ChainWeaver

Find where your agent no longer needs to reason. Review the evidence. Turn the accepted path into a governed deterministic capability.

Product thesis under validation — observe → prove → review → compile. ChainWeaver can inspect repeated tool behavior, surface candidates, and execute reviewed deterministic paths with typed contracts. Deterministic execution by itself is not the moat: if you already know the exact workflow, a normal Python function, LangGraph node, or provider-native tool may be simpler. The hypothesis being tested is that trace-derived evidence, useful rejection, governed promotion, security-boundary preservation, and drift detection make ChainWeaver worth adopting. See Product validation & adoption gates and #553.

Remove reasoning boundaries, never security boundaries. Compiling several tool calls into one capability must not silently aggregate privileges or erase child approval requirements. That invariant is tracked explicitly in #554.

Governance for deterministic tool paths. Typed I/O at every step, file-serializable flows, schema-drift detection, determinism attestation, property fuzzing, and structured audit traces provide a disciplined execution substrate for paths that have actually earned deterministic promotion.

Benchmarks are evidence about the executor, not proof of product-market fit. The repo's benchmark report is reproducible — regenerate it yourself with python benchmarks/report.py — and shows the deterministic core avoiding model-mediated transitions in its synthetic comparison. It does not establish that every repeated path should be compiled, or that ChainWeaver beats the obvious plain-Python implementation. The independent validation program requires that manual baseline explicitly.

from chainweaver import Tool, Flow, FlowStep, FlowRegistry, FlowExecutor
# (NumberInput, ValueOutput, double_fn defined in full example below)

# 1. Wrap any function as a schema-validated Tool
double = Tool(name="double", description="Doubles a number.",
              input_schema=NumberInput, output_schema=ValueOutput, fn=double_fn)
# 2. Wire tools into a Flow
flow = Flow(name="calc", description="Double a number.",
            steps=[FlowStep(tool_name="double", input_mapping={"number": "number"})])
# 3. Register and execute — zero LLM calls
registry = FlowRegistry()
registry.register_flow(flow)
executor = FlowExecutor(registry=registry)
executor.register_tool(double)
result = executor.execute_flow("calc", {"number": 5})
# result.final_output → {"number": 5, "value": 10}

See the full example below or run python examples/simple_linear_flow.py

Installation · Why ChainWeaver? · Is this for me? · Product validation · Quick Start · Architecture · Docs site · Roadmap

---

See it in 30 seconds

The deterministic executor solves a simple problem: once a path has been shown to need no intermediate reasoning, stop paying a model to re-decide the same plumbing on every run.

Before — a model-mediated path:

turn 1   ─►  LLM("plan")    ─►  search(query)         ─► 12 results
turn 2   ─►  LLM("next?")   ─►  extract(results)      ─► 8 facts
turn 3   ─►  LLM("next?")   ─►  validate(facts)       ─► 7 facts
turn 4   ─►  LLM("next?")   ─►  format(facts)         ─► answer

After review — the accepted path can run deterministically:

turn 1   ─►  LLM("plan")    ─►  search_summarize_flow(query)
                                  └─ search ─► extract ─► validate ─► format

The agent still decides which capability to invoke. The deterministic steps inside it run with strict Pydantic validation and no LLM involvement.

The harder product question comes before this diagram: should this path be compiled at all? A useful ChainWeaver analysis must be able to show why a candidate is recurrent and structurally safe and reject paths where semantic judgment, side effects, authorization, or approval boundaries still matter. That claim is currently being tested on independent traces in #553.

Copy-paste executor path:

pip install 'chainweaver[yaml]'
python examples/simple_linear_flow.py

The summary below is a condensed view of the real ExecutionResult the script produces:

flow=double_add_format success=True
final_output={'number': 5, 'value': 20, 'result': 'Final value: 20'}
step 0 double          {'value': 10}
step 1 add_ten         {'value': 20}
step 2 format_result   {'result': 'Final value: 20'}

---

Why ChainWeaver?

Why not just write a Python function?

Often, you should.

If your team already knows the workflow is fixed, a normal function or the workflow primitives in your existing framework are usually the lowest-complexity answer. ChainWeaver should earn another dependency only when its lifecycle adds meaningful value—for example:

• discovering non-obvious repeated model-mediated paths from real traces;
• showing evidence for recurrence, dataflow compatibility, and counterexamples;
• rejecting tempting paths that still require semantic judgment;
• preserving approval and authorization constraints during promotion;
• producing reproducible review evidence and artifact identity;
• detecting schema/safety/policy drift after promotion;
• exporting or executing the accepted capability without pretending that fewer model calls automatically means greater correctness.

Whether those advantages are strong enough in real teams is a falsifiable product hypothesis, not a README assumption. See docs/product-validation.md.

When an LLM-powered agent routes tools together — fetch_data → transform → store — a common pattern is to insert an LLM call between steps so the model can decide what to do next. For a path that has been demonstrated and reviewed as fully deterministic, those intermediate calls can add latency, cost, and variability without adding useful judgment.

ChainWeaver's executor can run an accepted deterministic path without any LLM involvement between steps:

User request
    │
    ▼
FlowExecutor ──► Tool A ──► Tool B ──► Tool C
    │
    ▼
Response

Criterion	Model-mediated path	ChainWeaver deterministic path
LLM calls between deterministic steps	potentially one or more	0
Reproducibility	depends on model decisions	deterministic path
Schema validation	framework/application dependent	Pydantic enforced
Observability	framework/application dependent	structured step logs
Reusability	application dependent	registered, versioned flows

How is this different from LangChain / LangGraph / Prefect / Dagster / Temporal?

