[Bug]: OpenClaw 2026.4.5 fails to fallback on Anthropic overloaded_error (503), retries indefinitely

Bug type

Behavior bug (incorrect output/state without crash)

Summary

OpenClaw 2026.4.5 fails to correctly utilize its configured model fallback chain when the primary Anthropic provider returns an overloaded_error (HTTP 503). Instead of attempting the next model in the fallback sequence (e.g., Gemini or OpenAI), OpenClaw retries the same overloaded Anthropic provider repeatedly until the session times out. This leads to persistent unavailability of Anthropic models and unnecessary session delays.

This issue is explicitly documented in previous GitHub issues #32533 and #49079, indicating it's a known, unaddressed bug.

Steps to reproduce

1. Configure Anthropic as the primary model with fallback models (e.g., Gemini) in openclaw.json.
2. Ensure Anthropic API key is valid and network connectivity is present.
3. Initiate conversations when the Anthropic API is under heavy load or returning overloaded_error (503).
4. Observe OpenClaw logs.

Expected behavior

When the Anthropic API returns an overloaded_error (503), OpenClaw should:

1. Immediately cease attempts to use the overloaded Anthropic provider.
2. Gracefully transition to the next available model in the configured fallbacks chain.
3. Continue the conversation with the fallback model without significant delay or session timeouts.

Actual behavior

OpenClaw exhibits the following behavior:

1. Upon receiving an overloaded_error (503) from Anthropic, OpenClaw logs WARN messages: error: "The AI service is temporarily overloaded. Please try again in a moment."
2. OpenClaw does not switch to a fallback model. It continues to retry the same Anthropic provider repeatedly.
3. These retries eventually exhaust the session timeout, leading to a FailoverError: Request was aborted. and the session reverting to a default model (e.g., Gemini) only after significant delay.
4. The fallback mechanism (which should escalate to other providers) is not triggered or is ignored for this specific error type.

Evidence from OpenClaw logs

{"0":"{\"subsystem\":\"agent/embedded\"}","1":{"event":"embedded_run_agent_end","tags":["error_handling","lifecycle","agent_end","assistant_error"],"runId":"cd02d139-a1cc-4adf-b68c-31a6fd2d6397","isError":true,"error":"The AI service is temporarily overloaded. Please try again in a moment.","failoverReason":"overloaded","model":"claude-sonnet-4-6","provider":"anthropic","rawErrorPreview":"{\"type\":\"error\",\"error\":{\"details\":null,\"type\":\"overloaded_error\",\"message\":\"Overloaded\"},\"request_id\":\"sha256:0e4608081233\"}","rawErrorHash":"sha256:1f16afbae93a","rawErrorFingerprint":"sha256:cab93443e944","providerErrorType":"overloaded_error","providerErrorMessagePreview":"Overloaded","requestIdHash":"sha256:0e4608081233"},"2":"embedded run agent end"}
{"0":"{\"subsystem\":\"model-fallback/decision\"}","1":{"event":"model_fallback_decision","tags":["error_handling","model_fallback","candidate_failed"],"runId":"cd02d139-a1cc-4adf-b68c-31a6fd2d6397","decision":"candidate_failed","requestedProvider":"anthropic","requestedModel":"claude-sonnet-4-6","candidateProvider":"anthropic","candidateModel":"claude-sonnet-4-6","attempt":1,"total":2,"reason":"overloaded","status":503,"errorPreview":"The AI service is temporarily overloaded. Please try again in a moment.","errorHash":"sha256:4003210ceba6","nextCandidateProvider":"openai","nextCandidateModel":"gpt-5.2","isPrimary":true,"requestedModelMatched":true,"fallbackConfigured":true},"2":"model fallback decision"}

Environment

• OpenClaw version: 2026.4.5 (3e72c03)
• OS: macOS 26.3.1 (arm64)
• Node: v22.22.1
• Install method: pnpm (global)
• Gateway: local, LaunchAgent
• Anthropic provider: Direct API key auth, baseUrl https://api.anthropic.com
• Affected models: All Anthropic models (claude-sonnet-4-6, claude-opus-4-6)
• Related issues: #32533, #49079 (highlighting this as a long-standing issue)
• Related Incident: This is part of a broader instability incident raw/incident-reports/OC-2026-0406-GPT5-404.md which also covers OpenAI GPT-5.x parameter compatibility issues.

Suggested fix

OpenClaw's model fallback logic needs to be updated to correctly interpret and handle overloaded_error (503) responses from providers like Anthropic. Upon receiving such an error, OpenClaw should immediately attempt the next model in the configured fallback chain, rather than retrying the same overloaded provider.