diff --git a/packages/core/src/core/openaiContentGenerator/converter.test.ts b/packages/core/src/core/openaiContentGenerator/converter.test.ts
index 453c476436..9c65b85320 100644
--- a/packages/core/src/core/openaiContentGenerator/converter.test.ts
+++ b/packages/core/src/core/openaiContentGenerator/converter.test.ts
@@ -544,6 +544,77 @@ describe('OpenAIContentConverter', () => {
       );
     });
 
+    it('rejects the recorded production unclosed <thinking> content leak (issue #6666)', () => {
+      // Production capture shape (sanitized): a hybrid-thinking model
+      // skipped the reasoning channel entirely and streamed its thinking as
+      // literal <thinking> text inside content — no reasoning_content on
+      // any chunk, no tool calls, and the tag is never closed before stop.
+      const stream = withStreamParser();
+      stream.responseParsingOptions = { contentOnlyThinkingTagLeaks: true };
+      const opening = converter.convertOpenAIChunkToGemini(
+        streamChunk('opening', { content: '<thi' }),
+        stream,
+      );
+      const body = converter.convertOpenAIChunkToGemini(
+        streamChunk('body', {
+          content:
+            'nking>\nThe user wants to query the compute resources for ' +
+            'project space 10088. Let me check the available APIs.',
+        }),
+        stream,
+      );
+
+      expect(opening.candidates?.[0]?.content?.parts).toEqual([]);
+      expect(body.candidates?.[0]?.content?.parts).toEqual([]);
+      expect(() => finishStream(stream, 'stop')).toThrowError(
+        expect.objectContaining({ type: 'PROTOCOL_TAG_LEAK' }),
+      );
+    });
+
+    it('holds a long confirmed opening tag until its closing tag arrives', () => {
+      const stream = withStreamParser();
+      stream.responseParsingOptions = { contentOnlyThinkingTagLeaks: true };
+      const text = `<thinking>${'x'.repeat(200)}</thinking>`;
+
+      const opening = converter.convertOpenAIChunkToGemini(
+        streamChunk('long-balanced', {
+          content: `<thinking>${'x'.repeat(200)}`,
+        }),
+        stream,
+      );
+      const closing = converter.convertOpenAIChunkToGemini(
+        streamChunk('long-balanced', { content: '</thinking>' }, 'stop'),
+        stream,
+      );
+
+      expect(opening.candidates?.[0]?.content?.parts).toEqual([]);
+      expect(closing.candidates?.[0]?.content?.parts).toEqual([{ text }]);
+    });
+
+    it('leaks the production <thinking> shape without provider provenance', () => {
+      // Control for the test above: without contentOnlyThinkingTagLeaks the
+      // same stream passes through verbatim — the defense is provider-gated,
+      // so endpoints whose provider does not opt in remain exposed.
+      const stream = withStreamParser();
+      const response = converter.convertOpenAIChunkToGemini(
+        streamChunk(
+          'literal',
+          {
+            content:
+              '<thinking>\nThe user wants to query the compute resources.',
+          },
+          'stop',
+        ),
+        stream,
+      );
+
+      expect(response.candidates?.[0]?.content?.parts).toEqual([
+        {
+          text: '<thinking>\nThe user wants to query the compute resources.',
+        },
+      ]);
+    });
+
     it.each([
       ['split literal block', ['<thi', 'nk>literal</think>']],
       ['empty block with a separate finish chunk', ['<think>\n\n</think>', '']],
@@ -551,6 +622,7 @@ describe('OpenAIContentConverter', () => {
         'two split valid blocks',
         ['<think>\n\n', '</think><thi', 'nk>literal</think>'],
       ],
+      ['long empty block', [`<thinking>${' '.repeat(128)}</thinking>`, '']],
     ])('preserves content-only %s', (_name, chunks) => {
       const stream = withStreamParser();
       stream.responseParsingOptions = { contentOnlyThinkingTagLeaks: true };
@@ -570,24 +642,33 @@ describe('OpenAIContentConverter', () => {
       expect(parts.every((part) => part.thought !== true)).toBe(true);
     });
 
