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Version: 2.32-unstable

Hooks

from_function​

FunctionHook​

Wraps a function (or a sync/async pair) into a serializable Hook.

Produced by the @hook decorator for the single-function case. To give a hook both an optimized sync and async path, construct it directly with both function and async_function set.

init​

python
__init__(
    function: Callable[[State], None] | None = None,
    async_function: Callable[[State], Awaitable[None]] | None = None,
) -> None

Initialize the hook with a synchronous function, an async function, or both.

Parameters:

  • function (Callable\[[State], None] | None) – The synchronous function invoked by run. Must be a regular function — coroutine functions should be passed to async_function instead. Either function or async_function (or both) must be set.
  • async_function (Callable\[[State], Awaitable[None]] | None) – Optional coroutine function awaited by run_async. When only async_function is set, run raises a RuntimeError. When only function is set, run_async calls function.

Raises:

  • ValueError – If neither is set, if function is a coroutine function, if async_function is not, or if a provided function does not declare a State-typed parameter.

run​

python
run(state: State) -> None

Run the synchronous function against the live State.

Parameters:

  • state (State) – The Agent's live State, mutated in place by the wrapped function.

Raises:

  • RuntimeError – If the hook only has an async_function; use the Agent's async run methods instead.

run_async​

python
run_async(state: State) -> None

Await the async function if set, otherwise call the synchronous function.

Parameters:

  • state (State) – The Agent's live State, mutated in place by the wrapped function.

to_dict​

python
to_dict() -> dict[str, Any]

Serialize the hook, storing each wrapped function as an importable reference.

Returns:

  • dict[str, Any] – A dictionary with the hook's type and the import paths of its sync/async functions.

from_dict​

python
from_dict(data: dict[str, Any]) -> FunctionHook

Deserialize the hook, resolving each function from its importable reference.

Parameters:

  • data (dict[str, Any]) – The serialized hook dictionary produced by to_dict.

Returns:

  • FunctionHook – The reconstructed FunctionHook.

hook​

python
hook(function: Callable[[State], None | Awaitable[None]]) -> FunctionHook

Wrap a function into a Hook the Agent can invoke during its run loop.

The decorated function receives the Agent's State and influences the run by mutating it in place. A coroutine function is wrapped as the hook's async path; a regular function as its sync path. To give a single hook both paths, construct a FunctionHook directly with both function and async_function.

Usage example​

python
from haystack.components.agents import Agent
from haystack.hooks import hook
from haystack.components.agents.state import State
from haystack.dataclasses import ChatMessage

@hook
def require_save(state: State) -> None:
    if state.get("tool_call_counts", {}).get("save", 0) == 0:
        state.set("messages", [ChatMessage.from_system("You must call `save` before finishing.")])
        state.set("continue_run", True)

agent = Agent(chat_generator=..., tools=[...], hooks={"on_exit": [require_save]})

Parameters:

  • function (Callable\[[State], None | Awaitable[None]]) – A callable taking the Agent's State and returning None (sync or async).

Returns:

  • FunctionHook – A FunctionHook wrapping the function.

protocol​

Hook​

Bases: Protocol

A callable the Agent invokes at a point in its run loop, receiving the live State.

A hook influences the run only by mutating State in place. At least messages (the conversation), step_count, token_usage and tool_call_counts are available; any additional keys defined in the Agent's state_schema are available too. The same hook object can be registered under multiple hook points.

Implement this protocol directly for stateful hooks (e.g. one wrapping a component), or use the @hook decorator to wrap a plain (State) -> None function.

A hook may additionally define async def run_async(self, state: State) -> None for true async behavior; when absent, the Agent calls run during async runs. It is left off this protocol on purpose so sync-only hooks don't have to implement it.

A hook may also implement the optional lifecycle methods warm_up / warm_up_async and close / close_async. The Agent calls them from its own warm_up / warm_up_async and close / close_async, so a hook can defer opening clients or reading credentials until warm-up and release them on close.

run​

python
run(state: State) -> None

Run the hook against the live State, mutating it in place.

to_dict​

python
to_dict() -> dict[str, Any]

Serialize the hook to a dictionary.

from_dict​

python
from_dict(data: dict[str, Any]) -> Hook

Deserialize the hook from a dictionary.

tool_result_offloading/hooks​

ToolResultOffloadHook​

Offload tool results to a ToolResultStore, replacing them in the conversation with a compact pointer.

