Native GemStone MCP Server

A Model Context Protocol server written natively in GemStone Smalltalk. It runs inside the image and executes tool calls directly — no Node.js process, no GCI/FFI bridge. The goal is to replace the GCI-based Jasper MCP server with one that any MCP client can reach over plain HTTP.

A note about versions: run-server.sh is safe to use on a 3.7.5 stone, but run-auth-server.sh needs 3.7.6 due to a bug in connecting to an external OIDC IdP. Instructions for installing and running are later in this document.

Transport

A single endpoint, /mcp, implementing the MCP Streamable HTTP transport with per-client sessions — each client gets its own isolated worker gem (see Per-client sessions):

• POST /mcp — body is a JSON-RPC 2.0 request; reply is an application/json JSON-RPC response (notifications get 202 Accepted, no body).
  • initialize opens a session and returns its id in the MCP-Session-Id response header.
  • Every other request must send that header back; a missing id → 400, an unknown/expired id → 404 (a compliant client then re-initializes).
• GET /mcp — opens the standalone server→client SSE stream (text/event-stream), held open with keepalive comments. No server-initiated messages yet, so it carries only keepalives.
• DELETE /mcp — ends the session named by MCP-Session-Id (closes its worker). Answers the same codes as the POST path: missing header → 400, unknown/already-ended id → 404, live session → 200.
• Any other method → 405.

ping is answered with an empty result on every session, as the spec requires.

Security (per the MCP spec): the server binds only to 127.0.0.1, session ids are cryptographically-random 128-bit tokens, and every request's Origin header is validated to prevent DNS-rebinding — a present Origin whose host is not loopback (localhost/127.0.0.1/[::1]) gets 403; an absent Origin (non-browser clients like curl) is allowed. Add a browser app's origin host by configuring the router instance — (McpRouter new allowedOriginHosts: #(...)) forkOnPort: 8000. For network-facing use, the McpAuthRouter subclass adds OAuth 2.1 / JWT bearer-token authentication (per-user worker gems), a WWW-Authenticate challenge + RFC 9728 Protected Resource Metadata, TLS (GsSecureSocket), and scope-based read-only sessions; the base McpRouter is the localhost, unauthenticated front end.

# initialize -- the response carries an MCP-Session-Id header
curl -si localhost:8000/mcp -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{}}'

# subsequent calls echo that id back
curl -s localhost:8000/mcp -H 'MCP-Session-Id: <id>' \
  -d '{"jsonrpc":"2.0","id":2,"method":"tools/list"}'

Works with the MCP Inspector / any MCP SDK client using the Streamable HTTP transport pointed at http://localhost:8000/mcp (such clients manage the MCP-Session-Id automatically).

Protocol conformance

The server speaks MCP revisions 2025-06-18 and 2025-11-25 (McpDispatcher class >> supportedProtocolVersions, the single source of truth for both initialize negotiation and the MCP-Protocol-Version header check, so the two cannot drift). initialize echoes the client's version when supported, otherwise answers 2025-11-25.

2025-03-26 is deliberately not supported: a server on that revision must accept JSON-RPC batches, which the single-object body parser does not. 2025-06-18 removed batching.

Requirement	Behavior
initialize version negotiation	echo if supported, else our latest
ping	empty result (spec MUST)
tools/list, tools/call	implemented; tools is the only declared capability
resources/*, prompts/*, logging/*, completion/*	undeclared and answered -32601
Notification POST	202 Accepted, no body
Invalid Origin	403
Invalid/unsupported MCP-Protocol-Version	400
Missing session id / unknown-expired session id	400 / 404, on both POST and DELETE
Malformed body	400 + -32700
Tool input schemas	JSON Schema 2020-12 (no $schema needed), closed with additionalProperties: false

serverInfo.title is sent when a deployment configures one (see Writing your own MCP server) and omitted otherwise, so a client falls back to displaying name. That is the server's title; a tool title is not implemented.

Not implemented, all optional at these revisions: pagination (tools/list returns every tool and no nextCursor), listChanged notifications, SSE resumability (Last-Event-ID), tool title / annotations / icons / outputSchema, server instructions, and tasks.

The draft 2026-07-28 revision is a different protocol era — no initialize, no sessions, no GET stream, per-request _meta, and a mandatory server/discover. It is not implemented, and supporting it will need a decision about how per-client worker-gem isolation survives a protocol with no session id to key it on.

