[Bug] Anthropic API Error: 529 Overloaded - Missing Exponential Backoff for Subagent Spawns

Bug Description
Title: Repeated 529 overloaded errors when spawning subagents (Opus model)

Description:
During a /gsd:plan-phase workflow, spawning a gsd-planner subagent with model: opus failed twice consecutively with 529 overloaded errors:

API Error: 529 {"type":"error","error":{"type":"overloaded_error","message":"Overloaded"}}

• Model: claude-opus-4-6 (1M context)
• Platform: Windows 11 Pro, Claude Code CLI
• Context: Subagent spawns via the Agent tool. The researcher agent (also opus) succeeded ~8 minutes earlier. Two back-to-back planner spawns failed before falling back to sonnet.
• Request IDs: req_011CZbe5iJ3VQtjAKPLD26Y1, req_011CZbegH8gh9JMW6XNY43PP
• Impact: ~10 minutes lost per failed attempt (each waited ~5 min before returning the error). No automatic retry with backoff — the user had to manually retry or switch models.

Expected: Graceful retry with exponential backoff, or faster failure (not 5 min timeout per attempt)

Environment Info

• Platform: win32
• Terminal: windows-terminal
• Version: 2.1.86
• Feedback ID: e6869e95-5f82-4ff6-a026-2e8289491510

Errors

[{"error":"EEXIST: file already exists, mkdir 'C:\\Users\\sswanson\\.claude'","timestamp":"2026-03-31T14:59:33.605Z"},{"error":"EEXIST: file already exists, mkdir 'C:\\Users\\sswanson\\.claude'","timestamp":"2026-03-31T14:59:36.856Z"},{"error":"AxiosError: Request failed with status code 401\n    at tp (B:/~BUN/root/src/entrypoints/cli.js:144:1194)\n    at <anonymous> (B:/~BUN/root/src/entrypoints/cli.js:149:12745)\n    at emit (node:events:92:22)\n    at endReadableNT (internal:streams/readable:861:50)\n    at processTicksAndRejections (native:7:39)\n    at request (B:/~BUN/root/src/entrypoints/cli.js:151:2467)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-03-31T14:59:43.471Z"},{"error":"AxiosError: Request failed with status code 401\n    at tp (B:/~BUN/root/src/entrypoints/cli.js:144:1194)\n    at <anonymous> (B:/~BUN/root/src/entrypoints/cli.js:149:12745)\n    at emit (node:events:92:22)\n    at endReadableNT (internal:streams/readable:861:50)\n    at processTicksAndRejections (native:7:39)\n    at request (B:/~BUN/root/src/entrypoints/cli.js:151:2467)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-03-31T14:59:43.487Z"},{"error":"EEXIST: file already exists, mkdir 'C:\\Users\\sswanson\\.claude'","timestamp":"2026-03-31T15:00:23.895Z"},{"error":"MaxFileReadTokenExceededError: File content (11702 tokens) exceeds maximum allowed tokens (10000). Use offset and limit parameters to read specific portions of the file, or search for specific content instead of reading the whole file.\n    at J1_ (B:/~BUN/root/src/entrypoints/cli.js:4337:5251)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-03-31T15:00:50.378Z"},{"error":"MaxFileReadTokenExceededError: File content (10459 tokens) exceeds maximum allowed tokens (10000). Use offset and limit parameters to read specific portions of the file, or search for specific content instead of reading the whole file.\n    at J1_ (B:/~BUN/root/src/entrypoints/cli.js:4337:5251)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-03-31T15:01:01.228Z"},{"error":"MaxFileReadTokenExceededError: File content (10028 tokens) exceeds maximum allowed tokens (10000). Use offset and limit parameters to read specific portions of the file, or search for specific content instead of reading the whole file.\n    at J1_ (B:/~BUN/root/src/entrypoints/cli.js:4337:5251)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-03-31T15:16:12.758Z"},{"error":"Error: File does not exist. Note: your current working directory is C:\\Users\\sswanson\\projects\\UserTool\\.claude\\worktrees\\agent-ad329c04.\n    at call (B:/~BUN/root/src/entrypoints/cli.js:4346:6593)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-03-31T17:24:11.807Z"},{"error":"Error: File does not exist. Note: your current working directory is C:\\Users\\sswanson\\projects\\UserTool\\.claude\\worktrees\\agent-afac6081.\n    at call (B:/~BUN/root/src/entrypoints/cli.js:4346:6593)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-03-31T17:33:34.215Z"},{"error":"Error: File does not exist. Note: your current working directory is C:\\Users\\sswanson\\projects\\UserTool\\.claude\\worktrees\\agent-afac6081.\n    at call (B:/~BUN/root/src/entrypoints/cli.js:4346:6593)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-03-31T17:33:34.334Z"},{"error":"Error: File does not exist. Note: your current working directory is C:\\Users\\sswanson\\projects\\UserTool\\.claude\\worktrees\\agent-afac6081.\n    at call (B:/~BUN/root/src/entrypoints/cli.js:4346:6593)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-03-31T17:33:35.733Z"},{"error":"Error: File…

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