Those frameworks can also execute deterministic code. ChainWeaver should not be selected because deterministic execution is impossible elsewhere. Its current product thesis is narrower: start from observed agent/tool behavior, establish which regions no longer need reasoning, make the evidence and rejections reviewable, then promote accepted paths into governed deterministic capabilities.

The execution substrate remains deliberately small and LLM-free between steps, but the project is testing whether the evidence/governance lifecycle—not the mere existence of another workflow runtime—is the part users value.

See docs/comparisons.md for the detailed, versioned comparison and docs/product-validation.md for the criteria that can falsify this positioning.

---

Is this for me?

ChainWeaver is built for one specific shape of problem. The full fit/non-fit page covers the nuances; the short version:

Use ChainWeaver when

• You have real agent/tool traces and suspect parts of the path are repeated plumbing rather than useful model judgment.
• You want evidence and review around which paths deserve deterministic promotion, not only a runtime for a workflow you already know.
• Determinism, strict schemas, auditability, and drift detection matter once a path is promoted.
• You are prepared to keep security and approval boundaries explicit rather than treating a macro-tool invocation as blanket child authorization.

Don't use ChainWeaver when

• You already know the workflow and a normal Python function or your existing framework expresses it clearly enough.
• Every step requires open-ended reasoning to pick the next one (use an agent framework: LangGraph, the OpenAI / Anthropic SDK tool-use loops).
• You need a general workflow engine for scheduled / durable jobs across time (use Prefect, Dagster, or Temporal).
• You expect the executor to call an LLM. It deliberately doesn't.
• You cannot preserve the authorization/approval semantics of a side-effecting path during compilation.

The product thesis, validation protocol, and kill/pivot criteria are public in docs/product-validation.md.

For the correctness argument behind the deterministic execution design, see docs/data-integrity.md.

Part of the Weaver Stack

ChainWeaver is the deterministic multi-step tool execution layer of the Weaver Stack — a family of small, composable SDKs that share weaver-spec's SelectableItem routing contract. On the request path a router picks which capability to invoke, ChainWeaver runs the deterministic tool path behind it, and downstream layers gate and guard the call:

flowchart LR
    req([Request]) --> ctx[contextweaver<br/>context assembly]
    ctx --> cw[<b>ChainWeaver</b><br/>deterministic flow execution]
    cw --> ak[agent-kernel<br/>capability gating]
    ak --> af[agentfence<br/>runtime guardrails]
    subgraph adjacent [Adjacent · use any subset]
        vg[vibeguard]
        lw[lessonweaver]
        se[skdr-eval]
    end

Loading

Use standalone or together. Each layer stands on its own — ChainWeaver's base install has no hard dependency on any sibling and works fully standalone. Real interop runs through the chainweaver[weaver-stack] extra, which pins the published weaver-contracts package: ChainWeaver consumes its SelectableItem / RoutingDecision / CapabilityToken types directly, so a router can hand a routing decision straight to resolve_flow_from_routing_decision() for deterministic execution. See the runnable Weaver Stack golden path (issue #234).

Layer	What it owns	Sibling project
Routing / capability selection	"Which named operation handles this request?"	weaver-spec (#91 — SelectableItem contract)
Context assembly	"What facts and tool descriptions belong in the prompt?"	contextweaver (#106)
Agent kernel	The model-mediated tool-use loop itself	agent-kernel (#89)
Deterministic flow execution	"Run this exact tool sequence with strict schemas, no LLM between steps"	ChainWeaver — this repo
Lessons & evaluation	Turning traces into reviewed operational guidance (how ChainWeaver feeds it)	lessonweaver (#210)

ChainWeaver does not replace an agent framework. It is meant to be called from one — see the LangGraph recipe (issue #205) and the OpenAI Agents SDK recipe (issue #206) for the canonical integration patterns.

For host-level expectations (when to invoke, how to store traces, side-effect tools, MCP parity), see the Runtime responsibilities page.

---

Installation

pip install chainweaver                  # base install — no extras
pip install 'chainweaver[yaml]'          # most common — needed for .flow.yaml files
pip install 'chainweaver[yaml,otel,mcp]' # combine extras with commas

The base install pulls only five runtime dependencies (deepdiff, packaging, pydantic, tenacity, typer) and has no transitive LLM SDK pinned. Pick extras for the integrations you actually use:

Extra	Use when	Pulls in
chainweaver[yaml]	Reading / writing .flow.yaml flow files (the CLI's run, validate, check, doctor commands need this)	pyyaml
chainweaver[otel]	Emitting OpenTelemetry spans for every flow run	opentelemetry-api
chainweaver[mcp]	Exposing flows over MCP via the chainweaver.mcp adapter	mcp
chainweaver[contrib]	Importing the curated standard tool library (see Standard tool library)	(no extra deps today)
chainweaver[langchain]	Bidirectional adapters between ChainWeaver and LangChain BaseTool	langchain-core
chainweaver[llamaindex]	Bidirectional adapters between ChainWeaver and LlamaIndex FunctionTool	llama-index-core
chainweaver[test]	Hypothesis-based property tests for your own flows	hypothesis, hypothesis-jsonschema
chainweaver[docs]	Building the docs site locally with mkdocs	mkdocs, mkdocs-material, mkdocstrings
chainweaver[weaver-stack]	Real Weaver Stack interop — consuming the shared routing/capability contract (weaver-spec #91, contextweaver #106, agent-kernel #89, #233)	weaver-contracts
chainweaver[integrations]	Every integration extra above at once — the composition CI exercises	the union of the integration rows above

Maintainer tooling (pytest, ruff, mypy, nbmake, ...) is not a published extra: it lives in PEP 735 dependency groups, installed with pip install -e ".[integrations]" --group dev (#550). The [dev] extra no longer exists.

