diff --git a/src/foundation/mem.c b/src/foundation/mem.c
index 67ef4d14e..b0a79f6c2 100644
--- a/src/foundation/mem.c
+++ b/src/foundation/mem.c
@@ -106,6 +106,21 @@ static void check_pressure(size_t rss) {
 
 /* ── Public API ────────────────────────────────────────────────── */
 
+#define RAM_FRACTION_DEFAULT 0.5
+#define RAM_FRACTION_16GB 0.25
+#define RAM_FRACTION_32GB 0.35
+#define RAM_BYTES_PER_GB (1024ULL * 1024 * 1024)
+
+double cbm_mem_ram_fraction_for_total(size_t total_ram_bytes) {
+    if (total_ram_bytes <= 16ULL * RAM_BYTES_PER_GB) {
+        return RAM_FRACTION_16GB;
+    }
+    if (total_ram_bytes <= 32ULL * RAM_BYTES_PER_GB) {
+        return RAM_FRACTION_32GB;
+    }
+    return RAM_FRACTION_DEFAULT;
+}
+
 void cbm_mem_init(double ram_fraction) {
     int expected = 0;
     if (!atomic_compare_exchange_strong(&g_initialized, &expected, 1)) {
diff --git a/src/foundation/mem.h b/src/foundation/mem.h
index 5001410a8..8c23c508a 100644
--- a/src/foundation/mem.h
+++ b/src/foundation/mem.h
@@ -11,6 +11,9 @@
 #include <stdbool.h>
 #include <stddef.h>
 
+/* Tiered default fraction for MCP startup: 25% on <=16GB, 35% on <=32GB, else 50%. */
+double cbm_mem_ram_fraction_for_total(size_t total_ram_bytes);
+
 /* Initialize memory budget = ram_fraction * total_physical_ram.
  * Thread-safe: only the first call takes effect.
  * Configures mimalloc options for reduced upfront memory. */
diff --git a/src/main.c b/src/main.c
index f2b72cba8..66cfb3430 100644
--- a/src/main.c
+++ b/src/main.c
@@ -30,8 +30,6 @@ enum {
     MAIN_MAX_PORT = 65536,
     PARENT_WATCHDOG_STACK_SIZE = 64 * CBM_SZ_1K, /* watchdog only polls — tiny stack suffices */
 };
-#define MAIN_RAM_FRACTION 0.5
-
 #define SLEN(s) (sizeof(s) - 1)
 #include "foundation/log.h"
 #include "foundation/diagnostics.h"
@@ -324,11 +322,11 @@ static int handle_subcommand(int argc, char **argv) {
             return 0;
         }
         if (strcmp(argv[i], "cli") == 0) {
-            cbm_mem_init(MAIN_RAM_FRACTION);
+            cbm_mem_init(cbm_mem_ram_fraction_for_total(cbm_system_info().total_ram));
             return run_cli(argc - i - SKIP_ONE, argv + i + SKIP_ONE);
         }
         if (strcmp(argv[i], "hook-augment") == 0) {
-            cbm_mem_init(MAIN_RAM_FRACTION);
+            cbm_mem_init(cbm_mem_ram_fraction_for_total(cbm_system_info().total_ram));
             return cbm_cmd_hook_augment();
         }
         if (strcmp(argv[i], "install") == 0) {
@@ -424,9 +422,7 @@ int main(int argc, char **argv) {
 #endif
 
     /* Default: MCP server on stdio */
-    cbm_mem_init(MAIN_RAM_FRACTION); /* 50% of RAM — safe now because mimalloc tracks ALL
-                                      * memory (C + C++ allocations) via global override.
-                                      * No more untracked heap blind spots. */
+    cbm_mem_init(cbm_mem_ram_fraction_for_total(cbm_system_info().total_ram));
     /* Store binary path for subprocess spawning + hook log sink */
     cbm_http_server_set_binary_path(argv[0]);
     cbm_log_set_sink(cbm_ui_log_append);
diff --git a/tests/test_mem.c b/tests/test_mem.c
index debb9b505..3130fbecf 100644
--- a/tests/test_mem.c
+++ b/tests/test_mem.c
@@ -140,6 +140,29 @@ TEST(mem_over_budget_low_rss) {
     PASS();
 }
 
+/* ── Tiered RAM fraction (host-size defaults) ─────────────────── */
+
+TEST(mem_ram_fraction_16gb_tier) {
+    size_t ram_16gb = 16ULL * 1024 * 1024 * 1024;
+    ASSERT_EQ(cbm_mem_ram_fraction_for_total(ram_16gb), 0.25);
+    ASSERT_EQ(cbm_mem_ram_fraction_for_total(ram_16gb - 1), 0.25);
+    PASS();
+}
+
+TEST(mem_ram_fraction_32gb_tier) {
+    size_t ram_32gb = 32ULL * 1024 * 1024 * 1024;
+    size_t ram_17gb = 17ULL * 1024 * 1024 * 1024;
+    ASSERT_EQ(cbm_mem_ram_fraction_for_total(ram_17gb), 0.35);
+    ASSERT_EQ(cbm_mem_ram_fraction_for_total(ram_32gb), 0.35);
+    PASS();
+}
+
+TEST(mem_ram_fraction_large_host) {
+    size_t ram_64gb = 64ULL * 1024 * 1024 * 1024;
+    ASSERT_EQ(cbm_mem_ram_fraction_for_total(ram_64gb), 0.5);
+    PASS();
+}
+
 /* ── RSS tracking tests ───────────────────────────────────────── */
 
 TEST(mem_rss_positive) {
@@ -638,6 +661,9 @@ SUITE(mem) {
     RUN_TEST(mem_worker_budget_one_worker);
     RUN_TEST(mem_worker_budget_many_workers);
     RUN_TEST(mem_over_budget_low_rss);
+    RUN_TEST(mem_ram_fraction_16gb_tier);
+    RUN_TEST(mem_ram_fraction_32gb_tier);
+    RUN_TEST(mem_ram_fraction_large_host);
     /* RSS tracking */
     RUN_TEST(mem_rss_positive);
     RUN_TEST(mem_peak_rss_gte_rss);