**Implements**: [SEP-2448: server execution telemetry]
(https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2448)
**Description:**
**The Observability Gap (The Problem)**
MCP clients propagate trace context to servers, but server-side
execution remains a black box. The client sees a single tools/call or
resources/read span; everything the server does (auth checks, policy
evaluation, API calls, sub-tool invocations) is invisible. In
cross-organization deployments, clients and servers use separate
observability backends with no shared collector access, making
traditional span export useless.
<img width="1015" height="450" alt="Screenshot 2026-03-23 at 3 43 21 PM"
src="https://github.com/user-attachments/assets/58c817b5-fee6-46a3-9877-d523a25368ad"
/>
**Server Execution Telemetry (The Solution)**
Servers advertise serverExecutionTelemetry and return a curated slice of
their execution spans directly in _meta.otel of the response. Clients
ingest these verbatim OTLP spans into their own collector, stitching
server-side execution into their distributed trace; no shared
infrastructure required. The black box becomes transparent.
<img width="945" height="574" alt="Screenshot 2026-03-23 at 3 43 44 PM"
src="https://github.com/user-attachments/assets/38d97c94-aa73-4e62-9b4e-3264600e5ed0"
/>
.
**Summary:**
Implement MCP serverExecutionTelemetry capability that enables
cross-organization distributed tracing by returning server-side
OpenTelemetry spans to clients inline via _meta.otel.traces.
Server-side (middleware):
- TelemetryPassbackMiddleware intercepts tools/call and resources/read
- ContextVarSpanCollector isolates span collection per-request via
ContextVar
- Propagates traceparent from client request for distributed trace
stitching
- Serializes collected spans to verbatim OTLP JSON (resourceSpans
format), directly POSTable to /v1/traces
- Top-level span filtering by default; full span tree via detailed
opt-in
- Middleware advertises capabilities via get_capabilities() on the
Middleware base class
- Provisional API: FutureWarning emitted until SEP-2448 is ratified
Client-side (reference agent):
- LangChain ReAct agent connects to MCP server via
streamable_http_client with OAuth 2.1
- Detects serverExecutionTelemetry capability at initialization
- Dynamically wraps discovered MCP tools with traceparent propagation
and _meta.otel span request
- Ingests returned server spans into Jaeger (OTLP JSON) and Galileo
(OTLP protobuf)
- Two-act demo: --no-passback (black box) vs default (full server-side
visibility)
Dependencies:
- opentelemetry-api and opentelemetry-sdk added to arcade-mcp-server
Bump arcade-mcp-server version to 1.18.0.
Here's the PR summary:
---
## Enforce semver validation for `MCPApp` versioning
### Problem
`MCPApp.__init__` accepted any string as `version` with no validation.
Invalid versions like `"1.0.0dev"` or `"latest"` silently propagated to
the Engine, where `compareToolVersions` fell back to lexicographic
`strings.Compare` instead of `semver.Compare` — causing incorrect
ordering (e.g. `1.10.0 < 1.9.0`).
### Solution
Validate and normalize `version` at `MCPApp` instantiation time using
the same acceptance rules as Go's `golang.org/x/mod/semver v0.31.0` (the
exact version used by the Engine).
### Changes
**`arcade_mcp_server/_validation.py`** (new file)
- Shared regex constants: `SEMVER_PATTERN` (semver.org spec),
`SHORT_VERSION_PATTERN`, `MAJOR_ONLY_PATTERN`
**`arcade_mcp_server/mcp_app.py`**
- Added `_validate_version()` mirroring the existing `_validate_name()`
pattern
- Added `version` property + setter (validates on mutation too)
- `__init__` now stores `self._version` via `_validate_version()`
**`arcade_mcp_server/settings.py`**
- Added `@field_validator("version")` on `ServerSettings` — covers the
`MCP_SERVER_VERSION` env var path
- Fixed default from `"0.1.0dev"` → `"0.1.0"` (the old default was
itself invalid)
**`pyproject.toml`** — bumped `arcade-mcp-server` `1.17.4` → `1.17.5`
### Normalization pipeline
All inputs are normalized to canonical `MAJOR.MINOR.PATCH` before
storage:
| Input | Stored as |
|-------|-----------|
| `v1.0.0` | `1.0.0` |
| `1.0` / `v1.0` | `1.0.0` |
| `1` / `v1` | `1.0.0` |
### Verification
Validated against `golang.org/x/mod/semver v0.31.0` (Engine's exact
pinned version) — 40/40 accept/reject cases match. The Engine's own
`store_test.go` uses `"1.0"` and `"1.1"` as `ToolkitVersion` values,
confirming short forms are intentionally supported.
