MoonshotAI · RealKai42 · Jul 16, 2026 · Jul 16, 2026
diff --git a/.changeset/remove-thinking-space-placeholder.md b/.changeset/remove-thinking-space-placeholder.md
@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Replay empty thinking content verbatim instead of substituting a placeholder space on Anthropic-compatible and Kimi preserved-thinking endpoints.
diff --git a/packages/agent-core-v2/src/app/llmProtocol/providers/anthropic.ts b/packages/agent-core-v2/src/app/llmProtocol/providers/anthropic.ts
@@ -252,20 +252,6 @@ function shouldPreserveUnsignedThinking(model: string): boolean {
   );
 }
 
-function shouldBackfillPreservedThinking(
-  model: string,
-  thinking: MessageCreateParams['thinking'] | undefined,
-  contextManagement: AnthropicContextManagement | undefined,
-): boolean {
-  return (
-    shouldPreserveUnsignedThinking(model) &&
-    thinking?.type !== 'disabled' &&
-    contextManagement?.edits.some(
-      (edit) => edit.type === CLEAR_THINKING_EDIT && edit.keep === 'all',
-    ) === true
-  );
-}
-
 const CACHEABLE_TYPES = new Set([
   'text',
   'image',
@@ -411,11 +397,7 @@ function toolResultToBlock(toolCallId: string, content: ContentPart[]): ToolResu
     content: blocks,
   } as ToolResultBlockParam;
 }
-function convertMessage(
-  message: Message,
-  model: string,
-  backfillPreservedThinking: boolean,
-): MessageParam {
+function convertMessage(message: Message, model: string): MessageParam {
   const role = message.role;
 
   if (role === 'system') {
@@ -438,26 +420,19 @@ function convertMessage(
   }
 
   const blocks: ContentBlockParam[] = [];
-  let hasThinkingPart = false;
-  let lastUnsignedThinkingBlockIndex: number | undefined;
-  let hasNonEmptyEmittedThinking = false;
   for (const part of message.content) {
     if (part.type === 'text') {
       blocks.push({ type: 'text', text: part.text } satisfies TextBlockParam);
     } else if (part.type === 'image_url') {
       blocks.push(imageUrlPartToAnthropic(part.imageUrl.url) as unknown as ContentBlockParam);
     } else if (part.type === 'think') {
-      hasThinkingPart = true;
       if (part.encrypted !== undefined) {
-        hasNonEmptyEmittedThinking ||= part.think.length > 0;
         blocks.push({
           type: 'thinking',
           thinking: part.think,
           signature: part.encrypted,
         } satisfies ThinkingBlockParam);
       } else if (shouldPreserveUnsignedThinking(model)) {
-        lastUnsignedThinkingBlockIndex = blocks.length;
-        hasNonEmptyEmittedThinking ||= part.think.length > 0;
         blocks.push({ type: 'thinking', thinking: part.think } as unknown as ThinkingBlockParam);
       }
     } else if (part.type === 'video_url') {
@@ -471,20 +446,6 @@ function convertMessage(
     }
   }
 
-  if (role === 'assistant' && backfillPreservedThinking) {
-    if (!hasThinkingPart) {
-      blocks.unshift({ type: 'thinking', thinking: ' ' } as unknown as ThinkingBlockParam);
-    } else if (
-      lastUnsignedThinkingBlockIndex !== undefined &&
-      !hasNonEmptyEmittedThinking
-    ) {
-      blocks[lastUnsignedThinkingBlockIndex] = {
-        type: 'thinking',
-        thinking: ' ',
-      } as unknown as ThinkingBlockParam;
-    }
-  }
-
   if (message.toolCalls.length > 0) {
     for (const tc of message.toolCalls) {
       let toolInput: Record<string, unknown> = {};
@@ -881,18 +842,12 @@ export class AnthropicChatProvider implements ChatProvider {
         ]
       : undefined;
 