Package metadata (pyproject.toml) publishes URLs for the documentation, the source, the changelog, and the issue tracker, so pip show chainweaver and the PyPI sidebar point users to the right place.

---

Quick Start

Define tools, build a flow, and execute it

from pydantic import BaseModel
from chainweaver import Tool, Flow, FlowStep, FlowRegistry, FlowExecutor

# --- 1. Declare schemas ---

class NumberInput(BaseModel):
    number: int

class ValueOutput(BaseModel):
    value: int

class ValueInput(BaseModel):
    value: int

class FormattedOutput(BaseModel):
    result: str

# --- 2. Implement tool functions ---

def double_fn(inp: NumberInput) -> dict:
    return {"value": inp.number * 2}

def add_ten_fn(inp: ValueInput) -> dict:
    return {"value": inp.value + 10}

def format_result_fn(inp: ValueInput) -> dict:
    return {"result": f"Final value: {inp.value}"}

# --- 3. Wrap as Tool objects ---

double_tool = Tool(
    name="double",
    description="Takes a number and returns its double.",
    input_schema=NumberInput,
    output_schema=ValueOutput,
    fn=double_fn,
)

add_ten_tool = Tool(
    name="add_ten",
    description="Takes a value and returns value + 10.",
    input_schema=ValueInput,
    output_schema=ValueOutput,
    fn=add_ten_fn,
)

format_tool = Tool(
    name="format_result",
    description="Formats a numeric value into a human-readable string.",
    input_schema=ValueInput,
    output_schema=FormattedOutput,
    fn=format_result_fn,
)

# --- 4. Define the flow ---

flow = Flow(
    name="double_add_format",
    description="Doubles a number, adds 10, and formats the result.",
    steps=[
        FlowStep(tool_name="double",        input_mapping={"number": "number"}),
        FlowStep(tool_name="add_ten",       input_mapping={"value": "value"}),
        FlowStep(tool_name="format_result", input_mapping={"value": "value"}),
    ],
)

# --- 5. Execute ---

registry = FlowRegistry()
registry.register_flow(flow)

executor = FlowExecutor(registry=registry)
executor.register_tool(double_tool)
executor.register_tool(add_ten_tool)
executor.register_tool(format_tool)

result = executor.execute_flow("double_add_format", {"number": 5})

print(result.success)       # True
print(result.final_output)  # {'number': 5, 'value': 20, 'result': 'Final value: 20'}

for record in result.execution_log:
    print(record.step_index, record.tool_name, record.outputs)
# 0 double {'value': 10}
# 1 add_ten {'value': 20}
# 2 format_result {'result': 'Final value: 20'}

You can also run the bundled examples directly:

python examples/simple_linear_flow.py   # simple arithmetic flow
python examples/etl_flow.py             # ETL flow: fetch → validate → normalize → enrich → store
python examples/mcp_search_flow.py      # MCP-style search → extract → format flow
python examples/naive_vs_compiled.py    # timing comparison: naive LLM calls vs ChainWeaver flow
python examples/coding_agent_pr_review.py    # deterministic PR-review checklist
python examples/coding_agent_changelog.py    # changelog generation workflow template
python examples/coding_agent_debug_log.py    # debug-log triage workflow template
python examples/mcp_style_before_after_demo.py        # before/after MCP-style flow demo
python examples/release_readiness_flow/release_readiness.py  # deterministic release-readiness gate
python examples/skdr_policy_eval_flow.py              # offline policy-evaluation workflow template
python examples/integrations/langgraph_node.py        # call a flow from a LangGraph node (needs chainweaver[langgraph])
python examples/integrations/openai_agents_tool.py    # expose a flow as an OpenAI Agents SDK tool (needs chainweaver[openai-agents])

The hosted docs also include a cookbook with paired scripts under examples/cookbook/, plus framework recipes and workflow templates (LangGraph, OpenAI Agents SDK, release-readiness, policy evaluation).

With the @tool decorator

The @tool decorator eliminates boilerplate by introspecting type hints to auto-generate input schemas:

from pydantic import BaseModel
from chainweaver import tool, Flow, FlowStep, FlowRegistry, FlowExecutor

class ValueOutput(BaseModel):
    value: int

class FormattedOutput(BaseModel):
    result: str

@tool(description="Doubles a number.")
def double(number: int) -> ValueOutput:
    return {"value": number * 2}

@tool(description="Adds ten.")
def add_ten(value: int) -> ValueOutput:
    return {"value": value + 10}

@tool(description="Formats the result.")
def format_result(value: int) -> FormattedOutput:
    return {"result": f"Final value: {value}"}

flow = Flow(
    name="double_add_format",
    description="Doubles a number, adds 10, and formats.",
    steps=[
        FlowStep(tool_name="double",        input_mapping={"number": "number"}),
        FlowStep(tool_name="add_ten",       input_mapping={"value": "value"}),
        FlowStep(tool_name="format_result", input_mapping={"value": "value"}),
    ],
)

registry = FlowRegistry()
registry.register_flow(flow)

executor = FlowExecutor(registry=registry)
executor.register_tool(double)
executor.register_tool(add_ten)
executor.register_tool(format_result)

result = executor.execute_flow("double_add_format", {"number": 5})
print(result.final_output)  # {'number': 5, 'value': 20, 'result': 'Final value: 20'}

Decorated tools are also directly callable:

print(double(number=5))  # {'value': 10}

See examples/decorator_tool.py for a runnable before/after comparison.

