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fix(rmcp): flatten Resource variant of PromptMessageContent #843

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fix(rmcp): flatten Resource variant of PromptMessageContent #843

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56 changes: 55 additions & 1 deletion crates/rmcp/src/model/prompt.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -158,7 +158,10 @@ pub enum PromptMessageContent {
image: ImageContent,
},
/// Embedded server-side resource
Resource { resource: EmbeddedResource },
Resource {
#[serde(flatten)]
resource: EmbeddedResource,
},
/// A link to a resource that can be fetched separately
ResourceLink {
#[serde(flatten)]
Expand Down Expand Up @@ -321,6 +324,57 @@ mod tests {
assert!(json.contains("\"name\":\"test.txt\""));
}

#[test]
fn test_prompt_message_resource_serialization_is_flat() {
// Regression test: PromptMessageContent::Resource must serialize to
// the spec-compliant flat shape `{ "type": "resource", "resource": { "uri", "mimeType", "text" } }`
// and NOT the double-nested shape `{ "type": "resource", "resource": { "resource": {...} } }`.
// See: https://modelcontextprotocol.io/specification/2025-06-18/server/prompts
let message = PromptMessage::new_resource(
PromptMessageRole::User,
"alc://packages/sc/narrative".to_string(),
Some("text/markdown".to_string()),
Some("# Hello".to_string()),
None,
None,
None,
);

let value: serde_json::Value = serde_json::to_value(&message).unwrap();

// Drill into content
let content = value.get("content").expect("content present");
assert_eq!(
content.get("type").and_then(|v| v.as_str()),
Some("resource")
);

let resource = content
.get("resource")
.expect("resource field present at content level");

// Spec-compliant: resource.uri / resource.mimeType / resource.text MUST be flat
assert_eq!(
resource.get("uri").and_then(|v| v.as_str()),
Some("alc://packages/sc/narrative"),
"expected flat resource.uri, got: {resource:#?}"
);
assert_eq!(
resource.get("mimeType").and_then(|v| v.as_str()),
Some("text/markdown")
);
assert_eq!(
resource.get("text").and_then(|v| v.as_str()),
Some("# Hello")
);

// Regression guard: content.resource MUST NOT contain a nested `resource` key.
assert!(
resource.get("resource").is_none(),
"double-nested resource detected (regression): {resource:#?}"
);
}

#[test]
fn test_prompt_message_content_resource_link_deserialization() {
let json = r#"{
Expand Down
55 changes: 22 additions & 33 deletions crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json
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Original file line number Diff line number Diff line change
Expand Up @@ -140,35 +140,6 @@
]
},
"Annotated2": {
"type": "object",
"properties": {
"_meta": {
"description": "Optional protocol-level metadata for this content block",
"type": [
"object",
"null"
],
"additionalProperties": true
},
"annotations": {
"anyOf": [
{
"$ref": "#/definitions/Annotations"
},
{
"type": "null"
}
]
},
"resource": {
"$ref": "#/definitions/ResourceContents"
}
},
"required": [
"resource"
]
},
"Annotated3": {
"description": "Represents a resource in the extension with metadata",
"type": "object",
"properties": {
Expand Down Expand Up @@ -244,7 +215,7 @@
"name"
]
},
"Annotated4": {
"Annotated3": {
"type": "object",
"properties": {
"annotations": {
Expand Down Expand Up @@ -1475,7 +1446,7 @@
"resourceTemplates": {
"type": "array",
"items": {
"$ref": "#/definitions/Annotated4"
"$ref": "#/definitions/Annotated3"
}
}
},
Expand All @@ -1502,7 +1473,7 @@
"resources": {
"type": "array",
"items": {
"$ref": "#/definitions/Annotated3"
"$ref": "#/definitions/Annotated2"
}
}
},
Expand Down Expand Up @@ -2142,8 +2113,26 @@
"description": "Embedded server-side resource",
"type": "object",
"properties": {
"_meta": {
"description": "Optional protocol-level metadata for this content block",
"type": [
"object",
"null"
],
"additionalProperties": true
},
"annotations": {
"anyOf": [
{
"$ref": "#/definitions/Annotations"
},
{
"type": "null"
}
]
},
"resource": {
"$ref": "#/definitions/Annotated2"
"$ref": "#/definitions/ResourceContents"
},
"type": {
"type": "string",
Expand Down
55 changes: 22 additions & 33 deletions crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -140,35 +140,6 @@
]
},
"Annotated2": {
"type": "object",
"properties": {
"_meta": {
"description": "Optional protocol-level metadata for this content block",
"type": [
"object",
"null"
],
"additionalProperties": true
},
"annotations": {
"anyOf": [
{
"$ref": "#/definitions/Annotations"
},
{
"type": "null"
}
]
},
"resource": {
"$ref": "#/definitions/ResourceContents"
}
},
"required": [
"resource"
]
},
"Annotated3": {
"description": "Represents a resource in the extension with metadata",
"type": "object",
"properties": {
Expand Down Expand Up @@ -244,7 +215,7 @@
"name"
]
},
"Annotated4": {
"Annotated3": {
"type": "object",
"properties": {
"annotations": {
Expand Down Expand Up @@ -1475,7 +1446,7 @@
"resourceTemplates": {
"type": "array",
"items": {
"$ref": "#/definitions/Annotated4"
"$ref": "#/definitions/Annotated3"
}
}
},
Expand All @@ -1502,7 +1473,7 @@
"resources": {
"type": "array",
"items": {
"$ref": "#/definitions/Annotated3"
"$ref": "#/definitions/Annotated2"
}
}
},
Expand Down Expand Up @@ -2142,8 +2113,26 @@
"description": "Embedded server-side resource",
"type": "object",
"properties": {
"_meta": {
"description": "Optional protocol-level metadata for this content block",
"type": [
"object",
"null"
],
"additionalProperties": true
},
"annotations": {
"anyOf": [
{
"$ref": "#/definitions/Annotations"
},
{
"type": "null"
}
]
},
"resource": {
"$ref": "#/definitions/Annotated2"
"$ref": "#/definitions/ResourceContents"
},
"type": {
"type": "string",
Expand Down