diff --git a/Cargo.lock b/Cargo.lock
index c308044c..83795bf5 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3626,6 +3626,9 @@ dependencies = [
 [[package]]
 name = "ocs_plugin_api"
 version = "0.1.0"
+dependencies = [
+ "acadrust",
+]
 
 [[package]]
 name = "once_cell"
diff --git a/Cargo.toml b/Cargo.toml
index 600d2f17..6f992d8c 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -24,7 +24,7 @@ solid3d = ["dep:truck-meshalgo", "dep:truck-shapeops", "dep:lzma-sys"]
 [dependencies]
 # Stable, dependency-free add-on contract (manifest + ribbon/CadModule types).
 # Plugin authors target this crate's semver, not OpenCADStudio internals.
-ocs_plugin_api = { path = "crates/ocs_plugin_api" }
+ocs_plugin_api = { path = "crates/ocs_plugin_api", features = ["host"] }
 # vtkio (pulled in transitively via truck-meshalgo) depends on xz2 → lzma-sys,
 # which by default links the system liblzma dynamically. On macOS that bakes a
 # Homebrew dylib path (/opt/homebrew/.../liblzma.5.dylib) into the binary, so
diff --git a/crates/ocs_plugin_api/Cargo.toml b/crates/ocs_plugin_api/Cargo.toml
index 4e2ea76d..535912cd 100644
--- a/crates/ocs_plugin_api/Cargo.toml
+++ b/crates/ocs_plugin_api/Cargo.toml
@@ -6,3 +6,12 @@ description = "Stable, dependency-free contract for Open CAD Studio add-on packa
 license = "GPL-3.0-only"
 
 [dependencies]
+# Pulled in only by the `host` feature, which adds the `acadrust`-typed
+# `HostApi` runtime surface. The default crate stays dependency-free so engine
+# crates and external tooling can depend on the manifest/ribbon contract cheaply.
+acadrust = { version = "0.3.4", optional = true }
+
+[features]
+# Enables the runtime host surface (`HostApi` trait). The OpenCADStudio binary
+# turns this on; pure-data consumers leave it off.
+host = ["dep:acadrust"]
diff --git a/crates/ocs_plugin_api/src/host.rs b/crates/ocs_plugin_api/src/host.rs
new file mode 100644
index 00000000..f7a3f502
--- /dev/null
+++ b/crates/ocs_plugin_api/src/host.rs
@@ -0,0 +1,92 @@
+//! Runtime host surface (`host` feature).
+//!
+//! [`HostApi`] is the `acadrust`-typed adapter a plugin uses at *dispatch* time
+//! — document access, entity creation, XDATA, undo, and the command line. It is
+//! the stable counterpart to the dependency-free manifest/ribbon contract: a
+//! plugin's `dispatch` receives `&mut dyn HostApi` rather than the host's
+//! concrete session type, so an out-of-tree add-on compiles against this crate
+//! alone.
+//!
+//! Per-tab plugin state is keyed by `manifest.id`. The trait exposes it in an
+//! object-safe `Any` form; use the [`plugin_state`], [`plugin_state_mut`] and
+//! [`ensure_plugin_state`] helpers for the ergonomic typed access.
+
+use std::any::Any;
+
+use acadrust::xdata::ExtendedDataRecord;
+use acadrust::{CadDocument, EntityType, Handle};
+
+/// The plugin-facing runtime surface for one active document tab.
+pub trait HostApi {
+    /// Index of the tab this session targets.
+    fn tab_index(&self) -> usize;
+
+    // ── Document ────────────────────────────────────────────────────────────
+    fn document(&self) -> &CadDocument;
+    fn document_mut(&mut self) -> &mut CadDocument;
+    /// Add an entity to the active document, returning its handle.
+    fn add_entity(&mut self, entity: EntityType) -> Handle;
+    /// Mark the scene geometry dirty so it is re-tessellated next frame.
+    fn bump_geometry(&mut self);
+
+    // ── XDATA ───────────────────────────────────────────────────────────────
+    /// Read the XDATA record for `app_name` on entity `handle`, if any.
+    fn read_record(&self, handle: Handle, app_name: &str) -> Option<&ExtendedDataRecord>;
+    /// Attach `record` to entity `handle`, replacing any existing record for the
+    /// same application and registering the APPID. Returns `false` if the entity
+    /// does not exist.
