{"record":{"id":"75bfaf14ad7a51d4","repo":"rohitg00/ai-engineering-from-scratch","slug":"stop-reason-tool-use-had-no-tool-use-block","errorCode":null,"errorMessage":"stop_reason tool_use had no tool_use block","messagePattern":"stop_reason tool_use had no tool_use block","errorType":"exception","errorClass":"ProtocolError","httpStatus":null,"severity":"error","filePath":"certifications/claude/lessons/08-messages-api-and-application-lifecycle/code/main.py","lineNumber":108,"sourceCode":"            {\"role\": \"user\", \"content\": [{\"type\": \"text\", \"text\": user_text}]}\n        ]\n\n        for turn in range(1, self.max_turns + 1):\n            response = self.transport.create(messages)\n            blocks = _validated_blocks(response)\n            stop_reason = response.get(\"stop_reason\")\n\n            # The assistant tool_use block must be retained before tool_result.\n            messages.append({\"role\": \"assistant\", \"content\": copy.deepcopy(blocks)})\n\n            if stop_reason == \"end_turn\":\n                return RunResult(_text_from_blocks(blocks), messages, turn)\n            if stop_reason != \"tool_use\":\n                raise ProtocolError(f\"unsupported stop_reason: {stop_reason!r}\")\n\n            tool_results = [self._execute_tool(block) for block in blocks if block[\"type\"] == \"tool_use\"]\n            if not tool_results:\n                raise ProtocolError(\"stop_reason tool_use had no tool_use block\")\n            messages.append({\"role\": \"user\", \"content\": tool_results})\n\n        raise ProtocolError(f\"maximum turn count {self.max_turns} exceeded\")\n\n    def _execute_tool(self, block: dict[str, Any]) -> dict[str, Any]:\n        tool_id = block.get(\"id\")\n        name = block.get(\"name\")\n        arguments = block.get(\"input\")\n        if not isinstance(tool_id, str) or not tool_id:\n            raise ProtocolError(\"tool_use requires a non-empty id\")\n        if not isinstance(name, str) or not isinstance(arguments, dict):\n            raise ProtocolError(\"tool_use requires name and object input\")\n\n        handler = self.tools.get(name)\n        if handler is None:\n            return {\n                \"type\": \"tool_result\",\n                \"tool_use_id\": tool_id,","sourceCodeStart":90,"sourceCodeEnd":126,"githubUrl":"https://github.com/rohitg00/ai-engineering-from-scratch/blob/39ea8a1c6d0b61f071226eff7ede4d4105fed820/certifications/claude/lessons/08-messages-api-and-application-lifecycle/code/main.py#L90-L126","documentation":"Consistency assertion in the turn loop: the response declared stop_reason 'tool_use' but its content blocks contain no tool_use block, so there is nothing to execute and no tool_result to send back. The protocol state machine treats this as a malformed response and stops rather than fabricating results.","triggerScenarios":"Thrown at certifications/claude/lessons/08-messages-api-and-application-lifecycle/code/main.py:108 when the library encounters an invalid state.","commonSituations":"See trigger scenarios.","solutions":["Include a well-formed tool_use block in the scripted assistant content","Fix block type typos in fixtures ('tool-use' vs 'tool_use')","Validate scripted responses against the expected stop_reason/content pairing in tests"],"exampleFix":null,"handlingStrategy":"validation","validationCode":null,"typeGuard":null,"tryCatchPattern":null,"preventionTips":[],"tags":[],"backgroundTag":null,"analyzedSha":"39ea8a1c6d0b61f071226eff7ede4d4105fed820","analyzedAt":"2026-08-26T03:13:46.626Z","contentChangedAt":null,"schemaVersion":2},"datasetVersion":"2026-09-23T08:17:48.524Z"}