Enable WebMCP by default and merge feature-list flags - #348
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Chromium 151 ships WebMCP (navigator.modelContext plus the CDP WebMCP domain). Enable it in default launch flags for headless and headful images via --enable-features=WebMCPTesting,DevToolsWebMCPSupport. MergeFlags now unions --enable-features / --disable-features values across base and runtime flag sources into single tokens. Chromium keeps only one value per switch, so without merging, a session-supplied --enable-features would silently drop the image defaults (or vice versa); --disable-features still wins per feature for opt-out.
Add --kernel-disable-features for per-session feature disabling. The launcher folds it into the single --disable-features switch after merging base and runtime flags; the in-image API also translates before writing /chromium/flags so stored files only contain switches Chromium understands. On images predating translation, Chromium ignores the unknown switch, so an opt-out degrades to a no-op instead of last-win clobbering boot-time disable lists.
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Summary
Enables WebMCP (Chromium's
navigator.modelContextAPI plus its CDP domain) in default browser launch flags, and makes flag merging feature-list-aware so per-session flags can't silently drop image defaults.Feature strings
Verified against the Chromium 151 sources:
chrome://flags/#enable-webmcp-testingmaps to base featureWebMCPTesting(disabled by default; implies the runtimeWebMCPfeature in the renderer), anddevtools-webmcp-supportmaps toDevToolsWebMCPSupport, which gates registration of the CDPWebMCPdomain handler. Browser-side use also requires theToolspermissions policy (EnableForSelf, depends on WebMCP), which top-level pages get by default.On Chromium 151+,
--enable-features=WebMCPTesting,DevToolsWebMCPSupportturns on both the JS API and the WebMCP CDP domain.What changed
server/cmd/wrapper/chromium.go) and headful defaults (images/chromium-headful/run-{unikernel,docker}.sh): add--enable-features=WebMCPTesting,DevToolsWebMCPSupport.chromiumflags.MergeFlags: union--enable-features/--disable-featuresCSV values across base and runtime sources into single tokens. Chromium keeps only one value per switch, so previously a session-supplied--enable-features=Xwould have silently dropped the image's entire default list (or vice versa). A feature present in both lists resolves to disabled inside Chromium, so sessions can still opt out with--disable-features=WebMCPTesting.chromiumflags.TranslateKernelDisableFeatures: folds a kernel-namespaced--kernel-disable-features=token into the single--disable-featuresswitch at flag-assembly time (both in the launcher and before/chromium/flagsis written). Chromium ignores unknown switches, so on images predating translation the token is inert — an opt-out degrades to a no-op instead of last-win clobbering boot-time disable lists.Tradeoffs
navigator.modelContextto pages, which stock Chrome 151 does not show without an origin trial or experimental-web-platform-features. Sites can detect that surface as a deviation. Callers that need stock-matching behavior can pass--disable-features=WebMCPTesting,DevToolsWebMCPSupport(or the kernel-namespaced token) through existing flag mechanisms; the control plane does this for stealth sessions.Testing
go test ./lib/chromiumflags/— extended table tests for feature-list merging (union across sources, opt-out via disable list, coexistence) and for kernel-token translation (fold into existing list, create list when absent, idempotency). All pass.go build ./...and fullgo test ./...run; remaining failures (display/OTLP/extension e2e tests requiring a running Chromium/X environment) fail identically on main.