mcp-mock-sim
MCP ServerFreeCLI tool for running, recording and replaying MCP tool-call scenarios
Capabilities5 decomposed
mcp tool-call scenario recording with request/response capture
Medium confidenceRecords live MCP tool invocations by intercepting and serializing the complete request-response cycle (tool name, arguments, results, errors) into a structured scenario file. Uses a middleware-style interception pattern that sits between the MCP client and server, capturing the exact state and side effects of each tool call without modifying the underlying tool implementations.
Implements MCP-specific recording by hooking into the protocol layer itself rather than wrapping individual tools, enabling capture of the exact tool schema, argument validation, and error responses as they flow through the MCP server
Captures MCP protocol semantics directly, whereas generic HTTP mocking tools would require manual translation of MCP messages into mock definitions
deterministic mcp scenario replay with request matching
Medium confidenceReplays recorded MCP tool-call scenarios by matching incoming tool requests against stored recordings and returning pre-recorded responses in sequence. Uses a state machine pattern that tracks replay position and validates that incoming requests match the recorded scenario structure (tool name, argument schema) before returning the corresponding response, enabling deterministic testing without live tool execution.
Implements replay as a stateful MCP server that validates incoming requests against the recorded scenario schema before returning responses, ensuring that replayed scenarios only match legitimate tool calls rather than accepting arbitrary requests
More precise than generic HTTP mocking because it understands MCP tool schemas and validates argument types, whereas tools like Nock or Sinon would require manual request matching logic
cli-driven scenario management and execution
Medium confidenceProvides command-line interface for recording, replaying, and managing MCP scenarios without requiring programmatic integration. Implements a CLI command parser that handles subcommands (record, replay, list, validate) and pipes scenario files through the recording/replay engines, with support for configuration files and environment variable overrides for server endpoints and scenario paths.
Wraps the recording/replay engines in a CLI layer that supports configuration files and environment variables, allowing scenario management without code changes — useful for teams that want to version control scenarios separately from test code
More accessible than programmatic APIs for non-developers and shell-based workflows, whereas libraries like jest-mock-extended require JavaScript/TypeScript knowledge
scenario validation and schema conformance checking
Medium confidenceValidates recorded scenario files against MCP protocol schema and tool definitions to ensure consistency and correctness. Implements a validation engine that checks that tool names match registered tools, arguments conform to declared schemas, and responses have the correct structure, reporting detailed validation errors that help developers identify malformed or stale scenarios.
Validates scenarios against live MCP tool schemas rather than static schema files, ensuring that recorded scenarios remain compatible as tool implementations evolve
More thorough than simple JSON schema validation because it understands MCP-specific semantics like tool argument constraints and error response formats
multi-scenario test suite execution with result aggregation
Medium confidenceExecutes multiple recorded scenarios in sequence or parallel, aggregating results and reporting pass/fail status for each scenario. Implements a test runner that loads scenario files, replays them against a mock MCP server, and compares actual responses against recorded expectations, with support for filtering scenarios by name or tag and generating test reports.
Implements test execution as a scenario replay engine with result comparison, rather than a generic test framework, enabling tight integration with MCP protocol semantics and scenario file formats
More specialized for MCP scenarios than generic test runners like Jest or Mocha, which would require custom adapters to understand scenario file formats and MCP protocol details
Capabilities are decomposed by AI analysis. Each maps to specific user intents and improves with match feedback.
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Best For
- ✓MCP server developers building tool-calling agents
- ✓teams implementing integration tests for MCP-based systems
- ✓developers debugging complex tool-call chains in production-like environments
- ✓MCP client developers testing agent logic in isolation
- ✓CI/CD pipelines that need fast, deterministic tool-call testing without external dependencies
- ✓developers reproducing and fixing bugs in tool-call handling logic
- ✓DevOps engineers and CI/CD pipeline builders
- ✓developers who prefer CLI tools over programmatic APIs
Known Limitations
- ⚠Recording captures only the MCP protocol layer — side effects outside the tool boundary (database writes, file system changes) are not automatically captured
- ⚠Large binary responses or streaming tool outputs may require custom serialization handlers
- ⚠No built-in deduplication of identical tool calls — recordings can become verbose with repeated invocations
- ⚠Replay is strictly sequential — out-of-order tool calls or branching logic that deviates from the recording will cause replay to fail or return incorrect responses
- ⚠No built-in support for parameterized scenarios — each unique set of arguments requires a separate recording
- ⚠Replay state is not persisted across sessions — restarting the replay requires re-initializing from the scenario file
Requirements
Input / Output
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CLI tool for running, recording and replaying MCP tool-call scenarios
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