ExMCP 1.0.0-rc.6

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1.0.0-rc.6 is the first published dual-era ExMCP release candidate and the modern-preferred soak candidate for stable 1.0. It implements MCP 2026-07-28 while retaining every legacy revision and the complete rc.5 public API.

What changes

  • New clients default to :prefer_modern: they probe with server/discover and fall back to legacy initialize only after positive compatibility evidence on a live transport.
  • Default servers accept both eras and advertise MCP 2026-07-28 first.
  • :legacy_only preserves the legacy protocol era connection policy (rc.7 still enforces server-issued sessions and newer lifecycle/security rules).
  • MCP 2026-07-28 adds stateless per-request context, typed result envelopes, multi-round tool results, subscriptions/listen, stateless Streamable HTTP, OAuth 2026 hardening, and the negotiated Tasks extension.
  • Deprecated Roots, Sampling, protocol Logging, legacy subscriptions, and HTTP+SSE remain available for supported legacy revisions throughout 1.x.

See the migration guide for rollout steps and the API diff for the public compatibility audit.

Post-rc.6 status (2026-08-12): 1.0.0-rc.7 is the next modern-preferred soak candidate. Current master after rc.6 includes legacy SSE persist/reconnect/replay, ACP Pi multi-chunk and subprocess isolation fixes, the 2026-08-12 security harden, and CI/compat follow-ups. rc.6 itself is not promoted to stable 1.0. See RELEASE_1_0_0_RC_7.md and the 2.0 roadmap backport lane.

Installation

{:ex_mcp, "~> 1.0.0-rc.6"}

Pin the rollout policy when it must not change with a package upgrade:

config :ex_mcp, protocol_mode: :prefer_modern
# Emergency rollback / legacy protocol era (not an exact rc.5 package rollback):
# config :ex_mcp, protocol_mode: :legacy_only

Security and dependencies

The release updates Mint, Mint WebSocket, Plug, Plug Cowboy, Cowboy, Cowlib, HPAX, Decimal, and related runtime dependencies. The locked dependency set has no critical or high-severity advisories.

Cowlib 2.19.0, the newest compatible release, retains two upstream advisories:

  • EEF-CVE-2026-43966 (medium) is mitigated because Plug rejects CR/LF/NUL in response-header values and Cowboy independently terminates invalid response headers before serialization.
  • EEF-CVE-2026-43969 (low) affects cow_cookie:cookie/1, which ExMCP and its Plug/Cowboy response path do not call.

These acknowledgements are exact IDs in the Hex audit configuration. CI still fails on every unacknowledged advisory and builds the real Hex archive.

Qualification evidence and ownership

The release tag identifies the exact source commit. Its GitHub Actions run is the durable evidence record; the external-conformance job uploads the complete runner output as mcp-2026-07-28-conformance.

GateOwnerrc.6 evidence
Legacy byte fixturesProtocol maintainermix test.suite compliance; version characterization and cross-version suites
Seven-row era matrixProtocol maintainerstdio/HTTP compatibility tests in the full CI and coverage jobs
Modern conformance and SDK v2 interopRelease maintainerexternal conformance artifact plus six isolated legacy/modern Node.js lanes
MRTR retry, tamper, replay, expiry, and rotationSecurity maintainerMRTR, request-state, replay-cache, integration, and coverage suites
Subscription pressure, isolation, reconnect, and cluster fanoutRuntime maintainersubscription, PubSub, multi-node, performance, stress, and coverage suites
OAuth/CIMD/SSRF/credential isolationSecurity maintainerauthorization matrix, redirect leakage, metadata fetch, and security suites
Modern-preferred soakRelease maintainerbegins when rc.6 is published; minimum seven calendar days
Security, resource bounds, and public APIRelease maintainermix hex.audit, Sobelow, performance/stress CI, API diff, and package build
Mixed-cluster rollback drillDeployment operatorrequired before stable 1.0; record the operator run in the stable release evidence

The external suite is pinned to @modelcontextprotocol/conformance@0.2.0-alpha.10 because no stable 2026-07-28-aware runner is published; its CI lane pins Node.js 22 because the harness imports fs.globSync. Official TypeScript SDK v2 interop pins @modelcontextprotocol/client, server, and node to 2.0.0. The legacy conformance baseline remains pinned independently to 0.1.16.

Load and regression budget

The qualifying workload is mix test.suite performance on OTP 27 / Elixir 1.17. It covers the committed microbenchmarks, concurrent serialization, high-volume tool request preparation, large payloads, a ten-second sustained load interval, security checks, schema compilation, subscription queue pressure, and slow-consumer behavior.

The rc.6 gate is:

  • zero errors or timeouts in the performance and stress suites;
  • no committed absolute performance threshold may regress;
  • shared legacy workloads may not regress more than 20% in median runtime or 10% in retained memory versus the rc.5 baseline on the same runner;
  • process and mailbox counts must return to their bounded post-cleanup state;
  • slow subscribers must coalesce or disconnect according to policy without cross-principal delivery.

Any unexplained breach blocks the release. Runner noise must be resolved by repeating both revisions on the same machine, not by widening a threshold in the release commit.

Stable 1.0 gates still open

Publishing rc.6 started, but did not complete, the stable-release clock. The post-rc.6 SSE lifecycle fix resets that clock. Stable 1.0.0 now requires:

  1. at least seven calendar days of final modern-preferred RC use without a release-blocking regression; and
  2. a successful mixed-version cluster rollback drill with active subscriptions and in-flight MRTR operations.

Any wire-design, public-API, or documented lifecycle change requires another RC and restarts the soak. Current master met that condition because of the legacy SSE persistence and disconnect-lifecycle fix described above.