---
name: omp-per-model-sampler-config
description: "Set per-model sampler params and preserve_thinking in OMP's models.yml via compat.extraBody, and verify the exact wire body with an echo-capture profile."
---

# OMP Per-Model Sampler Config (`compat.extraBody`)

`~/.omp/agent/models.yml` has **no per-model sampler keys**. Sampling is global (`config.yml`: `temperature`, `topP`, `topK`, `minP`, `presencePenalty`, `repetitionPenalty`; default `-1` = **not sent**). The per-model hook is `compat.extraBody`.

## Why it works
`applyOpenAIExtraBody` runs **last** in the openai-completions builder — after `applyChatCompletionsCompatPolicy` — via `Object.assign(params, extraBody)`. So extraBody overrides global settings and compat-policy output.

## Recipe
Use **wire names**, not OMP camelCase:

```yaml
- id: frank-large
  compat:
    reasoningEffortMap:
      max: xhigh
    extraBody:
      temperature: 1.0
      top_p: 0.95
      top_k: 20
      min_p: 0.0
      presence_penalty: 0.0
      repetition_penalty: 1.0
      preserve_thinking: true
```

## Two traps

1. **`preserve_thinking` is not user-settable.** `qwenPreserveThinking` is auto-derived in `buildOpenAICompat` from the *auto-detected* `thinkingFormat` + local baseUrl, **before** `applyCompatOverrides`. Setting `compat.thinkingFormat: qwen` in models.yml does **not** enable it. Custom-named models (`frank-large`) match no Qwen id pattern → use `extraBody: {preserve_thinking: true}`.
2. **Never put `chat_template_kwargs` in `extraBody`.** `Object.assign` is shallow — it replaces the whole object and silently drops the `reasoning_effort` kwarg the compat policy set. Top-level `preserve_thinking` is honoured by halogen; use that.

## Gotcha: silent greedy decode
halogen-flash-server has **no server-side sampler defaults** — `temperature` absent or `0` = greedy. If OMP sends nothing, the endpoint decodes greedy while looking configured.

## Verify the exact wire body (echo-capture)
Don't trust the YAML — capture what OMP actually emits.

1. Echo server on `127.0.0.1:18099` that dumps the JSON body to a file and returns a canned `chat.completion` (include `reasoning_content` so thinking paths exercise).
2. Copy the **real** models.yml into a throwaway profile with only the baseUrl swapped:
   ```bash
   sed 's|http://<host>:7770/v1|http://127.0.0.1:18099/v1|' \
     ~/.omp/agent/models.yml > ~/.omp/profiles/wiretest/agent/models.yml
   ```
3. Run and inspect:
   ```bash
   omp --profile wiretest --model "<provider>/<model>" -p "hi" \
     --no-tools --no-session --no-extensions --no-skills --no-rules
   jq 'del(.messages, .tools)' last-body.json
   ```
   Confirm every intended field is present with the right value.

## Verify a field is *honoured*, not just accepted
Unknown fields often return 200 anyway. Prove behavior with a **prompt-token delta**: replay an assistant turn carrying a long `reasoning_content`, then compare `usage.prompt_tokens` with the flag true vs false. A large delta means the field changed template rendering; zero delta means it was ignored.

## Cleanup
Trash the probe profile and echo server (`profiles/.AGENTTRASH`), never `rm`.