Anthropic Python SDK streaming TypeError — Pydantic union discriminator returns raw dict instead of typed model

What this error means

TypeError: Unexpected event runtime type, after deserialising twice — {'type': 'content_block_delta', 'index': 0, 'delta': {'type': 'text_delta', 'text': '...'}} — <class 'dict'> is a Anthropic API failure pattern reported for developers trying to fix anthropic python sdk streaming error typeerror unexpected event runtime type dict instead of typed model. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

GitHub PR #1542 in anthropics/anthropic-sdk-python fixes streaming TypeError when Pydantic union discriminator returns raw dict. accumulate_event raises TypeError at isinstance check because event.delta is a dict instead of TextDelta. Multiple prior PRs (#1167, #1287, #1448, #1467) attempted partial fixes but left nested fields as raw dicts. Category mapped to Anthropic API — this is an SDK bug affecting streaming responses.

Common causes

• GitHub PR #1542 in anthropics/anthropic-sdk-python fixes streaming TypeError when Pydantic union discriminator returns raw dict. accumulate_event raises TypeError at isinstance check because event.delta is a dict instead of TextDelta. Multiple prior PRs (#1167, #1287, #1448, #1467) attempted partial fixes but left nested fields as raw dicts. Category mapped to Anthropic API — this is an SDK bug affecting streaming responses.

Quick fixes

1. Confirm the exact error signature matches TypeError: Unexpected event runtime type, after deserialising twice — {'type': 'content_block_delta', 'index': 0, 'delta': {'type': 'text_delta', 'text': '...'}} — <class 'dict'>.
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-python/pull/1542

Evidence note: GitHub PR #1542 in anthropics/anthropic-sdk-python fixes streaming TypeError when Pydantic union discriminator returns raw dict. accumulate_event raises TypeError at isinstance check because event.delta is a dict instead of TextDelta. Multiple prior PRs (#1167, #1287, #1448, #1467) attempted partial fixes but left nested fields as raw dicts. Category mapped to Anthropic API — this is an SDK bug affecting streaming responses.

Related errors

• Anthropic API

FAQ

What should I check first?

Start with the exact TypeError: Unexpected event runtime type, after deserialising twice — {'type': 'content_block_delta', 'index': 0, 'delta': {'type': 'text_delta', 'text': '...'}} — <class 'dict'> 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 TypeError: Unexpected event runtime type, after deserialising twice — {'type': 'content_block_delta', 'index': 0, 'delta': {'type': 'text_delta', 'text': '...'}} — <class 'dict'>.