Proof of Concept - Python MCP SDK 2 Migration by gsmith-alvarez · Pull Request #10427 · marimo-team/marimo

gsmith-alvarez · 2026-08-01T04:00:43Z

feat(mcp): migrate server from FastMCP to MCPServer for MCP SDK 2.0
feat(mcp): migrate client and tools to snake_case fields for MCP SDK 2.0
fix(mcp): update tests for MCP SDK 2.0 compatibility
feat(mcp): deps updated, httpx2 adopted and MCP 1.0 dropped for 2.0 Support

📝 Summary

Seeing that my orginal pr was only part of the migration I wanted to see if I could do the rest of it for a small project.

I do not expect this to be merged as 2.0 largely breaks compatibility with 1.0.

I hope this pr serves more to assist when the migration eventually occurs.

Key changes:

FastMCP was renamed to MCPServer

The MCPServer object no longer take stateless_http, streamable_http_path, transport_security and these now must be passed into mcp.streamable_http_app().

The SDK has transitioned from preferring CamalCase to preferring snake_case.

The tests were mostly just updated to use snake_case for when calling the server and I updated the Tool type to use the snake_case preferred from the SDK.

Finally, I dropped the try/except for support for streamablehttp_client and adopted the httpx2 as advised from their docs.

Resources

Here is a picture to show that the server is responding.

NOTE:

I had to hardcore the exclude-newer tag for this to work.

Also, don't know how to fix the ruff formatting so that it doesn't clog up the git change.

📋 Pre-Review Checklist

For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
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✅ Merge Checklist

I have read the contributor guidelines.
Documentation has been updated where applicable, including docstrings for API changes.
Tests have been added for the changes made.

In MCP SDK 2.0, FastMCP was renamed to MCPServer and the import path changed from mcp.server.fastmcp to mcp.server.mcpserver. Transport-specific parameters (stateless_http, streamable_http_path, transport_security) were also moved off the MCPServer constructor and onto streamable_http_app(), as the constructor no longer accepts them. Changes: - Replace with in both code_server/main.py and server/main.py - Move stateless_http, streamable_http_path, and transport_security from MCPServer() constructor to streamable_http_app() calls - Remove redundant constructor arguments (stateless_http was a duplicate since it was also set at app creation time in v1)

MCP SDK 2.0 renamed all Pydantic model fields from camelCase to snake_case. The JSON wire format is unchanged (Pydantic aliases handle serialization), but Python attribute access now uses snake_case names. Key renames per the migration guide: - CallToolResult.isError -> is_error - Tool.inputSchema -> input_schema Changes in client.py: - _create_error_result(): use is_error=True (was isError=True) - is_error_result(): check both is_error and isError via getattr for cross-version compatibility - extract_text_content(): use self.is_error_result() helper instead of inline hasattr(result, "isError") check (eliminates duplication) - _add_server_tools(): read tool.input_schema with fallback to tool.inputSchema for cross-version compat; pass inputSchema to Tool() constructor which accepts both spellings via Pydantic populate_by_name Changes in tool_manager.py: - _convert_mcp_tool(): use mcp_tool.input_schema (was inputSchema)

@patch

MCP SDK 2.0 removed the deprecated streamablehttp_client function (renamed to streamable_http_client in v1.x). The legacy fallback test that patches streamablehttp_client now fails at import time because the attribute no longer exists on mcp.client.streamable_http. Changes: - test_http_connector_connect_legacy_fallback: remove @patch decorator for streamablehttp_client and instead check hasattr() at runtime, calling pytest.skip() when the legacy function is absent. This keeps the test working on MCP 1.x (where streamablehttp_client exists) while gracefully skipping on MCP 2.0+ where it was removed.

…upport

vercel · 2026-08-01T04:00:48Z

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github-actions · 2026-08-01T04:00:55Z

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mcp==2.0.0 was published 2026-07-28. The previous "7 days" window excluded it from dependency resolution, causing Vercel deploy failures.