+    fn write_record(&mut self, handle: Handle, record: ExtendedDataRecord) -> bool;
+    /// Remove the XDATA record for `app_name` from entity `handle`. Returns
+    /// `true` if a record was removed.
+    fn remove_record(&mut self, handle: Handle, app_name: &str) -> bool;
+
+    // ── Undo / dirty ────────────────────────────────────────────────────────
+    fn push_undo(&mut self, label: &str);
+    fn set_dirty(&mut self);
+
+    // ── Command line ────────────────────────────────────────────────────────
+    fn push_info(&mut self, msg: &str);
+    fn push_output(&mut self, msg: &str);
+    fn push_error(&mut self, msg: &str);
+
+    // ── Per-tab plugin state (object-safe; use the typed helpers below) ──────
+    fn plugin_state_any(&self, plugin_id: &str) -> Option<&(dyn Any + Send + Sync)>;
+    fn plugin_state_any_mut(&mut self, plugin_id: &str)
+        -> Option<&mut (dyn Any + Send + Sync)>;
+    /// Get the state for `plugin_id`, inserting `init()`'s result if absent.
+    fn ensure_plugin_state_any(
+        &mut self,
+        plugin_id: &'static str,
+        init: &mut dyn FnMut() -> Box<dyn Any + Send + Sync>,
+    ) -> &mut (dyn Any + Send + Sync);
+}
+
+/// Typed read of per-tab plugin state stored under `plugin_id`.
+pub fn plugin_state<'a, T: Any + Send + Sync>(
+    host: &'a dyn HostApi,
+    plugin_id: &str,
+) -> Option<&'a T> {
+    host.plugin_state_any(plugin_id)?.downcast_ref::<T>()
+}
+
+/// Typed mutable access to per-tab plugin state stored under `plugin_id`.
+pub fn plugin_state_mut<'a, T: Any + Send + Sync>(
+    host: &'a mut dyn HostApi,
+    plugin_id: &str,
+) -> Option<&'a mut T> {
+    host.plugin_state_any_mut(plugin_id)?.downcast_mut::<T>()
+}
+
+/// Typed get-or-insert of per-tab plugin state stored under `plugin_id`.
+pub fn ensure_plugin_state<'a, T: Any + Send + Sync>(
+    host: &'a mut dyn HostApi,
+    plugin_id: &'static str,
+    init: impl FnOnce() -> T,
+) -> &'a mut T {
+    let mut init = Some(init);
+    let any = host.ensure_plugin_state_any(plugin_id, &mut || {
+        Box::new((init.take().expect("init called once"))())
+    });
+    any.downcast_mut::<T>()
+        .expect("plugin state type mismatch for plugin_id")
+}
diff --git a/crates/ocs_plugin_api/src/lib.rs b/crates/ocs_plugin_api/src/lib.rs
index 7e217379..cff3749d 100644
--- a/crates/ocs_plugin_api/src/lib.rs
+++ b/crates/ocs_plugin_api/src/lib.rs
@@ -19,6 +19,10 @@
 pub mod manifest;
 pub mod ribbon;
 
+/// Runtime host surface — only built with the `host` feature (pulls `acadrust`).
+#[cfg(feature = "host")]
+pub mod host;
+
 pub use manifest::{ApiVersion, PluginManifest, API_VERSION};
 pub use ribbon::{
     CadModule, IconKind, ModuleEvent, RibbonGroup, RibbonItem, StyleKey, ToolDef,
diff --git a/docs/plugin-architecture.md b/docs/plugin-architecture.md
index cf976c08..0fc964aa 100644
--- a/docs/plugin-architecture.md
+++ b/docs/plugin-architecture.md
@@ -150,7 +150,7 @@ pub struct PluginManifest {
 pub trait BuiltinPlugin: Send + Sync {
     fn manifest(&self) -> &'static PluginManifest;
     fn ribbon(&self) -> Box<dyn CadModule>;
-    fn dispatch(&self, host: &mut HostSession<'_>, cmd: &str) -> bool;
+    fn dispatch(&self, host: &mut dyn HostApi, cmd: &str) -> bool;
 }
 ```
 
@@ -183,14 +183,20 @@ Plugins use `HostSession`, not `OpenCADStudio`:
 | Command line | `push_info`, `push_output`, `push_error`, `set_active_command` |
 | Undo / dirty | `push_undo`, `set_dirty` |
 
-**Status:** The dependency-free half of the contract — `PluginManifest` /
-`ApiVersion` (manifest) and `CadModule` + the ribbon types (`ToolDef`,
-`RibbonGroup`, …) — now lives in the standalone, semver-versioned
-[`crates/ocs_plugin_api`](../crates/ocs_plugin_api) crate. The host re-exports it
-(`crate::plugin::manifest`, `crate::modules`) so in-tree paths are unchanged.