-    it('releases a long undecided prefix before the stream finishes', () => {
+    it('releases a long unconfirmed prefix before the stream finishes', () => {
       const stream = withStreamParser();
       stream.responseParsingOptions = { contentOnlyThinkingTagLeaks: true };
-      const text = `<think>${' '.repeat(257)}`;
+      const text = `<think${' '.repeat(257)}`;
 
       const response = converter.convertOpenAIChunkToGemini(
         streamChunk('literal', { content: text }),
         stream,
       );
-      const continuation = converter.convertOpenAIChunkToGemini(
-        streamChunk('continuation', { content: 'literal' }, 'stop'),
+
+      expect(response.candidates?.[0]?.content?.parts).toEqual([{ text }]);
+    });
+
+    it('rejects an unclosed whitespace-only block at stream finish', () => {
+      const stream = withStreamParser();
+      stream.responseParsingOptions = { contentOnlyThinkingTagLeaks: true };
+      const response = converter.convertOpenAIChunkToGemini(
+        streamChunk('unclosed', {
+          content: `<thinking>${' '.repeat(128)}`,
+        }),
         stream,
       );
 
-      expect(response.candidates?.[0]?.content?.parts).toEqual([{ text }]);
-      expect(continuation.candidates?.[0]?.content?.parts).toEqual([
-        { text: 'literal' },
-      ]);
+      expect(response.candidates?.[0]?.content?.parts).toEqual([]);
+      expect(() => finishStream(stream, 'stop')).toThrowError(
+        expect.objectContaining({ type: 'PROTOCOL_TAG_LEAK' }),
+      );
     });
 
     it('preserves a leak-shaped literal without provider provenance', () => {
@@ -631,17 +712,38 @@ describe('OpenAIContentConverter', () => {
       expect(parts.map((part) => part.text).join('')).toBe(chunks.join(''));
     });
 
-    it('fails closed when a suspicious prefix exceeds the buffer limit', () => {
+    it('rejects an unclosed outer block containing a balanced nested block', () => {
+      const stream = withStreamParser();
+      stream.responseParsingOptions = { contentOnlyThinkingTagLeaks: true };
+
+      expect(() =>
+        converter.convertOpenAIChunkToGemini(
+          streamChunk(
+            'nested-unclosed',
+            {
+              content: '<thinking><thinking>inner</thinking>outer text',
+            },
+            'stop',
+          ),
+          stream,
+        ),
+      ).toThrowError(expect.objectContaining({ type: 'PROTOCOL_TAG_LEAK' }));
+    });
+
+    it('fails closed for a long suspicious prefix at stream finish', () => {
       const stream = withStreamParser();
       stream.responseParsingOptions = { contentOnlyThinkingTagLeaks: true };
       const content = '<think></think><think>9<think>' + 'x'.repeat(257);
 
-      expect(() =>
-        converter.convertOpenAIChunkToGemini(
-          streamChunk('long-leak', { content }),
-          stream,
-        ),
-      ).toThrowError(expect.objectContaining({ type: 'PROTOCOL_TAG_LEAK' }));
+      const response = converter.convertOpenAIChunkToGemini(
+        streamChunk('long-leak', { content }),
+        stream,
+      );
+
+      expect(response.candidates?.[0]?.content?.parts).toEqual([]);
+      expect(() => finishStream(stream, 'stop')).toThrowError(
+        expect.objectContaining({ type: 'PROTOCOL_TAG_LEAK' }),
+      );
     });
 
     it.each([
@@ -5079,6 +5181,7 @@ describe('OpenAIContentConverter', () => {
 
     it('should handle a single chunk delta with both reasoning_content and content simultaneously', () => {
       const ctx = withStreamParser();
+      ctx.responseParsingOptions = { contentOnlyThinkingTagLeaks: true };
       const part =
         converter.convertOpenAIChunkToGemini(
           {
@@ -5215,7 +5318,10 @@ describe('OpenAIContentConverter', () => {
             },
           ],
         } as unknown as OpenAI.Chat.ChatCompletion,
-        requestContext,
+        {
+          ...requestContext,
+          responseParsingOptions: { contentOnlyThinkingTagLeaks: true },
+        },
       );
 