With FlowBuilder

FlowBuilder provides a fluent, chainable API as a more Pythonic alternative to constructing Flow objects directly. It produces an identical Flow — it is syntax sugar, not a replacement:

from chainweaver import FlowBuilder

flow = (
    FlowBuilder("double_add_format", "Doubles a number, adds 10, and formats.")
    .step("double", number="number")
    .step("add_ten", value="value")
    .step("format_result", value="value")
    .build()
)

• .step(tool_name, **mapping) — adds a step; string values are context-key lookups, non-string values are literal constants, no kwargs = full-context passthrough.
• .step_from(flow_step) — appends a pre-built FlowStep for interop.
• .with_input_schema(Model) / .with_output_schema(Model) — optional flow-level Pydantic schema declarations.
• .with_trigger(conditions) — optional free-form trigger metadata.
• .build() — returns a validated Flow; raises FlowBuilderError if name or description is missing.

---

Interactive playground

Want to try ChainWeaver without installing anything locally? The playground/ directory ships a Streamlit app that lets you pick a pre-loaded flow, edit its JSON input, run it, and watch the step-by-step, LLM-free execution trace with a Mermaid diagram — the same FlowExecutor the library ships.

pip install -r playground/requirements.txt
streamlit run playground/app.py

It ships three example flows (arithmetic, a data flow, and an MCP-style search), produces shareable ?share=<token> links that round-trip a run through the URL, and is fully stateless so it deploys to Streamlit Community Cloud with no backend. See playground/README.md for local-run and deployment instructions.

---

Architecture

chainweaver/
├── __init__.py       # Public API
├── builder.py        # FlowBuilder — fluent API for flow construction
├── compat.py         # schema_fingerprint, check_flow_compatibility
├── compiler.py       # compile_flow — static schema flow validation
├── decorators.py     # @tool decorator for zero-boilerplate tool definition
├── tools.py          # Tool — named callable with Pydantic schemas
├── flow.py           # FlowStep + Flow + FlowStatus — ordered step definitions
├── registry.py       # FlowRegistry — multi-version flow catalogue
├── executor.py       # FlowExecutor — deterministic, LLM-free runner
├── exceptions.py     # Typed exceptions with traceable context
└── log_utils.py      # Structured per-step logging

Core abstractions

Tool

Tool(
    name="my_tool",
    description="...",
    input_schema=MyInputModel,   # Pydantic BaseModel
    output_schema=MyOutputModel, # Pydantic BaseModel
    fn=my_callable,
)

A tool wraps a plain Python callable together with Pydantic models for strict input/output validation.

FlowStep

FlowStep(
    tool_name="my_tool",
    input_mapping={
        "key_for_tool": "key_from_context",   # flat top-level lookup
        "city": "/user/address/city",         # RFC-6901 pointer into nested context
        "limit": 10,                          # non-string -> literal constant
    },
    output_mapping={"renamed": "value"},      # rename/prune outputs before merge
)

input_mapping maps keys from the accumulated execution context into the tool's input schema. String values are looked up in the context — a plain key is a top-level lookup, and a string starting with / is an RFC-6901 JSON pointer into the nested context (#387) — while non-string values are literal constants.

output_mapping (#386) optionally renames and prunes a tool's outputs before they merge into the context: {context_key: output_key} keeps only the listed output keys, each renamed. Omit it to merge every output key verbatim.

To inject per-request secrets that must never appear in a model-visible schema (auth tokens, account numbers), pass them at execute-time instead of in initial_input:

result = executor.execute_flow(
    "account_overview",
    {"query": "what's my balance?"},          # LLM-visible
    dynamic_params={"billingAccountNumber": "1.60007029"},  # hidden, injected (#316)
)

Flow

Flow(
    name="my_flow",
    version="0.1.0",             # SemVer string; defaults to "0.1.0" if omitted
    description="...",
    steps=[step_a, step_b, step_c],
    deterministic=True,          # metadata annotation; executor is always LLM-free
    trigger_conditions={"intent": "process data"},  # optional metadata
)

An ordered sequence of steps. See AGENTS.md §5 for the full field table (status, tool_schema_hashes, and the input_schema_ref / output_schema_ref string fields with their resolved-property accessors).

A FlowStep runs either a tool (tool_name) or a registered sub-flow (flow_name) — exactly one, never both. Referencing a sub-flow lets you compose reusable flows (issue #75):

fetch_validate = Flow(
    name="fetch_validate",
    description="Fetch and validate.",
    steps=[
        FlowStep(tool_name="fetch", input_mapping={"url": "url"}),
        FlowStep(tool_name="validate", input_mapping={"data": "data"}),
    ],
)
fetch_then_transform = Flow(
    name="fetch_then_transform",
    description="Reuse fetch_validate, then transform.",
    steps=[
        FlowStep(flow_name="fetch_validate", input_mapping={"url": "url"}),  # sub-flow
        FlowStep(tool_name="transform", input_mapping={"data": "data"}),
    ],
)

The executor runs the sub-flow with the step's resolved inputs, merges its output back into the parent context, and attaches the sub-flow's ExecutionResult to the parent StepRecord.sub_result. Sub-flow references are checked for cycles and a configurable max nesting depth (FlowExecutor(max_composition_depth=...), default 10) before execution, raising FlowCompositionError otherwise.

A deadline or CancellationToken passed to execute_flow is forwarded into composed sub-flows, so cancellation and the wall-clock budget are observed at the step boundaries inside a sub-flow — a long sub-flow stops between its own steps rather than only at the parent boundary. The cost report's steps_executed counts the tool invocations a composed step actually drove (recursively), so llm_calls_avoided reflects every tool that ran across the composition.

