diff --git a/CHANGELOG.md b/CHANGELOG.md
index 485e1334..28271b4e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,11 @@ loosely while pre-1.0 (breaking changes can land on minor bumps).
 ## [Unreleased]
 
 ### Fixed
+- **Intent LLM prompt text cap.** `build_llm_user_prompt` now truncates
+  request text at 64 KiB (UTF-8 safe, with a `[truncated]` marker) so
+  orchestrator hybrid/llm classify cannot forward an unbounded TUI,
+  chat, or event payload to the provider. Matches the existing
+  `POST /v1/intent` message limit.
 - **Remote TUI: recoverable SSE `error` is not a failed turn.** `RemoteSseTurnConsumer::finish` copied accumulated `error` event text even when the terminal `done` event had `ok: true` (e.g. catalog skip of a stored agent whose JSON failed validation). `done` is authoritative: success clears the result error; failure still prefers `done.error` and falls back to prior `error` events when that field is empty.
 - **Unreadable TUI sessions are not empty.** `session_json_is_empty` no
 - **Advise-gate cancel is not a bad bearer.** `POST /v1/advise/gate`
diff --git a/include/intent.h b/include/intent.h
index 89b92568..c7522a9e 100644
--- a/include/intent.h
+++ b/include/intent.h
@@ -16,12 +16,19 @@
 // Distinct from Constitution::MemoryConfig::intent_routing, which only
 // boosts /mem search types.
 
+#include <cstddef>
 #include <functional>
 #include <string>
 #include <vector>
 
 namespace arbiter {
 
+// HTTP POST /v1/intent already rejects messages above this size.  The
+// orchestrator ingress path does not, so the LLM prompt builder must
+// truncate here or a large TUI / chat / event payload is forwarded
+// wholesale to the provider.
+inline constexpr std::size_t kIntentLlmTextMaxBytes = 64 * 1024;
+
 struct IntentConfig {
     // off | heuristic | hybrid | llm
     // File-backed agents default off; master_constitution() sets hybrid.
diff --git a/src/intent.cpp b/src/intent.cpp
index 338e89fd..56767f7d 100644
--- a/src/intent.cpp
+++ b/src/intent.cpp
@@ -231,9 +231,24 @@ std::vector<std::string> cue_kinds(const std::string& norm) {
     return kinds;
 }
 
+std::string cap_intent_llm_text(std::string text) {
+    if (text.size() <= kIntentLlmTextMaxBytes) return text;
+    static constexpr char kMark[] = "\n[truncated]";
+    constexpr std::size_t kMarkLen = sizeof(kMark) - 1;
+    std::size_t keep = (kIntentLlmTextMaxBytes > kMarkLen)
+        ? (kIntentLlmTextMaxBytes - kMarkLen) : 0;
+    while (keep > 0 &&
+           (static_cast<unsigned char>(text[keep]) & 0xC0) == 0x80) {
+        --keep;
+    }
+    text.resize(keep);
+    text += kMark;
+    return text;
+}
+
 std::string build_llm_user_prompt(const IntentInput& in, const Intent& hint) {
     std::ostringstream q;
-    q << "[REQUEST]\n" << in.text << "\n[END REQUEST]\n\n";
+    q << "[REQUEST]\n" << cap_intent_llm_text(in.text) << "\n[END REQUEST]\n\n";
     q << "[ROSTER]\n";
     if (in.roster.empty()) {
         q << "(none)\n";
diff --git a/tests/test_intent.cpp b/tests/test_intent.cpp
index 28fafc23..fbf73d67 100644
--- a/tests/test_intent.cpp
+++ b/tests/test_intent.cpp
@@ -356,6 +356,62 @@ TEST_CASE("standalone classify defaults to heuristic, keeps master thresholds")
     CHECK(cfg.model == "advisor-model");
 }
 
+TEST_CASE("llm prompt truncates request text at 64 KiB") {
+    IntentInput in;
+    in.text.assign(kIntentLlmTextMaxBytes + 4096, 'x');
+    in.requested_agent = "index";
+    in.roster = starter_roster();
+    IntentConfig cfg;
+    cfg.mode = "llm";
+
+    std::string prompt;
+    auto llm = [&](const std::string& p) {
+        prompt = p;
+        return std::string{};
+    };
+    auto out = resolve_intent(in, cfg, llm);
+    CHECK(out.llm_used);
+    REQUIRE_FALSE(prompt.empty());
+
+    const auto req = prompt.find("[REQUEST]\n");
+    const auto end = prompt.find("\n[END REQUEST]");
+    REQUIRE(req != std::string::npos);
+    REQUIRE(end != std::string::npos);
+    REQUIRE(end > req);
+    const std::string body = prompt.substr(
+        req + std::string("[REQUEST]\n").size(),
+        end - (req + std::string("[REQUEST]\n").size()));
+    CHECK(body.size() <= kIntentLlmTextMaxBytes);
+    CHECK(body.find("[truncated]") != std::string::npos);
+    CHECK(body.find(std::string(kIntentLlmTextMaxBytes, 'x')) == std::string::npos);
+}
+
+TEST_CASE("llm prompt keeps request text at the 64 KiB boundary") {
+    IntentInput in;
+    in.text.assign(kIntentLlmTextMaxBytes, 'y');
+    in.requested_agent = "index";
+    in.roster = starter_roster();
+    IntentConfig cfg;
+    cfg.mode = "llm";
+
+    std::string prompt;
+    auto llm = [&](const std::string& p) {
+        prompt = p;
+        return std::string{};
+    };
+    resolve_intent(in, cfg, llm);
+
+    const auto req = prompt.find("[REQUEST]\n");
+    const auto end = prompt.find("\n[END REQUEST]");
+    REQUIRE(req != std::string::npos);
+    REQUIRE(end != std::string::npos);
+    const std::string body = prompt.substr(
+        req + std::string("[REQUEST]\n").size(),
+        end - (req + std::string("[REQUEST]\n").size()));
+    CHECK(body == in.text);
+    CHECK(body.find("[truncated]") == std::string::npos);
+}
+
 TEST_CASE("standalone heuristic does not invoke llm on unconfident text") {
     IntentInput in;
     in.text = "Hello there, please help";