diff --git a/apps/daemon/src/runtimes/defs/grok-build.ts b/apps/daemon/src/runtimes/defs/grok-build.ts
index 404cbf7f90..bc8a3576ee 100644
--- a/apps/daemon/src/runtimes/defs/grok-build.ts
+++ b/apps/daemon/src/runtimes/defs/grok-build.ts
@@ -49,11 +49,10 @@ export const grokBuildAgentDef = {
       label: 'grok-4.20-multi-agent (xAI · orchestration)',
     },
   ],
-  // Prompt delivered via stdin so Windows `spawn ENAMETOOLONG` and Linux
-  // `spawn E2BIG` can't truncate large composed prompts. `grok -p` with
-  // no positional argument reads from piped stdin.
-  buildArgs: (_prompt, _imagePaths, _extra = [], options = {}) => {
-    const args = ['-p'];
+  // Grok Build CLI v0.1.212 enforces `-p, --single <PROMPT>` as value-
+  // required — stdin piping no longer satisfies it. Inline the prompt.
+  buildArgs: (prompt, _imagePaths, _extra = [], options = {}) => {
+    const args = ['-p', prompt];
     if (options.model && options.model !== DEFAULT_MODEL_OPTION.id) {
       args.push('--model', options.model);
     }
@@ -69,7 +68,21 @@ export const grokBuildAgentDef = {
     { id: 'xhigh', label: 'xhigh' },
     { id: 'max', label: 'max' },
   ],
-  promptViaStdin: true,
+  promptViaStdin: false,
+  // Guard against prompts that would blow Windows' ~32 KB CreateProcess
+  // limit (or Linux MAX_ARG_STRLEN on extreme edges) before spawn. Same
+  // shape as the DeepSeek adapter — the previous stdin path is gone (CLI
+  // 0.1.212 enforces `-p <value>`), so the composed prompt now rides
+  // argv and a sufficiently large one — system text + history + skills/
+  // design-system content + user message — could surface as a generic
+  // spawn ENAMETOOLONG / E2BIG instead of a Grok-specific, user-
+  // actionable message. The /api/chat spawn path checks this byte
+  // budget against the composed prompt and emits AGENT_PROMPT_TOO_LARGE
+  // ("reduce skills/design-system context, or pick an adapter with
+  // stdin support") before calling `spawn`. 30_000 bytes leaves ~2.7 KB
+  // of argv headroom under the Windows command-line limit for `-p
+  // --model <id> --effort <level>` and internal quoting.
+  maxPromptArgBytes: 30_000,
   streamFormat: 'plain',
   installUrl: 'https://x.ai/cli',
   docsUrl: 'https://x.ai/cli',
diff --git a/apps/daemon/src/runtimes/prompt-budget.ts b/apps/daemon/src/runtimes/prompt-budget.ts
index ed8fa333e6..7dfc6ded43 100644
--- a/apps/daemon/src/runtimes/prompt-budget.ts
+++ b/apps/daemon/src/runtimes/prompt-budget.ts
@@ -10,6 +10,12 @@ function promptArgvBudgetMessage(
       'Reduce the selected skills/design-system context or conversation length, or use DeepSeek through an API/provider model connection for large contexts. Pick a stdin-capable adapter when the prompt must include large local context.'
     );
   }
+  if (def.id === 'grok-build') {
+    return (
+      `${def.name} requires the prompt as the value of -p / --single (xAI CLI 0.1.212+ no longer reads piped stdin), and this run's composed prompt exceeds the safe size (${bytes} > ${def.maxPromptArgBytes} bytes). ` +
+      'Reduce the selected skills/design-system context or conversation length, or pick an adapter with stdin support (e.g. claude, codex, hermes) when the prompt must include large local context.'
+    );
+  }
   return (
     `${def.name} requires the prompt as a command-line argument and this run's composed prompt exceeds the safe size (${bytes} > ${def.maxPromptArgBytes} bytes). ` +
     'Reduce the selected skills/design-system context, shorten the conversation, or pick an adapter with stdin support.'
