[Bug]: AI SDK ignores SSE event: field — custom server events crash Chat Completions stream with TypeValidationError #38106

Description

Тело:

Description

Summary

When a custom OpenAI-compatible provider emits named SSE events (using the
standard event: field from the SSE spec) inside a /v1/chat/completions
stream, OpenCode's AI SDK parser attempts to validate every data: line as a
chat.completion.chunk and throws a TypeValidationError, terminating the
agent turn.

Environment

• OpenCode version: opencode --version → 1.x.x
• Provider: custom OpenAI-compatible endpoint (Hermes Agent API server,
  @ai-sdk/openai-compatible)
• Model: any
• Runtime: ai-sdk

Reproducing

1. Provider config (opencode.json)json

{
"$schema": "https://opencode.ai/config.json",
"provider": {
"hermes": {
"npm": "@ai-sdk/openai-compatible",
"name": "Hermes Agent",
"options": {
"baseURL": "https://hermes-endpoint/v1"
},
"models": {
"glm-5": {
"name": "GLM-5",
"limit": { "context": 131072, "output": 8192 }
}
}
}
}
}

2. Trigger a tool call

Any prompt that causes the agent to call a tool, e.g.:bash
opencode run "search for X" --model hermes/glm-5

3. ErrorType validation failed: Value: {"tool":"web_search","emoji":"🔍","label":"...","toolCallId":"call_9e19...","status":"running"}.

Error message: [
{
"code": "invalid_union",
"errors": [
[
{
"expected": "array",
"code": "invalid_type",
"path": ["choices"],
"message": "Invalid input: expected array, received undefined"
}
],
[
{
"expected": "object",
"code": "invalid_type",
"path": ["error"],
"message": "Invalid input: expected object, received undefined"
}
]
],
"path": [],
"message": "Invalid input"
}
]

4. Root cause confirmed via curl

The Hermes API server emits named SSE events for tool-progress UX:bash
curl -N -s "https://hermes-endpoint/v1/chat/completions"
-H "Authorization: Bearer ***"
-H "Content-Type: application/json"
-d '{"model":"glm-5","messages":[{"role":"user","content":"search"}],"stream":true}'
The stream contains:data: {"id":"chatcmpl-...","object":"chat.completion.chunk","choices":[{"index":0,"delta":{"role":"assistant"},"finish_reason":null}]}

event: hermes.tool.progress
data: {"tool":"web_search","emoji":"🔍","label":"...","toolCallId":"call_...","status":"running"}

data: {"id":"chatcmpl-...","object":"chat.completion.chunk","choices":[{"index":0,"delta":{"content":"Here"},"finish_reason":null}]} (1/2)

The event: hermes.tool.progress block is a named SSE event per the
SSE specification:
data lines following an event: field must only be dispatched to listeners
registered for that event type, not to the default message handler.

The AI SDK's @ai-sdk/openai-compatible SSE parser ignores the event: field
entirely and passes every data: line to the Chat Completions chunk validator.
The tool-progress payload has no choices array and no error object, so the
Zod union fails → TypeValidationError.

5. Expected behavior

Per the SSE spec, data: lines preceded by event: foo should be:

• Ignored by consumers that only handle the default (unnamed) message event,
  OR
• Filtered out at the transport layer so they never reach the Chat
  Completions / Responses schema validator.

The fix should be in the AI SDK's SSE parser: respect the event: field
(event: without a matching listener → drop the frame), so custom provider
events don't crash the stream.

This is the same class of bug as #30916 (closed PR #30919 introduced a
configurable filterSseFrame option, but the PR was auto-closed by the cleanup
bot and never merged). A general fix — honoring the SSE event: field — would
resolve both that case and this one without requiring per-provider config.

6. Related

• Responses API SSE stream starts with a synthetic chatcmpl-dummy frame causing TypeValidationError #30916 — same root cause (non-standard SSE frame crashes validator), different
  symptom (synthetic chatcmpl-dummy on Responses API).
• PR fix(core): filter chatcmpl-dummy SSE frames from Responses API stream #30919 — proposed filterSseFrame config option, auto-closed, not merged.
• Add Provider : agentrouter.org #2784 — comment describing the same Type validation failed: Value: null
  pattern from a different provider's malformed SSE.

Operating System

Any

OpenCode version

Latest

Plugins

No response

OpenCode version

latest

Steps to reproduce

use hermes api integration

Screenshot and/or share link

No response

Operating System

macos 26

Terminal

opencode desktop, not terminal version