Tools (31 base + 2 optional Grail)

Execution

Tool	Arguments	Result
execute_code	code	printString of evaluating the Smalltalk source

Session / transaction

Tool	Arguments	Result
abort	–	abort the transaction, refresh the view
commit	–	commit the transaction
refresh	–	refresh the view to see other sessions' commits
status	–	session user, id, stone, uncommitted-changes flag

Listing

Tool	Arguments	Result
list_all_classes	–	every class across all dictionaries
list_classes	dictionaryName	classes in a dictionary
list_dictionaries	–	symbol dictionaries in lookup order
list_dictionary_entries	dictionaryName	every entry, tagged (class)/(global)

Browsing

Tool	Arguments	Result
describe_class	className	superclass, instance vars, selectors
export_class_source	className	full Topaz file-in (definition + methods)
get_class_definition	className	class-definition source expression
get_class_hierarchy	className	superclass chain + direct subclasses
get_method_source	className, selector, meta?	method source
list_methods	className	instance + class selectors grouped by category

Search

Tool	Arguments	Result
find_implementors	selector	methods implementing the selector
find_references_to	name	methods referencing a named global/class
find_senders	selector	methods sending the selector (capped at 200; note shows the true total)
search_method_source	pattern, dictionaryName?	methods whose source contains the substring (capped at 200)

Mutation

Tool	Arguments	Result
add_dictionary	dictionaryName	create + append a dictionary, commit
compile_class_definition	source, recompileMethods?	evaluate a class-definition expression, commit; the source must evaluate to a class (other expressions are rejected — use execute_code); on a shape change, by default recompiles the class's methods onto the new version and reports any that fail (refused if it has subclasses)
compile_method	className, source, category?, meta?	compile a method, commit
delete_class	className	remove a class, commit (destructive)
delete_method	className, selector, meta?	remove a method, commit (destructive)
remove_dictionary	dictionaryName	remove a dictionary, commit (destructive)
set_class_comment	className, comment	set the class comment, commit

Testing (SUnit)

Tool	Arguments	Result
describe_test_failure	className, selector	re-run one test in isolation, return the failure/error detail (exception class + description)
list_failing_tests	classNames?	failing/erroring methods (given classes, or all)
list_test_classes	–	all TestCase subclasses
run_test_class	className	run a test class, summary + failures
run_test_method	className, selector	run one test method

Python (optional — the McpGrailToolset)

These live in the optional McpGrailToolset, in its own source group (src/grail/) which only install.sh --grail loads — they reference ModuleAst and BaseException, so they cannot compile in an image without Grail. Once loaded the toolset joins the default tool surface automatically (McpServer class>>installedDefaultToolsetNames), or can be named explicitly in a router's toolsetNames.

Tool	Arguments	Result
compile_python	code	transpile Python source to Smalltalk via Grail (ModuleAst), return the generated source
eval_python	code	evaluate Python source via Grail (ModuleAst), return the printString of the result

Requirement: these tools call Grail's ModuleAst directly with no capability check, so they need an image with GemStone-Python installed.

Python errors are converted, not propagated. Grail models its exceptions outside the Smalltalk Error hierarchy (NameError is Exception < BaseException < Exception < AbstractException, and NameError inheritsFrom: Error is false), so the dispatcher's on: Error do: cannot see them — uncaught, a python error would take the whole worker gem down instead of answering the client. The toolset therefore catches BaseException and re-signals an McpError kinded pythonError, which comes back as an ordinary isError result. Verified 2026-08-18: an undefined name (NameError), a runtime error (1/0 → ZeroDivisionError) and a syntax error (def (: → SyntaxError) are all catchable and all reported this way. Both of the latter used to crash the gem, which is why the tests covering them were once switched off.

