feat(types): add disallowedTools, maxTurns, initialPrompt to AgentDefinition (#759)

Brings `AgentDefinition` to parity with the TypeScript SDK.

## New fields

| Field | Type | Purpose |
|---|---|---|
| `disallowedTools` | `list[str] \| None` | Tool names to explicitly
block for this agent |
| `maxTurns` | `int \| None` | Cap on agentic API round-trips before
stopping |
| `initialPrompt` | `str \| None` | Auto-submitted first turn when used
as main-thread agent |

## Changed

- `model` widened from `Literal["sonnet", "opus", "haiku", "inherit"]`
to `str` — TS accepts full model IDs (e.g. `claude-opus-4-5`) in
addition to aliases. Existing callers using the literal values are
unaffected.

## Not included

- `criticalSystemReminder_EXPERIMENTAL` — experimental, skipping for now
- Frontmatter-only fields (`background`, `permissionMode`, `hooks`,
`effort`, `isolation`) — neither SDK exposes these yet; separate effort

## Transport

Serialization in `client.py` already uses `asdict()` with a None-filter,
so new optional fields flow to the initialize request without transport
changes. Field names are camelCase to match the CLI's expected JSON keys
(same pattern as `mcpServers` from #684).

Author: qing-ant

src/claude_agent_sdk/types.py

@@ -67,11 +67,15 @@ class AgentDefinition:
     description: str
     prompt: str
     tools: list[str] | None = None
-    model: Literal["sonnet", "opus", "haiku", "inherit"] | None = None
+    disallowedTools: list[str] | None = None  # noqa: N815
+    # Model alias ("sonnet", "opus", "haiku", "inherit") or a full model ID.
+    model: str | None = None
     skills: list[str] | None = None
     memory: Literal["user", "project", "local"] | None = None
     # Each entry is a server name (str) or an inline {name: config} dict.
     mcpServers: list[str | dict[str, Any]] | None = None  # noqa: N815
+    initialPrompt: str | None = None  # noqa: N815
+    maxTurns: int | None = None  # noqa: N815
 
 
 # Permission Update types (matching TypeScript SDK)

tests/test_types.py

@@ -496,3 +496,45 @@ def test_mcp_servers_serializes_as_camelcase(self):
         assert "mcp_servers" not in payload
         assert payload["mcpServers"][0] == "slack"
         assert payload["mcpServers"][1]["local"]["command"] == "python"
+
+    def test_disallowed_tools_and_max_turns_serialize_as_camelcase(self):
+        """CLI expects ``disallowedTools`` and ``maxTurns`` (camelCase)."""
+        from claude_agent_sdk import AgentDefinition
+
+        agent = AgentDefinition(
+            description="test",
+            prompt="p",
+            disallowedTools=["Bash", "Write"],
+            maxTurns=10,
+        )
+        payload = self._serialize(agent)
+
+        assert payload["disallowedTools"] == ["Bash", "Write"]
+        assert "disallowed_tools" not in payload
+        assert payload["maxTurns"] == 10
+        assert "max_turns" not in payload
+
+    def test_initial_prompt_serializes_as_camelcase(self):
+        from claude_agent_sdk import AgentDefinition
+
+        agent = AgentDefinition(
+            description="test",
+            prompt="p",
+            initialPrompt="/review-pr 123",
+        )
+        payload = self._serialize(agent)
+
+        assert payload["initialPrompt"] == "/review-pr 123"
+        assert "initial_prompt" not in payload
+
+    def test_model_accepts_full_model_id(self):
+        from claude_agent_sdk import AgentDefinition
+
+        agent = AgentDefinition(
+            description="test",
+            prompt="p",
+            model="claude-opus-4-5",
+        )
+        payload = self._serialize(agent)
+
+        assert payload["model"] == "claude-opus-4-5"