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Playwright MCP Server

by devskido

start_codegen_session

Start recording browser interactions to automatically generate Playwright test code with configurable output options.

Instructions

Start a new code generation session to record Playwright actions

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
optionsYesCode generation options

Implementation Reference

  • Core implementation of the start_codegen_session tool. Defines the Tool object including name, description, input parameters schema, and the async handler function that merges options, ensures output directory, starts ActionRecorder session, and returns session details.
    export const startCodegenSession: Tool = {
      name: 'start_codegen_session',
      description: 'Start a new code generation session to record MCP tool actions',
      parameters: {
        type: 'object',
        properties: {
          options: {
            type: 'object',
            description: 'Code generation options',
            properties: {
              outputPath: { type: 'string' },
              testNamePrefix: { type: 'string' },
              includeComments: { type: 'boolean' }
            }
          }
        }
      },
      handler: async ({ options = {} }: { options?: CodegenOptions }) => {
        try {
          // Merge provided options with defaults
          const mergedOptions = { ...DEFAULT_OPTIONS, ...options };
          
          // Ensure output path is absolute and normalized
          const workspaceRoot = getWorkspaceRoot();
          const outputPath = path.isAbsolute(mergedOptions.outputPath) 
            ? mergedOptions.outputPath 
            : path.join(workspaceRoot, mergedOptions.outputPath);
          
          mergedOptions.outputPath = outputPath;
          
          // Ensure output directory exists
          try {
            await fs.mkdir(outputPath, { recursive: true });
          } catch (mkdirError: any) {
            throw new Error(`Failed to create output directory: ${mkdirError.message}`);
          }
          
          const sessionId = ActionRecorder.getInstance().startSession();
          
          // Store options with the session
          const recorder = ActionRecorder.getInstance();
          const session = recorder.getSession(sessionId);
          if (session) {
            session.options = mergedOptions;
          }
          
          return { 
            sessionId,
            options: mergedOptions,
            message: `Started codegen session. Tests will be generated in: ${outputPath}`
          };
        } catch (error: any) {
          throw new Error(`Failed to start codegen session: ${error.message}`);
        }
      }
    };
  • Registers and dispatches the start_codegen_session tool call in the main handleToolCall function by invoking its handler via handleCodegenResult. Imports the tool at lines 7-12.
    switch (name) {
      case 'start_codegen_session':
        return await handleCodegenResult(startCodegenSession.handler(args));
      case 'end_codegen_session':
        return await handleCodegenResult(endCodegenSession.handler(args));
      case 'get_codegen_session':
        return await handleCodegenResult(getCodegenSession.handler(args));
      case 'clear_codegen_session':
        return await handleCodegenResult(clearCodegenSession.handler(args));
    }
  • Defines the input schema for the start_codegen_session tool in createToolDefinitions(), used for MCP protocol tool definitions. Note: slightly stricter than the implementation schema (requires outputPath).
    {
      name: "start_codegen_session",
      description: "Start a new code generation session to record Playwright actions",
      inputSchema: {
        type: "object",
        properties: {
          options: {
            type: "object",
            description: "Code generation options",
            properties: {
              outputPath: { 
                type: "string", 
                description: "Directory path where generated tests will be saved (use absolute path)" 
              },
              testNamePrefix: { 
                type: "string", 
                description: "Prefix to use for generated test names (default: 'GeneratedTest')" 
              },
              includeComments: { 
                type: "boolean", 
                description: "Whether to include descriptive comments in generated tests" 
              }
            },
            required: ["outputPath"]
          }
        },
        required: ["options"]
      }
    },
  • src/tools.ts:485-486 (registration)
    Lists 'start_codegen_session' in the CODEGEN_TOOLS array, categorizing it for conditional handling.
    export const CODEGEN_TOOLS = [
      'start_codegen_session',

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observed

TDQS

C2.9/5.0
Behavior2/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

No annotations are provided, so the description carries the full burden of behavioral disclosure. It mentions recording Playwright actions, but lacks details on session lifecycle, permissions, rate limits, or what happens if a session is already active. For a tool that initiates a stateful process, this is a significant gap in transparency.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is a single, efficient sentence that directly states the tool's purpose without unnecessary words. It is front-loaded and appropriately sized, with every part contributing to understanding the tool's function.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness2/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the complexity of starting a code generation session with no annotations and no output schema, the description is incomplete. It doesn't explain what the tool returns, how the session interacts with other tools, or error conditions. For a state-initiating tool, more context is needed to guide effective use.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the schema fully documents the single parameter 'options' and its nested properties. The description adds no additional parameter semantics beyond what the schema provides, such as default values or usage examples. With high schema coverage, the baseline score of 3 is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the action ('Start a new code generation session') and the resource ('to record Playwright actions'), making the purpose evident. It distinguishes from siblings like 'clear_codegen_session' or 'end_codegen_session' by specifying the initiation of a session, though it doesn't explicitly contrast with all siblings like 'get_codegen_session'.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines2/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides no guidance on when to use this tool versus alternatives. It doesn't mention prerequisites, such as needing an active Playwright context, or when to choose this over other session-related tools like 'get_codegen_session'. Usage is implied only by the verb 'start', but no explicit context or exclusions are provided.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.