src/testing.rs

This file ships fakes for every moving part under the testing feature, so your agent logic runs end-to-end with zero network. It compiles only under the feature, so you carry no production impact.

Key items

MockApiClient is your scripted model: new(model), with_responses(vec![...]), with_text_response, with_tool_call, with_stop_reason, and with_error.

without_usage() is new in 0.4.0 and serves the script with no usage payload, which gives you the no-usage backend shape.

It emits the real provider event sequence with usage fixed at 50/25, and the last script entry repeats forever.

set_model works, which your model-switch tests rely on, and response_format is served with the canned text for parse-failure tests. A tool_constraint is rejected loudly.

Wire recording gives you captured_requests(), captured_stream_requests() from 0.4.0, and create_message_calls().

with_options_calls() counts both options-bearing transports since 0.4.0, and with_errors(Vec<Option<String>>) scripts failures per position.

MockResponse { text, tool_call, stop_reason } and MockToolCall { id, name, input } are your script shapes.

MockTool carries knobs for dispatch behavior: with_result, with_error, with_delay, with_concurrency_safe, with_read_only, and with_schema. It ignores its input.

Your fixtures are test_config(), test_message, test_assistant_message, and test_tool_use_message.

EnvGuard snapshots, sets, and restores environment variables across a test, serializes between tests, and restores even on panic.

Behavior notes

The mock ignores request content entirely, so “the model should have seen X” assertions belong to observers or the audit trail.

with_responses(vec![]) is a no-op and keeps the default script.

Beyond the mock, your repo pins wire contracts two more ways, both in tests/: the cassette corpus in cassettes/, which replays recorded real exchanges hermetically and is never weakened.

The second is golden request bytes in golden/, canonical-JSON snapshots of engine-constructed requests, regenerated only via UPDATE_GOLDENS=1.

Deep dive: Testing.