diff --git a/go.mod b/go.mod
index 682494a..5988d42 100644
--- a/go.mod
+++ b/go.mod
@@ -1,9 +1,9 @@
 module git.dev.alexdunmow.com/block/calcomblock
 
-go 1.26.4
+go 1.26.6
 
 require (
-	git.dev.alexdunmow.com/block/pluginsdk v0.3.5
+	git.dev.alexdunmow.com/block/pluginsdk v0.3.7
 	github.com/a-h/templ v0.3.1020
 	github.com/go-chi/chi/v5 v5.3.0
 	github.com/google/uuid v1.6.0
diff --git a/go.sum b/go.sum
index 01180ed..22bb311 100644
--- a/go.sum
+++ b/go.sum
@@ -1,7 +1,7 @@
 connectrpc.com/connect v1.20.0 h1:6TNDAB+WeNd2uolWNlYczB5E0KNNaVMNUEx8JEUsPmQ=
 connectrpc.com/connect v1.20.0/go.mod h1:A2ygJrukXwWy32vkCAAHNVguZrqZ+jeZ9rGRnGR4dN4=
-git.dev.alexdunmow.com/block/pluginsdk v0.3.5 h1:h8YZTgnB/+WQ1yPS5MbJr2aPKuv/kudr93nQowwH8nM=
-git.dev.alexdunmow.com/block/pluginsdk v0.3.5/go.mod h1:Z+eG+WZxAP0jfreLqlGcc0kkWKt8RWevzWyWn8d+dhM=
+git.dev.alexdunmow.com/block/pluginsdk v0.3.7 h1:PUT1YElhXre43m1UejDabSqRPAnx1BV7BEWKpG++7Rs=
+git.dev.alexdunmow.com/block/pluginsdk v0.3.7/go.mod h1:Z+eG+WZxAP0jfreLqlGcc0kkWKt8RWevzWyWn8d+dhM=
 github.com/BurntSushi/toml v1.6.0 h1:dRaEfpa2VI55EwlIW72hMRHdWouJeRF7TPYhI+AUQjk=
 github.com/BurntSushi/toml v1.6.0/go.mod h1:ukJfTF/6rtPPRCnwkur4qwRxa8vTRFBF0uk2lLoLwho=
 github.com/a-h/templ v0.3.1020 h1:ypAT/L5ySWEnZ6Zft/5yfoWXYYkhFNvEFOeeqecg4tw=
diff --git a/handler.go b/handler.go
index 1165a70..2f2bfd1 100644
--- a/handler.go
+++ b/handler.go
@@ -15,10 +15,10 @@ import (
 	"sync"
 	"time"
 
+	"git.dev.alexdunmow.com/block/calcomblock/internal/helpers"
 	"git.dev.alexdunmow.com/block/pluginsdk/auth"
 	"git.dev.alexdunmow.com/block/pluginsdk/plugin"
 	"git.dev.alexdunmow.com/block/pluginsdk/rbac"
-	"git.dev.alexdunmow.com/block/calcomblock/internal/helpers"
 	"github.com/go-chi/chi/v5"
 	"github.com/nyaruka/phonenumbers"
 )
@@ -500,7 +500,11 @@ func (h *CalcomHandler) HandleGetSlots(w http.ResponseWriter, r *http.Request) {
 				continue
 			}
 			timed = append(timed, timedSlot{at: local, slot: TimeSlot{
-				Start:       ts,
+				// UTC (…Z), never the event-timezone offset Cal.com may return:
+				// this value is carried raw in the slot button's query string, and
+				// a `+HH:MM` offset decodes to a space server-side, corrupting the
+				// timestamp Cal.com then rejects on booking.
+				Start:       t.UTC().Format(time.RFC3339),
 				DisplayTime: local.Format("3:04 PM"),
 				DateKey:     dateKey,
 			}})
diff --git a/plugin.mod b/plugin.mod
index 2e96ce2..4b86164 100644
--- a/plugin.mod
+++ b/plugin.mod
@@ -2,7 +2,7 @@
 name = "calcomblock"
 display_name = "Cal.com Booking"
 scope = "@ninja"
-version = "2.0.12"
+version = "2.0.13"
 description = "Embeddable Cal.com booking calendar block with custom styling, timezone-aware slot windowing, honeypot + captcha + rate-limited public booking endpoints, and webhook receiver."
 kind = "plugin"
 categories = ["forms"]
@@ -11,3 +11,5 @@ allowed_hosts = ["api.cal.com"]
 