diff --git a/apps/daemon/tests/runtimes/helpers/test-helpers.ts b/apps/daemon/tests/runtimes/helpers/test-helpers.ts
index 346f463108..4ed3249052 100644
--- a/apps/daemon/tests/runtimes/helpers/test-helpers.ts
+++ b/apps/daemon/tests/runtimes/helpers/test-helpers.ts
@@ -86,6 +86,7 @@ export const gemini = requireAgent('gemini');
 export const qoder = requireAgent('qoder');
 export const qwen = requireAgent('qwen');
 export const opencode = requireAgent('opencode');
+export const grokBuild = requireAgent('grok-build');
 export const deepseekMaxPromptArgBytes = (() => {
   assert.ok(
     deepseek.maxPromptArgBytes !== undefined,
@@ -93,6 +94,13 @@ export const deepseekMaxPromptArgBytes = (() => {
   );
   return deepseek.maxPromptArgBytes;
 })();
+export const grokBuildMaxPromptArgBytes = (() => {
+  assert.ok(
+    grokBuild.maxPromptArgBytes !== undefined,
+    'grok-build must define maxPromptArgBytes for argv budget tests',
+  );
+  return grokBuild.maxPromptArgBytes;
+})();
 const originalDisablePlugins = process.env.OD_CODEX_DISABLE_PLUGINS;
 const originalPath = process.env.PATH;
 const originalHome = process.env.HOME;
diff --git a/apps/daemon/tests/runtimes/prompt-budget.test.ts b/apps/daemon/tests/runtimes/prompt-budget.test.ts
index f3d5e56c32..c445d3a033 100644
--- a/apps/daemon/tests/runtimes/prompt-budget.test.ts
+++ b/apps/daemon/tests/runtimes/prompt-budget.test.ts
@@ -1,6 +1,6 @@
 import { test } from 'vitest';
 import {
-  assert, checkPromptArgvBudget, checkWindowsCmdShimCommandLineBudget, checkWindowsDirectExeCommandLineBudget, claude, deepseek, deepseekMaxPromptArgBytes, vibe,
+  assert, checkPromptArgvBudget, checkWindowsCmdShimCommandLineBudget, checkWindowsDirectExeCommandLineBudget, claude, deepseek, deepseekMaxPromptArgBytes, grokBuild, grokBuildMaxPromptArgBytes, vibe,
 } from './helpers/test-helpers.js';
 import type { TestAgentDef } from './helpers/test-helpers.js';
 
@@ -107,6 +107,64 @@ test('checkPromptArgvBudget gives DeepSeek-specific guidance for large contexts'
   assert.match(flagged.message, /stdin-capable adapter/);
 });
 
+// Grok Build CLI 0.1.212+ enforces `-p, --single <PROMPT>` as value-
+// required, so the prompt rides argv just like DeepSeek. Pin the budget
+// field and the byte-vs-codepoint guard so a future runtime-def edit
+// can't silently drop the guard or let it drift over the Windows
+// CreateProcess limit.
+test('grok-build declares a conservative argv-byte budget for the prompt', () => {
+  assert.equal(
+    typeof grokBuildMaxPromptArgBytes,
+    'number',
+    'grok-build must set maxPromptArgBytes so the spawn path can pre-flight oversized prompts before hitting CreateProcess / E2BIG',
+  );
+  assert.ok(
+    grokBuildMaxPromptArgBytes > 0 && grokBuildMaxPromptArgBytes < 32_768,
+    `grokBuildMaxPromptArgBytes must stay strictly under the Windows CreateProcess limit (~32 KB); got ${grokBuildMaxPromptArgBytes}`,
+  );
+});
+
+test('checkPromptArgvBudget flags oversized Grok Build prompts and lets short prompts through', () => {
+  const oversized = 'x'.repeat(grokBuildMaxPromptArgBytes + 1);
+  const flagged = checkPromptArgvBudget(grokBuild, oversized);
+  assert.ok(flagged, 'oversized prompts must trip the argv-byte guard');
+  assert.equal(flagged.code, 'AGENT_PROMPT_TOO_LARGE');
+  assert.equal(flagged.limit, grokBuildMaxPromptArgBytes);
+  assert.equal(flagged.bytes, grokBuildMaxPromptArgBytes + 1);
+  assert.match(flagged.message, /Grok Build/);
+  assert.match(flagged.message, /-p \/ --single/);
+  assert.match(flagged.message, /stdin/);
+
+  // Happy path: chat must keep working for normal-sized prompts.
+  assert.equal(checkPromptArgvBudget(grokBuild, 'hello'), null);
+
+  // Exact-budget edge: at-limit prompts pass; guard fires only on strict
+  // overrun.
+  const atLimit = 'x'.repeat(grokBuildMaxPromptArgBytes);
+  assert.equal(checkPromptArgvBudget(grokBuild, atLimit), null);
+
+  // Multi-byte UTF-8 (CJK = 3 bytes) must be byte-counted, not code-
+  // point-counted — mirrors the DeepSeek byte-count regression guard.
+  const cjkOversized = '汉'.repeat(
+    Math.ceil(grokBuildMaxPromptArgBytes / 3) + 1,
+  );
+  const cjkFlagged = checkPromptArgvBudget(grokBuild, cjkOversized);
+  assert.ok(cjkFlagged, 'byte-counted UTF-8 prompts must also trip the guard');
+  assert.equal(cjkFlagged.code, 'AGENT_PROMPT_TOO_LARGE');
+});
+
+test('checkPromptArgvBudget gives Grok-Build-specific guidance for large contexts', () => {
+  const oversized = 'x'.repeat(grokBuildMaxPromptArgBytes + 1);
+  const flagged = checkPromptArgvBudget(grokBuild, oversized);
+
+  assert.ok(flagged, 'oversized Grok Build prompts must return a diagnostic');
+  assert.match(flagged.message, /Grok Build/);
+  assert.match(flagged.message, /-p \/ --single/);
+  assert.match(flagged.message, /xAI CLI 0\.1\.212\+/);
+  assert.match(flagged.message, /no longer reads piped stdin/);
+  assert.match(flagged.message, /stdin support/);
+});
+
 // Adapters that ship the prompt over stdin (every other code agent
 // today) don't declare `maxPromptArgBytes` and must skip the guard
 // entirely — applying it to them would refuse perfectly valid huge