# How to Enable Prefix Caching for GLM-5.2 to Reduce Decode Latency

> Reduce GLM-5.2 decode latency by enabling prefix caching. Learn how to configure your serving framework like vLLM or SGLang to reuse KV-caches, skipping the prefill stage for faster token generation.

- Repository: [Z.ai/GLM-5](https://github.com/zai-org/GLM-5)
- Tags: how-to-guide
- Published: 2026-06-21

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**To enable prefix caching for GLM-5.2, configure your serving framework—such as vLLM with `enable_prefix_caching=True` or SGLang with `prefix_caching=True`—to reuse KV-caches for identical prompt prefixes, allowing the model to skip the prefill phase and jump directly to token generation.**

GLM-5.2 (also referred to as GLM-S.2) implements a **Prefill-Decode (PD) separation** architecture that processes initial prompts during the prefill phase before switching to autoregressive decode. When deploying this model from the `zai-org/GLM-5` repository, enabling prefix caching eliminates redundant computation by storing and reusing the key-value cache of common prompt prefixes, significantly reducing latency for subsequent requests.

## Understanding Prefill-Decode Separation and Prefix Caching

GLM-5.2 separates inference into two distinct phases: the **Prefill** phase, where the model processes the input prompt to build the initial KV-cache, and the **Decode** phase, where tokens are generated autoregressively. For deployment scenarios where multiple requests share identical prompt prefixes—such as chatbots with system prompts or API services with templated queries—recomputing the prefill for every request wastes accelerator resources.

Prefix caching addresses this by persisting the KV-cache produced during the first prefill. When a subsequent request arrives with a matching prefix, the serving framework loads the cached key-value tensors and bypasses the prefill computation entirely. According to the Ascend NPU deployment documentation in [`example/ascend.md`](https://github.com/zai-org/GLM-5/blob/main/example/ascend.md), this technique suppresses latency jitter and stabilizes throughput by eliminating the costly prefill step for cached prefixes.

## Enabling Prefix Caching in vLLM

The vLLM inference engine supports automatic prefix detection and KV-cache reuse through a configuration flag. When initialized with `enable_prefix_caching=True`, vLLM maintains a cache map of prompt prefixes and skips prefill computation for matching entries.

```python
from vllm import LLM, SamplingParams

# Initialize GLM-5.2 with prefix caching enabled

engine = LLM(
    model="zai-org/GLM-5.2",
    tensor_parallel_size=1,
    enable_prefix_caching=True,  # Activates KV-cache reuse

    max_cache_size=512 * 1024 * 1024,  # Optional: limit cache memory to 512MB

)

# First request: performs prefill and caches the prefix

prompt = "Explain quantum computing in simple terms."
outputs = engine.generate([prompt], SamplingParams(max_new_tokens=64))

# Subsequent identical requests: reuse cached prefix, skip prefill

outputs = engine.generate([prompt], SamplingParams(max_new_tokens=64))

```

Key parameters for vLLM configuration:

- **`enable_prefix_caching`**: Boolean flag to activate KV-cache reuse for identical prefixes.
- **`max_cache_size`**: Upper bound on total cache memory in bytes; essential for GPU memory management.
- **`cache_block_size`**: Controls granularity of cache slices; use default unless fine-grained control is required.

## Enabling Prefix Caching in SGLang

SGLang provides native prefix caching support through its engine builder API. Setting `prefix_caching=True` enables the runtime to maintain a per-prompt cache map and load stored KV tensors for matching requests.

```python
import sglang as sgl

# Build GLM-5.2 engine with prefix caching

engine = sgl.build_engine(
    model="zai-org/GLM-5.2",
    device="cuda",
    prefix_caching=True,  # Enable KV-cache reuse

)

# First request: creates the prefix cache

response = engine.chat("Summarize the theory of relativity.")
print(response)

# Identical request: loads cached KV-cache, reduces latency

response = engine.chat("Summarize the theory of relativity.")
print(response)

```

Key parameters for SGLang configuration:

- **`prefix_caching`**: Boolean flag to activate prefix KV-cache reuse.
- **`max_prefix_len`**: Optional maximum length of cached prefixes; defaults to the model's maximum context window.