-The `acadrust`-typed runtime surface in the table above (`document_mut`,
-`add_entity`, `set_active_command`, …) stays in the host binary for now; lifting
-it behind a `HostApi` trait in the same crate is the remaining phase-1b step.
+**Status:** The whole contract now lives in the standalone, semver-versioned
+[`crates/ocs_plugin_api`](../crates/ocs_plugin_api) crate:
+- **Dependency-free core** — `PluginManifest` / `ApiVersion` (manifest) and
+  `CadModule` + ribbon types (`ToolDef`, `RibbonGroup`, …). Engine crates and
+  external tooling depend on this cheaply.
+- **`host` feature** — the `acadrust`-typed `HostApi` trait (the runtime surface
+  in the table above). `HostSession` in the binary implements it; a plugin's
+  `dispatch` receives `&mut dyn HostApi`, so an out-of-tree add-on compiles
+  against this crate alone. Per-tab plugin state is reached through the
+  object-safe `plugin_state*` helpers. `set_active_command` (interactive
+  acquisition) stays host-side for now — see Command routing.
+
+The host re-exports both (`crate::plugin::manifest`, `crate::modules`,
+`crate::plugin::host::HostApi`) so in-tree paths are unchanged.
 
 ### Command routing
 
@@ -255,7 +261,7 @@ This mirrors QGIS: the application ships core menus; plugins add tabs/tools with
 - [x] Per-tab `plugin_state`
 - [x] Storm Sewer off `commands.rs` monolith
 - [x] Single registration (`plugin.toml` + `BuiltinPlugin::ribbon`)
-- [~] Extract `ocs_plugin_api` crate — manifest + ribbon/`CadModule` done; `acadrust`-typed host surface pending
+- [x] Extract `ocs_plugin_api` crate — dependency-free manifest + ribbon/`CadModule`, plus the `acadrust`-typed `HostApi` trait behind the optional `host` feature; `dispatch` takes `&mut dyn HostApi`
 - [x] Plugin manager UI (list installed, versions) — `PLUGINS` / `PLUGINMANAGER` command, or the Start-page "Plugins" button
 - [x] Enable/disable plugins from the manager — a disabled plugin drops its ribbon tab and command dispatch; persisted in `settings.txt` (`disabled_plugins=`)
 - [x] `ModuleEvent::PluginFileDialog` — a plugin tool requests a native file picker; the host opens it and dispatches `"<command> <path>"` back to the plugin with original case preserved (bypasses the command-line upper-casing)
diff --git a/docs/plugin-template/dispatch.rs b/docs/plugin-template/dispatch.rs
index 174f6b94..6c70a91c 100644
--- a/docs/plugin-template/dispatch.rs
+++ b/docs/plugin-template/dispatch.rs
@@ -1,8 +1,8 @@
-use crate::plugin::host::HostSession;
+use crate::plugin::host::HostApi;
 
 use super::manifest::PLUGIN_ID;
 
-pub fn handle(host: &mut HostSession<'_>, cmd: &str) -> bool {
+pub fn handle(host: &mut dyn HostApi, cmd: &str) -> bool {
     let _ = (host, PLUGIN_ID);
     match cmd {
         "MP_HELLO" => {
diff --git a/docs/plugin-template/plugin.rs b/docs/plugin-template/plugin.rs
index 6230b57d..92ca34e5 100644
--- a/docs/plugin-template/plugin.rs
+++ b/docs/plugin-template/plugin.rs
@@ -1,4 +1,4 @@
-use crate::plugin::host::{BuiltinPlugin, HostSession};
+use crate::plugin::host::{BuiltinPlugin, HostApi};
 use crate::plugin::manifest::PluginManifest;
 
 use super::dispatch;
@@ -15,7 +15,7 @@ impl BuiltinPlugin for MyPlugin {
         Box::new(super::MyPluginModule)
     }
 
-    fn dispatch(&self, host: &mut HostSession<'_>, cmd: &str) -> bool {
+    fn dispatch(&self, host: &mut dyn HostApi, cmd: &str) -> bool {
         dispatch::handle(host, cmd)
     }
 }
