compaction-threshold changedbuild-with-claude/compaction-threshold
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21 Sep 2026
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from line 2900, old and new numbered
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from line 2900
29002900
29012901On Claude Fable 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, remove the `thinking` and `redacted_thinking` blocks from any assistant turn you re-insert after the compaction block, or send `thinking.block_binding.prefix_mismatch_behavior: "drop_block"` with the `thinking-binding-controls-2026-08-01` [beta header](https://platform.claude.com/docs/en/api/beta-headers). Those blocks were produced when the full history was present, so they no longer pass the [conversation check](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation). Where the check is enforced, the continuation request is rejected with a 400 error. The preserved text and tool blocks can stay as they are. Letting the API summarize everything, without re-inserting earlier turns, avoids this. On Claude Sonnet 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`. With `between_tools`, remove the blocks instead. On Claude Haiku 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`, so with `thinking: {"type": "disabled"}`, remove the blocks instead.
29022902
2903Here's an example that uses `pause_after_compaction` to preserve the prior exchange and the current user message (three messages total) verbatim instead of summarizing them:
2903Here's an example that uses `pause_after_compaction` to preserve the prior exchange and the current user message (three messages total) instead of summarizing them. It keeps their text and tool blocks unchanged, removes the `thinking` and `redacted_thinking` blocks from the preserved assistant turn, and drops that turn if nothing else is left:
29042904
29052905<CodeGroup>
29062906 ```bash cURL
from line 2992
29922992 compaction_block = response.content[0]
29932993
29942994 # Preserve the prior exchange + current user message (3 messages)
2995 # by including them after the compaction block
2996 preserved_messages = messages[-3:] if len(messages) >= 3 else messages
2995 # by including them after the compaction block, without thinking
2996 # blocks; an assistant turn left with no content is dropped
2997 preserved_messages = [
2998 {
2999 **message,
3000 "content": [
3001 block
3002 for block in message["content"]
3003 if block.type not in ("thinking", "redacted_thinking")
3004 ],
3005 }
3006 if message["role"] == "assistant"
3007 else message
3008 for message in messages[-3:]
3009 ]
3010 preserved_messages = [m for m in preserved_messages if m["content"]]
29973011
29983012 # Build new message list: compaction + preserved messages
29993013 new_assistant_content = [compaction_block]
from line 3072
30583072 const compactionBlock = response.content[0];
30593073
30603074 // Preserve the prior exchange + current user message (3 messages)
3061 // by including them after the compaction block
3062 const preservedMessages = messages.length >= 3 ? messages.slice(-3) : [...messages];
3075 // by including them after the compaction block, without thinking
3076 // blocks; an assistant turn left with no content is dropped
3077 const preservedMessages = messages
3078 .slice(-3)
3079 .map((message) =>
3080 message.role === "assistant" && Array.isArray(message.content)
3081 ? {
3082 ...message,
3083 content: message.content.filter(
3084 (block) => block.type !== "thinking" && block.type !== "redacted_thinking"
3085 )
3086 }
3087 : message
3088 )
3089 .filter((message) => message.content.length > 0);
30633090
30643091 // Build new message list: compaction + preserved messages
30653092 const messagesAfterCompaction: Anthropic.Beta.Messages.BetaMessageParam[] = [
from line 3157
31303157 if (!response.Content[0].TryPickCompaction(out _))
31313158 throw new InvalidOperationException("Expected compaction block");
31323159
3133 var preserved = messages.Count >= 3
3134 ? messages.Skip(messages.Count - 3).ToList()
3135 : new List<BetaMessageParam>(messages);
3160 // Preserve the prior exchange + current user message (3 messages),
3161 // without thinking blocks; an assistant turn left empty is dropped
3162 var preserved = messages
3163 .Skip(Math.Max(0, messages.Count - 3))
3164 .Select(message => message.Content.TryPickBetaContentBlockParams(out var blocks)