FlowRegistry

registry = FlowRegistry()
registry.register_flow(flow)
registry.get_flow("my_flow")
registry.list_flows()
registry.match_flow_by_intent("process data")  # basic substring match

An in-memory catalogue of flows.

FlowExecutor

executor = FlowExecutor(registry=registry)
executor.register_tool(tool_a)
result = executor.execute_flow("my_flow", {"key": "value"})

# Version-targeted execution: run an exact registered version instead of the
# latest. Omitting `version` keeps the default (latest) behaviour. The version
# that actually ran is always recorded on `result.flow_version`, so routing,
# audit, and replay can correlate a result with the precise flow definition.
result = executor.execute_flow("my_flow", {"key": "value"}, version="1.2.0")
assert result.flow_version == "1.2.0"

Runs a flow step-by-step with full schema validation and structured logging. No LLM calls are made at any point.

ChainAnalyzer

from chainweaver import ChainAnalyzer, ToolChain

analyzer = ChainAnalyzer(tools=[tool_a, tool_b, tool_c])

# All schema-compatible pairs
matrix: dict[str, list[str]] = analyzer.compatibility_matrix()

# All valid tool sequences up to length 3
chains: list[ToolChain] = analyzer.find_chains(max_depth=3)

# Filter by start or end tool
chains = analyzer.find_chains(max_depth=3, start="tool_a", end="tool_c")

# Promote chains to ready-to-register Flow objects
flows = analyzer.suggest_flows(max_depth=3, min_depth=2)

Discovers schema-compatible tool combinations offline, before any flow is registered or executed. compatibility_matrix() checks that every required input field of a consumer tool appears in the output of the producer with a matching type. suggest_flows() auto-wires input_mapping by name-matching and returns Flow objects ready for FlowRegistry.register_flow().

Data flow

initial_input (dict)
       │
       ▼
 ┌─────────────────────────────────────────────┐
 │  Execution context (cumulative dict)        │
 │                                             │
 │  Step 0: resolve inputs → run tool → merge  │
 │  Step 1: resolve inputs → run tool → merge  │
 │  Step N: resolve inputs → run tool → merge  │
 └─────────────────────────────────────────────┘
       │
       ▼
 ExecutionResult.final_output (merged context)

---

MCP Integration Concept

ChainWeaver can sit between agent/tool observation and deterministic execution:

Agent / tool traces
   │  (observe repeated paths)
   ▼
Candidate analysis + human review
   │  (prove/reject; preserve security boundaries)
   ▼
Governed deterministic capability
   │  (FlowExecutor and/or supported export)
   ▼
MCP / host-framework invocation

MCP is an interoperability surface, not the product category. The current runtime can expose reviewed flows as MCP tools, while #555 explores whether portable outputs should let the same approved capability execute through other hosts without requiring ChainWeaver to own the runtime.

ChainWeaver is a library you embed, not the runtime that owns your trace store, identity system, or enterprise authorization control plane. Host authors should read docs/runtime-responsibilities.md.

---

Integrations

ChainWeaver plugs into the MCP ecosystem and major agent frameworks. Existing integrations remain supported; new adapter breadth is deliberately lower priority than independent product validation.

Integration	What it does	Entry point
MCP server (outbound)	Expose your flows as MCP tools — agents call a whole compiled flow as one deterministic tool	chainweaver serve · guide · FlowServer
MCP adapter (inbound)	Wrap tools advertised by an MCP server as ChainWeaver Tools	chainweaver.mcp.MCPToolAdapter
LangGraph	Call a flow from a LangGraph node	recipe · examples/integrations/langgraph_node.py
OpenAI Agents SDK	Expose a flow as an Agents SDK FunctionTool	recipe · examples/integrations/openai_agents_tool.py
LangChain / LlamaIndex	Bidirectional tool bridges	chainweaver.integrations.{langchain,llamaindex} (see below)
OpenCode	Observe tool runs, mine macro-flows, and expose reviewed flows back as MCP tools	recipe · chainweaver opencode
Claude Code	Capture PostToolUse hook traces, mine macro-flows, and expose reviewed flows back as MCP tools	recipe · chainweaver claude
VS Code / Copilot	Capture MCP tool traces (Copilot OTel) and expose reviewed flows via .vscode/mcp.json	recipe · chainweaver vscode
GitHub Action	Validate .flow.yaml / .flow.json files in CI with inline PR annotations	.github/actions/chainweaver · guide

Install the extra you need: pip install 'chainweaver[mcp]' (or langgraph, openai-agents, langchain, llamaindex). Importing any integration without its extra raises a clear ImportError.

Looking to publish or list ChainWeaver in the MCP registry / awesome-lists / framework directories? See docs/distribution.md. Broad distribution is intentionally gated behind the naming decision (#556) and validation evidence (#553).

