preserved-thinking changedbuild-with-claude/preserved-thinking
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184184 ```bash cURL
185185 curl https://api.anthropic.com/v1/messages \
186186 -H "x-api-key: $ANTHROPIC_API_KEY" \
187 -H "anthropic-version: 2023-06-01" \
187 -H "anthropic-version: 2026-10-21" \
188188 -H "anthropic-beta: thinking-binding-controls-2026-08-01" \
189189 -H "content-type: application/json" \
190190 -d '{
from line 1070
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10711071Each common prefix edit has a replacement that gives the model the same information and leaves earlier bytes unchanged, so later thinking stays valid. Find the edit your code makes today in the first column:
10721072
1073| Instead of | Use | Beta header |
1074| --------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
1075| Rebuilding the top-level `system` prompt | A [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#new-instructions) | None |
1076| Re-rendering the context in your first user message (environment, date, memory, project instructions) on each request | Render it once and resend it unchanged. When something changes, [put the new version in the newest turn](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#changing-context) | None |
1077| Clearing or shortening old `tool_result` content, or re-encoding old images, in place | Shorten a tool result or downscale an image before the first time you send it, not after. To clear old results later, [trim context on the server](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#server-side-trimming) with `clear_tool_uses_20250919` | `context-management-2025-06-27` |
1078| Injecting a reminder and deleting it on the next request | A [turn-scoped system message](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#per-turn-reminders) (`clear_at: "next_user_message"`) | `mid-conversation-system-clear-at-2026-08-21` |
1079| Adding or removing entries in `tools` | [`tool_addition` and `tool_removal` blocks](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#tool-changes) | `inline-tools-2026-09-15` (add `mcp-client-2026-09-15` when the tool comes from an MCP server connected through the MCP connector), or the older `mid-conversation-tool-changes-2026-07-01`, which works on the Claude API, Amazon Bedrock, Claude Platform on AWS, and Claude Models on Google Cloud (Google managed) |
1080| Changing top-level `output_config.effort` (restarts the cache, doesn’t affect thinking) | A [per-message `output_config`](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#effort-changes) | `mid-conversation-output-config-2026-07-01` |
1081| Dropping or summarizing old turns on the client | [On-demand compaction](https://platform.claude.com/docs/en/build-with-claude/compaction-on-demand) to keep the recent turns with their thinking, other server-side [compaction or context editing](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#server-side-trimming), or [client-side compaction](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#custom-compaction-on-the-client) that keeps no stale thinking | `compact-2026-09-04` |
1082| An image or document URL whose bytes change between requests | A [`file_id` from the Files API](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#files-by-id), or base64 | None |
1073| Instead of | Use | Beta header |
1074| --------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
1075| Rebuilding the top-level `system` prompt | A [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#new-instructions) | None |
1076| Re-rendering the context in your first user message (environment, date, memory, project instructions) on each request | Render it once and resend it unchanged. When something changes, [put the new version in the newest turn](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#changing-context) | None |
1077| Clearing or shortening old `tool_result` content, or re-encoding old images, in place | Shorten a tool result or downscale an image before the first time you send it, not after. To clear old results later, [trim context on the server](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#server-side-trimming) with `clear_tool_uses_20250919` | `context-management-2025-06-27` |
1078| Injecting a reminder and deleting it on the next request | A [turn-scoped system message](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#per-turn-reminders) (`clear_at: "next_user_message"`) | `mid-conversation-system-clear-at-2026-08-21` |
1079| Adding or removing entries in `tools` | [`tool_addition` and `tool_removal` blocks](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#tool-changes) | `inline-tools-2026-09-15` (add `mcp-client-2026-09-15` when the tool comes from an MCP server connected through the MCP connector), or `mid-conversation-tool-changes-2026-07-01`, which works on the Claude API, Amazon Bedrock, Claude Platform on AWS, and Claude Models on Google Cloud (Google managed) |
1080| Changing top-level `output_config.effort` (restarts the cache, doesn’t affect thinking) | A [per-message `output_config`](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#effort-changes) | `mid-conversation-output-config-2026-07-01` |
1081| Dropping or summarizing old turns on the client | [On-demand compaction](https://platform.claude.com/docs/en/build-with-claude/compaction-on-demand) to keep the recent turns with their thinking, other server-side [compaction or context editing](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#server-side-trimming), or [client-side compaction](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#custom-compaction-on-the-client) that keeps no stale thinking | `compact-2026-09-04` |
1082| An image or document URL whose bytes change between requests | A [`file_id` from the Files API](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#files-by-id), or base64 | None |
10831083
10841084All of these assume you [send assistant turns back exactly as returned](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#append-assistant-turns-exactly-as-returned). Mid-conversation system messages, turn-scoped system messages, and tool changes aren’t available on every model: [Mid-conversation system messages and tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) lists the models that accept them. If your code serves several models, keep editing the top-level `system` prompt for the models that don’t accept them.
