anthropic: Support alternative provider SSE formatting (#47847)

Raphael Lüthy and Ben Brandt created 1 month ago

The issue I ran into was that responses from anthropic compatible
providers, like Kimi for Coding, have no space after `data:`. This
change just adds a quick check to also allow for those providers to
work.

Before it just resolved but did not show any output:
<img width="50%" alt="CleanShot 2026-01-28 at 12 50 31@2x"
src="https://github.com/user-attachments/assets/c3c8fe27-348e-4b21-a5f1-25bcc82f3774"
width=50%/>

Now it returns the proper result:
<img width="50%" alt="CleanShot 2026-01-28 at 12 56 30@2x"
src="https://github.com/user-attachments/assets/4e524c1e-78ab-4956-bd65-a919d46adc59"
width=50%/>

Normal Anthropic models still work as expected:
<img width="50%" alt="CleanShot 2026-01-28 at 12 58 37@2x"
src="https://github.com/user-attachments/assets/5a2906aa-1183-45b6-939b-01a6830f3385"
/>

Config to test
```json
 "language_models": {
    "anthropic": {
      "api_url": "https://api.kimi.com/coding",
      "available_models": [
          {
            "name": "kimi-for-coding",
            "display_name": "Kimi 2.5 Coding",
            "max_tokens": 262144,
            "max_output_tokens": 32768,
          },
      ],
    },
}
```


TLDR:
- Accepts SSE data:{...} lines (no space) emitted by some alternative
Anthropic providers, in addition to the standard data: {...} format.

Release Notes:

- Fixed Anthropic streaming for alternative providers by accepting SSE data:{...} (no space) lines.

---------

Co-authored-by: Ben Brandt <[e-mail]>

Change summary

crates/anthropic/src/anthropic.rs | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

Detailed changes

crates/anthropic/src/anthropic.rs

@@ -781,7 +781,10 @@ pub async fn stream_completion_with_rate_limit_info(
             .filter_map(|line| async move {
                 match line {
                     Ok(line) => {
-                        let line = line.strip_prefix("data: ")?;
+                        let line = line
+                            .strip_prefix("data: ")
+                            .or_else(|| line.strip_prefix("data:"))?;
+
                         match serde_json::from_str(line) {
                             Ok(response) => Some(Ok(response)),
                             Err(error) => Some(Err(AnthropicError::DeserializeResponse(error))),