diff --git a/packages/contracts/src/assistantCitations.ts b/packages/contracts/src/assistantCitations.ts
index 71934d1684ab0..96572a4bfa656 100644
--- a/packages/contracts/src/assistantCitations.ts
+++ b/packages/contracts/src/assistantCitations.ts
@@ -4,13 +4,15 @@ import { EnvironmentId, MessageId, NonNegativeInt, ThreadId } from "./baseSchema
 export const ASSISTANT_CITATION_MAX_TEXT_LENGTH = 8_000;
 export const ASSISTANT_CITATION_MAX_COMMENT_LENGTH = 8_000;
 export const ASSISTANT_CITATION_CONTEXT_LENGTH = 32;
+// Provider message IDs nest and re-encode their thread and turn IDs, so they outgrow a few hundred characters.
+export const ASSISTANT_CITATION_MAX_ID_LENGTH = 4_096;
 
 /** Positions are UTF-16 offsets, not Markdown offsets. */
 export const AssistantCitation = Schema.Struct({
   version: Schema.Literal(1),
-  environmentId: EnvironmentId.check(Schema.isMaxLength(512)),
-  threadId: ThreadId.check(Schema.isMaxLength(512)),
-  messageId: MessageId.check(Schema.isMaxLength(512)),
+  environmentId: EnvironmentId.check(Schema.isMaxLength(ASSISTANT_CITATION_MAX_ID_LENGTH)),
+  threadId: ThreadId.check(Schema.isMaxLength(ASSISTANT_CITATION_MAX_ID_LENGTH)),
+  messageId: MessageId.check(Schema.isMaxLength(ASSISTANT_CITATION_MAX_ID_LENGTH)),
   text: Schema.String.check(
     Schema.isNonEmpty(),
     Schema.isMaxLength(ASSISTANT_CITATION_MAX_TEXT_LENGTH),
diff --git a/packages/shared/src/assistantCitations.test.ts b/packages/shared/src/assistantCitations.test.ts
index 7300de72e9d1c..7dbaaf9d8bdeb 100644
--- a/packages/shared/src/assistantCitations.test.ts
+++ b/packages/shared/src/assistantCitations.test.ts
@@ -2,6 +2,7 @@ import { describe, expect, it } from "vite-plus/test";
 import {
   ASSISTANT_CITATION_CONTEXT_LENGTH,
   ASSISTANT_CITATION_MAX_COMMENT_LENGTH,
+  ASSISTANT_CITATION_MAX_ID_LENGTH,
   ASSISTANT_CITATION_MAX_TEXT_LENGTH,
   EnvironmentId,
   MessageId,
@@ -126,9 +127,9 @@ describe("assistant citation references", () => {
   it("accepts complete 8k CJK quotes and comments with maximum-sized source selectors", () => {
     const largeCitation: AssistantCitation = {
       ...citation,
-      environmentId: EnvironmentId.make("環".repeat(512)),
-      threadId: ThreadId.make("線".repeat(512)),
-      messageId: MessageId.make("文".repeat(512)),
+      environmentId: EnvironmentId.make("環".repeat(ASSISTANT_CITATION_MAX_ID_LENGTH)),
+      threadId: ThreadId.make("線".repeat(ASSISTANT_CITATION_MAX_ID_LENGTH)),
+      messageId: MessageId.make("文".repeat(ASSISTANT_CITATION_MAX_ID_LENGTH)),
       text: "引".repeat(ASSISTANT_CITATION_MAX_TEXT_LENGTH),
       comment: "注".repeat(ASSISTANT_CITATION_MAX_COMMENT_LENGTH),
       start: Number.MAX_SAFE_INTEGER - ASSISTANT_CITATION_MAX_TEXT_LENGTH,
diff --git a/packages/shared/src/assistantCitations.ts b/packages/shared/src/assistantCitations.ts
index 64bb2b3904ba1..bb072e462a45e 100644
--- a/packages/shared/src/assistantCitations.ts
+++ b/packages/shared/src/assistantCitations.ts
@@ -1,5 +1,7 @@
 import {
+  ASSISTANT_CITATION_CONTEXT_LENGTH,
   ASSISTANT_CITATION_MAX_COMMENT_LENGTH,
+  ASSISTANT_CITATION_MAX_ID_LENGTH,
   ASSISTANT_CITATION_MAX_TEXT_LENGTH,
   AssistantCitation,
 } from "@t3tools/contracts";
@@ -8,9 +10,14 @@ import * as Schema from "effect/Schema";
 
 const CITATION_PROTOCOL = "t3-citation:";
 const CITATION_HREF_PREFIX = `${CITATION_PROTOCOL}//v1/`;
-// Percent encoding needs up to nine characters per UTF-16 code unit; 16k covers selectors.
+// Percent encoding needs up to nine characters per UTF-16 code unit; 1k covers keys and offsets.
 const MAX_CITATION_HREF_LENGTH =
-  9 * (ASSISTANT_CITATION_MAX_TEXT_LENGTH + ASSISTANT_CITATION_MAX_COMMENT_LENGTH) + 16_000;
+  9 *
+    (ASSISTANT_CITATION_MAX_TEXT_LENGTH +
+      ASSISTANT_CITATION_MAX_COMMENT_LENGTH +
+      3 * ASSISTANT_CITATION_MAX_ID_LENGTH +
+      2 * ASSISTANT_CITATION_CONTEXT_LENGTH) +
+  1_000;
 const CITATION_LINK = new RegExp(
   String.raw`\[Assistant quote\]\((${CITATION_HREF_PREFIX}[^\s)]{1,${MAX_CITATION_HREF_LENGTH - CITATION_HREF_PREFIX.length}})\)`,
   "g",