---

Error Handling

All errors are typed and traceable:

Exception	When it is raised
ToolNotFoundError	A step references an unregistered tool
FlowNotFoundError	The requested flow is not registered
FlowAlreadyExistsError	Registering a flow that already exists (without overwrite=True)
FlowStatusError	Executing a flow whose status is not ACTIVE (without force=True)
FlowCancelledError	A deadline passed or a CancellationToken was cancelled at a step boundary (carries the partial result)
InvalidFlowVersionError	A flow is registered with a version string that is not valid PEP 440
FlowSerializationError	A flow file (YAML/JSON) is malformed, has an unknown discriminator, or references an unresolvable class
SchemaValidationError	Input or output fails Pydantic validation
InputMappingError	A mapping key is not present in the context
FlowExecutionError	The tool callable raises an unexpected exception
ApprovalDeniedError	An execution-time approval callback denied a step, raised, or returned an invalid value — or strict_safety=True and a required-approval step has no callback
SafetyCeilingError	A step's ToolSafetyContract.side_effects exceeds the executor's configured max_side_effect_level
GuardrailViolationError	A registered guardrail_callback blocked a step at the input stage (content-safety / injection check)
ToolDefinitionError	The @tool decorator cannot build a tool from a function
DAGDefinitionError	A DAGFlow has a cycle, duplicate step_id, or unknown dependency
FlowCompositionError	A composed flow has a sub-flow cycle, exceeds max_composition_depth, or references an unregistered sub-flow
ToolTimeoutError	A Tool with timeout_seconds set exceeds the configured wall-clock cap
ToolOutputSizeError	A Tool with max_output_size set returns an output larger than the configured cap
FlowBuilderError	FlowBuilder.build() is called without a name or description
AttestationInputError	The attestation input generator cannot synthesize a value for a schema field
PluginDiscoveryError	Strict-mode plugin discovery (discover_tools(strict=True) / discover_flows(strict=True)) hits a misbehaving entry-point loader
ContribError	A chainweaver.contrib.tools tool hits a contract violation (missing JSON-pointer key, wrong predicate shape, assertion mismatch)
FixtureStaleError	A record_then_replay replay invocation cannot be matched to a recording (missing/stale fixture)
FuzzConfigError	A property-based fuzzing run is misconfigured (no properties, runs < 1, a flow with no input_schema and no base input, or an unsupported input-field type)
CostProfileError	A cost estimate is requested for a (provider, model) pair absent from the maintained PROVIDER_PRICES table
MCPMetadataError	A server-provided MCP tool name fails the adapter's MetadataPolicy (and on_invalid_name="error")
MCPSchemaDriftError	A pinned MCP tool's raw schema changed under MCPToolAdapter(on_drift="error")
FlowAuthenticationError	A network-exposed FlowServer authenticator returned None or raised; the call is refused before dispatch
RateLimitExceededError	A FlowServer rate limiter declined the call
FlowAuthorizationError	A FlowServer authorization callback denied the call (carries only a client-safe reason_code)
CheckpointVersionError	A resumed snapshot's snapshot_version is an incompatible MAJOR relative to the running library

All exceptions inherit from ChainWeaverError and carry a stable diagnostic code (e.g. CW-E006); the CLI prefixes it on error output and failing StepRecords expose it as error_code. See the full code table in docs/reference/error-table.md.

---

Standard tool library

chainweaver.contrib.tools ships a curated set of deterministic utility tools so that a new user can compose a meaningful flow on the first afternoon without writing any Tool boilerplate.

from chainweaver.contrib.tools import (
    assert_equal,
    filter_list,
    json_pluck,
    json_set,
    map_list,
    passthrough,
)

Tool	Purpose
passthrough	Identity — return the context unchanged.
json_pluck	Extract one value by RFC-6901 JSON pointer.
json_set	Set one value by RFC-6901 JSON pointer; returns a new dict.
assert_equal	Raise ContribError when two context keys differ.
map_list	Apply a registered sub-flow to each element of a list.
filter_list	Drop elements whose predicate sub-flow returns falsy.

The library is deterministic-only: no HTTP, file I/O, database access, RNG, or clocks. Anything stateful belongs in user code. Install with pip install 'chainweaver[contrib]'.

Runnable examples: examples/contrib_pluck_and_set.py, examples/contrib_map_filter.py.

---

Cost-avoided reporting

Every inter-step transition a naive agent delegates to an LLM is a routing call ChainWeaver eliminates. CostProfile / CostReport turn that into a dollar estimate, and the maintained PROVIDER_PRICES table (dated snapshots, no live HTTP lookup) lets you price it against a real model:

from chainweaver.cost import compute_cost_report

# Build a profile straight from the maintained price table.
report = compute_cost_report(
    steps_executed=6,                 # a six-tool flow
    actual_execution_ms=4.2,
    provider="anthropic",
    model="claude-opus-4-7",
)
print(report)

Cost Avoided Report (estimate)
──────────────────────────────
Steps executed:          6
LLM calls avoided:       5
Est. latency saved:      1500.0ms
Est. cost saved:         $0.1688
Actual execution time:   4.2ms
Priced against:          anthropic/claude-opus-4-7 (as of 2026-05-01)

Every report built from the table carries the snapshot's as_of date so stale prices are visible. Unknown (provider, model) pairs raise CostProfileError rather than guessing. Pass an explicit profile=CostProfile(...) when you have better per-call numbers, or set cost_profile= on FlowExecutor to attach a report to every ExecutionResult. Prices are refreshed by a maintainer-reviewed PR (.github/workflows/update-prices.yml) — never auto-merged.

These reports are estimates unless their inputs come from observed trace measurements. They must not be presented as evidence that a candidate should be compiled; #377 tracks calibration of assumed versus measured model mediation.

---

Export adapters

Hand a compiled flow off to any external agent framework via chainweaver.export:

from chainweaver.export import (
    flow_to_anthropic_tool,
    flow_to_callable,
    flow_to_openai_function,
)

openai_spec = flow_to_openai_function(flow, executor)
anthropic_spec = flow_to_anthropic_tool(flow, executor)
run = flow_to_callable(flow, executor)  # plain dict → dict callable

flow_to_openai_function emits the {"type": "function", "function": {…}} shape OpenAI's chat / responses APIs expect. flow_to_anthropic_tool emits Anthropic's tool_use shape. flow_to_callable wraps the flow as a Callable[[dict], dict] suitable for any framework that accepts arbitrary Python callables.

