`typescript
async function executeContextToolDiscard(ctx, toolCtx, hashes) {
const { state, logger } = ctx

// Validate hashes...

if (validHashes.length === 0) {
// Early return without fetching messages
const currentParams = getCurrentParams(state, [], logger) // ← BUG: Empty array
return "No valid hashes"
}

// Only fetch messages in success path
const messages = await client.session.messages(...)
}
`

✅ CORRECT:

`typescript
async function executeContextToolDiscard(ctx, toolCtx, hashes) {
const { client, state, logger } = ctx

// ALWAYS fetch messages first - required for thinking mode API compatibility
const messagesResponse = await client.session.messages({
path: { id: toolCtx.sessionID },
})
const messages = messagesResponse.data || messagesResponse

// ALWAYS initialize session - syncs reasoning_content
await ensureSessionInitialized(client, state, toolCtx.sessionID, logger, messages)

// Now validate hashes...

if (validHashes.length === 0) {
const currentParams = getCurrentParams(state, messages, logger) // ← Use actual messages
return "No valid hashes"
}
}
`

Why? Anthropic's thinking mode API requires reasoning_content on all assistant messages with tool calls. Skipping ensureSessionInitialized causes 400 errors.

---

$3

This function syncs reasoning_content from message parts to msg.info. Without it:

`
error, status code: 400, message: thinking is enabled but reasoning_content is missing
in assistant tool call message at index 2
`

Rule: Call ensureSessionInitialized at the START of every context_prune tool function, before any early returns.

---

$3

❌ WRONG:

`typescript
// Completely removing reasoning_content breaks API
state.prune.reasoningPartIds.push(partId)
// No replacement content → field removed → API error
`

✅ CORRECT:

`typescript
// Convert discard to distill with minimal placeholder
if (reasoningHashes.length > 0) {
const minimalSummaries = reasoningHashes.map(() => "—")
await executeContextReasoningDistill(
ctx,
toolCtx,
reasoningHashes.map((h, i) => [h, minimalSummaries[i]]),
)
}
`

Why? The reasoning_content field must exist (even if minimal) for thinking mode providers.

---

$3

Always test context_prune tool changes with:

1. Valid existing hashes (success path)
2. Non-existing hashes like "zzzzzz" (no-op path)
3. Mix of valid and invalid hashes

The no-op path is where most bugs hide because it's less tested.

---

$3

| Provider | Thinking Mode | reasoning_content Required | Notes |
| --------- | ----------------- | ---------------------------- | -------------------- |
| Anthropic | Extended thinking | ✅ Yes | Strict validation |
| DeepSeek | DeepThink | ✅ Yes | Similar to Anthropic |
| Kimi | K1 thinking | ✅ Yes | Similar to Anthropic |
| OpenAI | N/A | ❌ No | No thinking mode |
| Google | N/A | ❌ No | No thinking mode |

Always test with Anthropic when modifying context/pruning logic—it has the strictest API validation.

---

$3

`
User Message → Transform Hook → ensureReasoningContentSync → API Call
↑
Context tool runs HERE
`

If context_prune tool returns early (no-op), the transform hook may not have synced reasoning_content yet. That's why each tool function must call ensureSessionInitialized itself.

---

$3

When you see reasoning_content is missing errors:

1. Check the error index - "at index 2" means the 3rd message in the array
2. Verify message fetch - Is the function fetching messages before early returns?
3. Verify session init - Is ensureSessionInitialized called in ALL code paths?
4. Check the no-op path - Does the early return path have proper initialization?
5. Test with invalid hash - context_prune({ action: "discard", targets: [["zzzzzz"]] })

> Quick Pruning Reference:
>
> - Light pressure (<50%): No action
> - Moderate (50-75%): Prune old tool outputs using context_prune({ action: "discard", targets: [[hash]] })`
> - High (75-90%): Anchor critical info, then batch prune non-essential items
> - Critical (>90%): Aggressively discard all disposable context
>
> Always Protect: Active todos, current file edits, user requirements

Ask the user: "Would you like me to run a context optimization check?"