graph LR
    subgraph "Tool Lifecycle"
        LLM[LLM Decision] --> ToolUse[ToolUseBlock]
        ToolUse --> Validation{Input Validation}
        Validation -->|Pass| Permission{Permission Check}
        Validation -->|Fail| ErrorResult[Error Result]

        Permission -->|Allow| Execute["Tool.call()"]
        Permission -->|Deny| ErrorResult
        Permission -->|Ask| UserPrompt[User Dialog]

        UserPrompt -->|Allow| Execute
        UserPrompt -->|Deny| ErrorResult

        Execute --> Progress[Yield Progress]
        Progress --> Progress
        Progress --> Result[Yield Result]

        Result --> Transform[mapToolResult]
        Transform --> ToolResultBlock
        ErrorResult --> ToolResultBlock

        ToolResultBlock --> LLM
    end

The Tool Execution Pipeline: Async Generators All The Way Down

The most fascinating aspect of Claude Code's tool system is its use of async generators throughout the execution pipeline. This allows for streaming progress updates while maintaining clean error boundaries:

// The core tool execution function (reconstructed)
async function* executeTool(
  toolUse: ToolUseBlock,
  toolDef: ToolDefinition,
  context: ToolUseContext,
  permissionFn: PermissionGranter,
  assistantMessage: CliMessage
): AsyncGenerator<CliMessage, void, void> {
  // Phase 1: Input validation with Zod
  const validationStart = performance.now();
  const validation = toolDef.inputSchema.safeParse(toolUse.input);

  if (!validation.success) {
    // Format Zod errors for LLM consumption
    const errorMessage = formatZodError(validation.error);
    yield createToolResultMessage({
      tool_use_id: toolUse.id,
      content: [{
        type: 'text',
        text: `Input validation failed:\\\\n${errorMessage}`
      }],
      is_error: true
    });
    return;
  }

  // Phase 2: Permission check
  const permissionResult = await checkToolPermission(
    toolDef,
    validation.data,
    context.getToolPermissionContext(),
    permissionFn
  );

  if (permissionResult.behavior === 'deny') {
    yield createToolResultMessage({
      tool_use_id: toolUse.id,
      content: [{
        type: 'text',
        text: `Permission denied: ${permissionResult.message}`
      }],
      is_error: true
    });
    return;
  }

  if (permissionResult.behavior === 'ask') {
    // Yield UI event for permission dialog
    yield {
      type: 'permission_request',
      toolName: toolDef.name,
      input: validation.data,
      suggestions: permissionResult.ruleSuggestions
    };

    // Wait for user decision (handled by outer loop)
    const decision = await permissionFn(
      toolDef,
      validation.data,
      permissionResult
    );

    if (!decision.allowed) {
      yield createToolResultMessage({
        tool_use_id: toolUse.id,
        content: [{
          type: 'text',
          text: 'Tool execution cancelled by user'
        }],
        is_error: true
      });
      return;
    }
  }

  // Phase 3: Tool execution with progress tracking
  try {
    const executeStart = performance.now();
    let progressCount = 0;
    let finalResult = null;

    // Call the tool's async generator
    for await (const output of toolDef.call(
      validation.data,
      context,
      undefined, // mcpContext - skipping as requested
      assistantMessage
    )) {
      if (output.type === 'progress') {
        progressCount++;
        yield {
          type: 'progress',
          uuid: `progress-${toolUse.id}-${progressCount}`,
          timestamp: new Date().toISOString(),
          progress: {
            toolUseID: toolUse.id,
            data: output.data
          }
        };
      } else if (output.type === 'result') {
        finalResult = output.data;
      }
    }

    // Phase 4: Result transformation
    if (finalResult !== null) {
      const content = toolDef.mapToolResultToToolResultBlockParam(
        finalResult,
        toolUse.id
      );

      yield createToolResultMessage({
        tool_use_id: toolUse.id,
        content: Array.isArray(content) ? content : [content],
        is_error: false,
        executionTime: performance.now() - executeStart
      });
    }
  } catch (error) {
    // Error handling with rich context
    yield createToolResultMessage({
      tool_use_id: toolUse.id,
      content: formatToolError(error, toolDef),
      is_error: true
    });
  }
}