3165 ? new BetaMessageParam
3166 {
3167 Role = message.Role,
3168 Content = blocks
3169 .Where(block => block.Type.GetString() is not ("thinking" or "redacted_thinking"))
3170 .ToList()
3171 }
3172 : message)
3173 .Where(message => !message.Content.TryPickBetaContentBlockParams(out var blocks) || blocks.Count > 0)
3174 .ToList();
31363175
31373176 var messagesAfterCompaction = new List<BetaMessageParam>
31383177 {
from line 3254
32153254 if response.StopReason == "compaction" {
32163255 compactionParam := response.Content[0].ToParam()
32173256
3257 // Preserve the prior exchange + current user message (3 messages),
3258 // without thinking blocks; an assistant turn left empty is dropped
32183259 var preserved []anthropic.BetaMessageParam
3219 if len(messages) >= 3 {
3220 preserved = messages[len(messages)-3:]
3221 } else {
3222 preserved = messages
3260 for _, message := range messages[max(0, len(messages)-3):] {
3261 var content []anthropic.BetaContentBlockParamUnion
3262 for _, block := range message.Content {
3263 if block.OfThinking == nil && block.OfRedactedThinking == nil {
3264 content = append(content, block)
3265 }
3266 }
3267 if len(content) == 0 {
3268 continue
3269 }
3270 message.Content = content
3271 preserved = append(preserved, message)
32233272 }
32243273
32253274 messagesAfterCompaction := []anthropic.BetaMessageParam{
from line 3346
32973346 // Check if compaction occurred and paused
32983347 if (response.stopReason().isPresent()
32993348 && response.stopReason().get().equals(BetaStopReason.COMPACTION)) {
3300 // Preserve the prior exchange + current user message (3 messages)
3301 List<BetaMessageParam> preservedMessages = messages.size() >= 3
3302 ? new ArrayList<>(messages.subList(messages.size() - 3, messages.size()))
3303 : new ArrayList<>(messages);
3349 // Preserve the prior exchange + current user message (3 messages),
3350 // without thinking blocks; an assistant turn left empty is dropped
3351 List<BetaMessageParam> preservedMessages = new ArrayList<>();
3352 for (BetaMessageParam message : messages.subList(Math.max(0, messages.size() - 3), messages.size())) {
3353 if (message.content().isBetaContentBlockParams()) {
3354 message = message.toBuilder()
3355 .contentOfBetaContentBlockParams(message.content().asBetaContentBlockParams().stream()
3356 .filter(block -> !block.isThinking() && !block.isRedactedThinking())
3357 .toList())
3358 .build();
3359 if (message.content().asBetaContentBlockParams().isEmpty()) {
3360 continue;
3361 }
3362 }
3363 preservedMessages.add(message);
3364 }
33043365
33053366 // Build new message list: compaction + preserved messages
33063367 List<BetaMessageParam> messagesAfterCompaction = new ArrayList<>();
from line 3429
33683429 if ($response->stopReason === 'compaction') {
33693430 $compactionBlock = $response->content[0];
33703431
3371 $preserved = count($messages) >= 3
3372 ? array_slice($messages, -3)
3373 : $messages;
3432 // Preserve the prior exchange + current user message (3 messages),
3433 // without thinking blocks; an assistant turn left empty is dropped
3434 $preserved = array_values(array_filter(array_map(
3435 fn($message) => $message['role'] === 'assistant'
3436 ? array_merge($message, ['content' => array_values(array_filter(
3437 $message['content'],
3438 fn($block) => !in_array($block->type, ['thinking', 'redacted_thinking'], true)
3439 ))])
3440 : $message,
3441 array_slice($messages, -3)
3442 ), fn($message) => $message['content'] !== []));
33743443
33753444 $messagesAfterCompaction = array_merge(
33763445 [['role' => 'assistant', 'content' => [$compactionBlock]]],
from line 3500
34313500 if response.stop_reason == :compaction
34323501 compaction_block = response.content[0]
34333502
3434 preserved = messages.length >= 3 ? messages[-3..-1] : messages.dup
3503 # Preserve the prior exchange + current user message (3 messages),
3504 # without thinking blocks; an assistant turn left empty is dropped
3505 preserved = messages.last(3).map do |message|
3506 next message unless message[:role] == "assistant"
3507
3508 message.merge(content: message[:content].reject { |block| %i[thinking redacted_thinking].include?(block.type) })
3509 end.reject { |message| message[:content].empty? }
34353510
34363511 messages_after_compaction = [
34373512 { role: "assistant", content: [compaction_block] }
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