LLM Observability Testing Skill

Purpose

This skill helps you write comprehensive LLMObs tests that validate span events, messages, tokens, and metadata using category-appropriate strategies.

When to Use

• Writing tests for a new LLMObs plugin (ALWAYS check category first)
• Understanding category-specific test strategies
• Learning VCR cassettes (for LLM_CLIENT/MULTI_PROVIDER only)
• Learning assertion patterns for LLMObs spans

Core Testing Concepts

1. Test Structure

LLMObs tests use special helpers to validate span events.

Key components:

• useLlmObs() - Initializes LLMObs test environment
• getEvents() - Retrieves captured span events
• assertLlmObsSpanEvent() - Validates span structure with flexible matchers

Basic test flow:

1. Initialize test environment with useLlmObs({ plugin: 'name' })
2. Call instrumented method (chat completion, workflow execution, etc.)
3. Get captured span events with getEvents()
4. Validate span structure with assertLlmObsSpanEvent()

See references/test-structure.md for complete test file templates.

2. VCR Cassettes

VCR records real API calls and replays them in tests for deterministic testing without external dependencies.

Purpose:

• Record real LLM API responses once
• Replay deterministically in CI without API keys
• No external dependencies after recording

How it works:

1. Configure proxy baseURL: http://127.0.0.1:9126/vcr/{provider}
2. Run tests with real API keys (first time only)
3. VCR proxy records requests/responses to cassette files
4. Subsequent test runs replay from cassettes (no API keys needed)

Cassette location: test/llmobs/plugins/{integration}/cassettes/

When to use VCR:

• ✅ LlmObsCategory.LLM_CLIENT (Direct API wrappers)
• ✅ LlmObsCategory.MULTI_PROVIDER (Multi-provider frameworks)
• ❌ LlmObsCategory.ORCHESTRATION (Pure functions, no API calls)
• ❌ LlmObsCategory.INFRASTRUCTURE (Mock servers instead)

See references/vcr-cassettes.md for recording process and troubleshooting.

3. Category-Specific Test Strategies

Test strategy is determined by the LlmObsCategory enum.

LlmObsCategory.LLM_CLIENT & LlmObsCategory.MULTI_PROVIDER

Strategy: VCR with real API calls via proxy

Characteristics:

• Use VCR proxy baseURL
• Record cassettes with real API keys
• Tests make actual HTTP calls (recorded once)
• Validate LLM-specific data (messages, tokens, model info)

Span kind: Usually 'llm' for chat completions

See references/category-strategies.md for detailed patterns.

LlmObsCategory.ORCHESTRATION

Strategy: Pure function tests, NO VCR, NO real API calls

Characteristics:

• NO VCR cassettes
• NO HTTP calls to LLM providers
• Use library's native APIs with mock/test LLM responses
• Focus on workflow lifecycle, not API calls
• CRITICAL: Still test with actual LLM as orchestration node (not mocked completely)

Span kind: Usually 'workflow' or 'agent', NOT 'llm'

Example concept:

• LangGraph invokes nodes that call LLMs
• LangGraph itself doesn't make HTTP calls
• Test LangGraph's workflow execution, not the underlying LLM API

See references/category-strategies.md for orchestration test patterns.

LlmObsCategory.INFRASTRUCTURE

Strategy: Mock server tests

Characteristics:

• Mock server implementation
• Protocol-specific validation
• NO VCR

See references/category-strategies.md for infrastructure test patterns.

4. Assertion Patterns

assertLlmObsSpanEvent(actual, expected)

Validates span structure with flexible matchers for non-deterministic values.

Available matchers:

• MOCK_STRING - Matches any non-empty string (use for output text)
• MOCK_NOT_NULLISH - Matches any truthy value (use for token counts)
• MOCK_NUMBER - Matches any number
• MOCK_OBJECT - Matches any object (use for errors)

Assertable fields:

• spanKind (required) - Span type from LlmObsSpanKind enum
• name - Operation name
• modelName - Model identifier (for LLM spans)
• modelProvider - Provider name (for LLM spans)
• inputMessages - Input messages in [{content, role}] format
• outputMessages - Output messages in [{content, role}] format
• metrics - Token usage (input_tokens, output_tokens, total_tokens)
• metadata - Model parameters (temperature, max_tokens, etc.)
• error - Error object (if operation failed)

Partial validation: Only specified fields are checked, others ignored.

See references/assertion-helpers.md for complete API and patterns.

Test File Organization

Location: test/llmobs/plugins/{integration}/index.spec.js

Structure:

1. Import helpers from '../../util'
2. Initialize LLMObs test environment
3. Load modules in beforeEach() for fresh state
4. Group tests by method (describe('chat completions', ...))
5. Cover all instrumented methods
6. Test error cases

Standard imports:

useLlmObs, assertLlmObsSpanEvent, MOCK_STRING, MOCK_NOT_NULLISH, MOCK_NUMBER, MOCK_OBJECT

See references/test-structure.md for complete template.

Key Testing Points

Coverage Requirements

Test all instrumented methods with:

• ✅ Basic operation (single message/call)
• ✅ Multi-turn conversations (if applicable)
• ✅ Error cases
• ✅ All required span fields (spanKind, name, modelName, modelProvider)
• ✅ Message format validation ({content, role} structure)
• ✅ Metrics validation (token counts exist and are truthy)
• ✅ Metadata validation (parameters passed through)

Span Kind Validation

Match span kind to operation type using LlmObsSpanKind enum:

• Chat/completions → 'llm'
• Workflow execution → 'workflow'
• Agent runs → 'agent'
• Tool calls → 'tool'
• Embeddings → 'embedding'
• Retrieval → 'retrieval'

Error Handling

On errors, validate:

• Empty output messages: [{content: '', role: ''}]
• Error object exists: error: MOCK_OBJECT
• Span still created (not dropped)

Common Patterns by Category

LLM_CLIENT / MULTI_PROVIDER Pattern

• Use VCR proxy baseURL
• Test chat completions with various parameters
• Validate real API response structure
• Test streaming (if supported)
• Test error responses

ORCHESTRATION Pattern

• NO VCR
• Test workflow lifecycle methods (invoke, stream, run)
• Use mock LLM responses within workflow
• Focus on workflow span, not LLM spans
• Validate workflow-specific metadata (state, nodes, edges)

INFRASTRUCTURE Pattern

• Mock server setup
• Protocol-specific validation
• Connection/transport testing

Best Practices

1. Use MOCK_ for non-deterministic values* - Output text, token counts, error objects
2. Use exact values for inputs - You control input messages and parameters
3. Always validate spanKind - Required for every span
4. Match category to test strategy - VCR for clients, pure functions for orchestration
5. Test error paths - Verify empty outputs and error objects on failures
6. Group by method - Organize tests by instrumented method
7. Load modules fresh - Use beforeEach() to avoid state leakage
8. Cover edge cases - Empty messages, missing metadata, streaming

References

For detailed information, see:

• references/test-structure.md - Complete test file templates and organization
• references/vcr-cassettes.md - VCR recording process, cassette management, troubleshooting
• references/assertion-helpers.md - Complete assertLlmObsSpanEvent API, matchers, patterns
• references/category-strategies.md - Detailed test strategies for each LlmObsCategory

Key Principles

1. Category determines strategy - Always check LlmObsCategory to pick test approach
2. Orchestrators don't use VCR - They don't make direct API calls
3. Use matchers for variance - Real API responses vary, use MOCK_* matchers
4. Validate message format - Always check {content, role} structure
5. Test with real behavior - For orchestrators, use actual LLM as node (not fully mocked)