diff --git a/conformance/agent/express/compiler.yaml b/conformance/agent/express/compiler.yaml
index 65d6e1fb85..b88c2c99a5 100644
--- a/conformance/agent/express/compiler.yaml
+++ b/conformance/agent/express/compiler.yaml
@@ -463,6 +463,252 @@
             component: "Text"
             text: "Submit"
 
+- name: test_compile_express_event_user_message
+  description: A userMessage keyword argument on an Event compiles to the v1.0 event.userMessage field beside the name.
+  action: compile
+  args:
+    format: express
+    catalog: "test_data/catalogs/simplified_catalog_v1_0.json"
+  input: |-
+    root = Button(label, Event("reply", userMessage="Replying now"))
+    label = Text("Reply")
+  expect:
+    - version: "v1.0"
+      createSurface:
+        surfaceId: "default_surface"
+        catalogId: "conformance/simplified"
+        components:
+          - id: "root"
+            component: "Button"
+            child: "label"
+            action:
+              event:
+                name: "reply"
+                userMessage: "Replying now"
+          - id: "label"
+            component: "Text"
+            text: "Reply"
+
+- name: test_compile_express_event_user_message_binding_with_context
+  description: A userMessage given as a data binding compiles to a path beside a positional context map.
+  action: compile
+  args:
+    format: express
+    catalog: "test_data/catalogs/simplified_catalog_v1_0.json"
+  input: |-
+    root = Button(label, Event("reply", {threadId: "t1"}, userMessage=$/draft/message))
+    label = Text("Reply")
+  expect:
+    - version: "v1.0"
+      createSurface:
+        surfaceId: "default_surface"
+        catalogId: "conformance/simplified"
+        components:
+          - id: "root"
+            component: "Button"
+            child: "label"
+            action:
+              event:
+                name: "reply"
+                userMessage:
+                  "@path": "/draft/message"
+                context:
+                  threadId: "t1"
+          - id: "label"
+            component: "Text"
+            text: "Reply"
+
+- name: test_compile_express_event_user_message_function_call
+  description: A userMessage given as a function call compiles to a function call, since the v1.0 field is a DynamicString, which a string, a data binding or a function call satisfies.
+  action: compile
+  args:
+    format: express
+    catalog: "test_data/catalogs/simplified_catalog_v1_0.json"
+  input: |-
+    root = Button(label, Event("reply", userMessage=formatString("Replying to ${/thread/title}")))
+    label = Text("Reply")
+  expect:
+    - version: "v1.0"
+      createSurface:
+        surfaceId: "default_surface"
+        catalogId: "conformance/simplified"
+        components:
+          - id: "root"
+            component: "Button"
+            child: "label"
+            action:
+              event:
+                name: "reply"
+                userMessage:
+                  "@call": "formatString"
+                  args:
+                    value: "Replying to ${/thread/title}"
+          - id: "label"
+            component: "Text"
+            text: "Reply"
+
+- name: test_compile_express_event_user_message_that_is_not_a_dynamic_string_is_a_validation_error
+  description: A userMessage that is none of a string, a data binding or a function call, such as a number or a map literal, is a validation error.
+  action: compile
+  args:
+    format: express
+    catalog: "test_data/catalogs/simplified_catalog_v1_0.json"
+  input: |-
+    root = Button(label, Event("reply", userMessage={text: "Replying now"}))
+    label = Text("Reply")
+  expect_error:
+    category: "ValidationError"
+
+- name: test_compile_express_event_context_keyword
+  description: The context map of an Event can be given by keyword as well as positionally.
+  action: compile
+  args:
+    format: express
+    catalog: "test_data/catalogs/simplified_catalog_v1_0.json"
+  input: |-
+    root = Button(label, Event("submit", context={itemId: "42"}))
+    label = Text("Submit")
+  expect:
+    - version: "v1.0"
+      createSurface:
+        surfaceId: "default_surface"
+        catalogId: "conformance/simplified"
+        components:
+          - id: "root"
+            component: "Button"
+            child: "label"
+            action:
+              event:
+                name: "submit"
+                context:
+                  itemId: "42"
+          - id: "label"
+            component: "Text"
+            text: "Submit"
+
+- name: test_compile_express_event_context_keeps_a_function_call_a_plain_call
+  description: The entries of an Event's context are DynamicValues, not actions, so a function call among them compiles to a plain function call even though the Event itself sits in an action slot.
