[Bug] Anthropic API Error: Tool use concurrency validation (400)

Bug Description
API Error: 400 due to tool use concurrency

Reproduction:

• Fresh VS Code window
• New Claude Code chat
• Ask Claude to read a single file (~20 lines)
• File is read successfully
• Immediately after, API returns error 400 (tool use concurrency)

Notes:

• No multi-file request
• No write/refactor
• Auto workspace scan disabled
• This worked normally before and started recently

Impact:
Claude Code becomes unreliable even for basic single-file reads.

Environment Info

• Platform: darwin
• Terminal: vscode
• Version: 2.0.61
• Feedback ID: 2c61b59c-9ab5-4b68-a166-1c743ea59c01

Errors

[{"error":"AxiosError: Request failed with status code 403\n    at cS (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:171:1137)\n    at IncomingMessage.<anonymous> (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:172:9852)\n    at IncomingMessage.emit (node:events:525:35)\n    at IncomingMessage.emit (node:domain:489:12)\n    at endReadableNT (node:internal/streams/readable:1359:12)\n    at process.processTicksAndRejections (node:internal/process/task_queues:82:21)\n    at wHA.request (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:174:2130)\n    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)\n    at async _H0 (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:4654:3924)","timestamp":"2026-01-26T02:11:15.515Z"},{"error":"AxiosError: Request failed with status code 403\n    at cS (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:171:1137)\n    at IncomingMessage.<anonymous> (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:172:9852)\n    at IncomingMessage.emit (node:events:525:35)\n    at IncomingMessage.emit (node:domain:489:12)\n    at endReadableNT (node:internal/streams/readable:1359:12)\n    at process.processTicksAndRejections (node:internal/process/task_queues:82:21)\n    at wHA.request (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:174:2130)\n    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)\n    at async _H0 (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:4654:3924)","timestamp":"2026-01-26T02:11:31.419Z"},{"error":"AxiosError: Request failed with status code 403\n    at cS (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:171:1137)\n    at IncomingMessage.<anonymous> (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:172:9852)\n    at IncomingMessage.emit (node:events:525:35)\n    at IncomingMessage.emit (node:domain:489:12)\n    at endReadableNT (node:internal/streams/readable:1359:12)\n    at process.processTicksAndRejections (node:internal/process/task_queues:82:21)\n    at wHA.request (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:174:2130)\n    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)\n    at async _H0 (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:4654:3924)","timestamp":"2026-01-26T02:11:47.220Z"},{"error":"AxiosError: Request failed with status code 403\n    at cS (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:171:1137)\n    at IncomingMessage.<anonymous> (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:172:9852)\n    at IncomingMessage.emit (node:events:525:35)\n    at IncomingMessage.emit (node:domain:489:12)\n    at endReadableNT (node:internal/streams/readable:1359:12)\n    at process.processTicksAndRejections (node:internal/process/task_queues:82:21)\n    at wHA.request (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:174:2130)\n    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)\n    at async _H0 (file:///Users/thoith/.nvm/versions/node/v18.8.0/lib/node_modules/@anthropic-ai/claude-code/cli.js:4654:3924)","timestamp":"2026-01-26T02:12:03.004Z"}]