Add resource Link

src/mcp/types.py

@@ -733,14 +733,28 @@ class EmbeddedResource(BaseModel):
     model_config = ConfigDict(extra="allow")
 
 
-Content = TextContent | ImageContent | AudioContent | EmbeddedResource
+class ResourceLink(Resource):
+    """
+    A resource that the server is capable of reading, included in a prompt or tool call result.
+
+    Note: resource links returned by tools are not guaranteed to appear in the results of `resources/list` requests.
+    """
+
+    type: Literal["resource_link"]
+
+
+ContentBlock = TextContent | ImageContent | AudioContent | ResourceLink | EmbeddedResource
+"""A content block that can be used in prompts and tool results."""
+
+Content: TypeAlias = ContentBlock
+# """DEPRECATED: Content is deprecated, you should use ContentBlock directly."""
 
 
 class PromptMessage(BaseModel):
     """Describes a message returned as part of a prompt."""
 
     role: Role
-    content: Content
+    content: ContentBlock
     model_config = ConfigDict(extra="allow")
 
 
@@ -859,7 +873,7 @@ class CallToolRequest(Request[CallToolRequestParams, Literal["tools/call"]]):
 class CallToolResult(Result):
     """The server's response to a tool call."""
 
-    content: list[Content]
+    content: list[ContentBlock]
     isError: bool = False
 
 

tests/server/fastmcp/test_integration.py

@@ -37,6 +37,7 @@
     ProgressNotification,
     PromptReference,
     ReadResourceResult,
+    ResourceLink,
     ResourceListChangedNotification,
     ResourceTemplateReference,
     SamplingMessage,
@@ -147,6 +148,25 @@ async def tool_with_progress(message: str, ctx: Context, steps: int = 3) -> str:
     def echo(message: str) -> str:
         return f"Echo: {message}"
 
+    # Tool that returns ResourceLinks
+    @mcp.tool(description="Lists files and returns resource links", title="List Files Tool")
+    def list_files() -> list[ResourceLink]:
+        """Returns a list of resource links for files matching the pattern."""
+
+        # Mock some file resources for testing
+        file_resources = [
+            {
+                "type": "resource_link",
+                "uri": "file:///project/README.md",
+                "name": "README.md",
+                "mimeType": "text/markdown",
+            }
+        ]
+
+        result: list[ResourceLink] = [ResourceLink.model_validate(file_json) for file_json in file_resources]
+
+        return result
+
     # Tool with sampling capability
     @mcp.tool(description="A tool that uses sampling to generate content", title="Sampling Tool")
     async def sampling_tool(prompt: str, ctx: Context) -> str:
@@ -753,7 +773,17 @@ async def call_all_mcp_features(session: ClientSession, collector: NotificationC
     assert isinstance(tool_result.content[0], TextContent)
     assert tool_result.content[0].text == "Echo: hello"
 
-    # 2. Tool with context (logging and progress)
+    # 2. Test tool that returns ResourceLinks
+    list_files_result = await session.call_tool("list_files")
+    assert len(list_files_result.content) == 1
+
+    # Rest should be ResourceLinks
+    content = list_files_result.content[0]
+    assert isinstance(content, ResourceLink)
+    assert str(content.uri).startswith("file:///")
+    assert content.name is not None
+    assert content.mimeType is not None
+
     # Test progress callback functionality
     progress_updates = []