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migration changedmanaged-agents/migration

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### What you control ### What you still control

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4141 
4242## From a Messages API agent loop
4343 
44If you built an agent by calling `client.messages.create()` (csharp: `client.Messages.Create()`; go: `client.Messages.New()`; java: `client.messages().create()`; php: `$client->messages->create()`; cli: `ant messages create`; curl: `POST /v1/messages`) in a loop, running tool calls yourself, and appending results to the conversation history, most of that code goes away.
44If you built an agent by calling `client.messages.create()` (csharp: `client.Messages.Create()`; go: `client.Messages.New()`; java: `client.messages().create()`; php: `$client->messages->create()`; cli: `ant messages create`; curl: `POST /v1/messages`) in a loop, running tool calls yourself, and appending results to the conversation history, Claude Managed Agents replaces most of that loop code.
4545 
4646### What you stop managing
4747 
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530530 ```
531531</CodeGroup>
532532 
533### What you still control
533### What you control
534534 
535* **System prompt and model:** Same fields, now on the agent definition.
536* **Custom tools:** Still declared with JSON Schema. Execution moves from inline handling to responding to `agent.custom_tool_use` events. See [Session event stream](https://platform.claude.com/docs/en/managed-agents/events-and-streaming).
537* **Web search and web fetch settings:** Same `allowed_domains` (java, php: `allowedDomains`; csharp, go: `AllowedDomains`), `blocked_domains` (java, php: `blockedDomains`; csharp, go: `BlockedDomains`), `max_content_tokens` (java, php: `maxContentTokens`; csharp, go: `MaxContentTokens`), and `user_location` (java, php: `userLocation`; csharp, go: `UserLocation`) fields, now set once on the `web_search` and `web_fetch` entries of the agent toolset’s `configs` (csharp, go: `Configs`) array instead of on every request. The `max_uses` (java, php: `maxUses`; csharp, go: `MaxUses`), `citations` (csharp, go: `Citations`), and `cache_control` (java, php: `cacheControl`; csharp, go: `CacheControl`) fields are not available. See [Differences from the Messages API tools](https://platform.claude.com/docs/en/managed-agents/tools-web-restrictions#differences-from-the-messages-api-tools).
538* **Context:** You can still inject context through the system prompt, [file resources](https://platform.claude.com/docs/en/managed-agents/files), or [skills](https://platform.claude.com/docs/en/managed-agents/skills).
535* **System prompt and model:** Same fields, on the agent definition.
536* **Custom tools:** Declared with JSON Schema. Your client runs them when `agent.custom_tool_use` events arrive, not inside a request loop. See [Session event stream](https://platform.claude.com/docs/en/managed-agents/events-and-streaming).
537* **Web search and web fetch settings:** Same `allowed_domains` (java, php: `allowedDomains`; csharp, go: `AllowedDomains`), `blocked_domains` (java, php: `blockedDomains`; csharp, go: `BlockedDomains`), `max_content_tokens` (java, php: `maxContentTokens`; csharp, go: `MaxContentTokens`), and `user_location` (java, php: `userLocation`; csharp, go: `UserLocation`) fields, set once on the `web_search` and `web_fetch` entries of the agent toolset’s `configs` (csharp, go: `Configs`) array instead of on every request. The `max_uses` (java, php: `maxUses`; csharp, go: `MaxUses`), `citations` (csharp, go: `Citations`), and `cache_control` (java, php: `cacheControl`; csharp, go: `CacheControl`) fields are not available. See [Differences from the Messages API tools](https://platform.claude.com/docs/en/managed-agents/tools-web-restrictions#differences-from-the-messages-api-tools).
538* **Context:** You can inject context through the system prompt, [file resources](https://platform.claude.com/docs/en/managed-agents/files), or [skills](https://platform.claude.com/docs/en/managed-agents/skills).
