 #include <math.h>
 
 #include "puzzles.h"
@@ -92,6 +93,29 @@ game_params *dup_params(game_params *params)
     return ret;
 }
 
+game_params *decode_params(char const *string)
+{
+    game_params *ret = default_params();
+
+    ret->w = ret->h = atoi(string);
+    while (*string && isdigit(*string)) string++;
+    if (*string == 'x') {
+        string++;
+        ret->h = atoi(string);
+    }
+
+    return ret;
+}
+
+char *encode_params(game_params *params)
+{
+    char data[256];
+
+    sprintf(data, "%dx%d", params->w, params->h);
+
+    return dupstr(data);
+}
+
 config_item *game_configure(game_params *params)
 {
     config_item *ret;