> ## Documentation Index
> Fetch the complete documentation index at: https://docs.zespan.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Playground — compare a prompt across models side-by-side
> Run one versioned prompt against up to three models at once, compare cost, latency, and output, and save the winner as a new prompt version.
The Playground is where you compare **one prompt** across **different models** (or, per window, different prompt variants) to see what actually changes before you commit to it. The design is deliberately narrow: one shared prompt, up to three comparison windows, real execution against your own models — not a free-form chat toy.
Playground execution requires an [LLM connection](/platform/llm-connections) configured for your project — it always runs against your own connected provider, never a shared/managed key. If none is configured, you'll see a **Connect a provider** banner in place of the run controls.
## Opening the Playground
Navigate to **Playground** in the left sidebar for a blank session, or click **Test** in the header of a [prompt's detail view](/dashboard/prompts) to open it pre-filled with that prompt's text, version, and — if there's a recent failing generation — a one-click **Replay a failing trace** shortcut.
## The shared prompt rail
The left-hand rail holds **the prompt** — the single versioned artifact every comparison window runs by default. Edit it once and every window updates:
* **Single mode** — one template with `{{variable}}` placeholders.
* **Chat mode** — a system message plus a list of seed turns (few-shot examples, or the exact state a real conversation was in when it broke).
* **Tools & structured output** (collapsed by default) — shared tool definitions (JSON) and an optional structured-output schema, used by every window.
A **Save as new version** button on the rail versions this shared prompt directly, without needing to run it first.
This panel is what gets versioned. Everything in a comparison window below it — output, cost, latency — is disposable evidence of how the prompt behaves, not the artifact itself.
### Per-window override
Any window can break away from the shared prompt: click **Override** on a window to give it its own prompt text (a **CUSTOM PROMPT** badge marks it), or **Reset to shared** to go back to inheriting the rail. This is how you A/B two different prompt variants — not just two models — inside the same comparison.
## Comparison windows
Each window is an independent run: pick a model, run it, read the result. You can have up to **3 windows** open at once — beyond that stops being a comparison and starts being noise. Use the dashed **Add a window** tile to add one (it duplicates the active window's connection/model/toggles, but never its results), and duplicate or remove windows from each card's header.
### Connection-aware model selection
Each window has its own connection/model picker. The **model list is derived from the LLM connection you pick** — you can't accidentally select a Google model while only an OpenAI connection is configured. Switching a window's connection resets its model to a sensible default for that connection's provider (or keeps your current pick if it's still valid).
### Single vs. Chat mode
A page-level toggle switches every window between:
* **Single** — one request in, one output out. Shows output text, token counts, cost, latency, guardrail results, and (if the prompt calls tools) an interactive tool-call panel where you can submit synthetic tool results and continue the run.
* **Chat** — a live multi-turn conversation per window, seeded from the rail's system prompt and seed turns. Non-baseline windows (window 2 and 3) show a **divergence banner** the moment their tool-call pattern differs from window 1's corresponding turn — turn-by-turn, based on real transcript comparison, not a guess.
### Toggles
Each window has independent **Streaming** and **Guardrails** toggles (guardrails apply your project's configured policies to the run and show pass/fail banners), plus the temperature/max-tokens summary from the shared config.
### Reading results
Non-baseline windows (2 and 3) show latency and cost as a percentage delta against window 1, so you can see at a glance whether a model is faster/cheaper or slower/pricier — never a fabricated number when the baseline hasn't run yet. You can also run any configured evaluator against a window's output directly from its **Run Eval** control.
## The verdict banner
Once window 1 and the last window both have results, a verdict banner summarizes the comparison:
* If a guardrail blocked one window's output, the banner leads with that — a blocked output is a failure, not a saving, and isn't treated as a valid comparison.
* Otherwise it reports whether the outputs matched (single mode only), the latency/cost deltas, and — when the session was seeded from a prompt with known call volume — a projected daily cost delta at that volume.
* If any window is running on an overridden (non-shared) prompt, the banner flags that the comparison isn't a pure model comparison.
From the banner you can **save the candidate window as a new prompt version**, **add it to a dataset**, or **discard** it.
### Run over dataset
When the session was seeded from a prompt, the verdict banner's **Run over dataset** button takes you to that prompt's [Versions tab](/dashboard/prompts#the-quality-gate), where you run the candidate version over a full dataset and put it through the quality gate — a single Playground run is evidence, not proof.
## Importing input
The left rail also includes an **Import from trace** control: paste a trace ID and Zespan pulls that trace's prompt, provider, model, and config into the session, so you can reproduce a specific production call. If the session was seeded from a prompt with a recent failing generation, a **Replay a failing trace** shortcut does the same thing without typing an ID.
## Saving your work
* **Save as new version** (rail or window footer) creates a new, unlabeled draft prompt version from the shared rail or a specific window's effective prompt. It never deploys anything — you still promote it to a label from the prompt's Versions tab.
* **Add to dataset** (per window, or from the verdict banner) records the window's last run as a dataset item, provided that run came from an imported trace and the session was seeded from a prompt.
* **Share** copies a link that encodes the active window's prompt, provider, model, and config, so a teammate opens the same setup.
* **History** keeps your last 50 runs in this browser (per project) so you can reload a previous prompt/model/config combination.
* **Reset** clears all windows back to a single blank one.
Playground sessions and history are stored in your browser's local storage, scoped to the project. They aren't shared across devices or team members — use **Share** or **Save as new version** to hand off a result to someone else.