Tool-call safety

• Closed argument schemas — every tool's input schema sets additionalProperties: false plus its required list, so an unknown or missing argument is rejected up front, before the tool runs, rather than being silently dropped. Per MCP 2025-11-25 the rejection comes back as a tool execution error (isError: true, with structuredContent.error.kind = "invalidParams"), because that is the form a model can read and self-correct from. A malformed request — no tool name — and an unknown tool remain JSON-RPC protocol errors (-32602).
• Kernel-class guard — the mutation tools refuse to modify a base/kernel class (one that Globals binds under its own name); the refusal names the class and a remedy and carries kind = "refused". The test is deliberately by name and identity rather than via dictionaryAndSymbolOf:, which answers the first symbol-list dictionary binding a class by value under any key — in a Grail image Python binds kernel Object under an alias, ahead of Globals, which made Object read as unprotected and let the mutation tools through. Your own classes (in UserGlobals, or an application dictionary) stay freely mutable. execute_code is the deliberate escape hatch (and is itself gated in read-only mode).
• Structured error kinds — when a tool raises, the isError result keeps the human-readable message in content and carries structuredContent.error.kind, a short machine-readable classifier (compileError, refused, readOnly, notFound, invalidParams, other), so a client can branch on the kind instead of parsing prose.

Architecture

Class	Role
McpBase	abstract superclass of the router + worker; holds only the two shared helpers (parseBody:, log:)
McpRouter	front end: accept loop, HTTP, routing, the MCP-Session-Id → McpSession map, and the idle reaper. Owns the socket; never runs a tool
McpAuthRouter	network-facing McpRouter subclass: requires an OAuth/JWT bearer token, logs each worker in as its own GemStone user, serves the WWW-Authenticate challenge + RFC 9728 metadata, validates token claims/scopes, adds TLS, and (via writeScope) can open a session read-only
McpServer	per-client worker: the single-client MCP server that runs inside each worker gem — registry, dispatcher, the kernel guards, read-only gating, identity. The tools themselves belong to its toolsets, and which of those it registers is not fixed by the class. No socket
McpToolset	abstract tool pack: registerOn: (its tools + schemas), its tool_* handlers, toolNames, readOnlySafeToolNames (empty by default — fail closed), plus the shared schema builders, image-lookup helpers, and the kernel guards (which forward to the server's policy). Subclass this to add tools; a deployment picks the list
McpBrowsingToolset, McpExecutionToolset, McpListingToolset, McpMutationToolset, McpSearchToolset, McpSessionToolset, McpTestingToolset	the seven core toolsets, one per tool family. A deployment can expose any subset — or none of them, alongside its own
McpGrailToolset	optional Python toolset (eval_python, compile_python), filed in only on a Grail image. Needs nothing from the server, so it doubles as the worked example for a third-party toolset
McpSession	one client's isolated worker handle: a GsTsExternalSession gem + session id + last-activity + the worker class/toolsets/identity the front end resolved. prepareWorker sets the gem up in one call; forward: runs a request in it (<workerClass> handleJsonString: …); close stops it
McpHttpConnection	reads one HTTP/1.1 request, writes one JSON response (incl. MCP-Session-Id)
McpDispatcher	JSON-RPC 2.0 / MCP routing (initialize, tools/list, tools/call); read-only tool gating; structured error kinds
McpToolRegistry	name → McpTool map; produces tools/list descriptors
McpTool	one tool: name, description, JSON Schema, handler block; validates arguments against the schema
McpError	an error carrying a machine-readable kind (e.g. refused, readOnly) that the dispatcher surfaces in the tool-call error envelope

Built on existing image facilities: GsSocket (TCP), JsonParser parse: and Object>>asJson (JSON), and String>>evaluate (the execute_code engine).

Why a dedicated gem

Forked GsProcesses only run while the gem is actively executing Smalltalk. A GCI-driven session (like the Jasper VS Code session) is parked in the C client between commands, so a background accept loop forked there would be frozen and never serve requests. Therefore the server runs as the blocking main activity of a dedicated gem.

Configure a router instance and start it two ways:

• McpRouter new runOnPort: aPort — runs the accept loop as the calling session's blocking activity; never returns until stop. Use it to run the server in a foreground topaz.
• (McpRouter new … ) forkOnPort: aPort — spawns a separate gem via GsTsExternalSession and runs the loop there detached (forkAndDetachString:), returning immediately. The router's config travels to the child gem as JSON embedded in the fork string (configDict — host lists, file paths, and identifiers only, never key material), so nothing is committed and several differently-configured routers can run at once. The forked server is independent — it keeps serving after the launching session logs out. Stop it by port with ./stop-server.sh, or from anywhere with System stopSession: <id> / kill <pid> (both printed at fork). run-server.sh uses this.

The front end is always McpRouter, and it decides what each worker is — a worker never chooses for itself. Per session the router resolves the worker class (workerClassName, default McpServer) and the tool surface (toolsetNames, default the core toolsets plus McpGrailToolset when that file is loaded), then pushes both into the worker gem in one call at session open. Resolving per session rather than at boot means a Grail install that lands after startup reaches the next client — and it is what will later let an authenticated router narrow the surface per token, since the token is only visible on this side.

