Composing Custom Pipelines¶

ACE is built on a composable pipeline engine. Every runner (ACE, BrowserUse, LangChain, ClaudeCode, TraceAnalyser) is a thin wrapper around a Pipeline made of steps. You can compose your own pipelines by mixing and matching these steps — or writing custom ones.

Three Levels of ACE¶

Level	Pattern	Control
Zero-config	`ACELiteLLM.from_model("gpt-4o-mini")`	Roles + pipeline auto-created
Role customisation	`ACE.from_roles(agent=..., reflector=..., ...)`	Custom roles, pipeline auto-composed
Pipeline composition	`Pipeline([AgentStep(...), ...])`	Full control over step ordering

This guide covers Level 3 — composing pipelines directly.

Anatomy of an ACE Pipeline¶

Every ACE pipeline is a sequence of steps, each with a requires/provides contract that declares what context fields it reads and writes:

AgentStep ─────> EvaluateStep ─────> ReflectStep ─────> UpdateStep ─────> ApplyStep
  provides:        provides:           provides:          provides:         (mutates
  agent_output     trace                reflections       skill_manager     skillbook)
                                                                        _output

The pipeline validates these contracts at construction time — if a step requires a field that no earlier step provides, you'll get an error immediately.

Composing from Steps¶

All pipeline classes and ACE steps are importable from ace:

from ace import (
    # Pipeline engine
    Pipeline, Branch, MergeStrategy, StepProtocol, SampleResult,
    # ACE context
    ACEStepContext, SkillbookView,
    # Roles
    Agent, Reflector, SkillManager,
    # Steps
    AgentStep, EvaluateStep, learning_tail,
    # Types
    Sample, Skillbook, SimpleEnvironment,
)

skillbook = Skillbook()

pipe = Pipeline([
    AgentStep(Agent("gpt-4o-mini")),
    EvaluateStep(SimpleEnvironment()),
    *learning_tail(Reflector("gpt-4o-mini"), SkillManager("gpt-4o-mini"), skillbook),
])

Using `learning_tail()`¶

The learning_tail() helper returns the standard learning step sequence:

from ace import learning_tail, Reflector, SkillManager, Skillbook

steps = learning_tail(
    Reflector(llm),
    SkillManager(llm),
    Skillbook(),
    dedup_manager=my_dedup_manager,      # optional
    checkpoint_dir="/tmp/checkpoints",    # optional
)
# Returns: [ReflectStep, UpdateStep, ApplyStep,
#           DeduplicateStep, CheckpointStep]

Use it when building custom integrations that provide their own execute step but want the standard learning pipeline.

Inspecting Runner Presets with `build_steps()`¶

Every runner has a build_steps() classmethod that returns the step list it would use internally. This lets you inspect, modify, and recompose:

from ace import ACE, Pipeline, ACERunner, Skillbook

# Get the default steps
steps = ACE.build_steps(
    agent=my_agent,
    reflector=my_reflector,
    skill_manager=my_skill_manager,
    environment=my_env,
)

# Insert a custom step after EvaluateStep
steps.insert(2, MyLoggingStep())

# Build your own pipeline and runner
skillbook = Skillbook()
pipe = Pipeline(steps)
runner = ACERunner(pipeline=pipe, skillbook=skillbook)
results = runner.run(samples)

All runners support build_steps(): ACE, BrowserUse, ClaudeCode, LangChain, and TraceAnalyser.

Writing Custom Steps¶

A step is any object satisfying StepProtocol — no base class needed:

from ace import ACEStepContext

class MyLoggingStep:
    requires = frozenset({"agent_output"})
    provides = frozenset()

    def __call__(self, ctx: ACEStepContext) -> ACEStepContext:
        print(f"Agent answered: {ctx.agent_output.final_answer}")
        return ctx

Key rules:

requires: frozenset of context field names this step reads
provides: frozenset of context field names this step writes
__call__: receives and returns ACEStepContext (use ctx.replace(...) for updates)
Steps should be stateless — no internal counters

Mixing Integrations¶

You can compose steps from different integrations into one pipeline. For example, combining a browser-use execute step with custom learning:

from ace import Pipeline, learning_tail, Reflector, SkillManager, Skillbook
from ace.integrations.browser_use import BrowserExecuteStep, BrowserToTrace

skillbook = Skillbook()
pipe = Pipeline([
    BrowserExecuteStep(browser_llm),
    BrowserToTrace(),
    MyCustomFilterStep(),  # your custom step
    *learning_tail(Reflector(llm), SkillManager(llm), skillbook),
])

Integration steps live in ace.integrations since they have framework-specific dependencies.