None of these adapters imports openai or anthropic — they emit dicts and callables only. Runtime integration with those clients is the caller's job.

Runnable example: examples/export_openai_anthropic.py.

---

Ecosystem bridges (LangChain, LlamaIndex)

chainweaver.integrations.langchain and chainweaver.integrations.llamaindex ship thin bidirectional adapters so existing LangChain BaseTool / LlamaIndex FunctionTool instances can be pulled into ChainWeaver, and ChainWeaver Tool instances can be pushed back out.

from chainweaver.integrations.langchain import (
    from_langchain_tool,
    to_langchain_tool,
)

cw_tool = from_langchain_tool(my_langchain_tool)
lc_tool = to_langchain_tool(my_cw_tool)

Install with pip install 'chainweaver[langchain]' / 'chainweaver[llamaindex]'. Importing either module without the relevant extra raises a clear ImportError.

---

Plugin discovery

For third-party packages — chainweaver-aws, chainweaver-stripe, … — ChainWeaver follows the same entry-point convention used by pytest, Sphinx, MkDocs, and friends.

Publisher (pyproject.toml):

[project.entry-points."chainweaver.tools"]
aws = "chainweaver_aws:get_tools"

[project.entry-points."chainweaver.flows"]
aws = "chainweaver_aws:get_flows"

Consumer:

from chainweaver import FlowExecutor, FlowRegistry

# Auto-register every tool / flow advertised by an installed plugin.
registry = FlowRegistry(discover_plugins=True)
executor = FlowExecutor(registry=registry, discover_plugins=True)

Discovery is opt-in — importing chainweaver does not trigger plugin imports. Misbehaving plugins (raise on import, return the wrong type) are logged at WARNING and skipped; pass strict=True to discover_tools() / discover_flows() for the loud form.

Runnable example: examples/plugin_discovery.py.

---

Runtime learning

ChainWeaver can watch what an agent does and propose deterministic-flow candidates for repeated paths. A repeated sequence is not proof that the path is safe or valuable to compile; proposals require review, and the product validation program is explicitly measuring false positives, false negatives, and useful rejections.

from chainweaver import ChainObserver, FlowRegistry

observer = ChainObserver()

# Record tool calls as the agent makes them.
observer.record("fetch", {"url": "..."}, {"body": "..."})
observer.record("validate", {"body": "..."}, {"valid": True})
observer.record("transform", {"body": "..."}, {"records": [1, 2, 3]})
observer.end_trace()
# ... many traces later ...

registry = FlowRegistry()
for suggestion in observer.suggest_flows(min_occurrences=3):
    # Suggestions are proposals — review; never treat confidence as authorization.
    print(suggestion.flow.name, suggestion.confidence,
          suggestion.estimated_llm_calls_avoided)
    registry.register_flow(suggestion.flow)

• ChainObserver (#78) mines repeated tool sequences from runtime traces and emits ranked FlowSuggestions — never auto-registered.
• chainweaver record (#226) mines recorded JSONL traces and writes candidate flow files for explicit review/promotion.
• ChainWeaverService (#101) ties observation, static analysis, and optional offline proposals into an analyze → propose → govern → promote loop.

See Product validation & adoption gates before interpreting a suggestion as proof that a path should become deterministic.

---

Roadmap

The current roadmap is validation-first, not feature-count-first. The latest published release is v0.14.1; newer work on main remains unreleased until a subsequent release is cut.

Priority	Work	Why
P0	#553 independent product falsification	Establish whether trace-derived discovery/governance beats human inspection + a plain-function baseline.
P0	#554 authorization/approval preservation	Compilation may remove reasoning boundaries, never silently security boundaries.
P0	#522 stable/supported/experimental API tiers	Keep the compatibility promise smaller than the implementation surface.
P0	#519 release coherence	Source, package, tag, release, docs, and artifacts must agree.
P1	#527 privacy profiles	Trace analysis must work with minimized/local evidence.
Gate on #553	#334 canonical evidence architecture	Build the large lifecycle model only after users validate the job.
Gate on #553	#498 production golden path	Turn validated needs into one canonical end-to-end proof.
Explore if demanded	#555 portable compiled capabilities	If users value analysis but not FlowExecutor, make the accepted artifact portable.
Before broad distribution	#556 naming/search decision	Resolve discoverability/ambiguity while migration is still cheap.

Broad directory submissions, hosted-playground investment, and additional framework-adapter breadth are deliberately lower priority until these gates produce evidence.

v1.0.0 is also evidence-gated: independent workloads/adopters, a manual baseline, an external security review, repeated use, a downstream integration, and a 30-day RC compatibility soak are required by docs/v1-release-criteria.md.

---

Command-line interface

ChainWeaver ships a chainweaver console script with the following subcommands. Reading .flow.yaml files needs the YAML extra (pip install 'chainweaver[yaml]' — also listed in Installation). The run example below uses a flow shipped under examples/, so it should be invoked from the repository root.