       expect(response.candidates?.[0]?.content?.parts).toEqual([
diff --git a/packages/core/src/core/openaiContentGenerator/converter.ts b/packages/core/src/core/openaiContentGenerator/converter.ts
index 6585d0cc31..ffd8dfcfd6 100644
--- a/packages/core/src/core/openaiContentGenerator/converter.ts
+++ b/packages/core/src/core/openaiContentGenerator/converter.ts
@@ -1160,8 +1160,25 @@ function classifyContentOnlyThinkingTagPrefix(
   for (const closing of [false, true, false]) {
     const tagLength = consumeTag(rest, closing);
     if (tagLength === null) return 'pending';
-    if (tagLength === undefined) return 'clean';
+    if (tagLength === undefined) {
+      if (!closing) return 'clean';
+      break;
+    }
     rest = rest.slice(tagLength).trimStart();
+    if (closing && !rest) return 'clean';
+    // An opening tag followed by ordinary text is only a legitimate literal
+    // if a closing tag still balances it later. Without one, the turn is an
+    // unclosed thinking block — the exact shape of the recorded production
+    // leaks (issue #6666). Hold it mid-stream (a closing tag may still
+    // arrive) and reject it once the stream finishes. Whitespace-only tails
+    // stay undecided: they may still resolve into a closing tag.
+    if (
+      closing === false &&
+      /\S/.test(rest) &&
+      !/<\/think(?:ing)?\s*>/i.test(rest)
+    ) {
+      return streamFinished ? 'leaked' : 'pending';
+    }
   }
 
   let depth = 1;
@@ -1564,8 +1581,18 @@ export function convertOpenAIChunkToGemini(
         Boolean(choice.finish_reason) &&
         !closingTagName &&
         !/\S/.test(combinedCandidateText);
+      // The length cap releases undecided prefixes (e.g. a literal "<t" that
+      // never resolves) so ordinary content is not buffered forever. Once the
+      // candidate has committed to a complete opening tag, though, releasing
+      // it can leak the whole block — production thinking-tag leaks are longer
+      // than the cap (issue #6666). Keep those held until a closing tag arrives
+      // or the finished-stream check rejects an unclosed block.
+      const confirmedOpeningTagCandidate =
+        LEADING_THINKING_TAG_PATTERN.test(combinedCandidateText) &&
+        !combinedCandidateText.trimStart().startsWith('</');
       const releaseContentOnlyCandidate =
         contentOnlyThinkingState === 'pending' &&
+        !confirmedOpeningTagCandidate &&
         (Boolean(choice.finish_reason) ||
           combinedCandidateText.trimStart().length >
             MAX_THINKING_TAG_CANDIDATE_LENGTH);
@@ -1591,6 +1618,7 @@ export function convertOpenAIChunkToGemini(
         requestContext.pendingThinkingTagCandidate = undefined;
       } else if (isPossibleTag) {
         if (
+          !confirmedOpeningTagCandidate &&
           !closingTagName &&
           combinedCandidateText.trimStart().length >
             MAX_THINKING_TAG_CANDIDATE_LENGTH
diff --git a/packages/core/src/core/openaiContentGenerator/provider/dashscope.ts b/packages/core/src/core/openaiContentGenerator/provider/dashscope.ts
index 5f42ae9d09..d375990ba3 100644
--- a/packages/core/src/core/openaiContentGenerator/provider/dashscope.ts
+++ b/packages/core/src/core/openaiContentGenerator/provider/dashscope.ts
@@ -15,7 +15,6 @@ import type {
   ChatCompletionContentPartWithCache,
   ChatCompletionToolWithCache,
 } from './types.js';
-import type { OpenAIResponseParsingOptions } from '../responseParsingOptions.js';
 import { buildRuntimeFetchOptions } from '../../../utils/runtimeFetchOptions.js';
 import { createDebugLogger } from '../../../utils/debugLogger.js';
 import {
@@ -166,11 +165,6 @@ export class DashScopeOpenAICompatibleProvider extends DefaultOpenAICompatiblePr
     super(contentGeneratorConfig, cliConfig);
   }
 