-    const backfillPreservedThinking = shouldBackfillPreservedThinking(
-      this._model,
-      this._generationKwargs.thinking,
-      this._generationKwargs.contextManagement,
-    );
-
     const messages = mergeConsecutiveUserMessages(
       normalizeToolCallIdsForProvider(
         history.filter((msg) => !isToolDeclarationOnlyMessage(msg)),
         ANTHROPIC_TOOL_CALL_ID_POLICY,
       )
-        .map((msg) => convertMessage(msg, this._model, backfillPreservedThinking))
+        .map((msg) => convertMessage(msg, this._model))
         .filter(shouldKeepConvertedMessage),
       {
         isUser: (message) => message.role === 'user',

diff --git a/packages/agent-core-v2/src/app/llmProtocol/providers/kimi.ts b/packages/agent-core-v2/src/app/llmProtocol/providers/kimi.ts
@@ -157,10 +157,7 @@ function convertMessage(message: Message, preservedThinkingEnabled: boolean): Op
   }
 
   if (hasReasoningPart || (preservedThinkingEnabled && message.role === 'assistant')) {
-    result.reasoning_content =
-      preservedThinkingEnabled && message.role === 'assistant' && reasoningContent.length === 0
-        ? ' '
-        : reasoningContent;
+    result.reasoning_content = reasoningContent;
   }
 
   if (message.tools !== undefined && message.tools.length > 0) {

diff --git a/packages/agent-core-v2/test/app/llmProtocol/providers/anthropic-context-management.test.ts b/packages/agent-core-v2/test/app/llmProtocol/providers/anthropic-context-management.test.ts
@@ -97,7 +97,7 @@ describe('Anthropic withThinkingKeep context_management parity', () => {
     ]);
   });
 
-  it('backfills non-empty thinking when compatible text history is replayed with keep all', async () => {
+  it('replays compatible text history without injecting thinking with keep all', async () => {
     const history: Message[] = [
       { role: 'user', content: [{ type: 'text', text: 'Hi' }], toolCalls: [] },
       {
@@ -116,14 +116,11 @@ describe('Anthropic withThinkingKeep context_management parity', () => {
 
     expect(messages[1]).toEqual({
       role: 'assistant',
-      content: [
-        { type: 'thinking', thinking: ' ' },
-        { type: 'text', text: 'Hello' },
-      ],
+      content: [{ type: 'text', text: 'Hello' }],
     });
   });
 
-  it('backfills non-empty thinking before a compatible assistant tool call with keep all', async () => {
+  it('replays a compatible assistant tool call without injecting thinking with keep all', async () => {
     const history: Message[] = [
       {
         role: 'assistant',
@@ -143,7 +140,6 @@ describe('Anthropic withThinkingKeep context_management parity', () => {
     expect(messages[0]).toEqual({
       role: 'assistant',
       content: [
-        { type: 'thinking', thinking: ' ' },
         {
           type: 'tool_use',
           id: 'call_1',
@@ -155,7 +151,7 @@ describe('Anthropic withThinkingKeep context_management parity', () => {
     });
   });
 
-  it('replaces an existing empty thinking block when compatible history uses keep all', async () => {
+  it('preserves an existing empty thinking block when compatible history uses keep all', async () => {
     const history: Message[] = [
       {
         role: 'assistant',
@@ -176,7 +172,7 @@ describe('Anthropic withThinkingKeep context_management parity', () => {
     expect(messages[0]).toEqual({
       role: 'assistant',
       content: [
-        { type: 'thinking', thinking: ' ' },
+        { type: 'thinking', thinking: '' },
         { type: 'text', text: 'Hello', cache_control: { type: 'ephemeral' } },
       ],
     });

diff --git a/packages/agent-core-v2/test/app/llmProtocol/providers/empty-thinking-roundtrip.test.ts b/packages/agent-core-v2/test/app/llmProtocol/providers/empty-thinking-roundtrip.test.ts
@@ -157,7 +157,7 @@ describe('empty thinking round-trip', () => {
     expect(messages[0]).toHaveProperty('reasoning_content', '');
   });
 
-  it('Kimi backfills an assistant tool-call message when preserved thinking is active', async () => {
+  it('Kimi sends empty reasoning_content for an assistant tool-call message when preserved thinking is active', async () => {
     const history: Message[] = [
       {
         role: 'assistant',
@@ -172,10 +172,10 @@ describe('empty thinking round-trip', () => {
       provider.withExtraBody({ thinking: { type: 'enabled', keep: 'all' } }),
     );
 