Per-client sessions

Each MCP client gets its own worker gem so clients don't share uncommitted changes or transaction views. The port-owning gem runs McpRouter, a front end / router; it never runs tools itself (those run in the per-client McpServer workers):

• initialize → the front end opens a McpSession (a GsTsExternalSession worker gem, logged in as the current user via a one-time password), prepares it with a single prepareWorkerWithToolsets:readOnly:serverName:title:version: call — which sets read-only, resolves the named toolsets, applies the advertised identity, and pre-builds the server so the client's first request has no registration to do — assigns a server-side id, and returns it in the MCP-Session-Id response header. A worker class or toolset the worker gem cannot resolve fails here, at session open, where the error can say what to fix.
• Every other request must carry that header. The front end looks up the worker (map guarded by a mutex) and forwards the raw JSON-RPC body to it — worker executeString: '<workerClass> handleJsonString: ' , body printString, naming the class the router resolved — the worker runs the tool in its own session and returns the response, which the front end relays. Missing id → 400; unknown/expired → 404 (a compliant client re-initializes).
• DELETE closes the worker; and an idle reaper (a background GsProcess) closes any session idle beyond 30 minutes (sessionIdleTimeoutSeconds), so abandoned test gems don't pile up.

Isolation comes from each worker being a separate gem = a separate transaction view. Forwarding is blocking / serialized for now (a single front-end gem can't overlap blocking GCI calls, and the non-blocking poll path corrupts results); true cross-client concurrency is a deferred follow-up. The base McpRouter logs every worker in as the current (server) user; the network-facing McpAuthRouter instead logs each worker in as the token's own GemStone user via JWT.

Read-only mode

A router can refuse every state-changing tool, so a client can browse and search but not modify the image — primarily a localhost convenience so a single user cannot accidentally mutate or commit (it is a tool-gate, not an access-control boundary). Read-only is per-router: the router marks each worker read-only at session open, so two routers (one read-only, one not) can run at once with no shared state. A worker is read-only if either applies:

• The router is read-only — (McpRouter new readOnly: true) forkOnPort: 8000, or the shortcut GS_MCP_READONLY=1 ./run-server.sh. Every session that router opens is read-only.
• By OAuth scope (McpAuthRouter) — give the router a writeScope (e.g. ./run-auth-server.sh with MCP_WRITE_SCOPE=mcp:write): a token carrying that scope gets a read-write worker; a token lacking it gets a read-only worker for that session. For a client to actually request that scope, the router must also advertise it — and it does so automatically, so there is nothing to keep in sync. Advertising without requiring is the point: an entitled user is granted the scope and gets read-write, while an unentitled user (the authorization server withholds it) still connects read-only.

supportedScopes is the set published as scopes_supported (RFC 9728 metadata) and offered in the WWW-Authenticate challenge — what clients are told to request, as distinct from requiredScopes, what every token must carry. It is derived, not configured: the union of requiredScopes, the writeScope, and extraScopes. Because it is a union, a required scope is always advertised and the write scope is always requestable — neither can be left out by a configuration slip, and there is no subset rule to observe. Set extraScopes only for scopes the router itself does not gate on but the client still needs to ask for, such as an authorization server's own profile.

Configuring the authorization server (Keycloak). Keycloak's own MCP authorization server guide recommends the shape this router already expects: define mcp:* client scopes, and bind an audience mapper to them so a token carries the resource identifier this router checks as expectedAudience. It recommends binding the audience to a scope rather than to an RFC 8707 resource indicator, because Keycloak has not implemented resource indicators — so bind the mapper to a scope every client requests, i.e. one of the router's requiredScopes (say mcp:use), and every token comes out with the right audience without any per-client setup.

That choice interacts with extraScopes in one Keycloak-specific way worth knowing before you deploy. A client that registers dynamically (RFC 7591) and sends a scope field is assigned only the scopes it asked for — Keycloak drops the realm's default client scopes — so anything the token needs must be advertised or the client is never assigned it. In practice that means MCP_EXTRA_SCOPES="profile offline_access": profile because MCP_USERID_CLAIM is typically preferred_username on Keycloak (the claim defaults to sub) and the profile scope is what emits it, and offline_access because clients ask for it to get a refresh token. Advertising offline_access runs against MCP's SEP-2207, which says a resource SHOULD NOT list it since refresh tokens are not a resource requirement — but on Keycloak, advertising is the only mechanism by which a dynamically-registered client can come to hold that scope, so it is the supported configuration here rather than a workaround.

