diff --git a/apps/server/src/project/ProjectSetupScriptRunner.test.ts b/apps/server/src/project/ProjectSetupScriptRunner.test.ts
index 034b168e52c79..2dcf239cb04b5 100644
--- a/apps/server/src/project/ProjectSetupScriptRunner.test.ts
+++ b/apps/server/src/project/ProjectSetupScriptRunner.test.ts
@@ -1,16 +1,20 @@
 import * as NodeCrypto from "@effect/platform-node/NodeCrypto";
-import { describe, expect, it } from "@effect/vitest";
+import * as NodeServices from "@effect/platform-node/NodeServices";
+import { assert, describe, expect, it } from "@effect/vitest";
 import {
   ProjectId,
   TerminalNotRunningError,
   type TerminalEvent,
   type TerminalSessionSnapshot,
 } from "@t3tools/contracts";
+import * as HostProcess from "@t3tools/shared/HostProcess";
 import * as Deferred from "effect/Deferred";
 import * as Effect from "effect/Effect";
 import * as Fiber from "effect/Fiber";
 import * as Layer from "effect/Layer";
 import * as Queue from "effect/Queue";
+import * as Stream from "effect/Stream";
+import { ChildProcess, ChildProcessSpawner } from "effect/process";
 import { TestClock } from "effect/testing";
 
 import * as ServerSettings from "../serverSettings.ts";
@@ -164,7 +168,7 @@ describe("ProjectSetupScriptRunner", () => {
 
       yield* emit({ ...event, type: "activity", hasRunningSubprocess: false, label: "zsh" });
       yield* emit({ ...event, type: "output", data: "\r\n% \x1b[?2004h" });
-      expect(yield* Queue.take(writes)).toMatch(/^ \( vp i\r\);/);
+      expect(yield* Queue.take(writes)).toContain("vp i");
     }),
   );
 
@@ -172,7 +176,7 @@ describe("ProjectSetupScriptRunner", () => {
     Effect.gen(function* () {
       const { writes } = yield* setup();
       yield* TestClock.adjust("3 seconds");
-      expect(yield* Queue.take(writes)).toMatch(/^ \( vp i\r\);/);
+      expect(yield* Queue.take(writes)).toContain("vp i");
     }),
   );
 
@@ -194,7 +198,7 @@ describe("ProjectSetupScriptRunner", () => {
       yield* emit({ ...event, type: "restarted", snapshot: openSnapshot("") });
       // The restart lands inside a quiet window, so the next full window decides.
       yield* TestClock.adjust("6 seconds");
-      expect(yield* Queue.take(writes)).toMatch(/^ \( vp i\r\);/);
+      expect(yield* Queue.take(writes)).toContain("vp i");
     }),
   );
 
@@ -227,3 +231,110 @@ describe("ProjectSetupScriptRunner", () => {
       }),
   );
 });
+
+const runObservedSetup = Effect.fnUntraced(function* (command: string) {
+  const { writes, outputLines, emit, completion } = yield* setup({ command });
+  yield* emit({ ...event, type: "output", data: "\x1b[?2004h" });
+  const submitted = yield* Queue.take(writes);
+  const cwd = yield* HostProcess.WorkingDirectory;
+  const env = yield* HostProcess.Environment;
+
+  // Enter becomes a newline at the terminal boundary; transport assertions below
+  // cover line-editor submissions and canonical TTY limits that bash -c bypasses.
+  const child = yield* Effect.gen(function* () {
+    const spawner = yield* ChildProcessSpawner.ChildProcessSpawner;
+    const handle = yield* spawner.spawn(
+      ChildProcess.make(
+        "/bin/bash",
+        ["--noprofile", "--norc", "-c", submitted.replaceAll("\r", "\n")],
+        {
+          cwd,
+          env: { PATH: env.PATH },
+        },
+      ),
+    );
+    return yield* Effect.all(
+      {
+        stdout: handle.stdout.pipe(Stream.decodeText(), Stream.mkString),
+        stderr: handle.stderr.pipe(Stream.decodeText(), Stream.mkString),
+        exitCode: handle.exitCode,
+      },
+      { concurrency: "unbounded" },
+    );
+  }).pipe(Effect.scoped, Effect.provide(NodeServices.layer));
+  yield* emit({ ...event, type: "output", data: submitted + "\n" });
+  yield* emit({ ...event, type: "output", data: child.stdout + child.stderr });
+  yield* emit({ ...event, type: "closed" });
+  return {
+    submitted,
+    lines: yield* Queue.clear(outputLines),
+    exitCode: (yield* completion).exitCode,
+  };
+});
+
+describe.skipIf(HostProcess.Platform.defaultValue() === "win32")("POSIX setup completion", () => {
+  it.live("submits one line while preserving multiline shell syntax and literal text", () =>
+    Effect.gen(function* () {
+      const result = yield* runObservedSetup(
+        [
+          "printf '%s\\n' \"it's ready\" 'literal \\n $(printf wrong) `printf wrong` café'",
+          "cat <<'END'",
+          "$literal \\path 雪",
+          "END",
+          "printf '%s\\n' done # trailing comment",
+        ].join("\r\n"),
+      );
+      assert.equal(result.submitted.at(-1), "\r");
+      assert.notMatch(result.submitted.slice(0, -1), /[\r\n]/);
+      assert.deepEqual(result.lines, [
+        "it's ready",
+        "literal \\n $(printf wrong) `printf wrong` café",
+        "$literal \\path 雪",
+        "done",
+      ]);
+      assert.equal(result.exitCode, 0);
+    }),
+  );
+
+  it.live("completes long scripts without exceeding canonical terminal line limits", () =>
+    Effect.gen(function* () {
+      const expected = Array.from(
+        { length: 120 },
+        (_, index) => `line-${index}: café 雪 'quoted' \\path`,
+      );
+      const command = expected
+        .map((line) => `printf '%s\\n' "${line.replaceAll("\\", "\\\\")}"`)
+        .join("\n");
+      const utf8 = new TextEncoder();
+      assert.isAbove(utf8.encode(command).byteLength, 4_096);
+
+      const result = yield* runObservedSetup(command);
+      assert.deepEqual(result.lines, expected);
+      assert.equal(result.exitCode, 0);
+      assert.equal(result.submitted.at(-1), "\r");
+      assert.notInclude(result.submitted, "\n");
+      // The runner can write before the shell switches the terminal out of canonical mode.
+      for (const line of result.submitted.slice(0, -1).split("\r")) {
+        assert.isBelow(utf8.encode(line).byteLength, 1_024);
+      }
+    }),
+  );
+
+  it.live("preserves a trailing backslash-newline even when losing it would still exit zero", () =>
+    Effect.gen(function* () {
+      const result = yield* runObservedSetup("printf '%s\\n' direct\\\n");
+      assert.deepEqual(result.lines, ["direct"]);
+      assert.equal(result.exitCode, 0);
+    }),
+  );
+
+  it.live.each([
+    { name: "an explicit exit", command: "exit 7", exitCode: 7 },
+    { name: "a syntax error", command: "if then", exitCode: 2 },
+  ])("reports a nonzero completion for $name", ({ command, exitCode }) =>
+    Effect.gen(function* () {
+      const result = yield* runObservedSetup(command);
+      assert.equal(result.exitCode, exitCode);
+    }),
+  );
+});
diff --git a/apps/server/src/project/ProjectSetupScriptRunner.ts b/apps/server/src/project/ProjectSetupScriptRunner.ts
index a049bc89b01a1..7a5940818e0f0 100644
--- a/apps/server/src/project/ProjectSetupScriptRunner.ts
+++ b/apps/server/src/project/ProjectSetupScriptRunner.ts
@@ -177,7 +177,57 @@ function resolveCompletionShell(shell: string): CompletionShell | null {
   return "posix";
 }
 
