diff --git a/cmd/server/tests/podcast_test.go b/cmd/server/tests/podcast_test.go
index c4657977..40ebee36 100644
--- a/cmd/server/tests/podcast_test.go
+++ b/cmd/server/tests/podcast_test.go
@@ -2150,3 +2150,32 @@ func TestEpisodeCollectionsEmptySeedMatchesNothing(t *testing.T) {
 		"an empty-seed episodes collection matches nothing — a composer that degraded to the bare scope head would report the whole library here")
 	assert.Empty(t, out.Books)
 }
+
+// The create modal posts the seed as a FORM (urlencoded hx-post), and
+// echo's binder cannot set an any-typed struct field from a form
+// string — the first in-browser create 400'd "invalid request"
+// (2026-10-31). The form variant binds a dedicated string field and
+// rides the same parse path; this pins the form transport.
+func TestEpisodeCollectionsFormSeedCreate(t *testing.T) {
+	setup := setupTestServer(t)
+	regToken := setup.RegularToken
+	marker := fmt.Sprintf("formseed%d", time.Now().UnixNano()%100000)
+
+	form := url.Values{}
+	form.Set("collection_type", "smart")
+	form.Set("name", "test FormSeed "+marker)
+	form.Set("filter_tree", `{"scope":"episodes","mode":"all","children":[]}`)
+	code, body := podcastAuthedForm(t, setup, regToken, "POST", "/api/collections", form)
+	require.Equal(t, http.StatusCreated, code, "the FORM-encoded seed must bind and parse: %s", string(body))
+	var coll struct {
+		ID string `json:"id"`
+	}
+	require.NoError(t, json.Unmarshal(body, &coll))
+	t.Cleanup(func() {
+		podcastAuthed(t, setup, regToken, "DELETE", "/api/collections/"+coll.ID, "")
+	})
+
+	code, body = podcastAuthed(t, setup, regToken, "GET", "/api/collections/"+coll.ID, "")
+	require.Equal(t, http.StatusOK, code)
+	assert.Contains(t, string(body), `"scope":"episodes"`, "the seeded scope round-trips")
+}
diff --git a/internal/handlers/collections.go b/internal/handlers/collections.go
index d505c1ee..d8ce4137 100644
--- a/internal/handlers/collections.go
+++ b/internal/handlers/collections.go
@@ -46,8 +46,16 @@ type CreateCollectionRequest struct {
 	// smart membership runs through the qf engine. INVALID trees are a
 	// 400 (ParseFilterTree at the handler); a tree supersedes
 	// auto_assign_rules. Persisted as-is (any). RawMessage for
-	// round-trip safety.
-	FilterTree any `form:"filter_tree" json:"filter_tree"`
+	// round-trip safety. JSON bodies bind FilterTree; FORM bodies
+	// (the create modal's urlencoded hx-post) bind FilterTreeForm —
+	// echo's binder cannot set an any-typed field from a form string
+	// (the create-modal seed POST 400'd "invalid request" on it,
+	// 2026-10-31).
+	FilterTree any `json:"filter_tree"`
+	// FilterTreeForm mirrors filter_tree for application/x-www-form-
+	// urlencoded submissions: the raw JSON as a string, validated by
+	// the same parseRequestFilterTree path.
+	FilterTreeForm string `form:"filter_tree"`
 }
 
 // treeFilterJSON returns the request's filter tree as JSON storage
@@ -85,6 +93,16 @@ func parseRequestFilterTree(payload any) ([]byte, error) {
 	return b, nil
 }
 
+// resolveTreePayload — the tree payload from either transport: JSON
+// bodies carry the object in FilterTree, form bodies (the create
+// modal's urlencoded hx-post) the raw JSON string in FilterTreeForm.
+func resolveTreePayload(jsonTree any, formTree string) any {
+	if formTree != "" {
+		return formTree
+	}
+	return jsonTree
+}
+
 type UpdateCollectionRequest struct {
 	Name string `form:"name" json:"name" validate:"required"`
 	// Smart-collection match semantics (item 2.4): "any" (OR) | "all"
@@ -93,8 +111,11 @@ type UpdateCollectionRequest struct {
 	// The unified filter tree (item 3.2): when present the collection's
 	// smart membership runs through the qf engine. INVALID trees are a
 	// 400 (ParseFilterTree at the handler); a tree supersedes
-	// auto_assign_rules.
-	FilterTree      any                    `form:"filter_tree" json:"filter_tree"`
+	// auto_assign_rules. JSON bodies bind FilterTree; form bodies bind
+	// FilterTreeForm (echo's binder cannot set an any-typed field from
+	// a form string — see CreateCollectionRequest).
+	FilterTree      any                    `json:"filter_tree"`
+	FilterTreeForm  string                 `form:"filter_tree"`
 	Description     string                 `form:"description" json:"description"`
 	Color           string                 `form:"color" json:"color"`
 	Icon            string                 `form:"icon" json:"icon"`
@@ -186,7 +207,7 @@ func (h *CollectionHandler) CreateCollection(c *echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": err.Error()})
 	}
 
-	treeJSON, treeErr := parseRequestFilterTree(req.FilterTree)
+	treeJSON, treeErr := parseRequestFilterTree(resolveTreePayload(req.FilterTree, req.FilterTreeForm))
 	if treeErr != nil {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": treeErr.Error()})
 	}
@@ -507,7 +528,7 @@ func (h *CollectionHandler) UpdateCollection(c *echo.Context) error {
 		req.ViewSettings["show_up_next"] = *v
 	}
 
-	treeJSON, treeErr := parseRequestFilterTree(req.FilterTree)
+	treeJSON, treeErr := parseRequestFilterTree(resolveTreePayload(req.FilterTree, req.FilterTreeForm))
 	if treeErr != nil {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": treeErr.Error()})
 	}