What's gated: everything that can persist a change or run arbitrary code — execute_code, commit, and all the mutation tools. Everything else (browsing, listing, search, status/refresh/abort, and the test-runner tools) stays available.

Each toolset declares its own safe tools (McpToolset>>readOnlySafeToolNames), and the server answers their union, so a third-party toolset decides for its own tools without editing anything central. It is fail-closed: the default declaration is empty, so a newly added tool is gated until its toolset explicitly vouches for it. McpServer class>>coreReadOnlySafeToolNames remains as the audit list — the one place to read the whole core answer — and McpContractTest pins the union of the seven core toolsets against it, so a tool cannot quietly become "safe".

Screening happens at both levels, which matters because one family is mixed: McpSessionToolset holds abort/refresh/status (safe) and commit (not). A toolset that declares nothing safe — mutation, execution — is dropped whole; a mixed one keeps only its safe tools.

Two moments, too. When the router opens a read-only worker the gated tools are never registered (the flag is set before the server is built), which is a stronger gate than refusing them on call; the dispatcher's check still runs for a server whose flag was set afterwards. Either way a gated tool is hidden from tools/list and, if called by name, returns -32601 with error.data.kind = "readOnly" — deliberately not notFound, so a client can tell "exists but forbidden here" from "no such tool". A tool absent because its toolset was never loaded genuinely is notFound.

Install & run

export GEMSTONE=/path/to/GemStone64Bit3.7.x   # product dir
export GS_USER=DataCurator GS_PASS=...         # GemStone credentials

./install.sh                        # file in the classes (core + tests + auth) and commit
./install.sh --grail                # ...and the optional Grail/Python toolset (Grail image only)
GS_MCP_PORT=8000 ./run-server.sh    # fork a detached, independent localhost server gem and return
GS_MCP_READONLY=1 ./run-server.sh   # ...read-only (browse/search only; no accidental mutation)
GS_MCP_TOOLSETS="McpBrowsingToolset McpSearchToolset" ./run-server.sh   # ...only these tools
GS_MCP_WORKER_CLASS=MyMcpServer ./run-server.sh                        # ...a subclass as the worker
./run-auth-server.sh                # ...the OAuth/OIDC network-facing server (McpAuthRouter)

install.sh and the run-*.sh scripts use topaz; set GEMSTONE, GS_STONE, GS_USER, GS_PASS to match your environment. install.sh files the code in with topaz from load.gs — which on this branch includes src/auth/, since the OAuth front end is the point of it; --grail (or GS_MCP_WITH_GRAIL=1) files in load-grail.gs instead, which adds the src/grail/ group on top. run-server.sh builds a base McpRouter instance and calls its forkOnPort: (run-auth-server.sh builds an OIDC-configured McpAuthRouter — resource-server config as code, no commit), which launches a detached, independent front-end gem and returns; stop it with ./stop-server.sh (by port), or the System stopSession: <id> / kill <pid> line it prints. A loaded Grail toolset is picked up automatically, per session, by the front end.

Source layout

The classes live on disk as plain topaz file-outs — canonical Class>>fileOutClass output, grouped by area, with one loader per group:

src/core/    17 classes  the server itself: protocol, transport, dispatch, toolsets
src/tests/    9 classes  the SUnit suites and their fixtures
src/auth/     3 classes  the OAuth/OIDC front end McpAuthRouter + its two suites
src/grail/    2 classes  the optional GemStone-Python toolset + its suite
load.gs                  files in core + tests + auth, then commits
load-grail.gs            files in core + tests + auth + grail, then commits

Each group's load.gs names its files in dependency order, and every input path is relative to the repository root — install.sh cds there before starting topaz, so run any loader from the root too. There is no package manager in the loop: a .gs file-out files into any image topaz can log into, on any GemStone version, with no Rowan and no Tonel.

Each class keeps a category: matching its group (Mcp-Core, Mcp-Tests, Mcp-Auth, Mcp-Grail) — nothing reads it, but it groups the classes in a browser the same way the directories group them on disk.

