diff --git a/packages/shared/src/assistantCitations.ts b/packages/shared/src/assistantCitations.ts
index 9af6a29706724..765b5d3e6c0d8 100644
--- a/packages/shared/src/assistantCitations.ts
+++ b/packages/shared/src/assistantCitations.ts
@@ -131,6 +131,23 @@ export function assistantCitationsToPlainText(prompt: string): string {
   });
 }
 
+// Claude 5.1+ writes some user-facing updates as thinking blocks, which the adapter shows as
+// assistant messages whose percent-encoded item id ends in `:thinking:<n>`. The API rejects
+// that text coming back in a prompt as reasoning extraction, so only the comment goes out.
+const THINKING_DERIVED_MESSAGE_ID = /%3Athinking%3A\d+$/u;
+
+export function withholdThinkingCitationText(prompt: string): string {
+  let text = "";
+  let cursor = 0;
+  for (const { citation, start, end } of collectAssistantCitations(prompt)) {
+    if (!THINKING_DERIVED_MESSAGE_ID.test(citation.messageId)) continue;
+    const comment = citation.comment === undefined ? "" : ` User comment: ${citation.comment}`;
+    text += `${prompt.slice(cursor, start)}[Quote of one of your earlier updates, text withheld.${comment}]`;
+    cursor = end;
+  }
+  return text + prompt.slice(cursor);
+}
+
 export function expandAssistantCitationsForProvider(prompt: string): string {
   const matches = collectAssistantCitations(prompt);
   if (matches.length === 0) return prompt;
diff --git a/packages/shared/src/composerContextReferences.test.ts b/packages/shared/src/composerContextReferences.test.ts
index 7212a1b64a984..266a9cc91e6ab 100644
--- a/packages/shared/src/composerContextReferences.test.ts
+++ b/packages/shared/src/composerContextReferences.test.ts
@@ -2,10 +2,12 @@ import {
   ComposerContextId,
   type ComposerContextRecord,
   EnvironmentId,
+  MessageId,
   ThreadId,
 } from "@t3tools/contracts";
 import { describe, expect, it } from "vite-plus/test";
 
+import { serializeAssistantCitation } from "./assistantCitations.ts";
 import {
   collectComposerContextReferences,
   formatComposerContextHref,
@@ -156,6 +158,37 @@ describe("provider projection", () => {
     expect(projected).toContain("- font-size: 12px → 20px");
   });
 
+  it("withholds quoted text of thinking-derived messages but keeps the comment and other quotes", () => {
+    const quote = {
+      version: 1,
+      environmentId: EnvironmentId.make("env"),
+      threadId: ThreadId.make("thread"),
+      text: "private plan",
+      start: 0,
+      end: 12,
+      prefix: "before ",
+      suffix: " after",
+    } as const;
+    const thinking = serializeAssistantCitation({
+      ...quote,
+      messageId: MessageId.make("message:provider:claudeAgent:native-item:msg_01%3Athinking%3A1"),
+      comment: "fix this?",
+    });
+    const reply = serializeAssistantCitation({
+      ...quote,
+      text: "plain reply",
+      messageId: MessageId.make("msg_02"),
+    });
+    const projected = projectComposerContextForProvider({
+      text: `${thinking} and ${reply}`,
+      records: [],
+    });
+    expect(projected).toBe(
+      `[Quote of one of your earlier updates, text withheld. User comment: fix this?] and ${reply}`,
+    );
+    expect(projected).not.toContain("private");
+  });
+
   it("uses the payload kind when a reference disagrees with its record", () => {
     const projected = projectComposerContextForProvider({
       text: "[log](t3-context://v1/image/ctx_t)",
diff --git a/packages/shared/src/composerContextReferences.ts b/packages/shared/src/composerContextReferences.ts
index 88361606a9a64..a15b6e8719a63 100644
--- a/packages/shared/src/composerContextReferences.ts
+++ b/packages/shared/src/composerContextReferences.ts
@@ -9,6 +9,7 @@ import {
   type ScopedThreadRef,
   type ThreadContextRecord,
 } from "@t3tools/contracts";
+import { withholdThinkingCitationText } from "./assistantCitations.ts";
 
 const CONTEXT_PROTOCOL = "t3-context:";
 const COMPOSER_CONTEXT_HREF_PREFIX = `${CONTEXT_PROTOCOL}//v1/`;
@@ -312,13 +313,14 @@ export function projectComposerContextForProvider(input: {
   text: string;
   records: ReadonlyArray<ComposerContextRecord>;
 }): string {
-  const occurrences = collectComposerContextReferences(input.text);
-  if (occurrences.length === 0) return input.text;
+  const text = withholdThinkingCitationText(input.text);
+  const occurrences = collectComposerContextReferences(text);
+  if (occurrences.length === 0) return text;
   const recordsById = new Map<ComposerContextId, ComposerContextRecord | undefined>();
   for (const record of input.records) {
     recordsById.set(record.contextId, recordsById.has(record.contextId) ? undefined : record);
   }
-  const body = replaceComposerContextReferences(input.text, (occurrence) =>
+  const body = replaceComposerContextReferences(text, (occurrence) =>
     formatComposerContextProviderMarker(
       recordsById.get(occurrence.contextId)?.kind ?? occurrence.kind,
       occurrence.label,