 [compatibility]
 block_core = ">=0.3.4"
+admin_api = ">=0.1.2"
+plugin_sdk = ">=0.3.7"
diff --git a/slot_start_encoding_test.go b/slot_start_encoding_test.go
new file mode 100644
index 0000000..649001a
--- /dev/null
+++ b/slot_start_encoding_test.go
@@ -0,0 +1,64 @@
+package main
+
+import (
+	"html"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"regexp"
+	"testing"
+	"time"
+)
+
+// formHxGetRe extracts the hx-get URL of a slot button pointing at /form.
+var formHxGetRe = regexp.MustCompile(`hx-get="([^"]*/form[^"]*)"`)
+
+// TestHandleGetSlots_OffsetSlotStartSurvivesURLParsing pins the bidbuddy.com.au
+// (Australia/Perth) production failure: Cal.com returns slot starts in the
+// event's own timezone with a numeric `+08:00` offset. That value is carried in
+// the slot button's hx-get query string; a raw `+` in a query string decodes to
+// a SPACE server-side, corrupting the timestamp ("2026-06-11T10:00:00 08:00").
+// The corrupted value flows form → book → Cal.com, which rejects it with
+// "start must be a valid ISO 8601 date string". The slot's machine value must
+// therefore round-trip through URL query parsing as a valid RFC3339 timestamp.
+func TestHandleGetSlots_OffsetSlotStartSurvivesURLParsing(t *testing.T) {
+	// 2026-06-11T10:00:00+08:00 (Perth) == 2026-06-11T02:00:00Z.
+	fakeCalcom(t, "Australia/Perth", func(w http.ResponseWriter, _ *http.Request) {
+		_, _ = io.WriteString(w, `{
+			"status":"success",
+			"data":{"2026-06-11":[{"start":"2026-06-11T10:00:00+08:00"}]}
+		}`)
+	})
+	h, _ := newKeyedHandler(t)
+
+	req := httptest.NewRequest(http.MethodGet,
+		"/slots?username=alice&eventType=30min&date=2026-06-11&blockId=b1", nil)
+	rec := httptest.NewRecorder()
+	h.HandleGetSlots(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d (body: %s)", rec.Code, rec.Body.String())
+	}
+
+	m := formHxGetRe.FindStringSubmatch(rec.Body.String())
+	if m == nil {
+		t.Fatalf("no slot button hx-get to /form found in body: %q", rec.Body.String())
+	}
+	// The browser reads the attribute via the DOM, which unescapes & → &.
+	u, err := url.Parse(html.UnescapeString(m[1]))
+	if err != nil {
+		t.Fatalf("parse slot form URL %q: %v", m[1], err)
+	}
+
+	// u.Query() decodes exactly as net/http would on the wire — this is where a
+	// raw `+` becomes a space.
+	got := u.Query().Get("start")
+	parsed, err := time.Parse(time.RFC3339, got)
+	if err != nil {
+		t.Fatalf("slot start %q is not a valid ISO 8601 string after URL parsing (Cal.com would reject it): %v", got, err)
+	}
+	if want := time.Date(2026, 6, 11, 2, 0, 0, 0, time.UTC); !parsed.Equal(want) {
+		t.Errorf("slot start instant: got %s, want %s", parsed.UTC(), want)
+	}
+}