This after_tool Agent hook writes the full result to the store so the next LLM call sees a reference instead of the full result. Register it on an Agent under the after_tool hook point. Which tools offload, and under what condition, is controlled per tool by offload_strategies:

python
from haystack.components.agents import Agent
from haystack.components.generators.chat import OpenAIChatGenerator
from haystack.hooks.tool_result_offloading import (
    AlwaysOffload,
    FileSystemToolResultStore,
    NeverOffload,
    OffloadOverChars,
    ToolResultOffloadHook,
)

hook = ToolResultOffloadHook(
    store=FileSystemToolResultStore(root="tool_results"),
    offload_strategies={
        "web_search": AlwaysOffload(),          # force offload
        "get_time": NeverOffload(),             # opt out
        ("read_file", "list_dir"): OffloadOverChars(4000),  # tuple key: shared policy
        "*": OffloadOverChars(8000),            # wildcard default for any unlisted tool
    },
)
agent = Agent(
    chat_generator=OpenAIChatGenerator(model="gpt-5.4-nano"),
    tools=[web_search, get_time, read_file, list_dir],
    hooks={"after_tool": [hook]},
)

A key may be a single tool name, a tuple of tool names sharing one policy, or the wildcard "*" which applies to any tool without a more specific entry. More specific keys win. A tool with no matching key (and no "*") is not offloaded.

Only successful, text tool output is offloaded. Error results (including before_tool human-in-the-loop rejections) are always left in context. Non-text results (image or file content) are also left in context, and a warning is logged when such a result has a matching offload policy; supporting only text is a deliberate choice for now. Each result is offloaded at most once, even though the hook runs on every tool step.

The hook keeps no mutable state, so a single instance can be shared across concurrent runs. The constructor store, however, is shared by every run that does not override it — fine for single-user or local use, but in a multi-user server give each run its own isolated store (a per-session directory or sandbox) via hook_context under the key RESULT_STORE_CONTEXT_KEY (agent.run(messages=[...], hook_context={RESULT_STORE_CONTEXT_KEY: per_request_store})); it overrides the constructor store for that run. Isolating the store per run keeps concurrent users from colliding on store keys or reading each other's offloaded results — important especially when a bash/read tool is scoped to the store.

init​

python
__init__(
    store: ToolResultStore,
    offload_strategies: dict[str | tuple[str, ...], OffloadPolicy],
    *,
    preview_chars: int = 200
) -> None

Initialize the hook with a store and per-tool offload strategies.

Parameters:

  • store (ToolResultStore) – Where offloaded results are written. Can be overridden per run via hook_context.
  • offload_strategies (dict[str | tuple[str, ...], OffloadPolicy]) – Mapping of tool name (or a tuple of tool names, or the wildcard "*") to the OffloadPolicy that decides whether that tool's results are offloaded.
  • preview_chars (int) – Number of leading characters of the original result to include in the pointer left in the conversation, so the model knows roughly what was offloaded.

run​

python
run(state: State) -> None

Offload the freshly produced tool results in state.data["messages"] according to offload_strategies.

Considers only the trailing block of tool-result messages (the current step's results); earlier history is left untouched. Offloads each of those messages its policy opts in for, and writes the rewritten conversation back to messages only if at least one message changed.

Results are written to the store this run resolves to: a per-run store passed in state's hook_context under RESULT_STORE_CONTEXT_KEY if present, otherwise the store the hook was constructed with. Supply the per-run store when calling the Agent, e.g. agent.run(messages=[...], hook_context={RESULT_STORE_CONTEXT_KEY: per_request_store}). In a multi-user server, pass an isolated store per run this way so concurrent users write to separate locations and never read each other's results.

The hook keeps no mutable state, so a single instance is safe to share across concurrent runs; isolation comes entirely from giving each run its own store via hook_context.

Parameters:

  • state (State) – The Agent's live State. Reads the per-run store from hook_context and rewrites the offloaded tool-result messages back into messages.

Returns:

  • None – None. The hook mutates state in place.

to_dict​

python
to_dict() -> dict[str, Any]

Serialize the hook, including its store and per-tool offload strategies.

Returns:

  • dict[str, Any] – A dictionary representation of the hook.

from_dict​

python
from_dict(data: dict[str, Any]) -> ToolResultOffloadHook

Deserialize the hook, reconstructing its store and offload strategies.

Parameters:

  • data (dict[str, Any]) – A dictionary representation produced by to_dict.

Returns:

  • ToolResultOffloadHook – The deserialized ToolResultOffloadHook.

tool_result_offloading/policies​

AlwaysOffload​

Bases: OffloadPolicy

Offload every result of the tool it is assigned to.

should_offload​

python
should_offload(tool_name: str, result: str, state: State) -> bool

Decide whether to offload the given tool result.

Parameters:

  • tool_name (str) – The name of the tool that produced the result (unused; this policy always offloads).
  • result (str) – The tool result string (unused; this policy always offloads).
  • state (State) – The Agent's live State (unused; this policy always offloads).

Returns:

  • bool – Always True.

NeverOffload​

Bases: OffloadPolicy

Never offload; keep the tool's full result in context. Use to opt a tool out of a wildcard default.

should_offload​

python
should_offload(tool_name: str, result: str, state: State) -> bool

Decide whether to offload the given tool result.

Parameters:

  • tool_name (str) – The name of the tool that produced the result (unused; this policy never offloads).
  • result (str) – The tool result string (unused; this policy never offloads).
  • state (State) – The Agent's live State (unused; this policy never offloads).

