troubleshooting-tool-use changedagents-and-tools/tool-use/troubleshooting-tool-use
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## Parallel tool calls don’t work ## Parallel tool calls don't work
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1919| Symptom | Likely cause | Fix |
2020| ------------------------------------------- | ----------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------- |
21| Parameter that doesn't exist in your schema | Model over-generation without strict mode | Add `strict: true` if your schema is in the [supported subset](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use). |
21| Parameter that doesn’t exist in your schema | Model over-generation without strict mode | Add `strict: true` if your schema is in the [supported subset](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use). |
2222| Parameter values outside your enum | Missing strict mode or too-large enum | Shrink the enum or add `input_examples` showing valid choices. |
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24## Parallel tool calls don't work
24## Parallel tool calls don’t work
2525
2626| Symptom | Likely cause | Fix |
2727| ------------------------------------------------------------- | -------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
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4141| ---------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
4242| `tool_use ids were found without tool_result blocks immediately after` | Missing `tool_result` for some `tool_use` IDs, or `tool_result` is not the first content block in the user message | Return one `tool_result` for every `tool_use` block in the assistant response. Put `tool_result` blocks before any text. See [Handle tool calls](https://platform.claude.com/docs/en/agents-and-tools/tool-use/handle-tool-calls) and [Parallel tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/parallel-tool-use). |
4343| `was found without a corresponding <name>_tool_result block` | The previous assistant turn has a `server_tool_use` block with no result block (most often, Claude called it alongside a client tool), and either your next user message ended that turn (for example, with text after the `tool_result` blocks) or the resume request no longer defines that server tool (the message then ends with `but no <name> tool was provided`) | Send a user message containing only the `tool_result` blocks for the client `tool_use` IDs and keep the same `tools` array. See [Stop reasons and fallback](https://platform.claude.com/docs/en/build-with-claude/handling-stop-reasons#tool-use). |
44| `Unsupported regex feature in pattern field: ...` | A `pattern` in a strict tool's `input_schema` uses a regex feature that strict mode can't compile, such as a backreference, a lookaround, a word boundary, or a large `{n,m}` range | Simplify the pattern. Anchored patterns with basic quantifiers, character classes, and groups are supported; see [JSON Schema limitations](https://platform.claude.com/docs/en/build-with-claude/structured-outputs#json-schema-limitations). |
44| `Unsupported regex feature in pattern field: ...` | A `pattern` in a strict tool’s `input_schema` uses a regex feature that strict mode can’t compile, such as a backreference, a lookaround, a word boundary, or a large `{n,m}` range | Simplify the pattern. Anchored patterns with basic quantifiers, character classes, and groups are supported; see [JSON Schema limitations](https://platform.claude.com/docs/en/build-with-claude/structured-outputs#json-schema-limitations). |
4545| `All tools have defer_loading: true` | No tools visible to the model | At least one tool must be immediately loaded. The tool search tool itself must never have `defer_loading: true`. |
4646
4747## Error: thinking blocks cannot be modified
4848
49If a request fails with a 400 `invalid_request_error` whose message contains `` `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified `` when continuing a conversation after a tool call, your application is altering the assistant's thinking blocks before sending them back. Send the entire assistant message back unchanged, then append your `tool_result`.
49If a request fails with a 400 `invalid_request_error` whose message contains `` `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified `` when continuing a conversation after a tool call, your application is altering the assistant’s thinking blocks before sending them back. Send the entire assistant message back unchanged, then append your `tool_result`.
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5151See [Thinking blocks cannot be modified](https://platform.claude.com/docs/en/api/errors#thinking-blocks-cannot-be-modified) for the full error and fix steps.
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