diff --git a/README.md b/README.md
index 7f709b7..b33a5bf 100644
--- a/README.md
+++ b/README.md
@@ -154,6 +154,8 @@ printf '%s' "New findings that require more work..." \
 
 The send command does not report success from the HTTP response alone. It waits until the exact message is visible in T3's thread projection. Archived threads are rejected.
 
+By default, `send` refuses a busy thread with `THREAD_BUSY` and dispatches nothing. A thread is busy while a turn runs or while an earlier message still waits for its turn; `error.details` says which. Wait with `threads wait` and send again, or pass `--if-busy inject` to send into the running turn. The provider then folds the message into that turn or queues it, which `--wait` follows either way. The busy check is a snapshot, not a lock, so callers that send to the same thread at once must take turns themselves.
+
 Add `--wait` to wait for the turn that handles the message and print its reply:
 
 ```bash
diff --git a/skills/t3thread/SKILL.md b/skills/t3thread/SKILL.md
index 2aea6be..6446ade 100644
--- a/skills/t3thread/SKILL.md
+++ b/skills/t3thread/SKILL.md
@@ -90,7 +90,7 @@ Rules for sending:
 
 - A settled thread needs `--wake-settled`. The user's explicit instruction to message this thread authorizes it.
 - Archived threads cannot receive messages.
-- If the thread is mid-turn, the provider either folds the message into the running turn or queues a new turn. `--wait` handles both.
+- `send` refuses a busy thread with `THREAD_BUSY` (exit code 4): a turn runs or an earlier message waits. Wait with `threads wait`, then send. Pass `--if-busy inject` only when the user wants to steer the running turn; the provider then folds the message in or queues it, and `--wait` follows either way.
 - `THREAD_WAIT_TIMEOUT` (exit code 6) means the message was sent. Never resend it. Keep waiting with `t3code threads wait --thread <id> --timeout 540`.
 - `THREAD_TURN_NOT_VERIFIED` (exit code 5) means T3 has not shown the message yet. Do not retry automatically; read the thread first.
 
diff --git a/skills/use-t3code-cli/SKILL.md b/skills/use-t3code-cli/SKILL.md
index 14b0ca8..a0ccb73 100644
--- a/skills/use-t3code-cli/SKILL.md
+++ b/skills/use-t3code-cli/SKILL.md
@@ -89,7 +89,7 @@ printf '%s' "$THREAD_MESSAGE" \
   | t3code --json threads send --thread "$TARGET_THREAD_ID" --stdin
 ```
 
-Sending is an external state change. Keep the target and message within the caller's authorization. A settled thread requires interactive confirmation or `--wake-settled`; JSON and stdin workflows are non-interactive, so use that override only when waking the inspected target is authorized. Archived threads cannot receive a turn.
+Sending is an external state change. Keep the target and message within the caller's authorization. A settled thread requires interactive confirmation or `--wake-settled`; JSON and stdin workflows are non-interactive, so use that override only when waking the inspected target is authorized. Archived threads cannot receive a turn. A busy thread, with a running turn or a message waiting for its turn, gets `THREAD_BUSY` unless you pass `--if-busy inject` to send into the running turn.
 
 Add `--wait` to get the reply. Give the shell call a longer timeout than `--timeout`:
 
diff --git a/src/cli.ts b/src/cli.ts
index 737c955..5de6a33 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -202,6 +202,7 @@ interface SettingsCommandOptions {
 interface ThreadSendCommandOptions extends PromptOptions, ThreadWaitCommandOptions, SettingsCommandOptions {
   wakeSettled?: boolean;
   wait?: boolean;
+  ifBusy: "reject" | "inject";
 }
 
 interface ThreadRequestCommandOptions extends ThreadWaitCommandOptions {
@@ -592,7 +593,12 @@ addReplyOptions(
       .option("--prompt-file <path>", "Read the message from a UTF-8 file.")
       .option("--stdin", "Read the message from stdin.")
       .option("--wake-settled", "Explicitly allow this message to wake a settled thread.")
-      .option("--wait", "Wait for the turn that handles the message and print its reply."),
+      .option("--wait", "Wait for the turn that handles the message and print its reply.")
+      .addOption(
+        new Option("--if-busy <mode>", "reject: refuse while a turn runs or a message waits; inject: send into the running turn.")
+          .choices(["reject", "inject"])
+          .default("reject"),
+      ),
   ),
 ).action((options: ThreadSendCommandOptions) =>
   action(async () => {
@@ -605,6 +611,7 @@ addReplyOptions(
       ...(!context.json && !options.stdin ? { confirmSettled: confirmSettledThread } : {}),
       ...(options.wait ? { wait: waitOptions(options) } : {}),
       settings: settingsChange(options),
+      ifBusy: options.ifBusy,
     });
     const changed = result.settings ? describeChanges(result.settings) : "";
     const sent = `${changed ? `Changed ${changed}. ` : ""}Sent message ${result.message.messageId} to thread ${result.thread.id}; T3 accepted and projected the turn.`;
diff --git a/src/service.ts b/src/service.ts
index 473d359..a7ecc46 100644
--- a/src/service.ts
+++ b/src/service.ts
@@ -11,6 +11,7 @@ import { discoverRuntime } from "./runtime.js";
 import { T3ThreadApi, type ThreadSettlementState } from "./threadApi.js";
 import {
   changeSettingsWithApi,
+  busyState,
   hasSettingsChange,
   settingsSummary,
   type ThreadSettingsChange,
@@ -85,6 +86,11 @@ export interface ThreadSendOptions {
   wait?: ThreadWaitOptions;
   /** Change the thread's model, effort, speed, or modes before the message starts its turn. */
   settings?: ThreadSettingsChange;
+  /**
+   * What to do when the thread is busy: `reject` (the default) refuses to send; `inject` sends into the
+   * running turn, where the provider folds the message in or queues it.
+   */
+  ifBusy?: "reject" | "inject";
 }
 
 interface EffectiveT3Settings {
@@ -534,6 +540,23 @@ export async function sendThreadMessage(config: CliConfig, options: ThreadSendOp
       }
     }
 
