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The latest published-package check is the 2026-10-25 verification. Detailed 2026-10-22 interoperability findings cover the merged public API, context providers, protected HTTP, and observed client limitations. The dated verification below retains the earlier baseline. Verified on 2026-10-17 using Node 24.20.0 on macOS arm64. This report covers the dual-era working tree: MCP 2025-06-18 and 2026-07-28 tools over stdio and independent HTTP POSTs with JSON responses. Local reports record the checked-out base Git revision; verification can include uncommitted working-tree changes.

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Results

Codex’s normal test configuration contains neither features.mcp_2026_07_28 nor CODEX_MCP_PROTOCOL_VERSION. Captured traffic starts with initialize requesting 2025-06-18, then notifications/initialized, tools/list, and tools/call. Both transports return the expected identity and tool schema, ASCII and Unicode greetings, and isError: true for empty or numeric names. All six client/transport combinations across the three configurations pass, including 24 tool calls. The same HTTP example serves both eras. Modern captures confirm server/discover, per-request version/capability metadata, and matching HTTP routing headers. Legacy wire results omit modern resultType and cache fields; modern wire results retain them. CLI clients can normalize these fields before printing, so envelope assertions use raw captures and integration tests. After separating the modern and legacy modules on 2026-10-17, all client checks above pass again. A frozen build of a235d7e and the refactored build also produce identical results across 147 observations covering both eras, HTTP statuses and headers, malformed requests, stdio initialization, cancellation, and shutdown. Public server, transport, and HTTP declarations remain byte-identical. Modern modules have no direct or transitive dependency on legacy modules within MCP or HTTP; execution and transport behavior stay shared.

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Unified tool API verification — 2026-10-18

After moving tool authoring and execution into @toolgrain/mcp, Inspector 2.11.0 passes all 30 checks again over real stdio and plain Node HTTP, covering modern, legacy, and automatic discovery. Strict tool listings now also verify the title, read-only annotation, and object output schema. The Codex results above describe the earlier verification; Codex was not rerun for this API change. The library suites and six example applications pass 182 tests. Each example imports tool and createServer from @toolgrain/mcp and verifies metadata, output-schema discovery, and execution in both revisions. Additional server tests cover parsed/defaulted/transformed outputs, asynchronous validation, rich content, explicit errors, cancellation during output validation, and incompatible legacy metadata/output-schema omission. Compiled package/declaration checks pass, and vendor integrity remains unchanged. MCP coverage is 99.52% statements, 99.32% branches, 99.03% functions, and 100% lines. Structured error validation was also rechecked on 2026-10-18: both revisions now validate any supplied structuredContent against the declared output schema, including isError: true results. Text-only errors may omit structured data; output defaults do not create it. Regression tests cover parsed error defaults, transforms, async refinements, and 0, false, and null payloads. All 93 MCP and 38 HTTP tests pass, as do source type checks and compiled declaration checks for valid text-only errors and rejected incompatible structured errors. Review fixes on 2026-10-19 treat undefined optional mcpResult() envelope and content fields, and undefined optional properties after output parsing, as absent. Undefined values inside unparsed structured data are still rejected. Text that serializes the original structured data is replaced by the parsed serialization. Tools and rich results are recognized across duplicate package copies through registered-symbol brands; spreads still lose the brand. MCP now passes 99 tests.

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Normal Codex setup

Build the examples and configure the command or URL using normal MCP settings:
Use a Node version satisfying the repository’s runtime requirement. Start the HTTP example separately; Codex launches the stdio command itself. No modern feature flag or per-server protocol marker is required for the tested build. Persistent user configuration was not changed during verification.

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Explicit modern verification

These optional settings select 2026-07-28 in the tested Codex alpha:
HTTP uses the feature flag; stdio also needs the marker for modern mode. Without the marker, that Codex build still connects over legacy stdio. These settings are specific to the tested alpha and may change.