-    expect(messages[0]).toHaveProperty('reasoning_content', ' ');
+    expect(messages[0]).toHaveProperty('reasoning_content', '');
   });
 
-  it('Kimi backfills a text assistant message when keep=all omits thinking.type', async () => {
+  it('Kimi sends empty reasoning_content for a text assistant message when keep=all omits thinking.type', async () => {
     const history: Message[] = [
       {
         role: 'assistant',
@@ -188,15 +188,15 @@ describe('empty thinking round-trip', () => {
       provider.withExtraBody({ thinking: { keep: 'all' } }),
     );
 
-    expect(messages[0]).toHaveProperty('reasoning_content', ' ');
+    expect(messages[0]).toHaveProperty('reasoning_content', '');
   });
 
-  it('Kimi replaces an existing empty ThinkPart when preserved thinking is active', async () => {
+  it('Kimi replays an existing empty ThinkPart unchanged when preserved thinking is active', async () => {
     const messages = await captureKimiMessages(EMPTY_THINKING_TOOL_HISTORY, (provider) =>
       provider.withExtraBody({ thinking: { type: 'enabled', keep: 'all' } }),
     );
 
-    expect(messages[0]).toHaveProperty('reasoning_content', ' ');
+    expect(messages[0]).toHaveProperty('reasoning_content', '');
   });
 
   it('Kimi sends existing non-empty thinking verbatim when preserved thinking is active', async () => {
@@ -431,7 +431,7 @@ describe('empty thinking round-trip', () => {
     });
   });
 
-  it('Anthropic-compatible providers replace only the last empty unsigned thinking block', async () => {
+  it('Anthropic-compatible providers preserve all empty unsigned thinking blocks unchanged', async () => {
     const history: Message[] = [
       {
         role: 'assistant',
@@ -457,7 +457,7 @@ describe('empty thinking round-trip', () => {
 
     expect(messages[0]!.content).toEqual([
       { type: 'thinking', thinking: '' },
-      { type: 'thinking', thinking: ' ' },
+      { type: 'thinking', thinking: '' },
       { type: 'text', text: 'Done.' },
     ]);
   });
@@ -506,7 +506,7 @@ describe('empty thinking round-trip', () => {
     ]);
   });
 
-  it('Anthropic-compatible providers replace the last empty unsigned block after empty signed thinking', async () => {
+  it('Anthropic-compatible providers preserve an empty unsigned block after empty signed thinking', async () => {
     const history: Message[] = [
       {
         role: 'assistant',
@@ -526,7 +526,7 @@ describe('empty thinking round-trip', () => {
 
     expect(messages[0]!.content).toEqual([
       { type: 'thinking', thinking: '', signature: 'signed-thinking' },
-      { type: 'thinking', thinking: ' ' },
+      { type: 'thinking', thinking: '' },
     ]);
   });
 

diff --git a/packages/kosong/src/providers/anthropic.ts b/packages/kosong/src/providers/anthropic.ts
@@ -325,20 +325,6 @@ function shouldPreserveUnsignedThinking(model: string): boolean {
   );
 }
 
-function shouldBackfillPreservedThinking(
-  model: string,
-  thinking: MessageCreateParams['thinking'] | undefined,
-  contextManagement: AnthropicContextManagement | undefined,
-): boolean {
-  return (
-    shouldPreserveUnsignedThinking(model) &&
-    thinking?.type !== 'disabled' &&
-    contextManagement?.edits.some(
-      (edit) => edit.type === CLEAR_THINKING_EDIT && edit.keep === 'all',
-    ) === true
-  );
-}
-
 /**
  * Content block types that support cache_control injection.
  */
@@ -497,11 +483,7 @@ function toolResultToBlock(toolCallId: string, content: ContentPart[]): ToolResu
     content: blocks,
   } as ToolResultBlockParam;
 }
-function convertMessage(
-  message: Message,
-  model: string,
-  backfillPreservedThinking: boolean,
-): MessageParam {
+function convertMessage(message: Message, model: string): MessageParam {
   const role = message.role;
 
   // system role -> <system>...</system> wrapped user message
@@ -527,16 +509,12 @@ function convertMessage(
 
