[Bug] pi-ai/OpenAI Responses: data-only keep-alives reach JSON.parse

[xiangyuliu]

Summary

With a third-party OpenAI Responses-compatible gateway, data-only keep-alive
frames can reach the OpenAI SDK JSON parser and fail an otherwise healthy DSH
turn.

The gateway is not emitting fully conforming SSE, but a narrowly scoped
compatibility guard would let DSH work with this class of relay without hiding
other malformed payloads.

Environment

• macOS 26.6.1 (25G76)
• Node.js 24.19.0
• @deepseek-ai/dsh@0.1.5-rc.1
• @deepseek-ai/dsh-llm-pi-ai@0.1.5-rc.1
• @earendil-works/pi-ai@0.85.1
• openai@6.40.0
• custom GPT-family model through llm-pi-ai / openai-responses

Sanitized wire evidence

Two observed forms were:

data:

and:

: keep-alive
: keep-alive
data: :

The proper SSE comment lines are ignored, but the data-only lines are dispatched
as messages and passed to JSON.parse.

Actual behavior

The turn fails with messages equivalent to:

Could not parse message into JSON
From chunk: [ 'data: ' ]

Could not parse message into JSON: :
From chunk: [ ': keep-alive', ': keep-alive', 'data: :' ]

The Web UI then surfaces Unexpected end of JSON input or
Unexpected token ':', ":" is not valid JSON.

Expected behavior

When an SSE event has no explicit event type and its trimmed data payload is
exactly empty or a single colon, treat it as a malformed-but-empty keep-alive
and skip it. All other invalid JSON must still fail visibly.

Narrow workaround validated locally

At the streaming parser boundary, before JSON.parse:

const payload = sse.data.trim()
if (sse.event === null && (payload === '' || payload === ':')) continue

This was tested with:

1. empty data: skipped;
2. data: : skipped;
3. a following valid response.completed event still delivered;
4. data: not-json still throwing SyntaxError.

The best upstream location may be the pi-ai/OpenAI Responses compatibility
boundary rather than a direct modification of the vendored OpenAI SDK. The
important property is that the tolerance stays exact and does not become a
general “ignore malformed JSON” rule.

[xiangyuliu]

Additional independently observed keep-alive variant from the same class of
OpenAI Responses-compatible relay:

data: : PING

It failed twice before any model token was produced (inputTokens=0,
outputTokens=0) with:

Unexpected token ':', ": PING" is not valid JSON

This shows that matching only an empty payload or a single colon is too narrow.
I refined the local compatibility predicate to accept only the known
comment-shaped heartbeat vocabulary:

const payload = sse.data.trim()
const compatKeepAlive = payload === ''
  || /^:\s*(?:ping|keep[-_ ]?alive)?$/i.test(payload)
if (sse.event === null && compatKeepAlive) continue

The updated regression suite covers empty data:, data: :,
data: : PING, and data: : keep-alive, followed by a valid JSON event. It
also verifies that data: : ERROR and data: not-json still throw. This keeps
the workaround bounded to recognizable heartbeat frames instead of suppressing
arbitrary malformed provider output.