Merge pull request #170 from modelcontextprotocol/fix/152-resource-mime-type

fix: respect resource mime type in responses

Author: dsp-ant

CLAUDE.md

@@ -25,16 +25,31 @@ This document contains critical information about working with this codebase. Fo
    - New features require tests
    - Bug fixes require regression tests
 
-4. Version Control
-   - Commit messages: conventional format (fix:, feat:)
-   - PR scope: minimal, focused changes
-   - PR requirements: description, test plan
-   - Always include issue numbers
-   - Quote handling:
-     ```bash
-     git commit -am "\"fix: message\""
-     gh pr create --title "\"title\"" --body "\"body\""
-     ```
+- For commits fixing bugs or adding features based on user reports add:
+  ```bash
+  git commit --trailer "Reported-by:<name>"
+  ```
+  Where `<name>` is the name of the user.
+
+- For commits related to a Github issue, add
+  ```bash
+  git commit --trailer "Github-Issue:#<number>"
+  ```
+- NEVER ever mention a `co-authored-by` or similar aspects. In particular, never
+  mention the tool used to create the commit message or PR.
+
+## Pull Requests
+
+- Create a detailed message of what changed. Focus on the high level description of
+  the problem it tries to solve, and how it is solved. Don't go into the specifics of the
+  code unless it adds clarity.
+
+- Always add `jerome3o-anthropic` and `jspahrsummers` as reviewer.
+
+- NEVER ever mention a `co-authored-by` or similar aspects. In particular, never
+  mention the tool used to create the commit message or PR.
+
+## Python Tools
 
 ## Code Formatting
 
@@ -96,4 +111,4 @@ This document contains critical information about working with this codebase. Fo
    - Keep changes minimal
    - Follow existing patterns
    - Document public APIs
-   - Test thoroughly
+   - Test thoroughly

