Fault tolerance | LangChain Español

Fault tolerance middleware keeps your deep agent running when things go wrong. Not all errors should be handled the same way: transient failures (network timeouts, rate limits) should be retried automatically, errors the LLM can recover from (bad tool output, parsing failures) should be fed back to the model, and errors that need human input should pause the agent.

Error handling strategies

Different errors need different handling strategies:

Error type	Who fixes it	Strategy	Middleware or feature
Transient errors (network issues, rate limits)	System (automatic)	Retry with exponential backoff	ModelRetryMiddleware, ToolRetryMiddleware
LLM-recoverable errors (tool failures, parsing issues)	LLM	Convert to error `ToolMessage` and let the model adjust	ToolErrorMiddleware
User-fixable errors (missing information, unclear instructions)	Human	Pause with `interrupt()`	Human-in-the-loop
Provider outage	System (automatic)	Fall back to an alternative model	ModelFallbackMiddleware
Excessive calls (runaway loops)	System (automatic)	Cap model and tool calls per run	ModelCallLimitMiddleware, ToolCallLimitMiddleware
Unexpected errors	Developer	Let them bubble up	No middleware; let the exception propagate

The sections below cover each strategy with code examples.

Transient errors

LLM-recoverable

User-fixable

Provider outage

Excessive calls

Unexpected

Add retry middleware to automatically retry network issues and rate limits. Model calls and tool calls each have their own retry middleware with exponential backoff:

from langchain.agents import create_agent
from langchain.agents.middleware import ModelRetryMiddleware, ToolRetryMiddleware

agent = create_agent(
    model="google_genai:gemini-3.6-flash",
    tools=[search_tool, fetch_url_tool],
    middleware=[
        ModelRetryMiddleware(max_retries=3, backoff_factor=2.0, initial_delay=1.0),
        ToolRetryMiddleware(
            max_retries=2,
            tools=["search", "fetch_url"],
            retry_on=(TimeoutError, ConnectionError),
        ),
    ],
)

Use ToolErrorMiddleware to catch tool exceptions and convert them into error ToolMessages so the LLM can see what went wrong and try again:

Note

ToolErrorMiddleware requires langchain>=1.3.14.

from langchain.agents import create_agent
from langchain.agents.middleware import ToolErrorMiddleware


def on_error(exc: Exception, request: ToolCallRequest) -> str | None:
    if isinstance(exc, ValueError):
        return f"Tool `{request.tool_call['name']}` failed: {type(exc).__name__}. Fix the input and retry."
    # propagate everything else


agent = create_agent(
    model="google_genai:gemini-3.6-flash",
    tools=[search_tool],
    middleware=[ToolErrorMiddleware(on_error)],
)

Pause and collect information from the user when needed (like account IDs, order numbers, or clarifications). Use interrupt_on to pause the agent before specific tool calls:

from deepagents import create_deep_agent

agent = create_deep_agent(
    model="google_genai:gemini-3.6-flash",
    tools=[send_email_tool, delete_record_tool],
    interrupt_on={
        "send_email": True,
        "delete_record": True,
    },
)

For the full human-in-the-loop guide, see Human-in-the-loop.

If your primary model provider goes down entirely, use ModelFallbackMiddleware to switch to an alternative model:

from langchain.agents import create_agent
from langchain.agents.middleware import ModelFallbackMiddleware

agent = create_agent(
    model="google_genai:gemini-3.6-flash",
    tools=[search_tool],
    middleware=[
        ModelFallbackMiddleware("gpt-5.5"),
    ],
)

Without limits, a confused agent can burn through your LLM API budget in minutes by looping on the same tool call or making hundreds of model calls. Set caps on both model calls and tool executions per run:

from langchain.agents import create_agent
from langchain.agents.middleware import ModelCallLimitMiddleware, ToolCallLimitMiddleware

agent = create_agent(
    model="google_genai:gemini-3.6-flash",
    tools=[search_tool],
    middleware=[
        ModelCallLimitMiddleware(run_limit=50),
        ToolCallLimitMiddleware(run_limit=200),
    ],
)

