Configure Language Models with dspy.LM

dspy.LM is DSPy's unified interface for calling language models. It wraps LiteLLM so any provider -- OpenAI, Anthropic, Google, Together AI, Ollama, vLLM, and 100+ others -- works through one consistent API. You configure a model once, then every DSPy module uses it automatically.

Basic setup

import dspy

# Create an LM instance with a provider/model string
lm = dspy.LM("openai/gpt-4o-mini")  # or "anthropic/claude-sonnet-4-5-20250929", etc.

# Set it as the default for all DSPy modules
dspy.configure(lm=lm)

# Now any module uses this LM automatically
classify = dspy.ChainOfThought("text -> label")
result = classify(text="DSPy is great")
print(result.label)

The pattern is always: dspy.LM("provider/model") then dspy.configure(lm=lm).

Step 1 — Gather context

Before configuring an LM, confirm if not already clear:

1. Which provider? OpenAI, Anthropic, Google, Together AI, Groq, Azure, or local (Ollama/vLLM)?
2. Single model or mixed pipeline? One model everywhere, or different models per stage for cost/quality tradeoffs?
3. Reasoning model involved? (o1, o3, o4, DeepSeek-R1, Claude extended thinking) — these require temperature=1.0 or None and large max_tokens.
4. Local/offline requirement? Privacy or no-internet constraints point to Ollama or vLLM, which need extra setup.

Provider strings

DSPy uses the LiteLLM "provider/model-name" format. Here are the most common providers:

See LiteLLM provider docs for the full list.

Constructor parameters

lm = dspy.LM(
    model="openai/gpt-4o",           # Required: "provider/model-name"
    model_type="chat",                # "chat" (default), "text", or "responses"
    temperature=0.7,                  # Sampling temperature (default: provider default)
    max_tokens=1000,                  # Max output tokens (default: provider default)
    cache=True,                       # Enable built-in caching (default: True)
    num_retries=3,                    # Retry on transient failures (default: 3)
    use_developer_role=False,         # Use developer/system role (default: False)
    # Plus any extra kwargs passed to LiteLLM
)

Key parameters

• model (required) -- The provider/model string. This is the only required argument.
• temperature -- Controls randomness. Lower = more deterministic. Set to 0.0 for reproducible outputs. Reasoning models (o1, o3) require temperature=1.0 or None.
• maxtokens -- Maximum tokens in the response. Reasoning models require maxtokens >= 16000 or None.
• cache -- When True (the default), DSPy caches LM responses to reduce costs and speed up repeated calls. Set to False to disable.
• num_retries -- Number of retries with exponential backoff on transient failures.
• model_type -- Usually leave as "chat". Use "text" for completion-only models. Use "responses" for OpenAI responses API.

Per-module LM assignment

You do not have to use the same model for every step. Assign different LMs to different modules with set_lm():

expensive_lm = dspy.LM("openai/gpt-4o")
cheap_lm = dspy.LM("openai/gpt-4o-mini")

# Set a default
dspy.configure(lm=cheap_lm)

class MyPipeline(dspy.Module):
    def __init__(self):
        self.classify = dspy.Predict("text -> category")
        self.generate = dspy.ChainOfThought("text, category -> summary")

    def forward(self, text):
        category = self.classify(text=text)
        return self.generate(text=text, category=category.category)

pipeline = MyPipeline()

# Route: cheap model for classification, expensive for generation
pipeline.classify.set_lm(cheap_lm)
pipeline.generate.set_lm(expensive_lm)

When to use each override method

Temporary LM override with dspy.context

Use dspy.context to temporarily switch LMs for a block of code:

with dspy.context(lm=expensive_lm):
    # Everything inside uses expensive_lm
    result = pipeline(text="important document")

# Back to the default LM outside the block

Direct LM calls

You can call an LM instance directly for one-off prompts outside of DSPy modules:

lm = dspy.LM("openai/gpt-4o-mini")

# Pass a string prompt
response = lm("What is the capital of France?")
print(response)  # returns a list of strings

# Pass a messages list (chat format)
response = lm(messages=[
    {"role": "user", "content": "What is the capital of France?"}
])
print(response)  # returns a list of strings

Direct calls are useful for quick tests, but for structured tasks use DSPy modules and signatures -- they give you type checking, optimization, and caching.

Environment variables

Set API keys as environment variables. Never hardcode them.

# OpenAI
export OPENAI_API_KEY=sk-...

# Anthropic
export ANTHROPIC_API_KEY=sk-ant-...

# Together AI
export TOGETHER_API_KEY=...

# Google
export GEMINI_API_KEY=...

# Groq
export GROQ_API_KEY=...

# Azure OpenAI
export AZURE_API_KEY=...
export AZURE_API_BASE=https://your-resource.openai.azure.com/

DSPy (via LiteLLM) reads these automatically. You can also pass api_key directly to dspy.LM() if needed, but environment variables are preferred.

