diff --git a/CLAUDE.md b/CLAUDE.md
index 42d52b8..bfd3b83 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -188,7 +188,12 @@ fact bounded by both the probe and the board/device.
   read directly by our Digilent driver; unset → each driver's own default
   (FTDI 1000, Digilent 4000). Set it via `set`, a `probes.yaml` profile,
   or `defaults:`. (FTDI path hardware-validated; Digilent path untested.)
-- **B** — device `max_tck_khz` + resolution after `jtag_autoinit`.
+- **B (done)** — registry `max_tck_khz` caps the clock: `jtag_autoinit`,
+  after identifying the chain, clamps `JTAG_TCK_FREQ_KHZ` to the smallest
+  device max and re-opens the probe once (via the stored probe id) to
+  apply, then re-scans. Within-cap / unset just report the cap. Effective
+  `tck = min(request, device max)`; probe-max (`min(..., probe max)`) is
+  still TODO.
 - **C** — generalise the other link settings (reset/RTCK) and wire the
   `prog` method tag into backend dispatch (ties into the SVF player).
 
diff --git a/fpga_registry.yaml b/fpga_registry.yaml
index 456da89..0d18f7b 100644
--- a/fpga_registry.yaml
+++ b/fpga_registry.yaml
@@ -19,6 +19,9 @@
 #                     (omit if none is available yet)
 #   caveats           space/comma-separated flags: cclk_via_startup
 #                     (omit if none)
+#   max_tck_khz       max safe JTAG TCK in kHz for this part/board; if the
+#                     requested clock exceeds it, jtag_autoinit clamps and
+#                     re-opens at the cap (omit / 0 = unspecified)
 
 fpgas:
   # Xilinx Kintex UltraScale+ XCKU15P
diff --git a/modules/fpga/fpga.c b/modules/fpga/fpga.c
index 130fc0f..017f519 100644
--- a/modules/fpga/fpga.c
+++ b/modules/fpga/fpga.c
@@ -89,6 +89,7 @@ static void set_field(fpga_target *t, const char *key, const char *val)
     else if (!strcmp(key, "ir_isc_disable"))  t->ir_isc_disable = (unsigned int)strtoul(val, NULL, 0);
     else if (!strcmp(key, "proxy_bitstream")) { free((void *)t->proxy_bitstream); t->proxy_bitstream = xstrdup(val); }
     else if (!strcmp(key, "caveats"))         t->caveats        = parse_caveats(val);
+    else if (!strcmp(key, "max_tck_khz"))     t->max_tck_khz    = (int)strtol(val, NULL, 0);
     else fprintf(stderr, "fpga: unknown key '%s'\n", key);
 }
 
diff --git a/modules/fpga/fpga.h b/modules/fpga/fpga.h
index f3d52f0..e68cc0c 100644
--- a/modules/fpga/fpga.h
+++ b/modules/fpga/fpga.h
@@ -52,6 +52,7 @@ typedef struct {
 
     const char    *proxy_bitstream;  /* path under bscan_proxies/, NULL if not yet available */
     unsigned int   caveats;          /* FPGA_CAVEAT_* flags */
+    int            max_tck_khz;      /* max safe JTAG TCK for this part/board, 0 = unspecified */
 } fpga_target;
 
 /* Registry access. The YAML file is loaded lazily on first call to any
diff --git a/modules/script/script.c b/modules/script/script.c
index 6b88854..5dccbff 100644
--- a/modules/script/script.c
+++ b/modules/script/script.c
@@ -1474,7 +1474,13 @@ const char *cmd_autoinit_help[] = {
     "explanations line two",
     ""
 };
-static int cmd_autoinit(script_ctx *ctx, char *line)
+/* Id of the currently open probe (drv<<8|probe), -1 if none. Lets
+ * jtag_autoinit re-open the same probe to apply a clock re-tune. */
+static int g_probe_id = -1;
+
+/* Scan the chain and auto-load matching BSDLs. Returns the number of
+ * BSDLs loaded (>= 0), or a negative JTAG_CORE_* error. */
+static int autoinit_run(script_ctx *ctx)
 {
     jtag_core *jc;
     int number_of_devices, dev_nb;
@@ -1588,9 +1594,86 @@ static int cmd_autoinit(script_ctx *ctx, char *line)
         return JTAG_CORE_ACCESS_ERROR;
     }
 