Why each group loader pre-declares its class names. The classes reference each other in both directions (McpDispatcher asks McpServer for its name; McpServer builds an McpDispatcher), so no file order can put every class ahead of its first mention — the compiler would report undefined symbol and the file-in would stop. So each loader first binds its class names to nil in Published. That is enough, because the compiler binds a global by its association, and each class definition then fills that same association in; a method compiled before its referent still ends up pointing at the real class. Existing keys are left alone, so re-installing over a loaded image changes nothing.

Migrating an image that previously loaded the Rowan project. Filing these .gs files over classes the Rowan Mcp project had loaded fails at the first method with "Duplicate definition of signalKind:message: in McpError" (error 2318) — observed 2026-08-19 on a Rowan 3.5.0 + Grail image, while the identical file-in into a Rowan-free image loaded every class with no compiler errors. The mechanism is not pinned down (topaz's own removeallmethods / removeallclassmethods do clear the class when run on their own, and a plain compileMethod:dictionaries:category: recompiles happily), so treat it as a property of Rowan-managed classes rather than of the file-outs. Install into an image that never loaded the Rowan Mcp project, or remove the Mcp* keys from Published and commit before running install.sh.

To regenerate a file-out after changing a class in the image, have topaz write fileOutClass straight to its file — do not transcribe an export_class_source result, which drifts on trailing whitespace:

| s f |
s := McpServer fileOutClass.
f := GsFile openWriteOnServer: '/path/to/gs-mcp/src/core/McpServer.gs'.  "no mode: argument"
f nextPutAll: s; close.

Test

Two complementary suites:

Unit tests (in-image, no socket) — ./run-unit-tests.sh logs in via topaz and runs the base GsTestCase suites against the server's logic directly (milliseconds, no network), plus the Grail suite when McpGrailToolset is installed:

• McpToolTest — every tool_* handler called directly on its owning toolset (grouped by the tools - * categories). Tests operate on throwaway fixtures rather than on the production classes: a plain McpTestFixture and a McpTestSuiteFixture (a GsTestCase subclass with passing/ failing/erroring tests, for the test-runner tools), both classes in UserGlobals, plus a McpTestDict symbol dictionary of its own. All are cleaned up in tearDown.
• McpDispatcherTest — JSON-RPC routing/envelope: initialize, tools/list (31, alphabetical), success + error wrapping, -32601/-32602/-32700, notifications → nil, and the per-worker entry handleJsonString:.
• McpTransportTest — handleConnection: driven over a McpMockSocket wrapped in a real McpHttpConnection, so the genuine HTTP parsing/writing runs with no TCP. Covers the paths that spawn no worker gem: GET→SSE, DELETE→400/404, unknown verb→405, malformed→-32700, a session-less POST→400, chunked delivery, EOF, Content-Length. (initialize and a routed tool call spawn a real worker, so they're exercised by the integration test instead.)
• McpContractTest — contract / property tests over the tool surface, all driven through the real McpDispatcher>>handle: envelope: every tool schema is closed (additionalProperties:false), unknown/missing arguments → an isError tool execution error while a missing tool name / unknown tool stay -32602, ping → an empty result, a raised error carries a structured kind, kernel-class mutation is refused, and read-only hides + refuses the gated tools. Also the toolset invariants: the union of the core toolsets' read-only declarations equals the audit list, no toolset vouches for a tool it does not provide, toolNames matches what registerOn: registers, a server built from one toolset exposes only its tools, a read-only build drops an all-unsafe toolset whole, and the kernel guard survives a dictionary that shadows a kernel name. Socket-less and worker-less, so it runs in run-unit-tests.sh with the others above.
• McpExtensionTest — the extension story through two fixtures: McpFixtureToolset (a third-party toolset that owns its handler and vouches for its own read-only safety) and McpFixtureServer (a named worker subclass that names itself). Covers a vendor server exposing only its own tools, two independent toolsets composed on one server, a third-party tool surviving a read-only build, the worker entry answering as the named subclass, and the identity precedence — router config relabels a subclass's own default. McpStubSession lets it drive McpRouter>>openSessionCreating: (configure and prepare) with no login.
• McpAuthTest — the authenticated front end (McpAuthRouter): missing / non-bearer / garbage / valid tokens, RS-layer exp / issuer / audience / scope validation, and the write-scope read-only sessions. It commits a throwaway JWT user and spawns real worker gems (needs netldi), so — like test-tls.sh — it runs via the run_test_class tool or the scripts rather than the socket-less run-unit-tests.sh.
• McpGrailToolsetTest (Grail images only) — the optional Python toolset: eval_python→42, compile_python→___binOpMul___:, print→None, all three Python failure paths (undefined name, runtime, syntax) surfacing as isError with kind = "pythonError", a 33-tool tools/list check on core-plus-Grail, auto-detection into the default surface, and the toolset being dropped whole in a read-only session. The last two failure paths were switched-off tripwires while Grail crashed the gem on them; both run for real as of 2026-08-18.