## Performance Optimization and Memory Management

Prefix caching delivers the greatest latency reduction in high-concurrency serving environments where prompt prefixes are highly repetitive. However, implementation requires careful memory management:

**When to Use:**
- Online services with shared system prompts or few-shot examples
- Batch processing pipelines with standardized input templates
- Multi-turn conversations where the conversation history serves as a growing prefix

**Memory Considerations:**
- Cache size is constrained by available GPU or NPU memory; configure `max_cache_size` (vLLM) or `max_prefix_len` (SGLang) to prevent out-of-memory errors.
- Slight variations in whitespace or formatting invalidate cache hits; normalize prompts before submission to maximize cache utilization.
- The [`README.md`](https://github.com/zai-org/GLM-5/blob/main/README.md) in the `zai-org/GLM-5` repository lists supported inference frameworks and points to framework-specific documentation for GLM-5.2 deployment.

## Key Repository Files

Understanding the architectural context of prefix caching requires referencing specific documentation within the repository:

- **[`example/ascend.md`](https://github.com/zai-org/GLM-5/blob/main/example/ascend.md)**: Describes the PD separation architecture and prefix caching benefits for Ascend NPU deployments, specifically noting how cache reuse eliminates prefill latency.
- **[`README.md`](https://github.com/zai-org/GLM-5/blob/main/README.md)**: Lists supported serving frameworks (vLLM, SGLang) and provides pointers to their respective configuration guides for GLM-5.2.
- **[`skills/glm-master-skill/SKILL.md`](https://github.com/zai-org/GLM-5/blob/main/skills/glm-master-skill/SKILL.md)**: Documents the high-level API surface and deployment patterns for the GLM-5 model series.

The actual prefix caching implementations reside in the external serving frameworks rather than the core GLM-5 source tree, but these files provide the conceptual foundation for PD separation.

## Summary

- **Prefill-Decode separation** in GLM-5.2 creates distinct computational phases that allow for cache optimization.
- **Prefix caching** stores the KV-cache from the prefill phase, enabling subsequent requests to skip directly to decode.
- **vLLM users** activate caching via `enable_prefix_caching=True` in the `LLM` constructor.
- **SGLang users** enable the feature via `prefix_caching=True` in the engine builder.
- **Memory limits** should be configured via `max_cache_size` (vLLM) or `max_prefix_len` (SGLang) to prevent resource exhaustion.
- **Repository references** include [`example/ascend.md`](https://github.com/zai-org/GLM-5/blob/main/example/ascend.md) for architectural details and [`README.md`](https://github.com/zai-org/GLM-5/blob/main/README.md) for framework compatibility.

## Frequently Asked Questions

### What is Prefill-Decode separation in GLM-5.2?

Prefill-Decode separation is an architectural pattern where the model first processes the entire input prompt in a single forward pass (the Prefill phase) to build the key-value cache, then switches to autoregressive token generation (the Decode phase). This separation allows serving frameworks to cache the expensive prefill computation and reuse it across multiple generation calls, as documented in the Ascend deployment examples.

### How does prefix caching reduce decode latency?

Prefix caching eliminates the computational overhead of the prefill phase for repeated prompt prefixes. By storing the KV-cache tensors generated during the first prefill, subsequent requests with identical prefixes bypass the prefill computation and proceed directly to the decode loop. This avoids the memory bandwidth and compute costs associated with processing long input sequences, resulting in lower and more consistent response times.

### Which serving frameworks support prefix caching for GLM-5.2?

According to the [`README.md`](https://github.com/zai-org/GLM-5/blob/main/README.md) in the `zai-org/GLM-5` repository, GLM-5.2 supports deployment through vLLM and SGLang, both of which implement prefix caching mechanisms. vLLM uses the `enable_prefix_caching` parameter, while SGLang uses `prefix_caching`. Both frameworks automatically detect prefix matches and manage cache eviction policies based on memory constraints.

### How do I configure cache memory limits for production deployments?

In vLLM, set the `max_cache_size` parameter in bytes to cap the total memory allocated for prefix caches. In SGLang, use `max_prefix_len` to limit the maximum sequence length of cached prefixes. These settings prevent cache growth from consuming excessive GPU memory, particularly when serving long-context prompts or handling high request concurrency.