Performance Characteristics:

• Input validation: O(n) where n is input complexity, typically <1ms
• Permission check: O(rules) + potential user interaction time
• Tool execution: Varies wildly by tool (10ms to 30s)
• Result transformation: O(output size), typically <5ms

The Shell Parser: Claude Code's Secret Weapon

One of the most innovative components is the custom shell parser that enables passing JavaScript objects through shell commands:

// The shell parser implementation (reconstructed from decompilation)
class ShellParser {
  private static OPERATORS = /(\\\\|\\\\||&&|;;|\\\\|&|\\\\||<|>|>>|&|\\\\(|\\\\))/;
  private static SINGLE_QUOTE = /^'([^']*)'$/;
  private static DOUBLE_QUOTE = /^"([^"\\\\\\\\]*(\\\\\\\\.[^"\\\\\\\\]*)*)"$/;

  // The magic: random sentinel for object embedding
  private static SENTINEL = crypto.randomBytes(16).toString('hex');

  static parse(
    command: string,
    env: Record<string, any>,
    opts?: (token: string) => any
  ): ParsedCommand {
    // Phase 1: Variable expansion with object serialization
    const expandedCommand = this.expandVariables(command, env);

    // Phase 2: Tokenization
    const tokens = this.tokenize(expandedCommand);

    // Phase 3: Object rehydration if opts provided
    if (opts && typeof opts === 'function') {
      return tokens.map(token => {
        if (this.isSerializedObject(token)) {
          return this.deserializeObject(token);
        }
        return token;
      });
    }

    return tokens;
  }

  private static expandVariables(
    command: string,
    env: Record<string, any>
  ): string {
    return command.replace(
      /\\\\$\\\\{?(\\\\w+)\\\\}?/g,
      (match, varName) => {
        const value = env[varName];

        // The innovation: serialize objects with sentinel
        if (typeof value === 'object' && value !== null) {
          return this.SENTINEL + JSON.stringify(value) + this.SENTINEL;
        }

        return String(value || '');
      }
    );
  }

  private static tokenize(command: string): string[] {
    const tokens: string[] = [];
    let current = '';
    let inSingleQuote = false;
    let inDoubleQuote = false;
    let escape = false;

    for (let i = 0; i < command.length; i++) {
      const char = command[i];
      const next = command[i + 1];

      // Handle quotes and escapes
      if (!escape) {
        if (char === "'" && !inDoubleQuote) {
          inSingleQuote = !inSingleQuote;
          current += char;
          continue;
        }
        if (char === '"' && !inSingleQuote) {
          inDoubleQuote = !inDoubleQuote;
          current += char;
          continue;
        }
        if (char === '\\\\\\\\') {
          escape = true;
          current += char;
          continue;
        }
      } else {
        escape = false;
        current += char;
        continue;
      }

      // Handle operators when not in quotes
      if (!inSingleQuote && !inDoubleQuote) {
        const remaining = command.slice(i);
        const operatorMatch = remaining.match(/^(\\\\|\\\\||&&|;;|\\\\|&|\\\\||<|>|>>|&|\\\\(|\\\\))/);

        if (operatorMatch) {
          if (current) {
            tokens.push(current);
            current = '';
          }
          tokens.push(operatorMatch[1]);
          i += operatorMatch[1].length - 1;
          continue;
        }

        // Handle whitespace
        if (/\\\\s/.test(char)) {
          if (current) {
            tokens.push(current);
            current = '';
          }
          continue;
        }
      }

      current += char;
    }

    if (current) {
      tokens.push(current);
    }

    return tokens;
  }

  private static isSerializedObject(token: string): boolean {
    return token.startsWith(this.SENTINEL) &&
           token.endsWith(this.SENTINEL);
  }

  private static deserializeObject(token: string): any {
    const json = token.slice(
      this.SENTINEL.length,
      -this.SENTINEL.length
    );

    try {
      return JSON.parse(json);
    } catch {
      return token; // Fallback to string
    }
  }
}

This parser enables commands like:

# Where $CONFIG is a JavaScript object
mytool --config=$CONFIG --name="test"

# Becomes after parsing with rehydration:
['mytool', '--config', {setting: true, values: [1,2,3]}, '--name', 'test']

Core File Operation Tools

ReadTool: The Multimodal File Reader

// ReadTool implementation (reconstructed)
const ReadToolDefinition: ToolDefinition = {
  name: 'ReadFileTool',
  description: 'Read file contents with line numbers, supporting text and images',

  inputSchema: z.object({
    file_path: z.string().describe('Absolute path to the file'),
    offset: z.number().optional().describe('Starting line number (1-based)'),
    limit: z.number().optional().default(2000).describe('Maximum lines to read')
  }),

  async *call(input, context) {
    const { file_path, offset = 1, limit = 2000 } = input;

    // Progress: Starting read
    yield {
      type: 'progress',
      toolUseID: context.currentToolUseId,
      data: { status: `Reading ${path.basename(file_path)}...` }
    };

    // Check if file exists
    const stats = await fs.stat(file_path).catch(() => null);
    if (!stats) {
      throw new Error(`File not found: ${file_path}`);
    }

    // Detect file type
    const mimeType = await detectMimeType(file_path);

    if (mimeType.startsWith('image/')) {
      // Handle image files
      const imageData = await this.readImage(file_path, context);
      yield { type: 'result', data: imageData };
      return;
    }

    if (file_path.endsWith('.ipynb')) {
      // Handle Jupyter notebooks
      const notebookData = await this.readNotebook(file_path, offset, limit);
      yield { type: 'result', data: notebookData };
      return;
    }

    // Handle text files with streaming
    const content = await this.readTextFile(file_path, offset, limit);

    // Update file cache
    context.readFileState.set(file_path, {
      content: content.fullContent,
      timestamp: stats.mtimeMs
    });

    yield { type: 'result', data: content };
  },

  async readTextFile(filePath: string, offset: number, limit: number) {
    const stream = createReadStream(filePath, { encoding: 'utf8' });
    const lines: string[] = [];
    let lineNumber = 0;
    let truncated = false;

    for await (const chunk of stream) {
      const chunkLines = chunk.split('\\\\n');

      for (const line of chunkLines) {
        lineNumber++;

        if (lineNumber >= offset && lines.length < limit) {
          // Truncate long lines
          const truncatedLine = line.length > 2000
            ? line.substring(0, 2000) + '... (truncated)'
            : line;

          // Format with line numbers (cat -n style)
          lines.push(`${lineNumber}\\\\t${truncatedLine}`);
        }

        if (lines.length >= limit) {
          truncated = true;
          stream.destroy();
          break;
        }
      }
    }

    return {
      formattedContent: lines.join('\\\\n'),
      fullContent: await fs.readFile(filePath, 'utf8'),
      lineCount: lineNumber,
      truncated
    };
  },

  async readImage(filePath: string, context: ToolUseContext) {
    const buffer = await fs.readFile(filePath);
    const metadata = await sharp(buffer).metadata();

    // Resize if too large
    let processedBuffer = buffer;
    if (metadata.width > 1024 || metadata.height > 1024) {
      processedBuffer = await sharp(buffer)
        .resize(1024, 1024, {
          fit: 'inside',
          withoutEnlargement: true
        })
        .toBuffer();
    }

    return {
      type: 'image',
      mimeType: `image/${metadata.format}`,
      base64: processedBuffer.toString('base64'),
      dimensions: {
        original: { width: metadata.width, height: metadata.height },
        processed: { width: 1024, height: 1024 }
      }
    };
  },

  mapToolResultToToolResultBlockParam(result, toolUseId) {
    if (result.type === 'image') {
      return [{
        type: 'image',
        source: {
          type: 'base64',
          media_type: result.mimeType,
          data: result.base64
        }
      }];
    }

    // Empty file handling
    if (!result.formattedContent || result.formattedContent.trim() === '') {
      return [{
        type: 'text',
        text: '<system-reminder>Warning: the file exists but the contents are empty.</system-reminder>'
      }];
    }

    // Normal text result
    return [{
      type: 'text',
      text: result.formattedContent +
            (result.truncated ? '\\\\n... (content truncated)' : '')
    }];
  },

  isReadOnly: true
};

Performance Profile:

EditTool: Surgical File Modifications