\ No newline at end of file
diff --git a/src/app/plugin_host.rs b/src/app/plugin_host.rs
index 514ad21e..d96d8726 100644
--- a/src/app/plugin_host.rs
+++ b/src/app/plugin_host.rs
@@ -5,6 +5,7 @@ use std::any::{Any, TypeId};
 use acadrust::tables::AppId;
 use acadrust::xdata::ExtendedDataRecord;
 use acadrust::{CadDocument, EntityType, Handle};
+use ocs_plugin_api::host::HostApi;
 
 use super::OpenCADStudio;
 use crate::command::CadCommand;
@@ -167,6 +168,79 @@ impl<'a> HostSession<'a> {
     }
 }
 
+/// The stable contract a plugin's `dispatch` sees. Each method forwards to the
+/// inherent `HostSession` method of the same name (inherent methods take
+/// resolution priority, so this is plain delegation, not recursion). The
+/// per-tab plugin-state accessors expose the raw `Any` box; the typed
+/// `ocs_plugin_api::host::plugin_state*` helpers wrap them.
+impl HostApi for HostSession<'_> {
+    fn tab_index(&self) -> usize {
+        self.tab_index()
+    }
+    fn document(&self) -> &CadDocument {
+        self.document()
+    }
+    fn document_mut(&mut self) -> &mut CadDocument {
+        self.document_mut()
+    }
+    fn add_entity(&mut self, entity: EntityType) -> Handle {
+        self.add_entity(entity)
+    }
+    fn bump_geometry(&mut self) {
+        self.bump_geometry()
+    }
+    fn read_record(&self, handle: Handle, app_name: &str) -> Option<&ExtendedDataRecord> {
+        self.read_record(handle, app_name)
+    }
+    fn write_record(&mut self, handle: Handle, record: ExtendedDataRecord) -> bool {
+        self.write_record(handle, record)
+    }
+    fn remove_record(&mut self, handle: Handle, app_name: &str) -> bool {
+        self.remove_record(handle, app_name)
+    }
+    fn push_undo(&mut self, label: &str) {
+        self.push_undo(label)
+    }
+    fn set_dirty(&mut self) {
+        self.set_dirty()
+    }
+    fn push_info(&mut self, msg: &str) {
+        self.push_info(msg)
+    }
+    fn push_output(&mut self, msg: &str) {
+        self.push_output(msg)
+    }
+    fn push_error(&mut self, msg: &str) {
+        self.push_error(msg)
+    }
+    fn plugin_state_any(&self, plugin_id: &str) -> Option<&(dyn Any + Send + Sync)> {
+        self.app.tabs[self.tab]
+            .plugin_state
+            .get(plugin_id)
+            .map(|b| b.as_ref())
+    }
+    fn plugin_state_any_mut(
+        &mut self,
+        plugin_id: &str,
+    ) -> Option<&mut (dyn Any + Send + Sync)> {
+        self.app.tabs[self.tab]
+            .plugin_state
+            .get_mut(plugin_id)
+            .map(|b| b.as_mut())
+    }
+    fn ensure_plugin_state_any(
+        &mut self,
+        plugin_id: &'static str,
+        init: &mut dyn FnMut() -> Box<dyn Any + Send + Sync>,
+    ) -> &mut (dyn Any + Send + Sync) {
+        self.app.tabs[self.tab]
+            .plugin_state
+            .entry(plugin_id)
+            .or_insert_with(|| init())
+            .as_mut()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -202,4 +276,20 @@ mod tests {
         assert!(host.read_record(h, "DEMO_SURVEY").is_none());
         assert!(!host.remove_record(h, "DEMO_SURVEY"));
     }
+
+    #[test]
+    fn plugin_state_round_trips_through_hostapi_trait() {
+        use ocs_plugin_api::host::{self, HostApi};
+        let mut app = OpenCADStudio::new_for_test();
+        let mut session = HostSession::new(&mut app, 0);
+        let host: &mut dyn HostApi = &mut session;
+
+        // Absent before first use.
