Bridge era-mismatched client×backend cells in vMCP + live dual-era e2e

[jhrozek]

Part of #5756. Design: RFC THV-0083 (D3, Phase 4, Testing Strategy); arch doc §"The dual-era matrix" + §"Testing strategy". Mirrors the transport-proxy e2e issue #5837.

Bridge the two era-mismatched client×backend cells RFC-0081 defers to vMCP, and close with a live dual-era e2e suite. Both cells are single-request-scoped on one pod — no durable store required.

Changes

• Legacy-client → Modern-backend (easy cell): terminate the client session with existing machinery, re-originate stateless calls downstream (no Mcp-Session-Id).
• Modern-client → Legacy-backend (hard cell, confused-deputy boundary): synthesize and hold a backend session for the duration of the single request (pod-local; ephemeral for a bare tool call), initialized with the calling principal's per-request outgoing auth (existing OBO / header-injection / token-exchange strategies), partitioned by (principal, backend) — never shared across principals.
• Live dual-era e2e suite: out-of-process go-sdk v1.7 peers (do NOT import go-sdk v1.7 into the module — run separate binaries/containers over the wire, as the Add MCP conformance CI job for thv run proxy #5806 conformance job does) plus a raw-HTTP client; exercise all four matrix cells; concurrency- and confidentiality-focused.

Dependencies

#5910 (server-side Modern path) and #5911 (client-side per-backend dual protocol).

Verification

• Both bridge cells end-to-end.
• Hard cell: (principal, backend) partition prevents cross-principal session reuse; the synthesized initialize presents the calling principal's OBO, not a shared vMCP identity.
• Legacy↔Legacy and Modern↔Modern cells; concurrency cross-delivery safety.

Notes

Split into PRs: easy cell, hard cell, e2e suite. The hard cell is security-critical and should not share a PR with anything else.

[jhrozek]

Heads-up: coordinate with #5993 (go-sdk v1.7 adoption) — it rewrites the client-edge classification this issue builds on

PR #5993 (adopt-go-sdk-v1.7) is in review. It bumps toolhive-core to v0.0.34 (go-sdk v1.6.1 → v1.7.0-pre.3) and reworks client-edge backend-revision classification — touching the same files the bridge cells here will: pkg/vmcp/client/client.go, pkg/vmcp/client/modern.go, pkg/vmcp/session/internal/backend/mcp_session.go, pkg/mcp/revision.go. So the order these land in matters.

If #5993 merges before this work finishes:

• Detect backend era by whether server/discover advertises 2026-07-28 in supportedVersions (SEP-2575) — not the old "any clean server/discover ⇒ Modern" heuristic (which Support MCP 2026-07-28 spec (go-sdk v1.7 via toolhive-core) #5993 removes because under real v1.7 stateful 2025-11-25 backends also answer discover, negotiating down). Route the negotiate-down case through the new errModernNegotiatedDown sentinel + the existing reclassify-on-mismatch machinery.
• Reuse mcpparser.StripReservedModernMeta on the synthesized Legacy-backend egress in the Modern-client→Legacy-backend cell — don't hand-roll a second stripper (Support MCP 2026-07-28 spec (go-sdk v1.7 via toolhive-core) #5993 adds this helper).
• The module is on go-sdk v1.7 now, so the e2e no longer needs the out-of-process-only workaround adopted purely to avoid importing v1.7.

If this work finishes before #5993 merges:

• Do not bump go.mod / import go-sdk v1.7 — Support MCP 2026-07-28 spec (go-sdk v1.7 via toolhive-core) #5993 owns that bump; a second one is an MVS conflict.
• Key the bridge logic off the result of backend-era detection (a Revision value), not off probeRevision's current heuristic internals, so Support MCP 2026-07-28 spec (go-sdk v1.7 via toolhive-core) #5993's rework doesn't invalidate it. Expect a rebase.

Either way:

• The Modern-client→Legacy-backend cell relies on the per-backend era cache, which self-corrects on error but not on success (vMCP backend revision cache has no success/TTL re-probe (stale on stateless redeploy) #5992) — don't assume the cached label is fresh after a backend redeploy.
• Response-path _meta handling here overlaps Strip reserved io.modelcontextprotocol/* keys from backend response _meta (shared Legacy+Modern helper) #5986 (strip reserved io.modelcontextprotocol/* keys on backend→client) — coordinate so the two don't strip in different places.

Bottom line: the backend-classification design is changing under this issue via #5993, so isolate the bridge from classification internals and expect to re-key on supportedVersions.