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Repeat the Inspector checks

Use Node 24.20.0 or a newer supported Node version and Bun 1.3.14:
The command (also bun run verify:inspector) pins Inspector 2.11.0 through Bunx outside the workspace, runs the compiled examples using the invoking Node executable, and starts HTTP on an available loopback port. Configuration, storage, and secrets are temporary. It cleans up subprocess groups on POSIX systems, including timeouts and signals. No browser, authentication, inference, or npm publication is needed. An uncached Inspector needs registry access. To use a preinstalled copy:
There are 30 passing checks, 15 per transport:
  • In each era: discovery/connection, strict tool-schema listing, ASCII and Unicode greetings, empty-string and numeric-input errors, and an unlisted tool.
  • Automatic discovery selects modern mode on each transport.
Inspector rejects unlisted tools client-side. That check does not exercise the server’s unknown-tool error; package and process integration tests cover it. Inspector exits 5 for isError: true. Its connection-only CLI operation is named --method initialize in both eras; modern mode performs discovery. Interactive inspect scripts select modern mode; this verification script tests both. See the official protocol-era and CLI documentation. Runs write status, stdout/stderr, exit codes, versions, and base revision to reports/compatibility/inspector.json. Failures replace earlier success reports. This manual check is not added to CI and adds no MCP SDK dependency.

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Codex verification method

Tests launched the installed codex app-server --stdio with isolated temporary configuration and an ephemeral thread. An unused loopback model provider required no authentication; no inference turn started. The control sequence was:
  1. initialize with capabilities.experimentalApi: true, then initialized.
  2. thread/start with ephemeral: true, using the temporary test directory.
  3. mcpServerStatus/list for each server and returned threadId, using detail: "toolsAndAuthOnly"; assert connection and schema.
  4. mcpServer/tool/call with names Compatibility and Zoë 世界; assert greetings.
  5. Repeat with an empty and numeric name; assert isError: true.
The control protocol has its own initialize. Separate stdio/HTTP forwarding taps capture actual MCP traffic to distinguish it. All test-owned processes were stopped. A sanitized local summary is retained at reports/compatibility/codex.json.

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Protocol behavior and limits

  • Initialization negotiates 2025-06-18, including when a client proposes another revision. A client unable to use it must stop. This does not implement or claim the proposed revision. Modern discovery advertises the two supported revisions.
  • Modern metadata and HTTP routing headers remain strict. Contradictory versions are rejected. Stdio keeps legacy readiness per connection while allowing independent modern requests; HTTP selects the revision per POST without session IDs or one client’s negotiated version affecting another.
  • Legacy HTTP initialization may omit the version header; later POSTs use MCP-Protocol-Version: 2025-06-18. Optional routing/parameter headers are checked when present. Valid initialized/cancelled notifications return 202 empty; GET and DELETE remain 405.
  • Legacy HTTP cancellation is advisory and cannot cancel another invocation: a stateless notification has no client/session identity for safe routing. Disconnects and stdio cancellation suppress delivery and abort the handler’s ctx.signal; handlers stop early only if they observe it.
  • Legacy structured content contains objects only. Other JSON outputs retain their JSON text. Modern output behavior remains unchanged.
The tested Codex build connects with normal settings. Claude, Cursor, other versions, GUI flows, authentication, remote deployments, and real-client cancellation were not exercised. Inspector was tested through its CLI. Framework examples have real-process checks for both eras; independent host-client tests use plain Node HTTP. Greeting has no parameter-header annotations, so real-client parameter mirroring remains outside this report. The dual-era specification permits both behaviors. Legacy lifecycle and HTTP transport rules govern initialization-based calls. Root bun run test:package now also installs each packed library in its own npm consumer to check declared dependency resolution, runtime imports, and strict declarations. A separate full consumer checks both protocol revisions over stdio and HTTP under Node. This installation check uses the development dependency versions and offline JWT/Clerk initialization with test configuration; it does not extend the real-client compatibility claims above. Licensing and other proposed roadmap items remain separate work.