Run a single suite while a server is up via the run_test_class tool (e.g. run_test_class McpToolTest). ./run-unit-tests.sh runs the socket-less suites — McpToolTest, McpDispatcherTest, McpTransportTest, McpContractTest, McpExtensionTest (124 tests, 133 with the 9 in McpGrailToolsetTest on a Grail image) — and exits 0 when all pass. McpAuthTest (24) and McpAuthConformanceTest (25) live on the auth branch and need netldi, so run them there with run_test_class or the scripts — 173 tests in total on that branch.

Note: a test helper must never reuse a SUnit framework selector (run:, setUp, …) — doing so shadows the framework method and silently breaks suite run. The transport helper is named runRequest: for this reason.

Integration test (real socket) — ./test.sh starts the server in its own gem and drives the full Streamable HTTP transport with curl: it initializes, captures the MCP-Session-Id, and sends it on every subsequent request (tools/list of the 31 base tools, every core tool, a compile_method/commit round-trip, error paths, the SSE GET stream, DELETE), then shuts the server down. It targets the base server — run it against a base install. Uses port 8011 by default (set GS_MCP_PORT). Exit status 0 = all passed.

TLS test (real HTTPS socket) — ./test-tls.sh forks a TLS-enabled server and drives the same transport over HTTPS with curl -k: TLS handshake, the self-signed cert, the SSE GET stream, initialize, a routed tool call, the unknown-session 404, and a check that plaintext HTTP is refused on the TLS port. It generates a throwaway self-signed certs/ cert if none exists, and sets the cert/key only in the forked gem's session (never committed), so the repository's default stays plaintext — nothing to restore even if interrupted. Uses port 8443 by default (set GS_MCP_PORT). Exit status 0 = all passed.

Writing your own MCP server

You can ship an MCP server for your software on this transport — including one that exposes only your tools, with none of the Smalltalk-development surface. There are two extension points, and the first is the one you usually want.

To add tools, write a toolset. Subclass McpToolset, implement registerOn: (one name:description:inputSchema:do: send per tool, building schemas with the inherited objectSchema:required: / propString: / boolProperty: helpers), implement toolNames, and declare readOnlySafeToolNames for whichever of your tools cannot persist a change — the default is none, so an undeclared tool is gated in a read-only session. Write the handlers as instance methods on the same class, taking the parsed argument dictionary and returning a String; the inherited resolveClass:, dictNamed:, linesFrom: and capResult: helpers cover the usual image lookups and output capping. McpFixtureToolset (in src/tests/) and McpGrailToolset are small worked examples. A handler that mutates the image should pass through the inherited kernel guard (self assertMutableClass: cls) before it changes anything; that forwards to the server, because what counts as protected is one answer per deployment rather than each toolset's to invent, and a subclass can tighten it for every toolset at once. McpMutationToolset shows the pattern. Your toolset may layer a stricter guard of its own on top; a toolset built with no server refuses to mutate at all, fail-closed.

Errors raised inside a handler are caught by the dispatcher and returned as an MCP error result (isError: true) carrying a structured kind. If your tools can raise exceptions outside the Error hierarchy, catch them yourself and re-signal an McpError — that is what McpGrailToolset does for Python exceptions, and why it has to.

Then name your toolsets when you launch a router:

(McpRouter new
  toolsetNames: #('AcmeDbToolset');           "only your tools -- no execute_code, no mutation tools"
  serverName: 'acme-db-mcp'; serverVersion: '2.5.0';
  serverTitle: 'Acme Labels - sandbox')       "which INSTANCE this is, for a human"
    forkOnPort: 8000

Relabel the server when you configure one: serverName / serverVersion say which product this is, serverTitle says which instance a human is looking at — see Server identity below.