539539 
540540## From the Claude Agent SDK
541541 
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552552| `cwd`, `add_dirs` (typescript: `additionalDirectories`) point at local paths | Upload or mount [files](https://platform.claude.com/docs/en/managed-agents/files) as session resources. |
553553| `system_prompt` (typescript: `systemPrompt`) and the `CLAUDE.md` hierarchy | A single `system` (csharp, go: `System`; ruby: `system_`) string on the agent. Each update that changes the agent produces a new server-side version; pin sessions to a specific version to promote or roll back without a deploy. See [Define your agent](https://platform.claude.com/docs/en/managed-agents/agent-setup). |
554554| `mcp_servers` (typescript: `mcpServers`) configured and authenticated in one place | Declare servers on the agent; provide credentials through a [vault](https://platform.claude.com/docs/en/managed-agents/vaults) on the session. |
555| `permission_mode` (typescript: `permissionMode`), `can_use_tool` (typescript: `canUseTool`) | Per-tool [`permission_policy` (java, php: `permissionPolicy`; csharp, go: `PermissionPolicy`)](https://platform.claude.com/docs/en/managed-agents/permission-policies) (`always_allow`, `always_ask`, or `auto`); send `user.tool_confirmation` events for calls that pause for your approval. |
555| `permission_mode` (typescript: `permissionMode`), `can_use_tool` (typescript: `canUseTool`) | Per-tool [`permission_policy` (java, php: `permissionPolicy`; csharp, go: `PermissionPolicy`)](https://platform.claude.com/docs/en/managed-agents/permission-policies) (`always_allow`, `always_ask`, or `auto`). Send `user.tool_confirmation` events for calls that pause for your approval. |
556556 
557557### Code comparison
558558 
from line 1267
126712672. Port your system prompt and tool selection to an [agent definition](https://platform.claude.com/docs/en/managed-agents/agent-setup).
126812683. Replace your loop: [create a session](https://platform.claude.com/docs/en/managed-agents/sessions) with `client.beta.sessions.create()` (go: `client.Beta.Sessions.New()`; csharp: `client.Beta.Sessions.Create()`; java: `client.beta().sessions().create()`; php: `$client->beta->sessions->create()`; cli: `ant beta:sessions create`; curl: `POST /v1/sessions`) and [stream its events](https://platform.claude.com/docs/en/managed-agents/events-and-streaming) with `client.beta.sessions.events.stream()` (ruby: `client.beta.sessions.events.stream_events()`; go: `client.Beta.Sessions.Events.StreamEvents()`; csharp: `client.Beta.Sessions.Events.StreamStreaming()`; java: `client.beta().sessions().events().streamStreaming()`; php: `$client->beta->sessions->events->streamStream()`; cli: `ant beta:sessions:events stream`; curl: `GET /v1/sessions/{session_id}/events/stream`).
126912694. For any local files the agent reads, upload them through the [Files API](https://platform.claude.com/docs/en/managed-agents/files) and mount them as `resources` (csharp, go: `Resources`).
12705. For any custom tool handlers, move execution into your event loop as responses to `agent.custom_tool_use` events.
12716. Verify with a test session before pointing production traffic at the new flow.
12705. For any custom tool handlers, run them in your event loop as responses to `agent.custom_tool_use` events.
12716. Run a test session before you point production traffic at the migrated flow.
12721272 
12731273## Migrating between model versions
12741274 
from line 1370
13701370 
13711371Most model-level behavior changes documented in the [Migration guides](https://platform.claude.com/docs/en/about-claude/models/migration-guide) do not require action on your side:
13721372 
1373* **Request parameter changes** (`max_tokens` (java, php: `maxTokens`; csharp, go: `MaxTokens`) defaults, `thinking` (csharp, go: `Thinking`) configuration) are handled by the Claude Managed Agents runtime. These fields are not exposed on the agent definition.
1373* **Request parameter changes** (`max_tokens` (java, php: `maxTokens`; csharp, go: `MaxTokens`) defaults, `thinking` (csharp, go: `Thinking`) configuration) need no action: the Claude Managed Agents runtime handles them. The agent definition does not expose these fields.
13741374* **Assistant message prefilling** does not exist in the event-based session model, so moving to a model that rejects prefill needs no change from you.
1375* **Tool argument JSON escaping** is parsed by the runtime before you receive `agent.custom_tool_use` events. You see structured data, not raw strings.
1375* **Tool argument JSON escaping** needs no action: the runtime parses the arguments before you receive `agent.custom_tool_use` events. You see structured data, not raw strings.
13761376 
13771377The behavior descriptions in those guides (what the model does differently) still apply. The migration steps (how to change your request code) do not.
13781378 
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