> This page is for version v1.1.1.
> For other versions, use one of these documentation indexes:
> - Latest (v1.5.1) (default): https://docs.nvidia.com/dynamo/latest/llms.txt
> - dev: https://docs.nvidia.com/dynamo/dev/llms.txt
> - v1.5.1: https://docs.nvidia.com/dynamo/v1.5.1/llms.txt
> - v1.5.0: https://docs.nvidia.com/dynamo/v1.5.0/llms.txt
> - v1.4.2: https://docs.nvidia.com/dynamo/v1.4.2/llms.txt
> - v1.4.1: https://docs.nvidia.com/dynamo/v1.4.1/llms.txt
> - v1.4.0: https://docs.nvidia.com/dynamo/v1.4.0/llms.txt
> - v1.3.0: https://docs.nvidia.com/dynamo/v1.3.0/llms.txt
> - v1.2.1: https://docs.nvidia.com/dynamo/v1.2.1/llms.txt
> - v1.2.0: https://docs.nvidia.com/dynamo/v1.2.0/llms.txt
> - v1.1.1: https://docs.nvidia.com/dynamo/v1.1.1/llms.txt
> - v1.1.0: https://docs.nvidia.com/dynamo/v1.1.0/llms.txt
> - v1.0.2: https://docs.nvidia.com/dynamo/v1.0.2/llms.txt
> - v1.0.1: https://docs.nvidia.com/dynamo/v1.0.1/llms.txt
> - v1.0.0: https://docs.nvidia.com/dynamo/v1.0.0/llms.txt
> - v0.9.1: https://docs.nvidia.com/dynamo/v-0-9-1/llms.txt
> - v0.9.0: https://docs.nvidia.com/dynamo/v-0-9-0/llms.txt
> - v0.8.1: https://docs.nvidia.com/dynamo/v-0-8-1/llms.txt
> - v0.8.0: https://docs.nvidia.com/dynamo/v-0-8-0/llms.txt
> - v0.7.1: https://docs.nvidia.com/dynamo/v-0-7-1/llms.txt
> - v0.7.0: https://docs.nvidia.com/dynamo/v-0-7-0/llms.txt

> For clean Markdown content of this page, append .md to this URL. For the complete documentation index, see https://docs.nvidia.com/dynamo/llms.txt. For full content including API reference and SDK examples, see https://docs.nvidia.com/dynamo/llms-full.txt.

# Reasoning Parsing (Engine Fallback)

When Dynamo's registry does not list a reasoning parser for your model, fall
back to the upstream engine's parser via a **chat-processor swap**, which
keeps frontend tokenization and KV routing.

For Dynamo-native parsers, see [Reasoning Parsing (Dynamo)](/dynamo/v1.1.1/user-guides/reasoning/reasoning-parsing-dynamo). For
the equivalent tool-call fallback, see
[Tool Call Parsing (Engine Fallback)](/dynamo/v1.1.1/user-guides/tool-calling/tool-call-parsing-engine-fallback).

<Warning>
**Known Issue:** Engine-fallback reasoning parsing does not currently work
with [disaggregated serving](/dynamo/v1.1.1/user-guides/disaggregated-serving)
(support coming soon). Use the [Dynamo-native reasoning parser](/dynamo/v1.1.1/user-guides/reasoning/reasoning-parsing-dynamo)
for disaggregated deployments today.
</Warning>

## Configurations

| | Frontend flags | Worker flags | KV routing | Notes |
|---|---|---|---|---|
| **vLLM chat processor** | `--dyn-chat-processor vllm --reasoning-parser <name>` | *(none)* | Yes | Parsing runs in vLLM's Python preprocessor. See [vLLM Chat Processor](/dynamo/v1.1.1/backends/v-llm/frontend-processor-fallback). |
| **SGLang chat processor** | `--dyn-chat-processor sglang --reasoning-parser <name>` | *(none)* | Yes | Parsing runs in SGLang's Python preprocessor. See [SGLang Chat Processor](/dynamo/v1.1.1/backends/sg-lang/chat-processor). |
| **TRTLLM chat processor** | *(work in progress)* | *(work in progress)* | -- | Engine-fallback support for TRTLLM is in progress. Use the [Dynamo-native reasoning parser](/dynamo/v1.1.1/user-guides/reasoning/reasoning-parsing-dynamo) for TRTLLM today. |

<Note>
`--dyn-reasoning-parser` selects the **Dynamo-native** parser path, while
`--reasoning-parser` selects the **engine fallback** (vLLM or SGLang)
parser path. The accepted values for each flag come from a different
registry and may differ slightly based on the definitions from each
framework (e.g., vLLM's `nemotron_v3` vs Dynamo's `nemotron3`).
</Note>

## Examples

```bash
# vLLM chat processor
python -m dynamo.vllm ...
python -m dynamo.frontend --dyn-chat-processor vllm --reasoning-parser deepseek_r1

# SGLang chat processor
python -m dynamo.sglang ...
python -m dynamo.frontend --dyn-chat-processor sglang --reasoning-parser kimi_k25
```

## See Also

- [Reasoning Parsing (Dynamo)](/dynamo/v1.1.1/user-guides/reasoning/reasoning-parsing-dynamo) -- Dynamo-native parsers and common pairings
- [Tool Call Parsing (Engine Fallback)](/dynamo/v1.1.1/user-guides/tool-calling/tool-call-parsing-engine-fallback) -- Equivalent fallback for tool-call parsers
- [vLLM Chat Processor](/dynamo/v1.1.1/backends/v-llm/frontend-processor-fallback) -- vLLM chat-processor details
- [SGLang Chat Processor](/dynamo/v1.1.1/backends/sg-lang/chat-processor) -- SGLang chat-processor details
- [Frontend Configuration Reference](../components/frontend/configuration.md) -- Full CLI flag reference