+  action: compile
+  args:
+    format: express
+    catalog: "test_data/catalogs/simplified_catalog_v1_0.json"
+  input: |-
+    root = Button(label, Event("go", {greeting: formatString("Hi ${/name}")}))
+    label = Text("Go")
+  expect:
+    - version: "v1.0"
+      createSurface:
+        surfaceId: "default_surface"
+        catalogId: "conformance/simplified"
+        components:
+          - id: "root"
+            component: "Button"
+            child: "label"
+            action:
+              event:
+                name: "go"
+                context:
+                  greeting:
+                    "@call": "formatString"
+                    args:
+                      value: "Hi ${/name}"
+          - id: "label"
+            component: "Text"
+            text: "Go"
+
+- name: test_compile_express_event_unknown_keyword_is_a_validation_error
+  description: A keyword argument that Event does not define is a validation error rather than being dropped from the compiled action.
+  action: compile
+  args:
+    format: express
+    catalog: "test_data/catalogs/simplified_catalog_v1_0.json"
+  input: |-
+    root = Button(label, Event("reply", message="Replying now"))
+    label = Text("Reply")
+  expect_error:
+    category: "ValidationError"
+
+- name: test_compile_express_event_duplicate_context_is_a_validation_error
+  description: Giving the Event context both positionally and by keyword is a validation error.
+  action: compile
+  args:
+    format: express
+    catalog: "test_data/catalogs/simplified_catalog_v1_0.json"
+  input: |-
+    root = Button(label, Event("submit", {a: 1}, context={b: 2}))
+    label = Text("Submit")
+  expect_error:
+    category: "ValidationError"
+
+- name: test_compile_express_event_user_message_before_v1_is_a_validation_error
+  description: Before v1.0 the protocol's Action has no userMessage field, so an Event carrying one is a validation error instead of a silently dropped argument.
+  action: compile
+  args:
+    format: express
+    catalog: "test_data/catalogs/list_item_action_catalog_v09.json"
+  input: |-
+    root = Links([{label: "Reply", action: Event("reply", userMessage="Replying now")}])
+  expect_error:
+    category: "ValidationError"
+
+- name: test_compile_express_function_call_in_list_item_action_slot
+  description: A function call in an action slot nested inside a list item compiles to a functionCall action, the same as in a top-level action property, rather than to a bare function call the processor rejects.
+  action: compile
+  args:
+    format: express
+    catalog: "test_data/catalogs/list_item_action_catalog_v1_0.json"
+  input: |-
+    root = Links([{label: "Docs", action: openUrl("https://a2ui.org")}, {label: "Go", action: Event("go")}], primary=openUrl("https://a2ui.org/start"))
+  expect:
+    - version: "v1.0"
+      createSurface:
+        surfaceId: "default_surface"
+        catalogId: "conformance/list_item_action"
+        components:
+          - id: "root"
+            component: "Links"
+            items:
+              - label: "Docs"
+                action:
+                  functionCall:
+                    "@call": "openUrl"
+                    args:
+                      url: "https://a2ui.org"
+              - label: "Go"
+                action:
+                  event:
+                    name: "go"
+            primary:
+              functionCall:
+                "@call": "openUrl"
+                args:
+                  url: "https://a2ui.org/start"
+
+- name: test_compile_express_function_call_in_list_item_action_slot_through_variable
+  description: The nested action slot is found when the list item is reached through a variable, since the variable's value is compiled against the slot's schema.
