The twelve v1 @server.* decorator kinds are gone on v2. Their sites now
become add_request_handler / add_notification_handler calls at the
decorator's exact source position (registration there is when the v1
decorator ran, so execution order is preserved and the deprecated
capabilities land on the warning-free path), wired through generated
adapters that reproduce the v1 wrapper semantics: bare-list wrapping,
call_tool's any-exception-to-isError contract with jsonschema input and
output validation (tool lookup through the registered tools/list
handler, v1's own cache mechanism, so cross-module list_tools works),
read_resource content conversion, and the completion None-mapping.
Handler bodies are never touched. Shapes the adapter cannot serve
honestly -- a stacked decorator, an attribute receiver, a non-v1
signature, a non-literal decorator argument, a taken name -- are marked
with the reason. The suite migrates a six-registration server and
serves it to a v1-shaped ClientSession over the legacy protocol; the
templates are pinned against the installed v2 (method strings register,
params models exist, imports resolve, no 2026-era surface is emitted).
Also on the client surface: inline timedelta session timeouts convert
to float seconds and non-provable values are marked (the mismatch only
fails on the first request); cursor= on session list_* methods wraps
into params=PaginatedRequestParams(...); pydantic URL wrappers around
resource URIs are dropped where the target provably takes v2's plain
str and marked elsewhere; constructions of and pydantic method calls on
the v1 RootModel wrappers that became plain union aliases are marked
with the TypeAdapter fix; ._mcp_server and the type-keyed handler dicts
are marked with their v2 homes. Adapters honor an explicit `uri: str`
annotation and keep v1's AnyUrl otherwise, and keep the emitted code
insensitive to user return annotations so a wrong annotation cannot
manufacture type errors inside generated code.
Batch harness: seven more pinned repositories (two seven-decorator
servers, a multi-package lowlevel server, the method-local-server
marker path, two client libraries including a positional timedelta
timeout and the old streamablehttp spelling, and an exact ==1.6.0 pin).
Markers now cover the full statement they precede rather than a fixed
radius, Unknown-typed errors in files that carry markers classify as
cascade of a marked break, and the work directory is a dot-directory so
pytest never collects the cloned repositories' own suites. All eleven
repositories audit at zero uncovered errors.
An adversarial review round over the full change confirmed ten defects,
all fixed with regression tests: adapter imports now inject at the top
of the module (a mid-file import as the anchor left registration code
running before its imports bound); the rewrite gates now also block a
handler named like a template local, and any module-level non-import
binding of a name the adapter references (both were silent runtime
breaks past the gates); import injection dedup now reads the updated
module's top-level import binds, so conditional or function-local
imports no longer suppress a needed injection; list_* adapters pass a
returned full result model through instead of double-wrapping (v1's
runtime behavior); the blocked-progress marker names
add_notification_handler (a request-handler registration would never
fire); the timeout transform skips already-v2 shapes so re-runs stay
no-ops; the emitted name scheme is defined once and shared between
templates and gates; and the harness classifier no longer lets a
marker cover a whole def/class body or write off arbitrary
Unknown-typed errors (header-only spans; cascade restricted to
propagation rules and never detonators).
The goal is that migrated v1 code runs on v2 on its legacy paths, not
that it adopts v2 idioms. Applying that bar:
- Leave e.error.code / .message / .data chains alone: v2's MCPError
keeps a typed .error ErrorData, so the v1 spelling runs and
type-checks unchanged. The except-binding tracking goes with it.
- Rewrite one-argument McpError(...) calls to MCPError.from_error_data(...)
instead of flattening the inline ErrorData: the user's expression is
kept as written and the non-inline form no longer needs a marker.
- Convert v1 positional arguments on the lowlevel Server constructor to
keywords (v2 is keyword-only after name but kept v1's names and order),
pinned against the installed signature by a new ratchet test.
- Reword every marker message that pointed at replaced internals or at
the successor of the removed experimental tasks API; state removals
plainly instead of steering users onto new surfaces.
- Teach the batch harness that a reportArgumentType error naming a
detonating argument type (timedelta, AnyUrl) is a real break, never
v2 strictness drift, and ignore stale work/ directories.
Three additions to mcp-codemod, closing the gaps a comparison with the
TypeScript codemod surfaced:
Imports of module namespaces v2 deleted outright (the experimental tasks
namespaces, the WebSocket transports, `mcp.shared.progress`) are now
marked with replacement guidance. A new ratchet test freezes the 107
public modules v1 shipped and asserts every one imports on v2, is
renamed, or is in the removed table, so the whole v1 module namespace is
provably accounted for.
The codemod now also updates the `mcp` requirement in `pyproject.toml`
(PEP 621 tables and dependency groups) and `requirements*.txt` to
`>=2,<3` -- only where the current constraint cannot accept any v2
release, and only the version specifier: name, extras, environment
marker, and spacing keep the user's spelling. Poetry tables and the
removed `ws` extra are marked instead of guessed at, under the same
`# mcp-codemod:` contract as source markers.
`scripts/codemod-batch-test/` runs the codemod against pinned real
repositories and audits the marker contract end to end: it type-checks
the pristine clone against the latest v1 and the migrated copy against
this workspace's v2 with identical pyright settings, then requires every
error that exists only on the migrated side to sit next to a marker.
Across the four repos in the manifest every migration-surface error is
covered, and the audit caught two real bugs now fixed here: `Context`
imported from the old `.server` submodule is rehomed to the package (the
submodule holds the name at runtime, but a type checker treats a
non-re-exported name as private), and `request_context` on a receiver
the pre-pass proved holds a lowlevel `Server` is flagged again --
receiver-matched, so the live `ctx.request_context` idiom stays
untouched.
A new `mcp-codemod` workspace package (`uvx mcp-codemod v1-to-v2 ./src`)
that rewrites every v1 -> v2 change whose meaning is unambiguous from the
file alone, and inserts a `# mcp-codemod:` comment above every site it
recognized but would not guess at. Built on libCST.
Names are resolved through each file's imports, never matched as text, so
an aliased import or an unrelated symbol that shares a name with an SDK
one is never touched. The camelCase to snake_case rename is restricted to
the field names v1's `mcp.types` actually declared. Anything whose correct
rewrite depends on information that is not in the file -- the lowlevel
decorator to `on_*` relocation, the transport keywords on the `MCPServer`
constructor -- is left exactly as written and marked instead, so the
remaining work is one grep. Re-running on the output is a no-op.
The mapping tables are pinned against the installed v2 package by ratchet
tests so they cannot silently drift: every rename target must resolve,
every removed API must be provably absent, and no flagged constructor
keyword may survive on `MCPServer.__init__`. Measured against the example
files that exist on both `v1.x` and `main` (whose diff is the hand-written
migration), the codemod fully reproduces 13 of the 51 with a real
migration diff, improves 35 more, and makes none worse.
Also adds an "Automated migration" section to docs/migration.md, a mention
of the tool in README.v2.md, and the package to the publish workflow's
build step (the PyPI project and its trusted publisher must exist before a
release is tagged with this in it).
Max Isbey