# Prompt Cache Stability



Edit strategies change the message prefix, breaking prompt caching. Use `pin_editing_strategies_at_message` to keep the prefix stable.

The Problem [#the-problem]

```
Round 1: [a, b, c]
Round 2: [a(edited), b, c, d]           # cache miss
Round 3: [a(edited), b(edited), c, d, e] # cache miss
```

The Solution [#the-solution]

Pin where strategies are applied:

```
Round 1: [z(edited), a, b, c]           # edit_at_message_id = c
Round 2: [z(edited), a, b, c, d]        # stable prefix ✓
Round 3: [z(edited), a, b, c, d, e]     # stable prefix ✓
```

Usage [#usage]

<CodeGroup>
  ```python title="Python"
  from acontext import AcontextClient
  import os

  client = AcontextClient(api_key=os.getenv("ACONTEXT_API_KEY"))

  # First call - get the pin ID
  result = client.sessions.get_messages(
      session_id="session-uuid",
      edit_strategies=[{"type": "remove_tool_result", "params": {"keep_recent_n_tool_results": 3}}]
  )
  cache_pin_id = result.edit_at_message_id

  # Subsequent calls - pin to maintain cache
  result = client.sessions.get_messages(
      session_id="session-uuid",
      pin_editing_strategies_at_message=cache_pin_id,
      edit_strategies=[{"type": "remove_tool_result", "params": {"keep_recent_n_tool_results": 3}}]
  )
  ```

  ```typescript title="TypeScript"
  import { AcontextClient } from '@acontext/acontext';

  const client = new AcontextClient({
      apiKey: process.env.ACONTEXT_API_KEY,
  });

  // First call - get the pin ID
  let result = await client.sessions.getMessages("session-uuid", {
      editStrategies: [{ type: "remove_tool_result", params: { keep_recent_n_tool_results: 3 } }],
  });
  let cachePinId = result.editAtMessageId;

  // Subsequent calls - pin to maintain cache
  result = await client.sessions.getMessages("session-uuid", {
      pinEditingStrategiesAtMessage: cachePinId,
      editStrategies: [{ type: "remove_tool_result", params: { keep_recent_n_tool_results: 3 } }],
  });
  ```
</CodeGroup>

When to Reset [#when-to-reset]

When context grows too large, reset by omitting the pin:

<CodeGroup>
  ```python title="Python"
  if result.this_time_tokens > 50000:
      result = client.sessions.get_messages(
          session_id="session-uuid",
          edit_strategies=[
              {"type": "remove_tool_result", "params": {"keep_recent_n_tool_results": 3}},
              {"type": "token_limit", "params": {"limit_tokens": 30000}}
          ]
      )
      cache_pin_id = result.edit_at_message_id  # new pin
  ```

  ```typescript title="TypeScript"
  if (result.thisTimeTokens > 50000) {
      result = await client.sessions.getMessages("session-uuid", {
          editStrategies: [
              { type: "remove_tool_result", params: { keep_recent_n_tool_results: 3 } },
              { type: "token_limit", params: { limit_tokens: 30000 } },
          ],
      });
      cachePinId = result.editAtMessageId;  // new pin
  }
  ```
</CodeGroup>

Next Steps [#next-steps]

<Card title="Context Editing" icon="scissors" href="/engineering/editing">
  Available edit strategies
</Card>