   // user or assistant
   const blocks: ContentBlockParam[] = [];
-  let hasThinkingPart = false;
-  let lastUnsignedThinkingBlockIndex: number | undefined;
-  let hasNonEmptyEmittedThinking = false;
   for (const part of message.content) {
     if (part.type === 'text') {
       blocks.push({ type: 'text', text: part.text } satisfies TextBlockParam);
     } else if (part.type === 'image_url') {
       blocks.push(imageUrlPartToAnthropic(part.imageUrl.url) as unknown as ContentBlockParam);
     } else if (part.type === 'think') {
-      hasThinkingPart = true;
       // ThinkPart -> ThinkingBlockParam.
       //
       // Signed: emit the block with its signature. api.anthropic.com requires a
@@ -556,11 +534,8 @@ function convertMessage(
           thinking: part.think,
           signature: part.encrypted,
         } satisfies ThinkingBlockParam);
-        hasNonEmptyEmittedThinking ||= part.think.length > 0;
       } else if (shouldPreserveUnsignedThinking(model)) {
-        lastUnsignedThinkingBlockIndex = blocks.length;
         blocks.push({ type: 'thinking', thinking: part.think } as unknown as ThinkingBlockParam);
-        hasNonEmptyEmittedThinking ||= part.think.length > 0;
       }
     } else if (part.type === 'video_url') {
       blocks.push(videoUrlPartToAnthropic(part.videoUrl.url) as unknown as ContentBlockParam);
@@ -573,23 +548,6 @@ function convertMessage(
     }
   }
 
-  if (role === 'assistant' && backfillPreservedThinking) {
-    // Some compatible endpoints require every replayed assistant message to
-    // carry non-empty thinking. Keep the placeholder wire-only, and never
-    // alter signed blocks because their text is covered by the signature.
-    if (!hasThinkingPart) {
-      blocks.unshift({ type: 'thinking', thinking: ' ' } as unknown as ThinkingBlockParam);
-    } else if (
-      lastUnsignedThinkingBlockIndex !== undefined &&
-      !hasNonEmptyEmittedThinking
-    ) {
-      blocks[lastUnsignedThinkingBlockIndex] = {
-        type: 'thinking',
-        thinking: ' ',
-      } as unknown as ThinkingBlockParam;
-    }
-  }
-
   // Tool calls -> ToolUseBlockParam
   if (message.toolCalls.length > 0) {
     for (const tc of message.toolCalls) {
@@ -1006,12 +964,6 @@ export class AnthropicChatProvider implements ChatProvider {
         ]
       : undefined;
 
-    const backfillPreservedThinking = shouldBackfillPreservedThinking(
-      this._model,
-      this._generationKwargs.thinking,
-      this._generationKwargs.contextManagement,
-    );
-
     // Convert messages, then merge consecutive user messages into one. Strict
     // Anthropic-compatible backends reject consecutive user messages with HTTP
     // 400 ("roles must alternate"), and api.anthropic.com concatenates them
@@ -1031,7 +983,7 @@ export class AnthropicChatProvider implements ChatProvider {
         history.filter((msg) => !isToolDeclarationOnlyMessage(msg)),
         ANTHROPIC_TOOL_CALL_ID_POLICY,
       )
-        .map((msg) => convertMessage(msg, this._model, backfillPreservedThinking))
+        .map((msg) => convertMessage(msg, this._model))
         .filter(shouldKeepConvertedMessage),
       {
         isUser: (message) => message.role === 'user',

diff --git a/packages/kosong/src/providers/kimi.ts b/packages/kosong/src/providers/kimi.ts
@@ -168,12 +168,7 @@ function convertMessage(message: Message, preservedThinkingEnabled: boolean): Op
   }
 
   if (hasReasoningPart || (preservedThinkingEnabled && message.role === 'assistant')) {
-    // Keep the non-empty replay placeholder on the wire; canonical history
-    // continues to retain the original empty reasoning value.
-    result.reasoning_content =
-      preservedThinkingEnabled && message.role === 'assistant' && reasoningContent.length === 0
-        ? ' '
-        : reasoningContent;
+    result.reasoning_content = reasoningContent;
   }
 
   // Message-level tool declarations: a system message carrying `tools` loads