> ## Documentation Index
> Fetch the complete documentation index at: https://qitor.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Replace a provider and add context

> Learn QitOS with complete, executable notes-project code.

## Goal and prerequisites

A composition accepts a public model override and explicitly named extension factories. `ObservedFakeProvider` preserves the callable interface, counts actual requests and asserts the selected project context appears in each request. It then delegates to the visible deterministic provider.

The project contributor is a factory returning StaticContextContributor. Its registration name matches `context.contributors`. This avoids imports from arbitrary names embedded in YAML and keeps executable extension selection under application control.

This is a complete provider substitution and context integration example, not a new transport client. For a real provider, use the matching configuration and explicit credential resolver described in Configuration. Consult the extension index before replacing stores, sinks or sandboxes: preserving a Python method signature alone does not preserve their durability and security contracts.

Basic Python is required. The package declares Python ≥3.10; local qualification uses Python 3.12.7. Each chapter runs independently; reuse the environment and matching files when continuing your project. Commands below use a macOS/Linux shell.

## Prepare the project

```bash theme={null}
python3 -m venv .venv
source .venv/bin/activate
python -m pip install "qitos @ git+https://github.com/WhitzardAgent/WhitzardOS.git@60809b3be388d22ea40ea41b4aaa1f5540c76fda"
mkdir notes_lesson
cd notes_lesson
```

Save the complete files below in this directory. No repository clone, editable install, or copied tests are needed.

## Run and verify

```bash theme={null}
python provider_extension.py --root extension-run
```

Expected output fragments (generated IDs vary). Each run verifies tool results or persistence assertions and exits 0 on success.

```text theme={null}
provider replaced; context observed; requests=3
```

## Behavior and support boundaries

No provider is universally compatible. Codec, continuation, tool schema and loss policy must agree. Inspect typed failures; do not disable loss checks to accept an incompatible response.

## Exercise and answer

Rename the contributor key in both config and extensions. Leaving only one side changed must produce a configuration/extension failure.

## Common errors and cleanup

`ModuleNotFoundError`: activate the environment with the specified installation and save every file on this page. Existing run root: choose a new `--root` instead of overwriting evidence. Assertion failure: inspect the first failed tool or typed error, not only the final text. After all processes and the board stop, remove only this lesson’s newly created run directories if no longer needed; retain SQLite, journals and reports you want to debug.