+  action: compile
+  args:
+    format: express
+    catalog: "test_data/catalogs/list_item_action_catalog_v1_0.json"
+  input: |-
+    docs = {label: "Docs", action: openUrl("https://a2ui.org")}
+    root = Links([docs])
+  expect:
+    - version: "v1.0"
+      createSurface:
+        surfaceId: "default_surface"
+        catalogId: "conformance/list_item_action"
+        components:
+          - id: "root"
+            component: "Links"
+            items:
+              - label: "Docs"
+                action:
+                  functionCall:
+                    "@call": "openUrl"
+                    args:
+                      url: "https://a2ui.org"
+
 - name: test_compile_express_data_model_assignment
   description: A data path assignment beside a root fills the surface's initial data model.
   action: compile
diff --git a/conformance/agent/express/decompiler.yaml b/conformance/agent/express/decompiler.yaml
index 75418323e6..d575ce6ac9 100644
--- a/conformance/agent/express/decompiler.yaml
+++ b/conformance/agent/express/decompiler.yaml
@@ -602,6 +602,107 @@
     - 'Event("save", {userId: "u123", confirmed: true, count: 5})'
   expect_round_trip: true
 
+- name: test_decompile_express_event_action_with_user_message
+  description: An event action carrying a userMessage writes it out as a userMessage= keyword argument, after the context map when there is one, so the message survives a round trip instead of being dropped.
+  action: decompile
+  args:
+    format: express
+    catalog: "test_data/catalogs/simplified_catalog_v1_0.json"
+  messages:
+    - version: "v1.0"
+      createSurface:
+        surfaceId: "s1"
+        catalogId: "conformance/simplified"
+        components:
+          - id: "root"
+            component: "Column"
+            children: ["plain", "withContext"]
+          - id: "plain"
+            component: "Button"
+            child: "plainText"
+            action:
+              event:
+                name: "reply"
+                userMessage: "Replying now"
+          - id: "plainText"
+            component: "Text"
+            text: "Reply"
+          - id: "withContext"
+            component: "Button"
+            child: "withContextText"
+            action:
+              event:
+                name: "reply"
+                userMessage:
+                  "@path": "/draft/message"
+                context:
+                  threadId: "t1"
+          - id: "withContextText"
+            component: "Text"
+            text: "Reply from draft"
+  expect_contains:
+    - 'Event("reply", userMessage="Replying now")'
+    - 'Event("reply", {threadId: "t1"}, userMessage=$/draft/message)'
+  expect_round_trip: true
+
+- name: test_decompile_express_event_action_with_function_call_user_message
+  description: An event whose userMessage is a function call is still written as an Event, with the call as the userMessage= argument, so it compiles back to the same action.
+  action: decompile
+  args:
+    format: express
+    catalog: "test_data/catalogs/simplified_catalog_v1_0.json"
+  messages:
+    - version: "v1.0"
+      createSurface:
+        surfaceId: "s1"
+        catalogId: "conformance/simplified"
+        components:
+          - id: "root"
+            component: "Button"
+            child: "bText"
+            action:
+              event:
+                name: "reply"
+                userMessage:
+                  "@call": "formatString"
+                  args:
+                    value: "Replying to ${/thread/title}"
+          - id: "bText"
+            component: "Text"
+            text: "Reply"
+  expect_contains:
+    - 'Event("reply", userMessage=formatString("Replying to ${/thread/title}"))'
+  expect_round_trip: true
+
+- name: test_decompile_express_function_call_in_list_item_action_slot
+  description: A functionCall action nested inside a list item writes out as a plain function call in the item's map literal, which the compiler reads back into the same functionCall because the slot is an action.
+  action: decompile
+  args:
+    format: express
+    catalog: "test_data/catalogs/list_item_action_catalog_v1_0.json"
+  messages:
+    - version: "v1.0"
+      createSurface:
+        surfaceId: "s1"
+        catalogId: "conformance/list_item_action"
+        components:
+          - id: "root"
+            component: "Links"
+            items:
+              - label: "Docs"
+                action:
+                  functionCall:
+                    "@call": "openUrl"
+                    args:
+                      url: "https://a2ui.org"
+              - label: "Go"
+                action:
+                  event:
+                    name: "go"
+  expect_contains:
+    - 'root = Links([{label: "Docs", action: openUrl("https://a2ui.org")}, {label: "Go", action: Event("go")}])'
+  expect_round_trip: true
+
 - name: test_decompile_express_event_action_with_nested_context
   description: A nested map inside an event's context writes out as a nested Express map literal rather than being flattened into path assignments.