+        assert!(host::plugin_state::<u32>(&*host, "opencad.demo").is_none());
+        // Insert via ensure, then mutate.
+        *host::ensure_plugin_state(host, "opencad.demo", || 7u32) += 1;
+        assert_eq!(*host::plugin_state::<u32>(&*host, "opencad.demo").unwrap(), 8);
+        *host::plugin_state_mut::<u32>(host, "opencad.demo").unwrap() = 100;
+        assert_eq!(*host::plugin_state::<u32>(&*host, "opencad.demo").unwrap(), 100);
+    }
 }
\ No newline at end of file
diff --git a/src/modules/demo_plugin/dispatch.rs b/src/modules/demo_plugin/dispatch.rs
index d4344e0d..ddb2aab1 100644
--- a/src/modules/demo_plugin/dispatch.rs
+++ b/src/modules/demo_plugin/dispatch.rs
@@ -1,6 +1,6 @@
-use crate::plugin::host::HostSession;
+use crate::plugin::host::HostApi;
 
-pub fn handle(host: &mut HostSession<'_>, cmd: &str) -> bool {
+pub fn handle(host: &mut dyn HostApi, cmd: &str) -> bool {
     // "DP_IMPORT <path>" arrives from ModuleEvent::PluginFileDialog with the
     // path in its original case (the command line is bypassed).
     if let Some(path) = cmd.strip_prefix("DP_IMPORT ") {
diff --git a/src/modules/demo_plugin/plugin.rs b/src/modules/demo_plugin/plugin.rs
index 2d3e074e..1b9ec538 100644
--- a/src/modules/demo_plugin/plugin.rs
+++ b/src/modules/demo_plugin/plugin.rs
@@ -1,4 +1,4 @@
-use crate::plugin::host::{BuiltinPlugin, HostSession};
+use crate::plugin::host::{BuiltinPlugin, HostApi};
 use crate::plugin::manifest::PluginManifest;
 
 use super::dispatch;
@@ -15,7 +15,7 @@ impl BuiltinPlugin for DemoPlugin {
         Box::new(super::DemoPluginModule)
     }
 
-    fn dispatch(&self, host: &mut HostSession<'_>, cmd: &str) -> bool {
+    fn dispatch(&self, host: &mut dyn HostApi, cmd: &str) -> bool {
         dispatch::handle(host, cmd)
     }
 }
\ No newline at end of file
diff --git a/src/plugin/host.rs b/src/plugin/host.rs
index da1c53ad..f4ca3a50 100644
--- a/src/plugin/host.rs
+++ b/src/plugin/host.rs
@@ -1,6 +1,9 @@
-// Plugin traits — HostSession lives in `app::plugin_host` (same-crate field access).
+// Plugin traits — HostSession lives in `app::plugin_host` (same-crate field
+// access) and implements the stable `HostApi` contract plugins target.
 
 pub(crate) use crate::app::plugin_host::HostSession;
+/// The stable runtime surface a plugin's `dispatch` receives.
+pub use ocs_plugin_api::host::HostApi;
 
 use crate::modules::CadModule;
 
@@ -9,9 +12,11 @@ use super::manifest::PluginManifest;
 /// Add-on package entry point (phase 1: in-tree, in-process).
 ///
 /// One `PluginRegistration` per package — ribbon tab, manifest, and command
-/// dispatch are owned here. See `docs/plugin-architecture.md`.
+/// dispatch are owned here. `dispatch` receives `&mut dyn HostApi` (the stable
+/// `ocs_plugin_api` contract), not the host's concrete session type. See
+/// `docs/plugin-architecture.md`.
 pub trait BuiltinPlugin: Send + Sync {
     fn manifest(&self) -> &'static PluginManifest;
     fn ribbon(&self) -> Box<dyn CadModule>;
-    fn dispatch(&self, host: &mut HostSession<'_>, cmd: &str) -> bool;
+    fn dispatch(&self, host: &mut dyn HostApi, cmd: &str) -> bool;
 }
\ No newline at end of file