### Breaking change
Any user currently passing a non-semver version string (e.g.
`"1.0.0dev"`, `"latest"`) will get a `ValueError` on upgrade. This is
intentional — those versions were silently causing incorrect tool
ordering in the Engine.
Closes TOO-518
<!-- CURSOR_SUMMARY -->
---
> [!NOTE]
> **Medium Risk**
> Introduces stricter version validation/normalization that will raise
errors for previously-accepted non-semver inputs (including via env
vars), which may break existing consumers depending on lax version
strings.
>
> **Overview**
> **Enforces semver for server versioning** across both `MCPApp` and
`ServerSettings`, rejecting invalid strings and normalizing accepted
inputs (e.g., stripping leading `v`, expanding `1`/`1.2` to
`1.0.0`/`1.2.0`).
>
> Adds shared `normalize_version` logic in
`arcade_mcp_server/_validation.py`, updates `MCPApp` to validate on init
and via a new `version` property/setter, and adds a Pydantic `version`
validator so `MCP_SERVER_VERSION` is checked. Defaults are updated from
`0.1.0dev` to `0.1.0`, tests are expanded to cover accept/reject cases,
and the package version is bumped to `1.17.5`.
>
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
2ceabacb25372e67eef9720b901c1ee2b214868f. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->
---------
Co-authored-by: Eric Gustin <34000337+EricGustin@users.noreply.github.com>
# PR Description
`arcade.toml` was deprecated in favor of `credentials.yaml` in PR #116 .
This PR completely removes any references to handling the deprecation &
any references to `arcade.toml`
On the last few PRs I have noticed two problems:
1. `ruff format` fails even though it seems OK on our local machines
(sometimes, not always)
2. Nate's and Sam's machines kept flip-flopping a specific piece of
formatting back and forth, indicating a subtle difference of config
hiding somewhere
3. This was reproducible by running `ruff format` in the terminal,
followed by `make check`. The former would edit files, and then `make
check` would edit them back!
This PR addresses both issues, and further standardizes our editor &
linter configs to be super stable.
Specifically:
1. The main fix for the above, the pre-commit hook was pinned to a super
old version of ruff.
This resulted in subtle differences in behavior between our machines,
and on CI.
2. Moved ruff settings from `pyproject.toml` to `.ruff.toml`
pyproject files in subdirectories (e.g. `toolkits/**`) were overriding
the main pyproject file and erasing the custom ruff config we set at the
root. This meant that our ruff config was applied to `arcade` but not to
any of the other packages.
By moving the config to `.ruff.toml` at the root, all projects will
inherit the same ruff linting & formatting config.
4. Un-ignored the `.vscode/` directory so that we can share
vscode/cursor workspace settings.
This is valuable for standardizing settings like the default formatter
(ruff) and default test framework (pytest).
However, it's important that going forward we _only_ commit things here
that should apply across all of our machines.
5. To avoid any conflict between prettier and ruff, prettier now
explicitly ignores *.py files
6. Finally, `ruff format` and `make check` agree. A number of files are
newly auto-formatted.
* Add new tool to the arithmetic toolkit for summation of a range.
* Add ability to attach debugger to cli. Use `.vscode/launch.json`'s
"Debug arcade dev" to do so.
* Fix issue in cli's main that used the incorrect url.
This PR makes a few sweeping changes to the actor, cli, and overall
structure of the project.
- CLI commands skeleton
- ``arcade run``, ``arcade show``, and ``arcade new``
- Working package mangement solution (``arcade_`` packages)
- Actor approach for using frameworks other than FastAPI
- Client for calling Engine within ``arcade/core``
- beginning of the config interface.
---------
Co-authored-by: Nate Barbettini <nate@arcade-ai.com>