-  getResponseParsingOptions(): OpenAIResponseParsingOptions {
-    // ponytail: DashScope-only fallback; remove after provider output stabilizes.
-    return { contentOnlyThinkingTagLeaks: true };
-  }
-
   /**
    * Determines whether to use the DashScope-compatible provider.
    * Covers the official regional hosts (DASHSCOPE_REGIONAL_HOSTS),
diff --git a/packages/core/src/core/openaiContentGenerator/provider/default.test.ts b/packages/core/src/core/openaiContentGenerator/provider/default.test.ts
index ab8ce51253..052b2cfa61 100644
--- a/packages/core/src/core/openaiContentGenerator/provider/default.test.ts
+++ b/packages/core/src/core/openaiContentGenerator/provider/default.test.ts
@@ -93,6 +93,14 @@ describe('DefaultOpenAICompatibleProvider', () => {
     });
   });
 
+  describe('getResponseParsingOptions', () => {
+    it('enables leak handling without treating balanced tags as protocol', () => {
+      expect(provider.getResponseParsingOptions()).toEqual({
+        contentOnlyThinkingTagLeaks: true,
+      });
+    });
+  });
+
   describe('buildHeaders', () => {
     it('should build headers with User-Agent', () => {
       const headers = provider.buildHeaders();
diff --git a/packages/core/src/core/openaiContentGenerator/provider/default.ts b/packages/core/src/core/openaiContentGenerator/provider/default.ts
index 19000fc59c..6835bd38a3 100644
--- a/packages/core/src/core/openaiContentGenerator/provider/default.ts
+++ b/packages/core/src/core/openaiContentGenerator/provider/default.ts
@@ -4,6 +4,7 @@ import type { Config } from '../../../config/config.js';
 import type { ContentGeneratorConfig } from '../../contentGenerator.js';
 import { DEFAULT_MAX_RETRIES, resolveRequestTimeout } from '../constants.js';
 import type { OpenAICompatibleProvider } from './types.js';
+import type { OpenAIResponseParsingOptions } from '../responseParsingOptions.js';
 import { buildRuntimeFetchOptions } from '../../../utils/runtimeFetchOptions.js';
 import {
   tokenLimit,
@@ -123,6 +124,15 @@ export class DefaultOpenAICompatibleProvider
     return {};
   }
 
+  getResponseParsingOptions(): OpenAIResponseParsingOptions {
+    // Hybrid-thinking models occasionally bypass the reasoning channel and
+    // emit their thinking as literal <think>/<thinking> tags inside content
+    // (observed in production on qwen3-class models, issue #6666).
+    // Honored on the streaming path only; non-streaming responses are
+    // not classified.
+    return { contentOnlyThinkingTagLeaks: true };
+  }
+
   /**
    * Apply output token limit to a request's max_tokens parameter.
    *
diff --git a/packages/core/src/core/openaiContentGenerator/provider/minimax.ts b/packages/core/src/core/openaiContentGenerator/provider/minimax.ts
index 97008e1a45..49b0974792 100644
--- a/packages/core/src/core/openaiContentGenerator/provider/minimax.ts
+++ b/packages/core/src/core/openaiContentGenerator/provider/minimax.ts
@@ -38,7 +38,7 @@ export class MiniMaxOpenAICompatibleProvider extends DefaultOpenAICompatibleProv
     }
   }
 
-  getResponseParsingOptions(): OpenAIResponseParsingOptions {
+  override getResponseParsingOptions(): OpenAIResponseParsingOptions {
     return { taggedThinkingTags: true };
   }
 }