+    return loaded_bsdl;
+}
+
+/* Smallest device-declared max TCK (kHz) over the matched chain devices,
+ * or 0 if none of them declare one. */
+static int chain_max_tck_khz(jtag_core *jc)
+{
+    int i, n, cap = 0;
+    const fpga_target *t;
+
+    n = jtagcore_get_number_of_devices(jc);
+    for (i = 0; i < n; i++) {
+        t = fpga_lookup_by_idcode(jtagcore_get_dev_id(jc, i));
+        if (t && t->max_tck_khz > 0 && (cap == 0 || t->max_tck_khz < cap))
+            cap = t->max_tck_khz;
+    }
+    return cap;
+}
+
+static int cmd_autoinit(script_ctx *ctx, char *line)
+{
+    jtag_core *jc;
+    int loaded, cap, req;
+
+    (void)line;
+    jc = (jtag_core *)ctx->app_ctx;
+
+    loaded = autoinit_run(ctx);
+    if (loaded < 0)
+        return loaded;
+
+    // Phase B: resolve the JTAG clock against the devices now identified.
+    // The registry can cap the TCK for a part/board; if the requested
+    // clock exceeds that, clamp it and re-open the probe so it takes
+    // effect (the chain is then re-scanned at the safe clock).
+    cap = chain_max_tck_khz(jc);
+    req = jtagcore_getEnvVarValue(jc, "JTAG_TCK_FREQ_KHZ");
+    if (cap > 0)
+    {
+        if (req > 0 && req > cap)
+        {
+            char buf[24];
+            snprintf(buf, sizeof(buf), "%d", cap);
+            ctx->script_printf(ctx, MSG_WARNING,
+                "JTAG clock %d kHz exceeds the device max %d kHz; clamping.\n", req, cap);
+            setEnvVarDat((envvar_entry *)ctx->env, "JTAG_TCK_FREQ_KHZ", buf);
+            setEnvVarDat((envvar_entry *)ctx->env, "PROBE_FTDI_TCK_FREQ_KHZ", buf);
+            if (g_probe_id >= 0)
+            {
+                ctx->script_printf(ctx, MSG_INFO_0,
+                    "Re-opening at %d kHz and re-scanning.\n", cap);
+                if (jtagcore_select_and_open_probe(jc, g_probe_id) == JTAG_CORE_NO_ERROR)
+                {
+                    loaded = autoinit_run(ctx);
+                    if (loaded < 0)
+                        return loaded;
+                }
+                else
+                {
+                    ctx->script_printf(ctx, MSG_ERROR,
+                        "Re-open failed; the clock will apply on the next jtag_open.\n");
+                }
+            }
+            else
+            {
+                ctx->script_printf(ctx, MSG_WARNING,
+                    "Reopen the probe (jtag_close / jtag_open) to apply the clamped clock.\n");
+            }
+        }
+        else
+        {
+            ctx->script_printf(ctx, MSG_INFO_0,
+                "Device max JTAG clock: %d kHz%s.\n", cap,
+                (req > 0) ? "" : " (set JTAG_TCK_FREQ_KHZ to control the clock)");
+        }
+    }
+
     // Expose the count to scripts via last_data_value, but report success
     // (a non-zero return is treated as an error code by the engine).
-    ctx->last_data_value = loaded_bsdl;
+    ctx->last_data_value = loaded;
     return JTAG_CORE_NO_ERROR;
 }
 
@@ -1885,6 +1968,7 @@ static int cmd_open_probe(script_ctx *ctx, char *line)
         }
         else
         {
+            g_probe_id = id;
             ctx->script_printf(ctx, MSG_INFO_0, "Probe Ok !\n");
             return JTAG_CORE_NO_ERROR;
         }
@@ -1921,6 +2005,7 @@ static int cmd_close_probe(script_ctx *ctx, char *line)
         // chain looks probe-less until the next jtag_open.
         jc->io_functions.drv_DeInit(jc);
         memset(&jc->io_functions, 0, sizeof(jc->io_functions));
+        g_probe_id = -1;
         ctx->script_printf(ctx, MSG_INFO_0, "Probe released.\n");
     }
     else
@@ -2909,10 +2994,10 @@ static int cmd_fpga_list(script_ctx *ctx, char *line)
             i, t->idcode, t->idcode_mask,
             t->name, fpga_family_name(t->family));
         ctx->script_printf(ctx, MSG_NONE,
-            "      bsdl=%s ir=%d proxy=%s caveats=0x%x\n",
+            "      bsdl=%s ir=%d proxy=%s caveats=0x%x maxtck=%dkHz\n",
             t->bsdl_filename, t->ir_length,
             t->proxy_bitstream ? t->proxy_bitstream : "(none yet)",
-            t->caveats);
+            t->caveats, t->max_tck_khz);
     }
     return JTAG_CORE_NO_ERROR;
 }