diff --git a/AGENTS.md b/AGENTS.md
index 68e14a6..ef784dc 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -82,6 +82,10 @@ describe code behavior only, never a live deployment or interaction.
 - Tests for collaborative adapters must cover an unmentioned agent message in
   an established group DM, exact self-echo rejection, duplicate delivery, and
   rejection of an unestablished out-of-scope conversation.
+- Collaboration/history tools must preserve complete provider-sized individual
+  messages. Never silently clip a handoff and make the receiver infer the
+  missing suffix. If an aggregate context budget is necessary, omit older whole
+  messages with an explicit notice rather than truncating a shown message.
 
 ## Voice-control contract
 
diff --git a/src/tools/read-thread.ts b/src/tools/read-thread.ts
index 45144bf..df73165 100644
--- a/src/tools/read-thread.ts
+++ b/src/tools/read-thread.ts
@@ -106,7 +106,9 @@ function channelNameForEntry(entry: LogEntry): string {
 	return channel.startsWith("slack:#") ? channel.slice("slack:#".length) : entry.channelId || "unknown";
 }
 
-function normalizeText(text: unknown, maxLength = 1000): string {
+const DEFAULT_MAX_MESSAGE_CHARACTERS = 40_000;
+
+function normalizeText(text: unknown, maxLength = DEFAULT_MAX_MESSAGE_CHARACTERS): string {
 	if (typeof text !== "string") return "";
 	const normalized = text.replace(/\s+/g, " ").trim();
 	if (normalized.length <= maxLength) return normalized;
diff --git a/test/read-thread.test.ts b/test/read-thread.test.ts
index d02d1bf..c738f31 100644
--- a/test/read-thread.test.ts
+++ b/test/read-thread.test.ts
@@ -70,7 +70,7 @@ try {
 			channel: "slack:#tinyfat",
 			channelId: "C0123456789",
 			userName: "agent",
-			text: "I will check deploy QA in this thread.",
+			text: `I will check deploy QA in this thread. ${"context ".repeat(180)}COMPLETE HANDOFF END`,
 			isBot: true,
 		},
 		{
@@ -231,6 +231,8 @@ try {
 	assert(!first?.messages.some((m) => m.text.includes("product feedback")), "first transcript excludes second thread nuance");
 	assert(first?.messages[0]?.isRoot === true, "root message is marked root");
 	assert(first?.messages[1]?.isBot === true, "bot reply is marked Agent context");
+	assert(first?.messages[1]?.text.endsWith("COMPLETE HANDOFF END"), "read_thread preserves the end of provider-sized agent messages");
+	assert((first?.messages[1]?.text.length || 0) > 1000, "read_thread no longer clips messages at the old 1,000-character boundary");
 	assert(second?.messages.length === 2, "second thread transcript includes second thread messages");
 	assert(second?.messages.some((m) => m.text.includes("thread replies less noisy")), "second transcript preserves follow-up nuance");
 	assert(missing?.messages.length === 0, "valid but unseen thread returns an empty transcript");