diff --git a/tartarus/models.py b/tartarus/models.py
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+++ b/tartarus/models.py
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+"""Provider-neutral data types shared by the AgentLoop, Provider, and Broker.
+
+These shapes are deliberately independent of any vendor wire format. Each concrete
+Provider translates between these and its backend's request/response JSON, so the
+AgentLoop and Broker never branch on which backend is configured (PLAN.md §6.3).
+"""
+
+from dataclasses import dataclass
+from typing import Any
+
+
+@dataclass
+class ToolCall:
+    """A model's request to invoke one tool, normalized across providers."""
+
+    id: str
+    name: str
+    arguments: dict
+    # Set when the provider could not parse the model's raw arguments as a JSON
+    # object. The broker turns this into an error result so the model can retry,
+    # rather than the harness crashing on malformed output.
+    argument_error: str | None = None
+
+
+@dataclass
+class AssistantTurn:
+    """One assistant response, normalized across providers."""
+
+    text: str | None
+    tool_calls: list[ToolCall]
+    raw: dict[str, Any]  # provider-native message, appended to the transcript verbatim
+    stop_reason: str  # "tool_calls" | "end" | "length" | "error"
+
+
+@dataclass
+class ToolResult:
+    """The outcome of brokering one tool call.
+
+    The provider renders this into its backend's native tool-result message.
+    """
+
+    call_id: str
+    output: str
+    is_error: bool
+
+
+# --- Streaming events ---------------------------------------------------------
+#
+# A provider's stream() yields these as a turn is generated. The AgentLoop
+# forwards TextDelta to the UI as text arrives and uses the terminal
+# TurnComplete to broker any tool calls and extend the transcript. Keeping
+# these provider-neutral means the loop never branches on the backend.
+
+
+@dataclass
+class TextDelta:
+    """A chunk of assistant text produced mid-turn."""
+
+    text: str
+
+
+@dataclass
+class ToolOutputDelta:
+    """A chunk of command output produced while a tool is still running."""
+
+    call: ToolCall
+    text: str
+
+
+@dataclass
+class TurnComplete:
+    """Terminal stream event carrying the fully assembled turn."""
+
+    turn: AssistantTurn
+
+
+# What a provider's stream() yields.
+StreamEvent = TextDelta | TurnComplete