Caching

DSPy caches LM responses by default. This means:

• Repeated identical calls are free -- same prompt, same parameters, same model returns a cached result instantly with no API call.
• Development is faster -- re-running your script doesn't re-call the LM for already-seen inputs.
• Optimization is cheaper -- optimizers that re-evaluate examples benefit from cached results.

Controlling caching

# Caching enabled (default)
lm = dspy.LM("openai/gpt-4o-mini", cache=True)

# Disable caching for this LM
lm = dspy.LM("openai/gpt-4o-mini", cache=False)

# Configure cache settings globally (controls disk and memory tiers separately)
dspy.configure_cache(
    enable_disk_cache=True,        # Toggle on-disk caching
    enable_memory_cache=True,      # Toggle in-memory caching
    disk_size_limit_bytes=None,    # Optional: cap disk cache size
    memory_max_entries=None,       # Optional: cap in-memory entries
)

Cache is stored locally. If you need different responses for the same prompt (e.g., generating diverse examples), disable caching or use different temperature values.

Useful methods

Inspecting history

lm = dspy.LM("openai/gpt-4o-mini")
dspy.configure(lm=lm)

classify = dspy.Predict("text -> label")
classify(text="Hello world")

# See what was sent to the LM
dspy.inspect_history(n=1)

Connecting to local models

Ollama

# Start Ollama: ollama serve
# Pull a model: ollama pull llama3.1
lm = dspy.LM(
    "ollama_chat/llama3.1",
    api_base="http://localhost:11434",
    api_key="",
    temperature=0.7,
    num_ctx=8192,  # set context window explicitly — Ollama defaults to 4096
)
dspy.configure(lm=lm)

For full Ollama setup (model selection, GPU tuning, context window gotchas, optimization tips), see /dspy-ollama.

vLLM or any OpenAI-compatible server

# Start vLLM: vllm serve meta-llama/Llama-3.1-8B-Instruct
lm = dspy.LM(
    "openai/meta-llama/Llama-3.1-8B-Instruct",
    api_base="http://localhost:8000/v1",
    api_key="none",
)
dspy.configure(lm=lm)

For any server that exposes an OpenAI-compatible /v1/chat/completions endpoint, use the "openai/model-name" provider string with api_base pointing to your server.

For full vLLM setup (tensor parallelism, GPU sizing, quantization, production deployment), see /dspy-vllm.

Gotchas

1. Claude omits the provider prefix from the model string. Claude writes dspy.LM("gpt-4o-mini") instead of dspy.LM("openai/gpt-4o-mini"). While some models auto-detect the provider, the explicit "provider/model" format is required for reliable routing through LiteLLM. Always include the provider prefix.
2. Claude sets temperature=0 for reasoning models. OpenAI reasoning models (o1, o3, o4, gpt-5 families) require temperature=1.0 or None. Setting temperature=0 raises an error. Similarly, max_tokens must be >= 16000 or None for these models.
3. Claude calls dspy.configure(lm=lm) inside forward(). Configuration should happen once at the top of your script, not per-call. Calling dspy.configure inside forward() resets global state on every invocation and breaks caching. Use set_lm() or dspy.context() for per-module or temporary overrides instead.
4. Claude forgets apibase for local models. Ollama and vLLM require apibase pointing to the local server (http://localhost:11434 for Ollama, http://localhost:8000/v1 for vLLM). Without it, DSPy tries to reach the cloud API and fails with an authentication error.
5. Claude hardcodes API keys in source code. Set API keys as environment variables (OPENAIAPIKEY, ANTHROPICAPIKEY, etc.) — DSPy reads them automatically via LiteLLM. Passing api_key=... directly to dspy.LM() is supported but should be reserved for local dev overrides, not production code.

Related DSPy model classes

• dspy.Embedder — For computing text embeddings (RAG, semantic search). Uses the same LiteLLM provider string format: dspy.Embedder("openai/text-embedding-3-small"). Not an LM — cannot be passed to dspy.configure(lm=...). See dspy.ai/api/models/Embedder/.
• BaseLM (upcoming) — DSPy 3.3.0 (beta) introduces a typed BaseLM interface with LMRequest/LMResponse typed boundaries. Stable 3.2.1 uses dspy.LM as documented here.

Additional resources

• dspy.LM API docs
• dspy.Embedder API docs
• LiteLLM provider docs
• For API details, see [reference.md](reference.md)
• For worked examples, see [examples.md](examples.md)

Cross-references

Install any skill: npx skills add lebsral/DSPy-Programming-not-prompting-LMs-skills --skill <name>

• /dspy-signatures -- Define what your LM should do (inputs, outputs, types)
• /dspy-modules -- Wrap signatures with inference strategies (Predict, ChainOfThought, ReAct)
• /ai-switching-models -- Safely migrate between providers with re-optimization
• /ai-cutting-costs -- Reduce LM costs with per-module assignment and cheaper models
• Install /ai-do if you do not have it — it routes any AI problem to the right skill and is the fastest way to work: npx skills add lebsral/DSPy-Programming-not-prompting-LMs-skills --skill ai-do