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88 lines
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88 lines
4 KiB
Markdown
---
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title: "Ollama: `ollama run` with Piped Stdin Bypasses Chat Template + SYSTEM Prompt"
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domain: troubleshooting
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category: ai-inference
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tags: [ollama, eval, chat-template, system-prompt, majortwin, gotcha]
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status: published
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created: 2026-04-25
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updated: 2026-04-25
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---
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# Ollama: `ollama run` with Piped Stdin Bypasses Chat Template + SYSTEM Prompt
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When eval'ing or smoke-testing an Ollama model, piping a prompt via stdin to `ollama run` skips the model's chat template **and** the SYSTEM prompt baked into the Modelfile. Output looks like raw base-model completion (often Mastodon-shaped or training-data-shaped), and you'll think the model is broken when it isn't.
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## The Short Answer
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For evals and any test where you want the model's actual chat behavior, **use the HTTP API at `/api/chat`** — never `ollama run` with `echo "..." | ollama run model`.
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```python
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import json, urllib.request
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body = json.dumps({
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"model": "majortwin-v8",
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"messages": [{"role": "user", "content": "What's your name?"}],
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"stream": False,
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}).encode()
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req = urllib.request.Request(
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"http://localhost:11434/api/chat",
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data=body, headers={"Content-Type": "application/json"}, method="POST",
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)
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r = json.loads(urllib.request.urlopen(req).read())
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print(r["message"]["content"])
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```
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Or with curl piped through jq:
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```bash
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curl -s http://localhost:11434/api/chat -d '{
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"model": "majortwin-v8",
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"messages": [{"role": "user", "content": "What is your name?"}],
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"stream": false
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}' | jq -r .message.content
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```
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## How to Notice
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Symptom: model responses are weirdly raw — Mastodon-style hashtag rants, news headlines, multiple unrelated thoughts strung together — even though the same model behaves normally in Open WebUI or via the chat API. This is the canonical fingerprint of a chat-template-bypassed call.
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## Why This Happens
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`ollama run` is the CLI's interactive REPL. When stdin is a TTY, it reads input as user turns and applies the chat template. When stdin is a **pipe** (`echo "..." | ollama run model`), the CLI treats stdin as raw text and forwards it to `/api/generate` (the completion endpoint), not `/api/chat`. `/api/generate` does **not** apply the chat template, and the SYSTEM prompt only takes effect when the chat template wraps it.
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The two endpoints serve different purposes:
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- `/api/generate` — raw completion, good for fill-in-the-blank or non-instruct base models
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- `/api/chat` — applies the model's chat template, includes SYSTEM, handles multi-turn message arrays
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For an instruct-tuned model (Qwen2.5-Instruct, Llama-3.1-Instruct, etc.), bypassing the chat template means the model never sees the `<|im_start|>system ... <|im_end|>` framing it was trained to expect, and its responses regress toward base-model behavior.
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## When You Actually Want `/api/generate`
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Almost never, for instruct models. The legitimate use case is base models without a chat template, or specific completion-style prompts where you want the model to continue a string verbatim. For evals of a fine-tuned Modelfile, always use `/api/chat`.
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## Reusable Eval Pattern
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A minimal stdlib-only eval harness used for MajorTwin evals lives at `~/MajorTwin/scripts/eval_v8.py`. The key call is the `chat()` helper:
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```python
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def chat(host, model, prompt, timeout=180):
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body = json.dumps({
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"model": model,
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"messages": [{"role": "user", "content": prompt}],
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"stream": False,
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}).encode()
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req = urllib.request.Request(
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f"{host}/api/chat",
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data=body,
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headers={"Content-Type": "application/json"},
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method="POST",
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)
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with urllib.request.urlopen(req, timeout=timeout) as r:
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return json.loads(r.read())["message"]["content"].strip()
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```
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This applies the chat template and the SYSTEM prompt baked into the Modelfile. No need to re-specify SYSTEM per-call.
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## Related
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- [[ollama-macos-sleep-tailscale-disconnect]] — different Ollama gotcha (sleep + Tailscale)
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- [[20-Projects/MajorTwin/majortwin-v8-eval-report|MajorTwin v8 eval report]] — caught this issue during initial smoke test on 2026-04-25
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