fix(rmcp): flatten Resource variant of PromptMessageContent (#843)
The Resource variant of PromptMessageContent was missing #[serde(flatten)],
causing the embedded resource content block to serialize as a double-nested
shape `{ "type": "resource", "resource": { "resource": {...} } }` instead of
the spec-compliant flat shape `{ "type": "resource", "resource": {uri, mimeType, text} }`.
This caused Zod-based MCP clients (e.g. Claude Code) to reject prompts/get
responses containing embedded resource messages with InvalidUnion errors.
The Image and ResourceLink variants already use #[serde(flatten)] correctly;
only Resource was missing it.
Fix: add #[serde(flatten)] so EmbeddedResource (=Annotated<RawEmbeddedResource>)
fields _meta / annotations / resource are flattened to the content-block level,
matching the MCP spec for prompts embedded resources.
Regression test: test_prompt_message_resource_serialization_is_flat verifies
content.resource.uri is reachable and content.resource.resource is absent.
Schema snapshots regenerated via UPDATE_SCHEMA=1.
This commit is contained in:
Yutaka Nishimura
GitHub
parent
321ab14f67
commit
d83b1566d0
3 changed files with 99 additions and 67 deletions
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@ -158,7 +158,10 @@ pub enum PromptMessageContent {
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image: ImageContent,
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},
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/// Embedded server-side resource
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Resource { resource: EmbeddedResource },
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Resource {
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#[serde(flatten)]
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resource: EmbeddedResource,
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},
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/// A link to a resource that can be fetched separately
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ResourceLink {
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#[serde(flatten)]
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@ -321,6 +324,57 @@ mod tests {
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assert!(json.contains("\"name\":\"test.txt\""));
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}
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#[test]
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fn test_prompt_message_resource_serialization_is_flat() {
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// Regression test: PromptMessageContent::Resource must serialize to
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// the spec-compliant flat shape `{ "type": "resource", "resource": { "uri", "mimeType", "text" } }`
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// and NOT the double-nested shape `{ "type": "resource", "resource": { "resource": {...} } }`.
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// See: https://modelcontextprotocol.io/specification/2025-06-18/server/prompts
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let message = PromptMessage::new_resource(
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PromptMessageRole::User,
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"alc://packages/sc/narrative".to_string(),
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Some("text/markdown".to_string()),
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Some("# Hello".to_string()),
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None,
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None,
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None,
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);
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let value: serde_json::Value = serde_json::to_value(&message).unwrap();
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// Drill into content
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let content = value.get("content").expect("content present");
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assert_eq!(
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content.get("type").and_then(|v| v.as_str()),
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Some("resource")
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);
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let resource = content
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.get("resource")
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.expect("resource field present at content level");
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// Spec-compliant: resource.uri / resource.mimeType / resource.text MUST be flat
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assert_eq!(
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resource.get("uri").and_then(|v| v.as_str()),
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Some("alc://packages/sc/narrative"),
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"expected flat resource.uri, got: {resource:#?}"
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);
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assert_eq!(
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resource.get("mimeType").and_then(|v| v.as_str()),
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Some("text/markdown")
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);
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assert_eq!(
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resource.get("text").and_then(|v| v.as_str()),
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Some("# Hello")
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);
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// Regression guard: content.resource MUST NOT contain a nested `resource` key.
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assert!(
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resource.get("resource").is_none(),
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"double-nested resource detected (regression): {resource:#?}"
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);
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}
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#[test]
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fn test_prompt_message_content_resource_link_deserialization() {
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let json = r#"{
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@ -140,35 +140,6 @@
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]
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},
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"Annotated2": {
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"type": "object",
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"properties": {
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"_meta": {
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"description": "Optional protocol-level metadata for this content block",
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"type": [
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"object",
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"null"
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],
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"additionalProperties": true
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},
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"annotations": {
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"anyOf": [
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{
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"$ref": "#/definitions/Annotations"
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},
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{
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"type": "null"
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}
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]
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},
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"resource": {
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"$ref": "#/definitions/ResourceContents"
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}
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},
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"required": [
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"resource"
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]
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},
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"Annotated3": {
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"description": "Represents a resource in the extension with metadata",
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"type": "object",
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"properties": {
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@ -244,7 +215,7 @@
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"name"
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]
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},
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"Annotated4": {
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"Annotated3": {
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"type": "object",
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"properties": {
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"annotations": {
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@ -1481,7 +1452,7 @@
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"resourceTemplates": {
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"type": "array",
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"items": {
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"$ref": "#/definitions/Annotated4"
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"$ref": "#/definitions/Annotated3"
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}
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}
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},
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@ -1508,7 +1479,7 @@
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"resources": {
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"type": "array",
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"items": {
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"$ref": "#/definitions/Annotated3"
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"$ref": "#/definitions/Annotated2"
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}
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}
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},
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@ -2148,8 +2119,26 @@
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"description": "Embedded server-side resource",
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"type": "object",
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"properties": {
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"_meta": {
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"description": "Optional protocol-level metadata for this content block",
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"type": [
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"object",
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"null"
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],
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"additionalProperties": true
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},
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"annotations": {
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"anyOf": [
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{
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"$ref": "#/definitions/Annotations"
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},
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{
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"type": "null"
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}
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]
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},
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"resource": {
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"$ref": "#/definitions/Annotated2"
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"$ref": "#/definitions/ResourceContents"
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},
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"type": {
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"type": "string",
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@ -140,35 +140,6 @@
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]
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},
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"Annotated2": {
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"type": "object",
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"properties": {
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"_meta": {
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"description": "Optional protocol-level metadata for this content block",
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"type": [
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"object",
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"null"
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],
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"additionalProperties": true
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},
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"annotations": {
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"anyOf": [
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{
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"$ref": "#/definitions/Annotations"
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},
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{
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"type": "null"
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}
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]
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},
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"resource": {
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"$ref": "#/definitions/ResourceContents"
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}
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},
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"required": [
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"resource"
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]
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},
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"Annotated3": {
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"description": "Represents a resource in the extension with metadata",
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"type": "object",
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"properties": {
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@ -244,7 +215,7 @@
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"name"
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]
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},
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"Annotated4": {
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"Annotated3": {
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"type": "object",
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"properties": {
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"annotations": {
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@ -1481,7 +1452,7 @@
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"resourceTemplates": {
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"type": "array",
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"items": {
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"$ref": "#/definitions/Annotated4"
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"$ref": "#/definitions/Annotated3"
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}
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}
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},
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@ -1508,7 +1479,7 @@
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"resources": {
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"type": "array",
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"items": {
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"$ref": "#/definitions/Annotated3"
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"$ref": "#/definitions/Annotated2"
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}
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}
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},
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@ -2148,8 +2119,26 @@
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"description": "Embedded server-side resource",
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"type": "object",
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"properties": {
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"_meta": {
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"description": "Optional protocol-level metadata for this content block",
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"type": [
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"object",
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"null"
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],
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"additionalProperties": true
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},
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"annotations": {
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"anyOf": [
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{
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"$ref": "#/definitions/Annotations"
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},
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{
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"type": "null"
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}
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]
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},
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"resource": {
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"$ref": "#/definitions/Annotated2"
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"$ref": "#/definitions/ResourceContents"
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},
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"type": {
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"type": "string",
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