Returns:

  • bool – Always False.

OffloadOverChars​

Bases: OffloadPolicy

Offload a result only when its string length exceeds threshold characters.

init​

python
__init__(threshold: int) -> None

Initialize the policy with its character threshold.

Parameters:

  • threshold (int) – Offload the result when its length in characters is strictly greater than this value.

should_offload​

python
should_offload(tool_name: str, result: str, state: State) -> bool

Decide whether to offload the given tool result based on its length.

Parameters:

  • tool_name (str) – The name of the tool that produced the result (unused; only length is considered).
  • result (str) – The tool result string whose length is compared against the threshold.
  • state (State) – The Agent's live State (unused; only length is considered).

Returns:

  • bool – True when result is longer than threshold characters, otherwise False.

to_dict​

python
to_dict() -> dict[str, Any]

Serialize the policy, including its threshold.

Returns:

  • dict[str, Any] – A dictionary representation of the policy.

tool_result_offloading/stores​

FileSystemToolResultStore​

Bases: ToolResultStore

A ToolResultStore that writes offloaded tool results to files under a root directory on the local file system.

python
from haystack.hooks.tool_result_offloading import FileSystemToolResultStore

store = FileSystemToolResultStore(root="tool_results")
reference = store.write(key="search_1.txt", content="...")
store.read(reference)

init​

python
__init__(root: str | Path) -> None

Initialize the store with the root directory results are written under.

Parameters:

  • root (str | Path) – Directory under which result files are written. Created on first write if it does not exist.

write​

python
write(*, key: str, content: str) -> str

Write content to <root>/<key>, creating parent directories, and return the file path.

The resolved target must stay within the root directory: a key that escapes it (e.g. containing ../ or an absolute path) is rejected, so a tool-provided key cannot write outside the store.

Parameters:

  • key (str) – Relative file name for the result within the store root.
  • content (str) – The tool result to persist.

Returns:

  • str – The absolute path the content was written to, as a string, for use with read.

Raises:

  • ValueError – If key resolves to a location outside the store root.

read​

python
read(reference: str) -> str

Read back the content previously written to reference.

Parameters:

  • reference (str) – A path returned by write.

Returns:

  • str – The stored content.

to_dict​

python
to_dict() -> dict[str, Any]

Serialize the store, storing its root directory as a string.

Returns:

  • dict[str, Any] – A dictionary representation of the store.

from_dict​

python
from_dict(data: dict[str, Any]) -> FileSystemToolResultStore

Deserialize the store from a dictionary.

Parameters:

  • data (dict[str, Any]) – A dictionary representation produced by to_dict.

Returns:

  • FileSystemToolResultStore – The deserialized FileSystemToolResultStore.

tool_result_offloading/types/protocol​

ToolResultStore​

Bases: Protocol

A place a ToolResultOffloadHook writes offloaded tool results to, and reads them back from.

Implementations decide where and how the content lives (local disk, an isolated sandbox filesystem, object storage, ...). write returns an opaque reference string that the Agent puts in the conversation in place of the full result; read resolves that reference back to the original content.

Implement both to_dict and from_dict to make a custom store serializable; the default implementations below cover stores whose constructor takes no arguments.

write​

python
write(*, key: str, content: str) -> str

Persist content under key and return an opaque reference to it.

Parameters:

  • key (str) – A stable, per-result identifier the hook derives from the tool call (e.g. a file name).
  • content (str) – The tool result to persist.

Returns:

  • str – A reference string (e.g. a path or URI) that read can later resolve.

read​

python
read(reference: str) -> str

Return the content previously stored under reference.

to_dict​

python
to_dict() -> dict[str, Any]

Serialize the store to a dictionary.

from_dict​

python
from_dict(data: dict[str, Any]) -> ToolResultStore

Deserialize the store from a dictionary.

OffloadPolicy​

Bases: Protocol

Decides, per tool result, whether the ToolResultOffloadHook offloads it to the store or leaves it in context.

A ToolResultOffloadHook maps tool names to policies, so different tools can offload under different conditions (always, never, or a custom rule such as a size threshold).

Implement both to_dict and from_dict to make a custom policy serializable; the default implementations below cover policies whose constructor takes no arguments.

should_offload​

python
should_offload(tool_name: str, result: str, state: State) -> bool

Return whether the given tool result should be offloaded.

Parameters:

  • tool_name (str) – The name of the tool that produced the result.
  • result (str) – The tool result as a string (the content that would otherwise stay in the conversation).
  • state (State) – The Agent's live State, for policies that decide based on run context.

Returns:

  • bool – True to offload the result to the store, False to leave it in context.

to_dict​

python
to_dict() -> dict[str, Any]

Serialize the policy to a dictionary.

from_dict​

python
from_dict(data: dict[str, Any]) -> OffloadPolicy

Deserialize the policy from a dictionary.