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Pull request overview

Proof-of-concept migration of marimo’s MCP integration to the Python MCP SDK 2.x API surface (server/client/tool schema naming and transport wiring), along with dependency constraint updates to target MCP 2.x.

Changes:

Migrate MCP server setup from FastMCP to MCPServer, moving HTTP transport config into streamable_http_app(...).
Update MCP client/tool handling to prefer snake_case fields (e.g. is_error, input_schema) with selective backward-compat fallbacks.
Update dependency constraints to require mcp>=2 and adjust tests accordingly (including removing legacy fallback coverage).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/_server/ai/test_mcp.py	Removes legacy streamable HTTP fallback test aligned with dropping older MCP SDK behavior.
pyproject.toml	Updates MCP optional dependency constraints to MCP 2.x and adjusts formatting.
marimo/_server/ai/tools/tool_manager.py	Updates MCP tool schema field access to snake_case.
marimo/_server/ai/mcp/transport.py	Updates streamable HTTP connector to use MCP SDK 2 client + `httpx2`.
marimo/_server/ai/mcp/client.py	Updates error/tool schema handling for MCP SDK 2 with compatibility fallbacks.
marimo/_mcp/server/main.py	Migrates the main MCP server from `FastMCP` to `MCPServer` and updates HTTP app wiring.
marimo/_mcp/code_server/main.py	Migrates the code-mode MCP server from `FastMCP` to `MCPServer` and updates HTTP app wiring.

gsmith-alvarez · 2026-08-01T04:13:48Z

-                    timeout=server_def.timeout,
-                )
+        # Establish streamable HTTP connection
+        read, write = await exit_stack.enter_async_context(


The new streamable_http_client is a 2-tuple of read and write. So *_ is no longer needed.

        return ToolDefinition(
            name=namespaced_name or mcp_tool.name,
            description=mcp_tool.description or "No description available",
-            parameters=mcp_tool.inputSchema,
+            parameters=mcp_tool.input_schema,
            source="mcp",


gsmith-alvarez · 2026-08-01T04:16:26Z

 mcp = [
-    "mcp>=1.0.0,<2", # MCP 2.0 is a major rework, unsupported for now.
+    "mcp>=2",     # MCP 2.0 is a major rework, unsupported for now.
    "pydantic>2",
 ]


httpx2 is a transitive dependency of mcp>=2

gsmith-alvarez · 2026-08-01T04:15:15Z

        return CallToolResult(
-            isError=True,
+            is_error=True,
            content=[TextContent(type="text", text=error_message)],


gsmith-alvarez added 4 commits July 31, 2026 23:27

feat(mcp): deps updated, httpx2 adopted and MCP 1.0 dropped for 2.0 S…

abe9620

…upport

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[pre-commit.ci] auto fixes from pre-commit.com hooks

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vercel Bot had a problem deploying to Preview August 1, 2026 04:01 Failure

chore: bump uv exclude-newer to resolve mcp>=2.0.0

bf909bd

mcp==2.0.0 was published 2026-07-28. The previous "7 days" window excluded it from dependency resolution, causing Vercel deploy failures.

vercel Bot deployed to Preview August 1, 2026 04:05 View deployment

Copilot AI reviewed Aug 1, 2026

View reviewed changes

fix(mcp): Updating docstring to reference is_error

60c39c2

vercel Bot deployed to Preview August 1, 2026 04:13 View deployment

fix(mcp): update optional-dependencies snapshot for mcp>=2

91bdeb5

vercel Bot deployed to Preview August 1, 2026 04:32 View deployment

gsmith-alvarez marked this pull request as draft August 2, 2026 01:16

gsmith-alvarez marked this pull request as ready for review August 2, 2026 01:17

gsmith-alvarez mentioned this pull request Aug 2, 2026

Proof of Concept - Python SDK MCP v2.0 Adoption #10433

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Proof of Concept - Python MCP SDK 2 Migration - #10427

Proof of Concept - Python MCP SDK 2 Migration#10427
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marimo-team:mainfrom
gsmith-alvarez:feat/mcp-sdk-2-proof-of-concept

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📝 Summary

Key changes:

NOTE:

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