# Thinking encryption
<a name="claude-messages-thinking-encryption"></a>

Full thinking content is encrypted and returned in the signature field. This field is used to verify that thinking blocks were generated by Claude when passed back to the API. When streaming responses, the signature is added via a `signature_delta` inside a `content_block_delta` event just before the `content_block_stop` event.

**Note**  
It is only strictly necessary to send back thinking blocks when using tools with extended thinking. Otherwise, you can omit thinking blocks from previous turns, or let the API strip them for you if you pass them back.  
If sending back thinking blocks, we recommend passing everything back as you received it for consistency and to avoid potential issues.

## Thinking redaction in Claude 3.7 Sonnet
<a name="claude-messages-thinking-encryption-redaction"></a>

**Note**  
The following information applies specifically to Claude 3.7 Sonnet. Claude 4 models handle thinking differently and do not produce redacted thinking blocks.

In Claude 3.7 Sonnet, the following applies:
+ Occasionally Claude’s internal reasoning will be flagged by our safety systems. When this occurs, we encrypt some or all of the thinking block and return it to you as a redacted\$1thinking block. redacted\$1thinking blocks are decrypted when passed back to the API, allowing Claude to continue its response without losing context.
+ `thinking` and `redacted_thinking` blocks are returned before the text blocks in the response.

When building customer-facing applications that use extended thinking with Claude 3.7 Sonnet, consider the following:
+ Be aware that redacted thinking blocks contain encrypted content that isn’t human-readable.
+ Consider providing a simple explanation like: “Some of Claude’s internal reasoning has been automatically encrypted for safety reasons. This doesn’t affect the quality of responses.”
+ If you display thinking blocks to users, you can filter out redacted blocks while preserving normal thinking blocks.
+ Be transparent that using extended thinking features may occasionally result in some reasoning being encrypted.
+ Implement appropriate error handling to gracefully manage redacted thinking without breaking your UI.

Here’s an example showing both normal and redacted thinking blocks:

```
{
    "content": [
        {
            "type": "thinking",
            "thinking": "Let me analyze this step by step...",
            "signature":"WaUjzkypQ2mUEVM36O2TxuC06KN8xyfbJwyem2dw3URve/op91XWHOEBLLqIOMfFG/UvLEczmEsUjavL...."                },
        {
            "type": "redacted_thinking",
            "data":"EmwKAhgBEgy3va3pzix/LafPsn4aDFIT2Xlxh0L5L8rLVyIwxtE3rAFBa8cr3qpP..."
        },
        {
            "type": "text",
            "text": "Based on my analysis..."
        }
    ]
}
```

**Tip**  
Seeing redacted thinking blocks in your output is expected behavior. The model can still use this redacted reasoning to inform its responses while maintaining safety guardrails.  
If you need to test redacted thinking handling in your application, you can use this special test string as your prompt: `ANTHROPIC_MAGIC_STRING_TRIGGER_REDACTED_THINKING_46C9A13E193C177646C7398A98432ECCCE4C1253D5E2D82641AC0E52CC2876CB`

When passing `thinking` and `redacted_thinking` blocks back to the API in a multi-turn conversation, you must include the complete unmodified block back to the API for the last assistant turn. This is critical for maintaining the model’s reasoning flow. We suggest always passing back all thinking blocks to the API. For more details, see the [Extended thinking with tool usePreserve thinking blocks](claude-messages-extended-thinking.md#claude-messages-extended-thinking-tool-use-thinking-blocks).

The following example uses the **InvokeModelWithResponseStream** API to demonstrate the request and response structure when using thinking tokens with redactions.

When streaming is enabled, you’ll receive thinking content from the thinking\$1delta events. Here’s how to handle streaming with thinking:

**Request**

```
{
    "anthropic_version": "bedrock-2023-05-31",
    "max_tokens": 24000,
    "thinking": {
        "type": "enabled",
        "budget_tokens": 16000
    },
    "messages": [
        {
            "role": "user",
            "content": "What is 27 * 453?"
        }
    ]
}
```

**Response**

```
event: message_start
data: {"type": "message_start", "message": {"id": "msg_01...", "type": "message", "role": "assistant", "content": [], "model": "claude-3-7-sonnet-20250219", "stop_reason": null, "stop_sequence": null}}

event: content_block_start
data: {"type": "content_block_start", "index": 0, "content_block": {"type": "thinking", "thinking": ""}}

event: content_block_delta
data: {"type": "content_block_delta", "index": 0, "delta": {"type": "thinking_delta", "thinking": "Let me solve this step by step:\n\n1. First break down 27 * 453"}}

event: content_block_delta
data: {"type": "content_block_delta", "index": 0, "delta": {"type": "thinking_delta", "thinking": "\n2. 453 = 400 + 50 + 3"}}

// Additional thinking deltas...

event: content_block_delta
data: {"type": "content_block_delta", "index": 0, "delta": {"type": "signature_delta", "signature": "EqQBCgIYAhIM1gbcDa9GJwZA2b3hGgxBdjrkzLoky3dl1pkiMOYds..."}}

event: content_block_stop
data: {"type": "content_block_stop", "index": 0}

event: content_block_start
data: {"type": "content_block_start", "index": 1, "content_block": {"type": "text", "text": ""}}

event: content_block_delta
data: {"type": "content_block_delta", "index": 1, "delta": {"type": "text_delta", "text": "27 * 453 = 12,231"}}

// Additional text deltas...

event: content_block_stop
data: {"type": "content_block_stop", "index": 1}

event: message_delta
data: {"type": "message_delta", "delta": {"stop_reason": "end_turn", "stop_sequence": null}}

event: message_stop
data: {"type": "message_stop"}
```