   action: decompile
diff --git a/conformance/test_data/catalogs/list_item_action_catalog_v09.json b/conformance/test_data/catalogs/list_item_action_catalog_v09.json
new file mode 100644
index 0000000000..d3027f363a
--- /dev/null
+++ b/conformance/test_data/catalogs/list_item_action_catalog_v09.json
@@ -0,0 +1,67 @@
+{
+  "$id": "https://a2ui.org/conformance/test_data/catalogs/list_item_action_catalog_v09.json",
+  "catalogId": "conformance/list_item_action",
+  "protocolVersion": "0.9",
+  "components": {
+    "Text": {
+      "type": "object",
+      "allOf": [
+        { "$ref": "common_types.json#/$defs/ComponentCommon" }
+      ],
+      "properties": {
+        "component": { "const": "Text" },
+        "text": { "$ref": "common_types.json#/$defs/DynamicString" }
+      },
+      "required": [ "component", "text" ]
+    },
+    "Links": {
+      "type": "object",
+      "allOf": [
+        { "$ref": "common_types.json#/$defs/ComponentCommon" }
+      ],
+      "properties": {
+        "component": { "const": "Links" },
+        "items": {
+          "type": "array",
+          "items": {
+            "type": "object",
+            "properties": {
+              "label": { "type": "string" },
+              "action": { "$ref": "common_types.json#/$defs/Action" }
+            },
+            "required": [ "label", "action" ]
+          }
+        },
+        "primary": { "$ref": "common_types.json#/$defs/Action" }
+      },
+      "required": [ "component", "items" ]
+    }
+  },
+  "functions": {
+    "openUrl": {
+      "type": "object",
+      "description": "Opens a URL.",
+      "returnType": "void",
+      "properties": {
+        "call": { "const": "openUrl" },
+        "args": {
+          "type": "object",
+          "properties": {
+            "url": { "type": "string" }
+          },
+          "required": [ "url" ]
+        }
+      },
+      "required": [ "call", "args" ]
+    }
+  },
+  "$defs": {
+    "anyComponent": {
+      "oneOf": [
+        { "$ref": "#/components/Text" },
+        { "$ref": "#/components/Links" }
+      ],
+      "discriminator": { "propertyName": "component" }
+    }
+  }
+}
diff --git a/conformance/test_data/catalogs/list_item_action_catalog_v1_0.json b/conformance/test_data/catalogs/list_item_action_catalog_v1_0.json
new file mode 100644
index 0000000000..ba7ccdc4e2
--- /dev/null
+++ b/conformance/test_data/catalogs/list_item_action_catalog_v1_0.json
@@ -0,0 +1,58 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://a2ui.org/conformance/test_data/catalogs/list_item_action_catalog_v1_0.json",
+  "protocolVersion": "1.0",
+  "title": "Conformance List Item Action Catalog (v1.0)",
+  "description": "A catalog whose component carries actions inside list items, for testing that an Express function call in a nested action slot compiles to a functionCall action.",
+  "catalogId": "conformance/list_item_action",
+  "components": {
+    "Text": {
+      "type": "object",
+      "description": "Displays a run of text.",
+      "properties": {
+        "component": { "const": "Text" },
+        "text": { "$ref": "common_types.json#/$defs/DynamicString" }
+      },
+      "required": ["component", "text"]
+    },
+    "Links": {
+      "type": "object",
+      "description": "A list of labelled links, each firing an action when chosen.",
+      "properties": {
+        "component": { "const": "Links" },
+        "items": {
+          "type": "array",
+          "items": {
+            "type": "object",
+            "properties": {
+              "label": { "type": "string" },
+              "action": { "$ref": "common_types.json#/$defs/Action" }
+            },
+            "required": ["label", "action"]
+          }
+        },
+        "primary": { "$ref": "common_types.json#/$defs/Action" }
+      },
+      "required": ["component", "items"]
+    }
+  },
+  "functions": {
+    "openUrl": {
+      "type": "object",
+      "description": "Opens a URL.",
+      "returnType": "void",
+      "properties": {
+        "@call": { "const": "openUrl" },
+        "args": {
+          "type": "object",
+          "properties": {
+            "url": { "type": "string" }
+          },
+          "required": ["url"],
+          "unevaluatedProperties": false
+        }
+      },
+      "required": ["@call", "args"]
+    }
+  }
+}