Harness Testkit Feature

The testkit provides deterministic utilities for exercising inherently nondeterministic model and tool workflows.

Source Inspiration

LangChain keeps fake models, standard provider tests, trajectory-style agent tests, event-stream tests, cache tests, and callback tests:

• fake chat models: https://github.com/langchain-ai/langchain/blob/master/libs/core/langchain_core/language_models/fake_chat_models.py
• standard integration tests: https://github.com/langchain-ai/langchain/tree/master/libs/standard-tests
• core unit tests: https://github.com/langchain-ai/langchain/tree/master/libs/core/tests/unit_tests
• v1 agent tests: https://github.com/langchain-ai/langchain/tree/master/libs/langchain_v1/tests

TinyAgents should ship a reusable testkit so application code and provider adapters can assert behavior without live providers.

Responsibilities

• Provide fake chat models.
• Provide deterministic fake embedding models.
• Provide scripted streaming models.
• Provide fake tools.
• Provide failing and flaky tools/models.
• Provide in-memory stores and event collectors.
• Provide deterministic clocks and id generators.
• Provide trajectory assertions for agent loops.
• Provide graph trajectory assertions for state-graph runs.
• Provide conformance tests for provider adapters.
• Provide replay fixtures from recorded events.
• Provide helpers for structured-output validation tests.
• Provide in-memory vector stores and record managers for retrieval/indexing tests.

Core Utilities

pub struct ScriptedModel {
    pub script: Vec<ModelScriptStep>,
}

pub enum ModelScriptStep {
    Respond(AssistantMessage),
    Stream(Vec<ModelStreamItem>),
    Fail(TinyAgentsError),
}

pub struct EventCollector {
    pub events: Arc<Mutex<Vec<HarnessEvent>>>,
}

pub struct DeterministicFakeEmbedding {
    pub dimensions: usize,
    pub seed_namespace: String,
}

Trajectory Assertions

Tests should be able to assert:

• number of model calls
• number of tool calls
• exact tool names and arguments
• final assistant message
• structured output value
• emitted event kinds
• retry attempts
• fallback selections
• cache hit/miss behavior
• summary creation
• usage and cost records
• no unexpected provider calls
• retrieved document ids and scores
• indexed document hashes
• graph run status
• graph node transition sequence
• checkpoint labels and state snapshots
• HITL interrupt kind/options/resume node

Provider Conformance

Provider adapters should run a shared suite that covers:

• text invoke
• text stream
• system messages
• multiple user/assistant turns
• Unicode
• multimodal input when profile declares support
• tool binding
• tool-call id preservation
• no-argument tool calls
• streamed tool-call chunks when profile declares support
• structured output in provider mode
• structured output in tool mode
• usage metadata
• cancellation
• timeout
• rate-limit error classification

State-graph conformance should cover:

• linear route
• conditional route
• fork and merge reducer
• cycle guarded by max steps
• cancellation
• node error propagation
• HITL pause/resume/reject
• in-memory and durable checkpointer round trip
• run list pagination and newest-first order
• per-agent blueprint validation
• live turn graph tool advertisement, allowlist enforcement, malformed argument handling, and final output depending on actual tool execution
• provider options passthrough

Tests requiring live providers should be opt-in and documented with environment variables. Unit-level conformance should run with fakes in normal CI.