Running the Pipeline¶

With a runner¶

The simplest way to run a custom pipeline is through ACERunner:

from ace import ACERunner, Sample, Skillbook

runner = ACERunner(pipeline=pipe, skillbook=skillbook)
results = runner.run(
    [Sample(question="What is 2+2?", ground_truth="4")],
    epochs=1,
)

Directly¶

You can also run the pipeline directly by constructing contexts yourself:

from ace import Pipeline, ACEStepContext, SkillbookView, Sample, Skillbook

ctx = ACEStepContext(
    sample=Sample(question="What is 2+2?", ground_truth="4"),
    skillbook=SkillbookView(skillbook),
)

results = pipe.run([ctx])
pipe.wait_for_background()  # wait for async learning steps

Branching (Parallel Steps)¶

The pipeline engine supports parallel branches for steps that can run concurrently:

from ace import Pipeline, Branch, MergeStrategy

pipe = Pipeline([
    AgentStep(agent),
    Branch(
        [EvaluateStep(env_a), EvaluateStep(env_b)],
        merge=MergeStrategy.LAST,
    ),
    *learning_tail(reflector, skill_manager, skillbook),
])

See the Pipeline Engine docs for full branching and merge strategy details.

Using RRStep (Recursive Reflector)¶

RRStep satisfies both StepProtocol and ReflectorLike, so it can be used in two ways:

As a drop-in reflector replacement¶

Pass it anywhere a Reflector is expected:

from ace import ACELiteLLM
from ace.rr import RRStep, RRConfig

ace = ACELiteLLM.from_model("gpt-4o-mini", reflector=RRStep("gpt-4o-mini", config=RRConfig(max_llm_calls=10)))

As a pipeline step¶

Place it directly in a pipeline (it provides reflections):

from ace import Pipeline, learning_tail, SkillManager, Skillbook
from ace.rr import RRStep, RRConfig

skillbook = Skillbook()
rr = RRStep("gpt-4o-mini", config=RRConfig(max_llm_calls=15))

pipe = Pipeline([
    MyExecuteStep(),
    MyToTrace(),
    rr,  # replaces ReflectStep — provides "reflections"
    *learning_tail(None, SkillManager("gpt-4o-mini"), skillbook)[1:],  # skip ReflectStep
])

With a separate sub-agent model¶

Route sub-agent calls to a smaller/faster model:

from ace.rr import RRStep, RRConfig

rr = RRStep(
    "gpt-4o",
    config=RRConfig(max_llm_calls=40, subagent_model="gpt-4o-mini"),
)

See RR_DESIGN.md for the full architecture, sandbox API, configuration reference, and trace schema.

Available Steps¶

All steps are importable from ace:

Step	Purpose
`AgentStep`	Execute Agent role
`EvaluateStep`	Run TaskEnvironment evaluation
`ReflectStep`	Run Reflector role (async boundary)
`UpdateStep`	Run SkillManager to generate updates
`ApplyStep`	Apply updates to skillbook
`DeduplicateStep`	Merge near-duplicate skills
`CheckpointStep`	Save skillbook to disk
`LoadTracesStep`	Load JSONL trace files
`ExportSkillbookMarkdownStep`	Export skillbook as markdown
`ObservabilityStep`	Generic observability hook
`PersistStep`	Persist step output
`OpikStep`	Log traces to Opik
`RRStep`	Recursive Reflector

Integration steps (in ace.integrations):

Step	Integration
`BrowserExecuteStep` / `BrowserToTrace`	browser-use
`LangChainExecuteStep` / `LangChainToTrace`	LangChain
`ClaudeCodeExecuteStep` / `ClaudeCodeToTrace`	Claude Code
`ClaudeSDKExecuteStep` / `ClaudeSDKToTrace`	Anthropic Python SDK
`OpenClawToTraceStep`	OpenClaw