Let them bubble up for debugging. Do not catch what you cannot handle. ToolErrorMiddleware only surfaces exceptions you explicitly return content for; everything else propagates unchanged:

def on_error(exc: Exception, request: ToolCallRequest) -> str | None:
    if isinstance(exc, (ValueError, KeyError)):
        # Surface known, recoverable errors to the model
        return f"Tool `{request.tool_call['name']}` failed: {type(exc).__name__}."
    # Everything else (unexpected errors) propagates and halts the run

Transient errorsLLM-recoverableUser-fixableProvider outageExcessive callsUnexpected

Add retry middleware to automatically retry network issues and rate limits. Model calls and tool calls each have their own retry middleware with exponential backoff:

from langchain.agents import create_agent
from langchain.agents.middleware import ModelRetryMiddleware, ToolRetryMiddleware

agent = create_agent(
    model="google_genai:gemini-3.6-flash",
    tools=[search_tool, fetch_url_tool],
    middleware=[
        ModelRetryMiddleware(max_retries=3, backoff_factor=2.0, initial_delay=1.0),
        ToolRetryMiddleware(
            max_retries=2,
            tools=["search", "fetch_url"],
            retry_on=(TimeoutError, ConnectionError),
        ),
    ],
)

Use ToolErrorMiddleware to catch tool exceptions and convert them into error ToolMessages so the LLM can see what went wrong and try again:

Note

ToolErrorMiddleware requires langchain>=1.3.14.

from langchain.agents import create_agent
from langchain.agents.middleware import ToolErrorMiddleware


def on_error(exc: Exception, request: ToolCallRequest) -> str | None:
    if isinstance(exc, ValueError):
        return f"Tool `{request.tool_call['name']}` failed: {type(exc).__name__}. Fix the input and retry."
    # propagate everything else


agent = create_agent(
    model="google_genai:gemini-3.6-flash",
    tools=[search_tool],
    middleware=[ToolErrorMiddleware(on_error)],
)

Pause and collect information from the user when needed (like account IDs, order numbers, or clarifications). Use interrupt_on to pause the agent before specific tool calls:

from deepagents import create_deep_agent

agent = create_deep_agent(
    model="google_genai:gemini-3.6-flash",
    tools=[send_email_tool, delete_record_tool],
    interrupt_on={
        "send_email": True,
        "delete_record": True,
    },
)

For the full human-in-the-loop guide, see Human-in-the-loop.

If your primary model provider goes down entirely, use ModelFallbackMiddleware to switch to an alternative model:

from langchain.agents import create_agent
from langchain.agents.middleware import ModelFallbackMiddleware

agent = create_agent(
    model="google_genai:gemini-3.6-flash",
    tools=[search_tool],
    middleware=[
        ModelFallbackMiddleware("gpt-5.5"),
    ],
)

from langchain.agents import create_agent
from langchain.agents.middleware import ModelCallLimitMiddleware, ToolCallLimitMiddleware

agent = create_agent(
    model="google_genai:gemini-3.6-flash",
    tools=[search_tool],
    middleware=[
        ModelCallLimitMiddleware(run_limit=50),
        ToolCallLimitMiddleware(run_limit=200),
    ],
)

Let them bubble up for debugging. Do not catch what you cannot handle. ToolErrorMiddleware only surfaces exceptions you explicitly return content for; everything else propagates unchanged:

def on_error(exc: Exception, request: ToolCallRequest) -> str | None:
    if isinstance(exc, (ValueError, KeyError)):
        # Surface known, recoverable errors to the model
        return f"Tool `{request.tool_call['name']}` failed: {type(exc).__name__}."
    # Everything else (unexpected errors) propagates and halts the run

Rate limiting

There are two complementary ways to limit resource usage: controlling the request rate to your model provider, and capping the total number of calls per run.