## Complete files: save in the project root

```python title="notes.py" theme={null}
"""Synthetic notes; fake model, real tools, composition, Session and journal."""
import argparse
from dataclasses import replace
import json
from pathlib import Path

from qitos.config import build_agent_composition, load_agent_config
from qitos.core.function_tool_decorator import function_tool
from qitos.engine.runtime import LifecyclePolicy

# docs:start fixture
NOTES = (
    "Session: A durable session can resume after a process exits.",
    "Artifact: Large tool outputs can be retained outside model context.",
)


@function_tool(read_only=True, concurrency_safe=True)
def summarize_note(index: int) -> dict:
    """Extract a title and word count from a synthetic in-memory note."""
    text = NOTES[index]
    return {"title": text.split(":", 1)[0], "words": len(text.split())}
# docs:end fixture


# docs:start provider
class FakeProvider:
    """Scripted responses; this does not summarize or reason like a real model."""
    model = "notes-fake"
    qitos_protocol = "react_text_v1"

    def __init__(self, start=0):
        self.stage = start

    def call_raw(self, messages, **options):
        if self.stage < len(NOTES):
            content = f"Thought: inspect a note\nAction: summarize_note(index={self.stage})"
        else:
            content = "Final Answer: Indexed 2 notes: Session, Artifact."
        self.stage += 1
        return {"choices": [{"message": {"content": content}}]}
# docs:end provider


class PauseAfterTool(LifecyclePolicy):
    policy_id = "notes.pause_after_tool"

    def should_pause(self, context):
        return context.step_id == 0


# docs:start composition
def configuration(root):
    config = load_agent_config(Path(__file__).with_name("agent.yaml"))
    return replace(config, runtime=replace(
        config.runtime, data_root=str(root / "data"),
        environment=replace(config.runtime.environment, workspace=str(root)),
        session=replace(config.runtime.session, path=str(root / "sessions.sqlite3")),
        trajectory=replace(config.runtime.trajectory, output=str(root / "trajectory.journal")),
    ))


def compose(root, *, start=0, pause=False):
    config = configuration(root)
    if pause:
        config = replace(config, lifecycle={"policy": "pause"})
    composition = build_agent_composition(
        config, model_override=FakeProvider(start), extensions={"pause": PauseAfterTool},
    )
    composition.tool_registry.register(summarize_note)
    return composition
# docs:end composition


# docs:start run
def run(root):
    root.mkdir(parents=True, exist_ok=False)
    with compose(root) as composition:
        session = composition.session("Index both synthetic notes")
        result = session.run()
        outputs = [action.output for record in result.records for action in record.action_results
                   if action.tool_name == "summarize_note"]
        assert [output["title"] for output in outputs] == ["Session", "Artifact"]
        assert result.state.final_result == "Indexed 2 notes: Session, Artifact."
        config = composition.config.to_dict()
        config["runtime"]["environment"] = {"type": "unsafe_host", "workspace": str(root)}
        (root / "agent.json").write_text(json.dumps(config), encoding="utf-8")
        control = {"session_id": session.session_id.value, "run_id": result.run_id}
        (root / "control.json").write_text(json.dumps(control), encoding="utf-8")
        print(json.dumps({**control, "result": result.state.final_result, "outputs": outputs}))
# docs:end run


if __name__ == "__main__":
    parser = argparse.ArgumentParser(description=__doc__)
    parser.add_argument("--root", type=Path, default=Path("notes-run"))
    run(parser.parse_args().root.resolve())
```

```yaml title="agent.yaml" theme={null}
schema: qitos.agent
agent:
  name: notes_agent
  protocol: react_text_v1
model:
  provider: openai_compatible
  model: notes-fake
  credential:
    ref: notes-provider
  request:
    max_tokens: 512
    timeout_seconds: 30
    retries: 0
tools:
  preset: none
runtime:
  environment:
    type: unsafe_host
    workspace: .
  session:
    mode: durable
    store: sqlite
    path: ./notes-run/sessions.sqlite3
  trajectory:
    enabled: true
    output: ./notes-run/trajectory.journal
budgets:
  max_steps: 6
  max_requests: 6
  max_runtime_seconds: 30
failure_policy:
  tool: fail_closed
```

```python title="provider_extension.py" theme={null}
"""Replace the provider and inject context, without changing the kernel."""
import argparse
from dataclasses import replace
from pathlib import Path

from notes import FakeProvider, configuration, summarize_note
from qitos.config import build_agent_composition
from qitos.core.context import StaticContextContributor


class ObservedFakeProvider(FakeProvider):
    """Keep the public provider call shape and validate selected context."""
    def __init__(self):
        super().__init__()
        self.requests = 0

    def call_raw(self, messages, **options):
        assert "notes-project-context" in str(messages)
        self.requests += 1
        return super().call_raw(messages, **options)


def run(root):
    root.mkdir(parents=True, exist_ok=False)
    config = replace(configuration(root), context={"contributors": ["project"]})
    provider = ObservedFakeProvider()
    with build_agent_composition(config, model_override=provider, extensions={
        "project": lambda: StaticContextContributor("notes.project", "project", "notes-project-context"),
    }) as composition:
        composition.tool_registry.register(summarize_note)
        result = composition.session("Index both notes").run()
        assert result.state.final_result == "Indexed 2 notes: Session, Artifact."
        assert provider.requests == 3
    print("provider replaced; context observed; requests=3")


if __name__ == "__main__":
    parser = argparse.ArgumentParser(description=__doc__)
    parser.add_argument("--root", type=Path, required=True)
    run(parser.parse_args().root.resolve())
```

## Next step and API

[API Reference](/reference/api) · [Configuration](/reference/configuration) · [Learning path](/tutorials/index) · [Next](/quickstart)

[Source file](https://github.com/WhitzardAgent/WhitzardOS/blob/master/examples/tutorials/notes/provider_extension.py) (optional; all required code is already on this page).