[Bug] OpenCode and Antigravity emit Content-Length frames; Antigravity cannot read NDJSON

Package

Other

Description

Affected packages: @microsoft/agent-governance-opencode 5.0.0 and @microsoft/agent-governance-antigravity-cli 5.0.0.

The MCP stdio transport specifies newline-delimited JSON, but both bundled servers write responses using LSP-style Content-Length framing:

• OpenCode: agent-governance-opencode/server/agt-mcp.mjs:116.
• Antigravity: agent-governance-antigravity-cli/assets/extensions/agt-global-policy/mcp/server.mjs:295.

OpenCode accepts both newline-delimited JSON and Content-Length-framed input, but its responses are not newline-delimited JSON. Antigravity's drainInputBuffer waits for \r\n\r\n before processing a request (mcp/server.mjs:52–67), so newline-delimited requests are never processed. In the Antigravity runtime check, a newline-delimited initialize request produced no output and the process exited with status 0 on EOF; the same request with Content-Length framing received a Content-Length-framed response.

A spec-conformant MCP client therefore cannot complete initialization with either server. Antigravity also cannot process subsequent newline-delimited requests. This prevents access to their tools and to server/discover, which landed in #3642; #3642 did not introduce the framing defect.

imran-siddique identified this in the review of #3642:
https://github.com/microsoft/agent-governance-toolkit/pull/3642#pullrequestreview-5098013134

The claude-code server has the same write-side defect, but its fix is covered by #3833 and is outside this issue's scope.

The planned follow-up is one PR covering OpenCode's write path and both Antigravity transport directions, within the boundary described in the proposed ADR-0027. That PR will follow #3833 so transport and discovery can be tested together.

How does this impact your work?

End-to-end use of these two servers is blocked for spec-conformant MCP clients: initialization cannot complete, so their governance and policy tools cannot be reached. The workaround is a client that supports LSP-style Content-Length framing.

Timeline

No hard deadline. The fix PR is planned after #3833 lands.

Steps to Reproduce

1. Clone the repository and check out the revision used for verification:

   git clone https://github.com/microsoft/agent-governance-toolkit.git
   cd agent-governance-toolkit
   git checkout c63c51e
   

2. Send Antigravity a newline-delimited initialize request:

   request='{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2026-07-28","capabilities":{},"clientInfo":{"name":"probe","version":"0"}}}'
   printf '%s\n' "$request" |
     node agent-governance-antigravity-cli/assets/extensions/agt-global-policy/mcp/server.mjs
   

   Observed: no output; the server exits with status 0 on EOF without processing the request.

   Expected: the server processes the request and writes a newline-delimited JSON response.

3. Send the same request using Content-Length framing:

   length=$(node -e 'process.stdout.write(String(Buffer.byteLength(process.argv[1], "utf8")))' "$request")
   printf 'Content-Length: %s\r\n\r\n%s' "$length" "$request" |
     node agent-governance-antigravity-cli/assets/extensions/agt-global-policy/mcp/server.mjs
   

   Observed: the server responds, but the response also uses Content-Length framing.

4. Inspect OpenCode's transport implementation:

   • agent-governance-opencode/server/agt-mcp.mjs:116 serializes responses as Content-Length: ${Buffer.byteLength(body, "utf8")}\r\n\r\n${body}.
   • Its reader supports both Content-Length frames and newline-delimited JSON, as documented in agent-governance-opencode/README.md:64.

   OpenCode was verified by source inspection only; its runtime reproduction was not run.

Environment

• OS: macOS 26.6.2 (Darwin 25.6)
• Node.js: v26.3.0
• Repository revision: c63c51e on main (2026-09-15)
• OpenCode package: @microsoft/agent-governance-opencode 5.0.0
• Antigravity package: @microsoft/agent-governance-antigravity-cli 5.0.0
• Declared Node.js requirements: OpenCode >=22.0.0; Antigravity >=20.19.0
• Verification: Antigravity runtime checks and OpenCode source inspection

Logs / Error Output

Antigravity — newline-delimited initialize request:
(no stdout output; process exits with status 0 on EOF)

Antigravity — the same initialize request with Content-Length framing:
Content-Length: 154

{"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2024-11-05","capabilities":{"tools":{}},"serverInfo":{"name":"agt-global-policy","version":"3.3.0"}}}

Code of Conduct

• I agree to follow the Microsoft Open Source Code of Conduct

Start with agent-governance-opencode/server/agt-mcp.mjs:116 and agent-governance-antigravity-cli/assets/extensions/agt-global-policy/mcp/server.mjs:52–67, 295; run the provided Antigravity initialize probes at revision c63c51e. Trace both read and write paths, then verify that newline-delimited MCP requests receive newline-delimited responses for initialization and later requests, including server/discover.