inspect_variables

Inspect variables in current scope to debug NodeJS server code by examining local or global values during execution.

Instructions

Inspects variables in current scope

Input Schema

TableJSON Schema

Name	Required	Description	Default
`scope`	No	Scope to inspect (local/global)

Implementation Reference

src/mcp-server.js:529-613 (handler)

The handler function that implements the core logic of the 'inspect_variables' tool. It handles both global and local scope inspection using the Node.js debugger protocol. For local scopes when paused, it iterates through the scope chain of the top call frame, fetches properties using getProperties, and formats the variables for output.

async ({ scope = 'local' }) => {
  try {
    // Ensure debugger is enabled
    if (!inspector.debuggerEnabled) {
      await inspector.enableDebugger();
    }
    
    if (scope === 'global' || !inspector.paused) {
      // For global scope or when not paused, use Runtime.globalProperties
      const response = await inspector.send('Runtime.globalLexicalScopeNames', {});
      
      // Get global object properties for a more complete picture
      const globalObjResponse = await inspector.send('Runtime.evaluate', {
        expression: 'this',
        contextId: 1,
        returnByValue: true
      });
      
      return {
        content: [{
          type: "text",
          text: JSON.stringify({
            lexicalNames: response.names,
            globalThis: globalObjResponse.result.value
          }, null, 2)
        }]
      };
    } else {
      // For local scope when paused, get variables from the current call frame
      if (inspector.currentCallFrames.length === 0) {
        return {
          content: [{
            type: "text",
            text: "No active call frames. Debugger is not paused at a breakpoint."
          }]
        };
      }
      
      const frame = inspector.currentCallFrames[0]; // Get top frame
      const scopeChain = frame.scopeChain;
      
      // Create a formatted output of variables in scope
      const result = {};
      
      for (const scopeObj of scopeChain) {
        const { scope, type, name } = scopeObj;
        
        if (type === 'global') continue; // Skip global scope for local inspection
        
        const objProperties = await inspector.getProperties(scope.object.objectId);
        const variables = {};
        
        for (const prop of objProperties.result) {
          if (prop.value && prop.configurable) {
            if (prop.value.type === 'object' && prop.value.subtype !== 'null') {
              variables[prop.name] = `[${prop.value.subtype || prop.value.type}]`;
            } else if (prop.value.type === 'function') {
              variables[prop.name] = '[function]';
            } else if (prop.value.value !== undefined) {
              variables[prop.name] = prop.value.value;
            } else {
              variables[prop.name] = `[${prop.value.type}]`;
            }
          }
        }
        
        result[type] = variables;
      }
      
      return {
        content: [{
          type: "text",
          text: JSON.stringify(result, null, 2)
        }]
      };
    }
  } catch (err) {
    return {
      content: [{
        type: "text",
        text: `Error inspecting variables: ${err.message}`
      }]
    };
  }
}

src/mcp-server.js:523-614 (registration)

The registration of the 'inspect_variables' tool using server.tool(), including name, description, input schema, and handler reference.

server.tool(
  "inspect_variables",
  "Inspects variables in current scope",
  {
    scope: z.string().optional().describe("Scope to inspect (local/global)")
  },
  async ({ scope = 'local' }) => {
    try {
      // Ensure debugger is enabled
      if (!inspector.debuggerEnabled) {
        await inspector.enableDebugger();
      }
      
      if (scope === 'global' || !inspector.paused) {
        // For global scope or when not paused, use Runtime.globalProperties
        const response = await inspector.send('Runtime.globalLexicalScopeNames', {});
        
        // Get global object properties for a more complete picture
        const globalObjResponse = await inspector.send('Runtime.evaluate', {
          expression: 'this',
          contextId: 1,
          returnByValue: true
        });
        
        return {
          content: [{
            type: "text",
            text: JSON.stringify({
              lexicalNames: response.names,
              globalThis: globalObjResponse.result.value
            }, null, 2)
          }]
        };
      } else {
        // For local scope when paused, get variables from the current call frame
        if (inspector.currentCallFrames.length === 0) {
          return {
            content: [{
              type: "text",
              text: "No active call frames. Debugger is not paused at a breakpoint."
            }]
          };
        }
        
        const frame = inspector.currentCallFrames[0]; // Get top frame
        const scopeChain = frame.scopeChain;
        
        // Create a formatted output of variables in scope
        const result = {};
        
        for (const scopeObj of scopeChain) {
          const { scope, type, name } = scopeObj;
          
          if (type === 'global') continue; // Skip global scope for local inspection
          
          const objProperties = await inspector.getProperties(scope.object.objectId);
          const variables = {};
          
          for (const prop of objProperties.result) {
            if (prop.value && prop.configurable) {
              if (prop.value.type === 'object' && prop.value.subtype !== 'null') {
                variables[prop.name] = `[${prop.value.subtype || prop.value.type}]`;
              } else if (prop.value.type === 'function') {
                variables[prop.name] = '[function]';
              } else if (prop.value.value !== undefined) {
                variables[prop.name] = prop.value.value;
              } else {
                variables[prop.name] = `[${prop.value.type}]`;
              }
            }
          }
          
          result[type] = variables;
        }
        
        return {
          content: [{
            type: "text",
            text: JSON.stringify(result, null, 2)
          }]
        };
      }
    } catch (err) {
      return {
        content: [{
          type: "text",
          text: `Error inspecting variables: ${err.message}`
        }]
      };
    }
  }
);

src/mcp-server.js:526-528 (schema)
Zod input schema defining the optional 'scope' parameter for local or global variable inspection.
```
{
  scope: z.string().optional().describe("Scope to inspect (local/global)")
},
```

src/mcp-server.js:307-318 (helper)

Helper method in the Inspector class used by the tool to retrieve detailed properties of scope objects during local variable inspection.

async getProperties(objectId, ownProperties = true) {
	try {
		return await this.send('Runtime.getProperties', {
			objectId,
			ownProperties,
			accessorPropertiesOnly: false,
			generatePreview: true
		});
	} catch (err) {
		throw err;
	}
}

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