refactor: standardize resource response format

Introduce ReadResourceContents type to properly handle MIME types in resource responses. Breaking change in FastMCP read_resource() return type.

Github-Issue:#152

tests/issues/test_152_resource_mime_type.py

@@ -6,6 +6,7 @@ from pydantic import AnyUrl
 from mcp import types
 from mcp.server.fastmcp import FastMCP
 from mcp.server.lowlevel import Server
+from mcp.server.lowlevel.helper_types import ReadResourceContents
 from mcp.shared.memory import (
     create_connected_server_and_client_session as client_session,
 )
@@ -98,9 +99,11 @@ async def test_lowlevel_resource_mime_type():
     @server.read_resource()
     async def handle_read_resource(uri: AnyUrl):
         if str(uri) == "test://image":
-            return (base64_string, "image/png")
+            return ReadResourceContents(content=base64_string, mime_type="image/png")
         elif str(uri) == "test://image_bytes":
-            return (bytes(image_bytes), "image/png")
+            return ReadResourceContents(
+                content=bytes(image_bytes), mime_type="image/png"
+            )
         raise Exception(f"Resource not found: {uri}")
 
     # Test that resources are listed with correct mime type

tests/server/fastmcp/servers/test_file_server.py

@@ -88,10 +88,10 @@ async def test_list_resources(mcp: FastMCP):
 
 @pytest.mark.anyio
 async def test_read_resource_dir(mcp: FastMCP):
-    files, mime_type = await mcp.read_resource("dir://test_dir")
-    assert mime_type == "text/plain"
+    res = await mcp.read_resource("dir://test_dir")
+    assert res.mime_type == "text/plain"
 
-    files = json.loads(files)
+    files = json.loads(res.content)
 
     assert sorted([Path(f).name for f in files]) == [
         "config.json",
@@ -102,8 +102,8 @@ async def test_read_resource_dir(mcp: FastMCP):
 
 @pytest.mark.anyio
 async def test_read_resource_file(mcp: FastMCP):
-    result, _ = await mcp.read_resource("file://test_dir/example.py")
-    assert result == "print('hello world')"
+    res = await mcp.read_resource("file://test_dir/example.py")
+    assert res.content == "print('hello world')"
 
 
 @pytest.mark.anyio
@@ -119,5 +119,5 @@ async def test_delete_file_and_check_resources(mcp: FastMCP, test_dir: Path):
     await mcp.call_tool(
         "delete_file", arguments=dict(path=str(test_dir / "example.py"))
     )
-    result, _ = await mcp.read_resource("file://test_dir/example.py")
-    assert result == "File not found"
+    res = await mcp.read_resource("file://test_dir/example.py")
+    assert res.content == "File not found"

tests/server/fastmcp/test_server.py

@@ -581,8 +581,8 @@ class TestContextInjection:
 
         @mcp.tool()
         async def tool_with_resource(ctx: Context) -> str:
-            data, mime_type = await ctx.read_resource("test://data")
-            return f"Read resource: {data} with mime type {mime_type}"
+            r = await ctx.read_resource("test://data")
+            return f"Read resource: {r.content} with mime type {r.mime_type}"
 
         async with client_session(mcp._mcp_server) as client:
             result = await client.call_tool("tool_with_resource", {})

tests/server/test_read_resource.py

@@ -0,0 +1,109 @@
+from pathlib import Path
+from tempfile import NamedTemporaryFile
+
+import pytest
+from pydantic import AnyUrl, FileUrl
+
+import mcp.types as types
+from mcp.server.lowlevel.server import ReadResourceContents, Server
+
+
+@pytest.fixture
+def temp_file():
+    """Create a temporary file for testing."""
+    with NamedTemporaryFile(mode="w", delete=False) as f:
+        f.write("test content")
+        path = Path(f.name).resolve()
+    yield path
+    try:
+        path.unlink()
+    except FileNotFoundError:
+        pass
+
+
+@pytest.mark.anyio
+async def test_read_resource_text(temp_file: Path):
+    server = Server("test")
+
+    @server.read_resource()
+    async def read_resource(uri: AnyUrl) -> ReadResourceContents:
+        return ReadResourceContents(content="Hello World", mime_type="text/plain")
+
+    # Get the handler directly from the server
+    handler = server.request_handlers[types.ReadResourceRequest]
+
+    # Create a request
+    request = types.ReadResourceRequest(
+        method="resources/read",
+        params=types.ReadResourceRequestParams(uri=FileUrl(temp_file.as_uri())),
+    )
+
+    # Call the handler
+    result = await handler(request)
+    assert isinstance(result.root, types.ReadResourceResult)
+    assert len(result.root.contents) == 1
+
+    content = result.root.contents[0]
+    assert isinstance(content, types.TextResourceContents)
+    assert content.text == "Hello World"
+    assert content.mimeType == "text/plain"
+
+
+@pytest.mark.anyio
+async def test_read_resource_binary(temp_file: Path):
+    server = Server("test")
+
+    @server.read_resource()
+    async def read_resource(uri: AnyUrl) -> ReadResourceContents:
+        return ReadResourceContents(
+            content=b"Hello World", mime_type="application/octet-stream"
+        )
+
+    # Get the handler directly from the server
+    handler = server.request_handlers[types.ReadResourceRequest]
+
+    # Create a request
+    request = types.ReadResourceRequest(
+        method="resources/read",
+        params=types.ReadResourceRequestParams(uri=FileUrl(temp_file.as_uri())),
+    )
+
+    # Call the handler
+    result = await handler(request)
+    assert isinstance(result.root, types.ReadResourceResult)
+    assert len(result.root.contents) == 1
+
+    content = result.root.contents[0]
+    assert isinstance(content, types.BlobResourceContents)
+    assert content.mimeType == "application/octet-stream"
+
+
+@pytest.mark.anyio
+async def test_read_resource_default_mime(temp_file: Path):
+    server = Server("test")
+
+    @server.read_resource()
+    async def read_resource(uri: AnyUrl) -> ReadResourceContents:
+        return ReadResourceContents(
+            content="Hello World",
+            # No mime_type specified, should default to text/plain
+        )
+
+    # Get the handler directly from the server
+    handler = server.request_handlers[types.ReadResourceRequest]
+
+    # Create a request
+    request = types.ReadResourceRequest(
+        method="resources/read",
+        params=types.ReadResourceRequestParams(uri=FileUrl(temp_file.as_uri())),
+    )
+
+    # Call the handler
+    result = await handler(request)
+    assert isinstance(result.root, types.ReadResourceResult)
+    assert len(result.root.contents) == 1
+
+    content = result.root.contents[0]
+    assert isinstance(content, types.TextResourceContents)
+    assert content.text == "Hello World"
+    assert content.mimeType == "text/plain"