Claude Opus 4.6 and Sonnet 4.6 Fail to Make Parallel Tool Calls in Batch API — Regression from Opus 4.5

What this error means

Claude Opus 4.6 / Sonnet 4.6 only calls 1 tool in Batch API with 46 tools, while Opus 4.5 correctly calls 39+ tools in parallel is a Anthropic API failure pattern reported for developers trying to fix claude opus 4.6 / sonnet 4.6 not making parallel tool calls in message batches api. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

Regression in 4.6 models’ parallel tool calling in batch mode. Verified with same request structure, tools, and document. Opus 4.5 works correctly with 64K max_tokens. Affects medical document extraction pipelines and other high-throughput workflows.

Common causes

• Opus 4.6 and Sonnet 4.6 only produce 1 tool call per response in Batch API despite having 46 tool definitions, while Opus 4.5 correctly makes 39+ parallel tool calls. Breaks production document extraction pipelines.
• Regression in 4.6 models’ parallel tool calling in batch mode. Verified with same request structure, tools, and document. Opus 4.5 works correctly with 64K max_tokens. Affects medical document extraction pipelines and other high-throughput workflows.

Quick fixes

1. Confirm the exact error signature matches Claude Opus 4.6 / Sonnet 4.6 only calls 1 tool in Batch API with 46 tools, while Opus 4.5 correctly calls 39+ tools in parallel.
2. Check the Anthropic API account, local tool state, and provider configuration involved in the failing workflow.
3. Compare the failing environment with a known working setup, then change one configuration value at a time.

Platform/tool-specific checks

• Verify the command, editor, extension, or API client that produced the error.
• Compare local settings with CI, deployment, or editor-level settings when the error appears in only one environment.
• Avoid deleting credentials, local model data, or project settings until the failing scope is clear.

Step-by-step troubleshooting

1. Capture the exact error message and the command, editor action, or request that triggered it.
2. Check whether the failure is account/auth, quota/rate, model/provider, local runtime, or deployment configuration.
3. Review the source evidence below and compare it with your environment.
4. Apply one change at a time and rerun the smallest failing action.
5. Keep the working fix documented for the team or deployment environment.

How to prevent it

• Keep provider/tool configuration documented.
• Record non-secret diagnostics such as tool version, provider name, model name, and command path.
• Add a lightweight check before CI or production workflows depend on the tool.

Sources checked

• https://github.com/anthropics/anthropic-sdk-typescript/issues/956

Evidence note: Regression in 4.6 models’ parallel tool calling in batch mode. Verified with same request structure, tools, and document. Opus 4.5 works correctly with 64K max_tokens. Affects medical document extraction pipelines and other high-throughput workflows.

Related errors

• Anthropic Batch API tool_choice ignored
• Claude 4.6 tool calling regression
• Anthropic messages.batches.create parallel tool calls

FAQ

What should I check first?

Start with the exact Claude Opus 4.6 / Sonnet 4.6 only calls 1 tool in Batch API with 46 tools, while Opus 4.5 correctly calls 39+ tools in parallel text and the smallest action that reproduces it.

Can I ignore this error?

No. Treat it as a failed Anthropic API workflow until the root cause is understood.

Is this guaranteed to have one fix?

No. The imported evidence supports the troubleshooting path above, but tool behavior can vary by account, plan, version, provider, and local configuration.

How do I know the fix worked?

Rerun the same command, editor action, or request. The fix is working when that action completes without Claude Opus 4.6 / Sonnet 4.6 only calls 1 tool in Batch API with 46 tools, while Opus 4.5 correctly calls 39+ tools in parallel.