-// A block with its own closing line protects the sentinel from trailing comments, heredocs and scripts reading stdin.
+const utf8 = new TextEncoder();
+// Canonical TTY input (before a line editor starts reading) caps a line below 1 KiB on Linux.
+const POSIX_MAX_LINE_BYTES = 1_023;
+const POSIX_APPEND = '__t3_command="$__t3_command"';
+
+function encodePosixScript(text: string): string {
+  let encoded = "";
+  for (const character of text) {
+    const code = character.charCodeAt(0);
+    if (character === "'") {
+      encoded += "'\\''";
+    } else if (character === "\\" || code <= 0x1f || code === 0x7f) {
+      encoded += `\\0${code.toString(8).padStart(3, "0")}`;
+    } else {
+      encoded += character;
+    }
+  }
+  return encoded;
+}
+
+// Line editors run each submitted line on its own, so every line must be a complete command.
+function wrapPosixCommand(command: string, sentinel: string): string {
+  const normalized = command.replace(/\r\n/g, "\n");
+  // The `_` suffix keeps trailing newlines; the subshell contains exits and syntax errors.
+  const evaluate = (payload: string) =>
+    `( __t3_command=$(printf '%b_' ${payload}); eval "\${__t3_command%_}" ); printf '\\n${sentinel}%s\\n' "$?"`;
+  const inline = evaluate(`'${encodePosixScript(normalized)}'`);
+  if (utf8.encode(` ${inline}\r`).byteLength <= POSIX_MAX_LINE_BYTES) return inline;
+
+  const maxChunkBytes = POSIX_MAX_LINE_BYTES - utf8.encode(` ${POSIX_APPEND}''\r`).byteLength;
+  const lines: string[] = [];
+  let prefix = "__t3_command=";
+  let chunk = "";
+  let chunkBytes = 0;
+  for (const character of normalized) {
+    const encoded = encodePosixScript(character);
+    const bytes = utf8.encode(encoded).byteLength;
+    if (chunkBytes + bytes > maxChunkBytes) {
+      lines.push(`${prefix}'${chunk}'`);
+      prefix = POSIX_APPEND;
+      chunk = "";
+      chunkBytes = 0;
+    }
+    chunk += encoded;
+    chunkBytes += bytes;
+  }
+  lines.push(`${prefix}'${chunk}'`, `${evaluate('"$__t3_command"')}; unset __t3_command`);
+  // The leading space gives every line the history suppression a short command gets.
+  return lines.join("\r ");
+}
+
 function wrapCommandForCompletion(
   command: string,
   shell: CompletionShell,
@@ -204,7 +254,7 @@ function wrapCommandForCompletion(
       ].join("; ");
     }
     case "posix":
-      return `( ${body}\r); printf '\\n${sentinel}%s\\n' "$?"`;
+      return wrapPosixCommand(command, sentinel);
   }
 }