diff --git a/docs/adr/0004-plugin-manifests-declare-admin-api-compatibility.md b/docs/adr/0004-plugin-manifests-declare-admin-api-compatibility.md
new file mode 100644
index 0000000..1c069fd
--- /dev/null
+++ b/docs/adr/0004-plugin-manifests-declare-admin-api-compatibility.md
@@ -0,0 +1,37 @@
+# Plugin manifests declare admin API compatibility
+
+Plugin admin bundles consume the versioned `@block-ninja/api` browser package.
+The existing `block_core` compatibility constraint describes the host plugin
+runtime, but it cannot tell a registry or CMS whether an admin bundle is safe
+to load against the host's browser API. Treating those surfaces as one version
+would couple independent release cycles and allow an otherwise compatible
+plugin to fail only after its admin page loads.
+
+Decision: `[compatibility]` gains an optional `admin_api` string. Its value is
+a semantic-version constraint for the host-provided browser admin API, for
+example `admin_api = ">=0.1.2"`. The manifest/parser layer preserves the value
+verbatim; registry and host resolvers own constraint validation and matching.
+
+An omitted or empty value means the plugin has not declared browser API
+compatibility. It is not equivalent to an unconstrained wildcard. Automated
+latest-compatible resolution that filters by a host admin API version must
+therefore fail closed for an undeclared value. This does not change the
+separate `block_core` constraint or imply that every plugin has an admin
+bundle.
+
+The field is part of the shared manifest model and the CLI's hand-written
+`plugin.mod` serializer so version bumps, tag edits, and initialization cannot
+silently discard it.
+
+Consequences:
+
+- Registries and hosts can select plugin releases compatible with both the
+  runtime ABI and browser admin API.
+- Headless plugins can continue to omit `admin_api`.
+- Plugin authors express a range rather than pinning a single browser package
+  version.
+- Constraint syntax and matching remain the resolver's responsibility rather
+  than being duplicated in manifest parsers.
+
+Keywords: plugin.mod, compatibility, admin_api, AdminAPI, @block-ninja/api,
+semantic version, browser API, fail closed, plugin resolver
diff --git a/plugin/mod.go b/plugin/mod.go
index 07e9fa5..515edf3 100644
--- a/plugin/mod.go
+++ b/plugin/mod.go
@@ -91,6 +91,7 @@ type ModPublicRoute struct {
 
 type ModCompat struct {
 	BlockCore string `toml:"block_core"`
+	AdminAPI  string `toml:"admin_api"`
 }
 
 type ModRequirement struct {
diff --git a/plugin/mod_test.go b/plugin/mod_test.go
index 7eaa9ce..ab5ab83 100644
--- a/plugin/mod_test.go
+++ b/plugin/mod_test.go
@@ -278,6 +278,7 @@ version = "0.2.0"
 
 [compatibility]
 block_core = ">=1.5 <2.0"
+admin_api = ">=0.1.2 <0.2.0"
 
 [[requires]]
 name = "@blockninja/smartblock"
@@ -291,8 +292,14 @@ version = ">=1.2"
 	if err != nil {
 		t.Fatalf("ParseModFull err: %v", err)
 	}
-	if m.Compatibility == nil || m.Compatibility.BlockCore != ">=1.5 <2.0" {
-		t.Errorf("Compat = %+v", m.Compatibility)
+	if m.Compatibility == nil {
+		t.Fatal("Compatibility is nil")
+	}
+	if m.Compatibility.BlockCore != ">=1.5 <2.0" {
+		t.Errorf("Compat.BlockCore = %q", m.Compatibility.BlockCore)
+	}
+	if m.Compatibility.AdminAPI != ">=0.1.2 <0.2.0" {
+		t.Errorf("Compat.AdminAPI = %q", m.Compatibility.AdminAPI)
 	}
 	if len(m.Requires) != 2 {
 		t.Fatalf("Requires len = %d, want 2", len(m.Requires))