diff --git a/docs/config_schema.md b/docs/config_schema.md
index f7b10eee..f751ed51 100644
--- a/docs/config_schema.md
+++ b/docs/config_schema.md
@@ -983,6 +983,12 @@ race) agree on `cyc`, `mmio` and `mc` on every frame. They also end with
 identical ordered write hashes. The only differences are one-frame VBlank
 straddles of `wc`/`sc`, as documented for batched code.
 
+The scale and selected batching profile are fixed at session startup and
+included in the existing netplay compatibility fingerprint. Peers with
+different batching choices cannot start together. Both paths use portable C;
+this does not authorize compatibility between otherwise different builds or
+replace the existing game/core/config identity checks.
+
 Gates (all present gates must be open; with none the scale always applies):
 
 - `guest_cycle_scale_gate` — RAM predicates on aligned main-RAM words
diff --git a/runtime/include/psx_cycles.h b/runtime/include/psx_cycles.h
index 2c4d267e..15c67998 100644
--- a/runtime/include/psx_cycles.h
+++ b/runtime/include/psx_cycles.h
@@ -124,6 +124,13 @@ static inline uint32_t psx_gcs_recip_for(uint32_t scale) {
 }
 /* [timing] guest_cycle_scale_batch: batch scaled CPU charges (psx_cyc.h). */
 extern int g_psx_gcs_batch;
+/* Fixed session timing profile. Both implementations are portable C and are
+ * present in every build; peers must agree on the selected one before frame 0.
+ * Preserve the existing scale-only tag for the unbatched path. */
+static inline uint32_t psx_gcs_session_profile(uint32_t scale, int batch) {
+    if (scale <= 1u) return 0u;
+    return 0x6C000000u | (batch ? 0x00800000u : 0u) | scale;
+}
 static inline uint32_t psx_gcs_scale(uint32_t cpu_cycles) {
     uint64_t t = (uint64_t)cpu_cycles * g_psx_gcs_recip_q16 + g_psx_gcs_frac;
     g_psx_gcs_frac = (uint32_t)(t & 0xFFFFu);
diff --git a/runtime/src/main.cpp b/runtime/src/main.cpp
index 9e66770d..4765f44a 100644
--- a/runtime/src/main.cpp
+++ b/runtime/src/main.cpp
@@ -19631,7 +19631,8 @@ session_reboot:
         // exactly as before.
         if (psx_guest_cycle_scale_config() != 1u) {
             char tag[9];
-            std::snprintf(tag, sizeof tag, "%08x", 0x6C000000u | psx_guest_cycle_scale_config());
+            std::snprintf(tag, sizeof tag, "%08x",
+                          psx_gcs_session_profile(psx_guest_cycle_scale_config(), g_psx_gcs_batch));
             if (std::strlen(net_cfg.content_fingerprint) != 64)
                 std::snprintf(net_cfg.content_fingerprint, sizeof(net_cfg.content_fingerprint),
                               "%s", "0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c00000000");
@@ -19640,8 +19641,8 @@ session_reboot:
                 auto hv = [](char c) { return c <= '9' ? c - '0' : (c | 0x20) - 'a' + 10; };
                 *d = "0123456789abcdef"[hv(*d) ^ hv(tag[i])];
             }
-            std::printf("psxrecomp: netplay requires guest_cycle_scale %u on every peer\n",
-                        psx_guest_cycle_scale_config());
+            std::printf("psxrecomp: netplay requires guest_cycle_scale %u, batch profile %d on every peer\n",
+                        psx_guest_cycle_scale_config(), g_psx_gcs_batch != 0);
         }
         const int nrc = psx_netplay_start(&net_cfg);
         if (nrc != 0) {
diff --git a/runtime/tests/test_psx_guest_cycle_scale.c b/runtime/tests/test_psx_guest_cycle_scale.c
index 29072e7e..17349008 100644
--- a/runtime/tests/test_psx_guest_cycle_scale.c
+++ b/runtime/tests/test_psx_guest_cycle_scale.c
@@ -129,6 +129,11 @@ static void check_batch_ignores_host_stack(uint32_t scale, uint32_t charges, uin
 }
 
 int main(void) {
+    assert(psx_gcs_session_profile(1, 0) == 0u);
+    assert(psx_gcs_session_profile(1, 1) == 0u); /* no scaled path */
+    assert(psx_gcs_session_profile(2, 0) == (0x6C000000u | 2u));
+    assert(psx_gcs_session_profile(2, 1) != psx_gcs_session_profile(2, 0));
+    assert(psx_gcs_session_profile(3, 1) != psx_gcs_session_profile(2, 1));
     assert(psx_gcs_recip_for(1) == 65536u);
     assert(psx_gcs_recip_for(0) == 65536u);          /* never slower than faithful */
     assert(psx_gcs_recip_for(2) == 32768u);