+    const busy = busyState(thread);
+    if (busy && (options.ifBusy ?? "reject") === "reject") {
+      throw new CliError(
+        "THREAD_BUSY",
+        `Thread ${threadId} ${busy.turnRunning ? "is running a turn" : "has a message waiting for its turn"}. Wait for it with threads wait, or pass --if-busy inject to send into the running turn.`,
+        {
+          exitCode: 4,
+          details: {
+            threadId,
+            ...busy,
+            sessionStatus: thread.session?.status ?? null,
+            latestTurnState: thread.latestTurn?.state ?? null,
+          },
+        },
+      );
+    }
+
     const settings = hasSettingsChange(options.settings)
       ? await changeSettingsWithApi(api, adapter, thread, options.settings)
       : null;
diff --git a/src/threadControls.ts b/src/threadControls.ts
index 7b4b66d..b56c673 100644
--- a/src/threadControls.ts
+++ b/src/threadControls.ts
@@ -22,7 +22,7 @@ import {
   type ThreadWaitOptions,
   type ThreadWaitView,
 } from "./threadSupport.js";
-import { pendingRequests, type PendingQuestion, type PendingRequest } from "./transcript.js";
+import { pendingRequests, queuedMessages, type PendingQuestion, type PendingRequest } from "./transcript.js";
 import type { CliConfig, InteractionMode, ModelSelection, RuntimeMode, T3Thread } from "./types.js";
 
 /** Settings a caller asks to change on an existing thread; anything left out stays as it is. */
@@ -65,6 +65,13 @@ function liveSession(thread: T3Thread): boolean {
   return thread.session != null && thread.session.status !== "stopped";
 }
 
+/** What keeps a thread busy: a running turn, or messages that still wait for their turn. */
+export function busyState(thread: T3Thread): { turnRunning: boolean; queuedMessages: number } | null {
+  const running = turnRunning(thread);
+  const queued = queuedMessages(thread).length;
+  return running || queued > 0 ? { turnRunning: running, queuedMessages: queued } : null;
+}
+
 /** T3 binds a conversation to its provider once the thread has a session or any history. */
 function conversationStarted(thread: T3Thread): boolean {
   return thread.session != null || thread.latestTurn != null || (thread.messages?.length ?? 0) > 0;
diff --git a/src/threads.test.ts b/src/threads.test.ts
index c575833..4c7ea18 100644
--- a/src/threads.test.ts
+++ b/src/threads.test.ts
@@ -742,6 +742,38 @@ describe("thread controls", () => {
     expect(harness.commands[0]).toMatchObject({ type: "thread.meta.update" });
   });
 
+  it("refuses to send into a busy thread unless the caller injects", async () => {
+    const harness = await testHarness([makeThread("target", running())]);
+
+    await expect(sendThreadMessage(harness.config, { threadId: "target", prompt: "Also check the docs" })).rejects.toMatchObject({
+      code: "THREAD_BUSY",
+      exitCode: 4,
+      details: { turnRunning: true, queuedMessages: 0 },
+    });
+    expect(harness.commands).toEqual([]);
+
+    const injected = await sendThreadMessage(harness.config, { threadId: "target", prompt: "Also check the docs", ifBusy: "inject" });
+
+    expect(harness.commands).toEqual([expect.objectContaining({ type: "thread.turn.start", threadId: "target" })]);
+    expect(injected.verification).toMatchObject({ accepted: true });
+  });
+
+  it("counts a message waiting for its turn as busy", async () => {
+    const harness = await testHarness([makeThread("target", {
+      latestTurn: { turnId: "turn-1", state: "completed", requestedAt: at(0), startedAt: at(0), completedAt: at(2), assistantMessageId: null },
+      messages: [
+        ...running().messages,
+        { id: "answer", role: "assistant", text: "Done", turnId: "turn-1", streaming: false, createdAt: at(1), updatedAt: at(1) },
+        { id: "queued", role: "user", text: "Next", turnId: null, streaming: false, createdAt: at(5), updatedAt: at(5) },
+      ],
+    })]);
+
+    await expect(sendThreadMessage(harness.config, { threadId: "target", prompt: "And then this" })).rejects.toMatchObject({
+      code: "THREAD_BUSY",
+      details: { turnRunning: false, queuedMessages: 1 },
+    });
+  });
+
   it("refuses to send with new settings while a turn runs", async () => {
     const harness = await testHarness([makeThread("target", running())], { catalog: CATALOG });
 
diff --git a/tests/cli.test.mjs b/tests/cli.test.mjs
index 23c4ade..d5077b5 100644
--- a/tests/cli.test.mjs
+++ b/tests/cli.test.mjs
@@ -84,6 +84,16 @@ describe("CLI parsing", () => {
     });
   });
 
+  it("rejects an unknown busy-thread mode", async () => {
+    const result = await run(["--json", "threads", "send", "--thread", "thread-1", "--prompt", "x", "--if-busy", "queue"]);
+
+    expect(result.code).toBe(2);
+    expect(JSON.parse(result.stderr).error).toEqual({
+      code: "INVALID_USAGE",
+      message: "option '--if-busy <mode>' argument 'queue' is invalid. Allowed choices are reject, inject.",
+    });
+  });
+
   it("rejects an unknown read detail", async () => {
     const result = await run(["--json", "threads", "read", "--thread", "thread-1", "--detail", "verbose"]);