Toolsets compose — #('AcmeDbToolset' 'McpBrowsingToolset') gives your tools plus class browsing, and two unrelated vendors' toolsets can be combined. This is the reason tools live in toolsets rather than in McpServer subclasses: single inheritance could never express it.

To change behavior, subclass McpServer — the kernel guards, the worker entry, dispatcher wiring, or the advertised identity. Name your subclass in workerClassName (nothing auto-detects it):

(McpRouter new workerClassName: 'AcmeDbServer'; toolsetNames: #('AcmeDbToolset')) forkOnPort: 8000

Server identity

The initialize result's serverInfo carries three fields, and they answer different questions:

Field	Means	Set by
name	which software this is	the product: override class-side defaultServerName, or set router config serverName for a toolset-composed server with no McpServer subclass
version	which release of that software	same
title	which instance this is, for a human	the operator: router config serverTitle

name is the programmatic identifier and title is the display string (MCP BaseMetadata); when there is no title a client displays the name. So the two shapes are:

"same software, three stones -- name stays truthful, humans can tell them apart"
(McpRouter new serverTitle: 'GemStone - geode teststone 3.7.6') forkOnPort: 8000
(McpRouter new readOnly: true; serverTitle: 'GemStone (read-only)') forkOnPort: 8001
"a different product assembled from toolsets, with no McpServer subclass"
(McpRouter new toolsetNames: #('AcmeDbToolset');
   serverName: 'acme-db-mcp'; serverVersion: '2.5.0';
   serverTitle: 'Acme Labels - sandbox') forkOnPort: 8002

To name your product, override the class-side defaultServerName / defaultServerVersion. That keeps the name a default a deployment can still relabel through router config — the path for a server assembled from toolsets that never subclasses McpServer. Overriding the instance-side serverName instead wins over config, which is a deliberate lock rather than the normal path.

There is no default title: class-side defaultServerTitle answers nil and the title key is then left out of serverInfo entirely (not sent as null or ''). A title being present therefore means a human deliberately labeled that instance. A product that wants its own display name overrides defaultServerTitle; per-box labeling stays the operator's serverTitle.

Where your classes must live: a worker gem may log in as a different user than the front end (under McpAuthRouter, as the token's own GemStone user), so your toolsets and any worker subclass must be in a symbol dictionary in the worker's symbol list — Published, not the operator's UserGlobals.

Concurrency & robustness

Each accepted connection is handled in its own forked GsProcess, so a slow or stalled client cannot block the accept loop (the forked handlers run during the loop's accept waits). McpHttpConnection>>readRequest also bails after an 8s read timeout, so a client that connects but never sends a complete request is dropped rather than wedging the server. Each client's requests run in its own worker gem (a separate session), so there's no shared transaction to protect; a Semaphore (mutex) guards only the MCP-Session-Id → session map. Forwarding to a worker is a blocking call, so forwarding across clients is serialized for now — true concurrent cross-client execution is deferred.

Status

Streamable HTTP transport (POST→JSON, GET→SSE stream, DELETE) with per-client sessions — each client gets its own isolated worker gem, routed by MCP-Session-Id (missing→400, unknown→404), reaped after 30 min idle. 31 base tools in seven composable toolsets (execution, session, listing, browsing, search, mutation, testing) — plus 2 Python tools in the optional McpGrailToolset (filed in by install.sh --grail); per-connection forking + read timeout. Verified end-to-end with curl (initialize / MCP-Session-Id routing / tools/call / two-client isolation / 400 / 404 / SSE GET / DELETE, and stalled-connection load) and by the in-image unit tests. Since the first release it has also gained: OAuth 2.1 / JWT authentication + TLS (the McpAuthRouter subclass), per-router read-only mode (a router toggle plus per-token write-scope sessions), closed argument schemas + a kernel-class guard + structured error kinds, and a selectable tool surface: tools live in McpToolsets, the front end resolves the worker class and toolset list per session and pushes them into the worker gem, and a server can announce its own name/version — so a third party can ship an MCP server for their own software, exposing only their tools. The Python tools delegate to Grail's ModuleAst and require a Grail-equipped image (see the Python note above).

Future work: true concurrent cross-client forwarding; server-initiated SSE messages (which would let notifications/tools/list_changed announce a surface change); an external OIDC identity provider; mapping OAuth scopes to toolsets, so a token's scopes select what it may see rather than only whether it may write; and an optional serverInfo.title for a per-deployment label distinct from the product name.