What causes "FUSION Panel ${model} returned empty content" in decolua/9router?

Inspect the panel's raw response (enable debug logging) to see where the content actually sits

FUSION Panel ${model} returned empty content

Error message

FUSION Panel ${model} returned empty content

What it means

After a panel model returns a 2xx response, the FUSION service parses its JSON and extracts text via extractPanelText. If the payload parses but contains no usable content (empty choices/message content, whitespace-only text), the panel is skipped with this warning and excluded from fusion.

Source

Thrown at open-sse/services/combo.js:601

  log.info("FUSION", `fan-out collected in ${Date.now() - t0}ms`);

  // 2. Collect successful answers.
  const answers = [];
  for (let i = 0; i < settled.length; i++) {
    const res = settled[i];
    const model = panel[i];
    if (!res) { log.warn("FUSION", `Panel ${model} dropped (straggler/timeout)`); continue; }
    if (res.__timeout) { log.warn("FUSION", `Panel ${model} timed out`); continue; }
    if (res.__error) { log.warn("FUSION", `Panel ${model} threw`, { error: res.__error?.message || String(res.__error) }); continue; }
    if (!res.ok) { log.warn("FUSION", `Panel ${model} failed`, { status: res.status }); continue; }
    try {
      const json = await res.clone().json();
      const text = extractPanelText(json);
      if (text) {
        answers.push({ model, text });
        log.info("FUSION", `Panel ${model} ok (${text.length} chars)`);
      } else {
        log.warn("FUSION", `Panel ${model} returned empty content`);
      }
    } catch (e) {
      log.warn("FUSION", `Panel ${model} unparseable`, { error: e.message || String(e) });
    }
  }

  // 3. Degrade gracefully when the panel is too thin to fuse.
  if (answers.length === 0) {
    log.warn("FUSION", "All panel models failed");
    return new Response(
      JSON.stringify({ error: { message: "All fusion panel models failed" } }),
      { status: 503, headers: { "Content-Type": "application/json" } }
    );
  }
  if (answers.length === 1) {
    log.info("FUSION", `Only ${answers[0].model} succeeded — answering directly (no fusion)`);
    return handleSingleModel(body, answers[0].model);
  }

View on GitHub (pinned to 90b52e06ff)

Solutions

Inspect the panel's raw response (enable debug logging) to see where the content actually sits
Raise max_tokens for the request so the model can produce non-empty output
Confirm the model returns standard OpenAI-style choices[0].message.content, or add a direct translator for that provider
Retry the prompt — one-off empty content (safety filter) usually resolves with rephrasing
Check whether a content filter on the provider account is suppressing output

Example fix

// before
defaults: { max_tokens: 1 }
// after
defaults: { max_tokens: 4096 }

Defensive patterns

Strategy: validation

Validate before calling

const json = await res.clone().json();
const text = extractPanelText(json);
if (typeof text !== 'string' || !text.trim()) throw new Error(`panel ${model} returned empty content`);

Type guard

const hasContent = (json) =>
  typeof json?.choices?.[0]?.message?.content === 'string' &&
  json.choices[0].message.content.trim().length > 0;

Try / catch

try {
  const text = extractPanelText(await res.clone().json());
  if (!text) throw new Error('empty content');
  return text;
} catch (e) {
  console.warn(`panel unusable: ${e.message}`); // excluded from fusion, request still served
  return null;
}

Prevention

Set a sane max_tokens floor so completions are never truncated to empty
Rephrase prompts that trip provider safety filters
Use direct translators for nonstandard providers so content lands where extractPanelText reads
Log the raw JSON when extraction fails to catch provider shape drift early

When it happens

Trigger: Upstream returned 200 with an empty body content — e.g. the model returned only a refusal stripped of content, a filter removed the output, max_tokens was set so low the completion is empty, or a provider returns a nonstandard success shape extractPanelText doesn't recognize.

Common situations: max_tokens=1 or tiny budget yields empty text; safety filters blank the content; a translator maps the provider's response into a shape where content lands in a field extractPanelText doesn't read (nonstandard model); streaming-only endpoints return an empty non-stream body.

Related errors

AI-assisted analysis of decolua/9router@90b52e06ff (2026-08-30). Data as JSON: /api/errors/2c2daa3735cdca07. Report an issue: GitHub.