10851085
from line 1185
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11861186### Add or remove tools with `tool_addition` and `tool_removal`
11871187
1188Editing the `tools` array mid-session invalidates preserved thinking blocks. Leave the array as you first sent it, and change which tools the model can use by appending a `role: "system"` message that carries `tool_addition` or `tool_removal` blocks. These are [mid-conversation tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#mid-conversation-tool-changes) and need the beta header `inline-tools-2026-09-15`, which is available on the Claude API and Claude Platform on AWS. The older `mid-conversation-tool-changes-2026-07-01` header still works for changes that name a tool by reference, on the Claude API, Amazon Bedrock, Claude Platform on AWS, and Claude Models on Google Cloud (Google managed).
1188Editing the `tools` array mid-session invalidates preserved thinking blocks. Leave the array as you first sent it, and change which tools the model can use by appending a `role: "system"` message that carries `tool_addition` or `tool_removal` blocks. These are [mid-conversation tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#mid-conversation-tool-changes) and need the beta header `inline-tools-2026-09-15`, which is available on the Claude API and Claude Platform on AWS. The `mid-conversation-tool-changes-2026-07-01` header works for changes that name a tool by reference, on the Claude API, Amazon Bedrock, Claude Platform on AWS, and Claude Models on Google Cloud (Google managed).
11891189
11901190You have two ways to use these blocks:
11911191
from line 1247
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12481248If the API connects to the MCP server for you through the [MCP connector](https://platform.claude.com/docs/en/agents-and-tools/mcp-connector), also send `mcp-client-2026-09-15`. It covers everything `mcp-client-2025-11-20` does, so send it instead of that one. The block’s `definition` can then be an `mcp_toolset` for a server listed in `mcp_servers`. When the API has to fetch a server’s tool list, the response starts with an `mcp_tool_listing` block for that server. Send it back unchanged with the rest of the assistant turn, and keep sending `mcp-client-2026-09-15` on every later request that carries it. The block pins the toolset to that list, so the API doesn’t contact the server again for it. These MCP connector features are available on the Claude API.
12491249
1250With only the older header, you can still append a tool you learn about mid-session to `tools` with `defer_loading: true`, then offer it with a `tool_addition` block. That’s safe because the prefix check ignores a deferred tool until a `tool_addition` block references it. Adding a tool without `defer_loading: true` changes the prefix and invalidates earlier thinking.
1250With only the `mid-conversation-tool-changes-2026-07-01` header, you can append a tool you learn about mid-session to `tools` with `defer_loading: true`, then offer it with a `tool_addition` block. That’s safe because the prefix check ignores a deferred tool until a `tool_addition` block references it. Adding a tool without `defer_loading: true` changes the prefix and invalidates earlier thinking.
12511251
12521252The `role: "system"` messages that carry these blocks join the prefix for later thinking. Leave them in place on later requests.
12531253
from line 1313
13131313 curl https://api.anthropic.com/v1/messages \
13141314 -H "content-type: application/json" \
13151315 -H "x-api-key: $ANTHROPIC_API_KEY" \
1316 -H "anthropic-version: 2023-06-01" \
1316 -H "anthropic-version: 2026-10-21" \
13171317 -H "anthropic-beta: thinking-binding-controls-2026-08-01" \
13181318 -d '{
13191319 "model": "claude-fable-5-1",
No line in this hunk matches that.