# Run a flow from disk — no Python required.
chainweaver run examples/double_add_format.flow.yaml \
    --tools examples.simple_linear_flow \
    --input '{"number": 5}'

# Serve a flow as MCP tools (needs chainweaver[mcp]) — agents call the whole
# compiled flow as one deterministic tool. See docs/mcp-server.md.
chainweaver serve examples/double_add_format.flow.yaml \
    --tools examples.simple_linear_flow

# Validate a flow file (used by CI gates and editor tooling).
chainweaver validate flows/etl.flow.yaml
chainweaver check flows/                  # whole-directory variant

# Scaffold a runnable first flow project (tools + flow file + run script).
chainweaver init my-first-flow --template linear --with-tests

# Render a flow as ASCII, Graphviz DOT, or Mermaid. Discover it from a directory
# of flow files, an installed package's entry points, or the default registry.
chainweaver viz my_flow --discover-dir flows/ --format dot | dot -Tpng -o my_flow.png
chainweaver viz my_flow --discover-dir flows/ --format mermaid
chainweaver viz --result trace.json --format mermaid   # overlay a real run

# Explain a flow deterministically (LLM-free) for review — paste into a PR.
chainweaver explain my_flow --discover-dir flows/ > flow-review.md

# Inspect a flow's structure (table or JSON). `flows list` previews what is
# discoverable so you can see what `inspect`/`viz` can target.
chainweaver inspect my_flow --discover-dir flows/ --format json
chainweaver flows list --discover-dir flows/

# Check that your environment is ready before running anything.
chainweaver doctor flow --profile first-run

# Inspect a coding-agent workspace's MCP / observe setup (read-only).
chainweaver doctor vscode --workspace .

# Install tab-completion for your shell (bash/zsh/fish).
chainweaver --install-completion

# Analyze ExecutionResult traces — bottlenecks, p50/p95/p99 across runs,
# and per-step / per-tool retry / skip / fallback / failure aggregates.
chainweaver profile trace_a.json trace_b.json --format json

# Compare two ExecutionResult JSON files step-by-step.
chainweaver diff baseline.json current.json --perf-tolerance 25

# Observed-determinism attestation: run N inputs × M repeats.
chainweaver attest flows/etl.flow.yaml --tools my_pkg.tools --runs 50 --repeats 3

# Advisory optimization suggestions for a saved flow.
chainweaver suggest flows/etl.flow.yaml --tools my_pkg.tools --trace trace_a.json

# Mine candidate flows from a recorded JSONL tool trace (offline, no LLM).
chainweaver record examples/agent_tool_trace.jsonl --output-dir candidates/
chainweaver flows promote candidates/suggested__fetch__validate.flow.yaml --to reviewed
chainweaver flows promote candidates/suggested__fetch__validate.flow.yaml --to active

# Run one continuous-analysis service pass and report flow proposals.
chainweaver service --tools my_pkg.tools --trace trace.jsonl

# Check saved flows for tool schema drift against the live registry.
chainweaver doctor flow flows/ --check-drift --tools my_pkg.tools

# Property-based fuzzing: generate cases, check invariants, save/minimize failures.
chainweaver fuzz flows/etl.flow.yaml --tools my_pkg.tools \
  --property my_pkg.props:no_unauthorized_action --runs 1000 --seed 42 \
  --minimize --save-failures failures/

run is the fastest path from a fresh install to seeing a flow execute: point it at a .flow.yaml/.flow.json file, pass --tools <module> (the import path of a Python module that exposes Tool instances at top level), and supply the initial input as JSON. Hand-authored flow files must declare a type: Flow (or type: DAGFlow) discriminator at the top — see the flow file format reference. Most reporting subcommands also accept --format json for machine consumption (inspect, validate, check, run, profile, diff, attest, suggest, doctor); the exceptions are viz, which uses --format ascii|dot|mermaid, explain, which uses --format md|text, and dump-schema, which writes a raw JSON Schema and has no --format flag. The result-producing commands (inspect, validate, check, profile, diff, attest) wrap their --format json output in a stable, versioned envelope ({"schema_version", "status", "data", "errors"}) so automation can branch on status / error codes — see machine-readable output. All subcommands share the same exit-code contract (0 success, 1 business-logic error, 2 file-not-found / argument error), and the CLI ships tab-completion (chainweaver --install-completion).

inspect and viz resolve flows from disk or a registry. Pass --file <path>, --discover-dir <dir>, or --discover-entry-points to resolve a flow without writing any Python (issue #381); chainweaver flows list previews what is discoverable. With no discovery flag they fall back to a process-scoped, in-memory registry installed programmatically — running chainweaver inspect my_flow with neither a flag nor a configured registry exits 1 with No registry configured. Call chainweaver.cli.set_default_registry(...) before invoking the CLI.. To wire the default-registry path, use a small entry script:

# my_cli_entry.py
from chainweaver import FlowRegistry
from chainweaver.cli import main, set_default_registry
from my_app import build_registry  # returns a populated FlowRegistry

set_default_registry(build_registry())
main()

See docs/cli.md § Programmatic registration for the full pattern, including why the split exists (file-oriented commands stay zero-config, registry-oriented commands stay introspection-friendly).

---

Development

New contributors: see Your first contribution in CONTRIBUTING.md for the good-first-issue / good-first-ai-issue onramp and the step-by-step path to your first PR.

# Install with the integration extras and the maintainer tooling group
pip install --upgrade pip          # --group needs pip >= 25.1
pip install -e ".[integrations]" --group dev

# Run tests
python -m pytest tests/ -v

# Run the examples
python examples/simple_linear_flow.py   # simple arithmetic flow
python examples/etl_flow.py             # ETL flow
python examples/mcp_search_flow.py      # MCP-style search & summarize flow
python examples/naive_vs_compiled.py    # naive vs compiled timing comparison
python examples/coding_agent_pr_review.py
python examples/coding_agent_changelog.py
python examples/coding_agent_debug_log.py

---

License

This project is licensed under the Apache License 2.0 - see the LICENSE file for details.