README.md

@@ -218,7 +218,7 @@ async def long_task(files: list[str], ctx: Context) -> str:
     for i, file in enumerate(files):
         ctx.info(f"Processing {file}")
         await ctx.report_progress(i, len(files))
-        data = await ctx.read_resource(f"file://{file}")
+        data, mime_type = await ctx.read_resource(f"file://{file}")
     return "Processing complete"
 ```
 
@@ -436,7 +436,7 @@ async def run():
             tools = await session.list_tools()
 
             # Read a resource
-            resource = await session.read_resource("file://some/path")
+            content, mime_type = await session.read_resource("file://some/path")
 
             # Call a tool
             result = await session.call_tool("tool-name", arguments={"arg1": "value"})

src/mcp/server/fastmcp/server.py

@@ -20,6 +20,7 @@
 from mcp.server.fastmcp.utilities.logging import configure_logging, get_logger
 from mcp.server.fastmcp.utilities.types import Image
 from mcp.server.lowlevel import Server as MCPServer
+from mcp.server.lowlevel.helper_types import ReadResourceContents
 from mcp.server.sse import SseServerTransport
 from mcp.server.stdio import stdio_server
 from mcp.shared.context import RequestContext
@@ -197,14 +198,16 @@ async def list_resource_templates(self) -> list[MCPResourceTemplate]:
             for template in templates
         ]
 
-    async def read_resource(self, uri: AnyUrl | str) -> str | bytes:
+    async def read_resource(self, uri: AnyUrl | str) -> ReadResourceContents:
         """Read a resource by URI."""
+
         resource = await self._resource_manager.get_resource(uri)
         if not resource:
             raise ResourceError(f"Unknown resource: {uri}")
 
         try:
-            return await resource.read()
+            content = await resource.read()
+            return ReadResourceContents(content=content, mime_type=resource.mime_type)
         except Exception as e:
             logger.error(f"Error reading resource {uri}: {e}")
             raise ResourceError(str(e))
@@ -606,7 +609,7 @@ async def report_progress(
             progress_token=progress_token, progress=progress, total=total
         )
 
-    async def read_resource(self, uri: str | AnyUrl) -> str | bytes:
+    async def read_resource(self, uri: str | AnyUrl) -> ReadResourceContents:
         """Read a resource by URI.
 
         Args:

src/mcp/server/lowlevel/helper_types.py

@@ -0,0 +1,9 @@
+from dataclasses import dataclass
+
+
+@dataclass
+class ReadResourceContents:
+    """Contents returned from a read_resource call."""
+
+    content: str | bytes
+    mime_type: str | None = None

src/mcp/server/lowlevel/server.py

@@ -74,6 +74,7 @@ async def main():
 from pydantic import AnyUrl
 
 import mcp.types as types
+from mcp.server.lowlevel.helper_types import ReadResourceContents
 from mcp.server.models import InitializationOptions
 from mcp.server.session import ServerSession
 from mcp.server.stdio import stdio_server as stdio_server
@@ -252,25 +253,45 @@ async def handler(_: Any):
         return decorator
 
     def read_resource(self):
-        def decorator(func: Callable[[AnyUrl], Awaitable[str | bytes]]):
+        def decorator(
+            func: Callable[[AnyUrl], Awaitable[str | bytes | ReadResourceContents]],
+        ):
             logger.debug("Registering handler for ReadResourceRequest")
 
             async def handler(req: types.ReadResourceRequest):
                 result = await func(req.params.uri)
+
+                def create_content(data: str | bytes, mime_type: str | None):
+                    match data:
+                        case str() as data:
+                            return types.TextResourceContents(
+                                uri=req.params.uri,
+                                text=data,
+                                mimeType=mime_type or "text/plain",
+                            )
+                        case bytes() as data:
+                            import base64
+
+                            return types.BlobResourceContents(
+                                uri=req.params.uri,
+                                blob=base64.urlsafe_b64encode(data).decode(),
+                                mimeType=mime_type or "application/octet-stream",
+                            )
+
                 match result:
-                    case str(s):
-                        content = types.TextResourceContents(
-                            uri=req.params.uri,
-                            text=s,
-                            mimeType="text/plain",
+                    case str() | bytes() as data:
+                        warnings.warn(
+                            "Returning str or bytes from read_resource is deprecated. "
+                            "Use ReadResourceContents instead.",
+                            DeprecationWarning,
+                            stacklevel=2,
                         )
-                    case bytes(b):
-                        import base64
-
-                        content = types.BlobResourceContents(
-                            uri=req.params.uri,
-                            blob=base64.urlsafe_b64encode(b).decode(),
-                            mimeType="application/octet-stream",
+                        content = create_content(data, None)
+                    case ReadResourceContents() as contents:
+                        content = create_content(contents.content, contents.mime_type)
+                    case _:
+                        raise ValueError(
+                            f"Unexpected return type from read_resource: {type(result)}"
                         )
 
                 return types.ServerResult(

tests/issues/test_152_resource_mime_type.py

@@ -0,0 +1,146 @@
+import base64
+
+import pytest
+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,
