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fix: don't cache import map in dev apps bundle (#4106)

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Jeremiah Lowin 2026-05-07 11:46:13 -04:00 • committed by GitHub
commit 3d476ca01e
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1 changed files with 41 additions and 30 deletions
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73
src/fastmcp/cli/apps_dev.py
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@ -1265,41 +1265,28 @@ def _fetch_app_bridge_bundle_sync(
ext-apps app-bridge.js imports the SDK via bare specifiers
(``@modelcontextprotocol/sdk/types.js`` etc.) that the browser cannot
resolve. We rewrite them to concrete esm.sh URLs before serving.
Caching
-------
``app_bridge_js`` is cached on disk because it is static for a given
ext-apps + SDK version pair (downloaded from npm).
The import map is NOT cached. It contains the concrete zod version that
esm.sh resolves ``zod@^x`` to at fetch time, and that version can change
when esm.sh publishes a new zod release. A stale cached import map would
redirect the old version URL while the browser loads the new version URL,
so the redirect would not apply and ``t.custom`` would be undefined.
"""
cache_path = (
Path(tempfile.gettempdir())
/ f"fastmcp-ext-apps-{version}-sdk-{sdk_version}-bundle.json"
)
if cache_path.exists():
cached = json.loads(cache_path.read_text())
return cached["app_bridge_js"], cached["import_map_json"]
# -- Download and patch app-bridge.js -----------------------------------
npm_url = f"https://registry.npmjs.org/@modelcontextprotocol/ext-apps/-/ext-apps-{version}.tgz"
with httpx.Client(timeout=30.0) as client:
resp = client.get(npm_url, follow_redirects=True)
resp.raise_for_status()
data = resp.content
with tarfile.open(fileobj=io.BytesIO(data), mode="r:gz") as tar:
member = tar.extractfile("package/dist/src/app-bridge.js")
if member is None:
raise RuntimeError("app-bridge.js not found in ext-apps tarball")
app_bridge_js = member.read().decode()
# Rewrite bare SDK module specifiers to concrete esm.sh URLs
sdk_base = f"https://esm.sh/@modelcontextprotocol/sdk@{sdk_version}"
for sdk_path in ("types.js", "shared/protocol.js"):
app_bridge_js = app_bridge_js.replace(
f'from"@modelcontextprotocol/sdk/{sdk_path}"',
f'from"{sdk_base}/{sdk_path}"',
)
# -- Detect the broken Zod v4.mjs URL -----------------------------------
# -- Detect the broken Zod v4.mjs URL (always fresh, never cached) ------
# The SDK's types module imports zod/v4 via a version-range URL like
# /zod@^4.3.5/v4?target=es2022. That wrapper re-exports from the
# version-specific v4.mjs (e.g. /zod@4.3.6/es2022/v4.mjs) which is
# broken. We fetch the wrapper to discover the exact version.
#
# We do this before the (potentially cached) app-bridge download so that
# any network error is surfaced early and clearly.
types_url = f"{sdk_base}/types.js"
with httpx.Client(timeout=30.0) as client:
resp = client.get(types_url, follow_redirects=True)
@ -1336,10 +1323,34 @@ def _fetch_app_bridge_bundle_sync(
import_map_json = json.dumps({"imports": {broken_url: fixed_url}})
# -- Cache and return ----------------------------------------------------
cache_path.write_text(
json.dumps({"app_bridge_js": app_bridge_js, "import_map_json": import_map_json})
# -- Download and patch app-bridge.js (cached per ext-apps + SDK version) -
app_bridge_cache = (
Path(tempfile.gettempdir()) / f"fastmcp-ext-apps-{version}-sdk-{sdk_version}.js"
)
if app_bridge_cache.exists():
app_bridge_js = app_bridge_cache.read_text(encoding="utf-8")
return app_bridge_js, import_map_json
npm_url = f"https://registry.npmjs.org/@modelcontextprotocol/ext-apps/-/ext-apps-{version}.tgz"
with httpx.Client(timeout=30.0) as client:
resp = client.get(npm_url, follow_redirects=True)
resp.raise_for_status()
data = resp.content
with tarfile.open(fileobj=io.BytesIO(data), mode="r:gz") as tar:
member = tar.extractfile("package/dist/src/app-bridge.js")
if member is None:
raise RuntimeError("app-bridge.js not found in ext-apps tarball")
app_bridge_js = member.read().decode()
# Rewrite bare SDK module specifiers to concrete esm.sh URLs
for sdk_path in ("types.js", "shared/protocol.js"):
app_bridge_js = app_bridge_js.replace(
f'from"@modelcontextprotocol/sdk/{sdk_path}"',
f'from"{sdk_base}/{sdk_path}"',
)
app_bridge_cache.write_text(app_bridge_js, encoding="utf-8")
return app_bridge_js, import_map_json