Provider rate limiting

Chat model providers impose a limit on the number of invocations that can be made in a given time period. To control the rate at which requests are made, initialize your model with a rate_limiter:

from langchain.rate_limiters import InMemoryRateLimiter
from langchain.chat_models import init_chat_model

rate_limiter = InMemoryRateLimiter(
    requests_per_second=0.1,  # 1 request every 10s
    check_every_n_seconds=0.1,  # Check every 100ms whether allowed to make a request
    max_bucket_size=10,  # Controls the maximum burst size
)

model = init_chat_model(
    model="google_genai:gemini-3.6-flash",
    rate_limiter=rate_limiter,  # [!code highlight]
)

agent = create_deep_agent(model=model, tools=[search_tool])

For the full configuration, see Rate limiting.

Call limits

from deepagents import create_deep_agent
from langchain.agents.middleware import ModelCallLimitMiddleware, ToolCallLimitMiddleware

agent = create_deep_agent(
    model="google_genai:gemini-3.6-flash",
    middleware=[
        ModelCallLimitMiddleware(run_limit=50),
        ToolCallLimitMiddleware(run_limit=200),
    ],
)

Use run_limit to cap calls within a single invocation (resets each turn). Use thread_limit to cap calls across an entire conversation (requires a checkpointer). See ModelCallLimitMiddleware and ToolCallLimitMiddleware for the full configuration.

Retries

Transient failures (network timeouts, rate limits) should be retried automatically. Model calls and tool calls each have their own retry middleware with exponential backoff:

from deepagents import create_deep_agent
from langchain.agents.middleware import ModelRetryMiddleware, ToolRetryMiddleware

agent = create_deep_agent(
    model="google_genai:gemini-3.6-flash",
    middleware=[
        # Retry model calls on rate limits, timeouts, and 5xx errors
        ModelRetryMiddleware(max_retries=3, backoff_factor=2.0, initial_delay=1.0),
        # Retry specific tools that hit external APIs (not all tools)
        ToolRetryMiddleware(
            max_retries=2,
            tools=["search", "fetch_url"],
            retry_on=(TimeoutError, ConnectionError),
        ),
    ],
)

Scope ToolRetryMiddleware to specific tools rather than retrying everything. A filesystem read_file that fails will not benefit from a retry, but a web search that times out probably will. See ModelRetryMiddleware for the full configuration.

Note

Major integration packages raise standard exception types (ModelAuthenticationError, ModelRateLimitError, ModelTimeoutError, and others) that carry an is_retryable flag the retry middleware respects by default. See Model exceptions for the full list.

Fallbacks

If your primary model provider goes down entirely, the fallback middleware switches to an alternative model:

from deepagents import create_deep_agent
from langchain.agents.middleware import ModelFallbackMiddleware

agent = create_deep_agent(
    model="google_genai:gemini-3.6-flash",
    middleware=[
        # If the primary model is fully down, fall back to an alternative
        ModelFallbackMiddleware("gpt-5.5"),
    ],
)

See ModelFallbackMiddleware for the full configuration.

Error handling

When a tool raises an exception during execution, the agent run halts by default. Use ToolErrorMiddleware to catch specific exceptions and convert them into error ToolMessages that the model can see and recover from, instead of crashing the run.

Note

ToolErrorMiddleware requires langchain>=1.3.14.

from deepagents import create_deep_agent
from langchain.agents.middleware import ToolErrorMiddleware


def on_error(exc: Exception, request: ToolCallRequest) -> str | None:
    if isinstance(exc, ValueError):
        return f"`{request.tool_call['name']}` failed with {type(exc).__name__}."
    # propagate everything else


agent = create_deep_agent(
    model="google_genai:gemini-3.6-flash",
    middleware=[ToolErrorMiddleware(on_error)],
)

For the full configuration options and usage patterns, including async handlers and composing with retry middleware, see Prebuilt middleware.