A truncated streamed reply is accepted as a complete answer

[AbirAbbas]

Seen on dev@debffabf6.

What happened

A stream that sends a content delta and then closes without finish_reason or [DONE] is returned as a complete answer. In the 2026-09-24 hand test, the chat finalized Partial ans. The same happened on #1410 and its dev base.

Replication

Deterministic (no model). Save as internal/provider/stream_cut_test.go and run go test ./internal/provider -run TestAStreamCutBeforeItsEndIsNotAnAnswer -count=1:

package provider

import (
	"context"
	"net/http"
	"testing"
)

// A stream that ends with neither a finish_reason nor [DONE] was cut; it is not an answer.
func TestAStreamCutBeforeItsEndIsNotAnAnswer(t *testing.T) {
	handler := http.HandlerFunc(func(writer http.ResponseWriter, _ *http.Request) {
		writer.Header().Set("Content-Type", "text/event-stream")
		_, _ = writer.Write([]byte(`data: {"choices":[{"index":0,"delta":{"content":"Partial ans"}}]}` + "\n\n"))
	})
	client, err := NewClient(Config{
		APIKey: "k", BaseURL: "http://provider.test", Model: "sim/model", HTTPClient: handlerClient(handler),
	})
	if err != nil {
		t.Fatal(err)
	}
	ctx := WithStreamObserver(context.Background(), func(StreamEvent) {})
	response, err := client.CompleteWithMessages(ctx, userMessages("hi"))
	if err == nil {
		t.Fatalf("a stream cut before its end was returned as a complete answer: %q", response.Text())
	}
}

Today it fails with a stream cut before its end was returned as a complete answer: "Partial ans". The existing TestStreamedTextAnswerCarriesNoToolCalls, whose stream ends with finish_reason:"stop" and [DONE], is the control.

Through the real door (no model). This stub answers every request with one chunk and closes:

# stub.py: a provider whose every reply is one content chunk.
#   python3 stub.py cut   -> then the connection closes (no finish_reason, no [DONE])
#   python3 stub.py hold  -> then the connection stays open for ten minutes
import json, sys, time
from http.server import ThreadingHTTPServer, BaseHTTPRequestHandler
MODE = sys.argv[1]
MODEL = "deepseek/deepseek-v4-flash"
class H(BaseHTTPRequestHandler):
    protocol_version = "HTTP/1.1"
    def log_message(self, *a): pass
    def do_GET(self):  # the model listing
        b = json.dumps({"object": "list", "data": [{"id": MODEL, "object": "model"}]}).encode()
        self.send_response(200); self.send_header("Content-Type", "application/json")
        self.send_header("Content-Length", str(len(b))); self.end_headers(); self.wfile.write(b)
    def do_POST(self):
        self.rfile.read(int(self.headers.get("Content-Length", "0")))
        self.send_response(200); self.send_header("Content-Type", "text/event-stream")
        self.send_header("Connection", "close"); self.end_headers()
        chunk = {"id": "c", "object": "chat.completion.chunk", "model": MODEL,
                 "choices": [{"index": 0, "delta": {"content": "Partial ans"}, "finish_reason": None}]}
        self.wfile.write(("data: " + json.dumps(chunk) + "\n\n").encode()); self.wfile.flush()
        if MODE == "hold": time.sleep(600)
        self.close_connection = True
srv = ThreadingHTTPServer(("", 0), H)
print("http://0.0.0.0:%d/v1" % srv.server_address[1], flush=True)
srv.serve_forever()

export CODEAF_HOME=$(mktemp -d) HOME=$(mktemp -d) OPENROUTER_API_KEY=<any non-empty value>
U=$(mktemp); python3 stub.py cut > "$U" & sleep 1
R=$(mktemp -d) && git -C "$R" init -q && cd "$R"
CODEAF_BASE_URL="$(cat "$U")" codeaf chat --no-host

Send reply with the word OK. Today the chat settles Partial ans as the reply.

Where

internal/provider/sse.go:129 returns EOF at transport close; internal/provider/client.go:1741 breaks its stream loop on that EOF even when no terminal marker was seen.

The fix

Track terminal completion separately from transport EOF. Treat a premature close as a cut eligible for the existing retry policy; do not journal the partial text as a finished answer.

Acceptance

• e2e: codeaf chat against the cut stub above (or an httptest.NewServer stream in the tagged suite) that closes after one delta does not finalize Partial ans; the terminal-frame control does.
• Unit: EOF with and without finish_reason or [DONE] has distinct outcomes.
• Update the stream-failure manual and add the former EOF acceptance to invalidates.