+)
+
+pytestmark = pytest.mark.anyio
+
+
+async def test_fastmcp_resource_mime_type():
+    """Test that mime_type parameter is respected for resources."""
+    mcp = FastMCP("test")
+
+    # Create a small test image as bytes
+    image_bytes = b"fake_image_data"
+    base64_string = base64.b64encode(image_bytes).decode("utf-8")
+
+    @mcp.resource("test://image", mime_type="image/png")
+    def get_image_as_string() -> str:
+        """Return a test image as base64 string."""
+        return base64_string
+
+    @mcp.resource("test://image_bytes", mime_type="image/png")
+    def get_image_as_bytes() -> bytes:
+        """Return a test image as bytes."""
+        return image_bytes
+
+    # Test that resources are listed with correct mime type
+    async with client_session(mcp._mcp_server) as client:
+        # List resources and verify mime types
+        resources = await client.list_resources()
+        assert resources.resources is not None
+
+        mapping = {str(r.uri): r for r in resources.resources}
+
+        # Find our resources
+        string_resource = mapping["test://image"]
+        bytes_resource = mapping["test://image_bytes"]
+
+        # Verify mime types
+        assert (
+            string_resource.mimeType == "image/png"
+        ), "String resource mime type not respected"
+        assert (
+            bytes_resource.mimeType == "image/png"
+        ), "Bytes resource mime type not respected"
+
+        # Also verify the content can be read correctly
+        string_result = await client.read_resource(AnyUrl("test://image"))
+        assert len(string_result.contents) == 1
+        assert (
+            getattr(string_result.contents[0], "text") == base64_string
+        ), "Base64 string mismatch"
+        assert (
+            string_result.contents[0].mimeType == "image/png"
+        ), "String content mime type not preserved"
+
+        bytes_result = await client.read_resource(AnyUrl("test://image_bytes"))
+        assert len(bytes_result.contents) == 1
+        assert (
+            base64.b64decode(getattr(bytes_result.contents[0], "blob")) == image_bytes
+        ), "Bytes mismatch"
+        assert (
+            bytes_result.contents[0].mimeType == "image/png"
+        ), "Bytes content mime type not preserved"
+
+
+async def test_lowlevel_resource_mime_type():
+    """Test that mime_type parameter is respected for resources."""
+    server = Server("test")
+
+    # Create a small test image as bytes
+    image_bytes = b"fake_image_data"
+    base64_string = base64.b64encode(image_bytes).decode("utf-8")
+
+    # Create test resources with specific mime types
+    test_resources = [
+        types.Resource(
+            uri=AnyUrl("test://image"), name="test image", mimeType="image/png"
+        ),
+        types.Resource(
+            uri=AnyUrl("test://image_bytes"),
+            name="test image bytes",
+            mimeType="image/png",
+        ),
+    ]
+
+    @server.list_resources()
+    async def handle_list_resources():
+        return test_resources
+
+    @server.read_resource()
+    async def handle_read_resource(uri: AnyUrl):
+        if str(uri) == "test://image":
+            return ReadResourceContents(content=base64_string, mime_type="image/png")
+        elif str(uri) == "test://image_bytes":
+            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
+    async with client_session(server) as client:
+        # List resources and verify mime types
+        resources = await client.list_resources()
+        assert resources.resources is not None
+
+        mapping = {str(r.uri): r for r in resources.resources}
+
+        # Find our resources
+        string_resource = mapping["test://image"]
+        bytes_resource = mapping["test://image_bytes"]
+
+        # Verify mime types
+        assert (
+            string_resource.mimeType == "image/png"
+        ), "String resource mime type not respected"
+        assert (
+            bytes_resource.mimeType == "image/png"
+        ), "Bytes resource mime type not respected"
+
+        # Also verify the content can be read correctly
+        string_result = await client.read_resource(AnyUrl("test://image"))
+        assert len(string_result.contents) == 1
+        assert (
+            getattr(string_result.contents[0], "text") == base64_string
+        ), "Base64 string mismatch"
+        assert (
+            string_result.contents[0].mimeType == "image/png"
+        ), "String content mime type not preserved"
+
+        bytes_result = await client.read_resource(AnyUrl("test://image_bytes"))
+        assert len(bytes_result.contents) == 1
+        assert (
+            base64.b64decode(getattr(bytes_result.contents[0], "blob")) == image_bytes
+        ), "Bytes mismatch"
+        assert (
+            bytes_result.contents[0].mimeType == "image/png"
+        ), "Bytes content mime type not preserved"

tests/server/fastmcp/servers/test_file_server.py

@@ -88,8 +88,10 @@ async def test_list_resources(mcp: FastMCP):
 
 @pytest.mark.anyio
 async def test_read_resource_dir(mcp: FastMCP):
-    files = await mcp.read_resource("dir://test_dir")
-    files = json.loads(files)
+    res = await mcp.read_resource("dir://test_dir")
+    assert res.mime_type == "text/plain"
+
+    files = json.loads(res.content)
 
     assert sorted([Path(f).name for f in files]) == [
         "config.json",
@@ -100,8 +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
@@ -117,5 +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 @@ def test_resource() -> str:
 
         @mcp.tool()
         async def tool_with_resource(ctx: Context) -> str:
-            data = await ctx.read_resource("test://data")
-            return f"Read resource: {data}"
+            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", {})