Summary

Goose's Anthropic SSE stream parser requires data: (with a trailing space) before JSON payloads, but the SSE specification treats the space as optional. Providers that send data:{"type":...} instead of data: {"type":...} are silently ignored, resulting in "Stream yielded no message" and an empty session.

Affected providers

• Kimi Code (api.kimi.com/coding) — Anthropic-compatible endpoint sends data: without space. This is the provider that exposed the issue.
• Potentially any third-party Anthropic-compatible provider.

Steps to reproduce

1. Configure a custom Anthropic-compatible provider pointing to Kimi's API:

   {
     "name": "kimi",
     "engine": "anthropic",
     "base_url": "https://api.kimi.com/coding",
     "models": [{"name": "kimi-for-coding", "context_limit": 262144}],
     "supports_streaming": true,
     "requires_auth": true
   }

2. Run goose run --text "Hello" --no-session
3. Goose connects, receives a valid 200 response with SSE events, but displays no output.

Root cause

In crates/goose/src/providers/formats/anthropic.rs line 575:

if line.trim().is_empty() || !line.starts_with("data: ") {
    continue;
}

This skips all lines that start with data: but not data: . The same issue exists in the OpenAI, OpenAI Responses, and Snowflake format parsers.

Expected behavior

Both data: {...} and data:{...} should be parsed, per the SSE spec.

Debug log

WARN: "🔍 Anthropic no final usage to yield"
WARN: "Failed to generate session description: Execution error: Stream yielded no message"

Environment

• Goose 1.27.2
• macOS