Opt Modern backends into log notifications via logLevel _meta

[JAORMX]

Summary

The 2026-07-28 (Modern) MCP revision removed the logging/setLevel RPC — the per-request io.modelcontextprotocol/logLevel _meta key replaces it. modelcontextprotocol/go-sdk#1116 confirmed go-sdk's SetLoggingLevel is wont-fix-by-design on Modern sessions, so a Modern-classified backend never emitted notifications/message through vMCP: the Legacy-only enableBackendLogging RPC doesn't run on the Modern path, and the Modern tools/call never set the logLevel opt-in (a conformant backend MUST NOT emit logs without it).

This adds Modern logging parity with the Legacy path, in two halves (opt-in alone would elicit notifications that were then silently dropped):

• Opt-in: when server→client forwarding is bound, modernCallTool overlays logLevel: debug onto the request _meta it mints — the Modern analogue of enableBackendLogging. Unbound, the key is omitted so the backend does not emit. All other modernCall sites (discover, list, resources/read, prompts/get, completion) are unchanged.
• Relay: modernCall/readModernSSE accept an onNotification callback; interleaved SSE notifications are handed to it instead of dropped. modernCallTool wires it to a new newModernNotificationForwarder, relaying notifications/message → NotifyLog and notifications/progress → NotifyProgress downstream. A nil listener preserves the prior drop behavior, so non-tool callers are unaffected.

Also exports mcp.MetaKeyLogLevel for the overlay, and reframes two pin comments that attributed the -32020 rejection to a transient upstream bug — it is the standing contract for a removed RPC.

Closes #6130

Type of change

• Bug fix

Test plan

• Unit tests (task test)
• Linting (task lint-fix)

New unit coverage: TestModernCall_LogLevelMeta (key injected / omitted), TestModernCall_SSENotificationRelay (notification delivered AND response still matched), TestModernCallTool_LogLevelGating (bound vs unbound forwarders). Full pkg/vmcp/... and pkg/mcp suites pass.

Changes

File	Change
pkg/vmcp/client/modern.go	modernCall/readModernEnvelope/readModernSSE/mergeModernMeta take logLevel + onNotification; relay interleaved SSE notifications
pkg/vmcp/client/client.go	modernCallTool injects logLevel: debug + wires the notification forwarder when forwarding is bound; other callers pass ""/nil
pkg/vmcp/client/forwarding.go	New newModernNotificationForwarder (raw method+params → NotifyLog/NotifyProgress)
pkg/mcp/revision.go	Export MetaKeyLogLevel
pkg/vmcp/server/*_integration_test.go	Reframe the two logging/setLevel pin comments to go-sdk#1116's wont-fix resolution
docs/arch/10-virtual-mcp-architecture.md	Extend the logging limitation with the Modern dimension

Does this introduce a user-facing change?

Yes. vMCP now relays backend log (notifications/message) and progress (notifications/progress) notifications from Modern (2026-07-28) backends to the downstream client when server→client forwarding is bound — previously these were never emitted (no logLevel opt-in) and dropped (no relay). Behavior for Legacy backends and for unbound deployments is unchanged.

Special notes for reviewers

• Why two halves: injecting logLevel only asks the backend to emit; without the relay, readModernSSE dropped every interleaved notification (env.Method != ""). The opt-in without the relay is a silent no-op, so both land together.
• SSE-only delivery: the relay fires only on the text/event-stream path; a single JSON 200 body carries no notifications. This is the same inherent limitation as the Legacy standalone-stream design and is documented in the modernCall doc comment.
• Level is always debug when bound: mirrors the documented limitation (docs/arch/10) that downstream level preferences are not yet applied to the relayed stream — vMCP strips the downstream per-hop logLevel and overlays its own.

[codecov]

Codecov Report

❌ Patch coverage is 44.89796% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.46%. Comparing base (0a0cbd9) to head (9736750).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/vmcp/client/forwarding.go	7.69%	23 Missing and 1 partial ⚠️
pkg/vmcp/client/client.go	75.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6140      +/-   ##
==========================================
- Coverage   72.56%   72.46%   -0.10%     
==========================================
  Files         734      734              
  Lines       75912    75952      +40     
==========================================
- Hits        55084    55042      -42     
- Misses      16920    17015      +95     
+ Partials     3908     3895      -13     

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