diff --git a/src/agent/prompt-sizes.test.ts b/src/agent/prompt-sizes.test.ts
new file mode 100644
index 000000000..71bbf9be0
--- /dev/null
+++ b/src/agent/prompt-sizes.test.ts
@@ -0,0 +1,177 @@
+import { describe, expect, test } from "bun:test";
+import { DIRECTOR_REGISTRY } from "./directors/registry.js";
+import { DIRECTOR_IDS, type DirectorId } from "./directors/types.js";
+import {
+  canonicalToolNamesForDirector,
+  directorPromptSizeTable,
+  formatPromptSizeTable,
+  type PromptSizeFamily,
+} from "./prompt-sizes.js";
+
+/**
+ * Prompt size budget (CL-7664). Numeric asserts only — copy edits must not
+ * fail this test. Baselines were captured from the canonical fixture in
+ * src/agent/prompt-sizes.ts with a +2000 char / +3000 byte allowance; bytes
+ * get the larger headroom because multibyte copy can shift them faster.
+ */
+const CHAR_BUDGET: Record<DirectorId, number> = {
+  skywalker: 27000,
+  builder: 48800,
+  explorer: 14200,
+  counsel: 52700,
+  intern: 16800,
+  critic: 54400,
+  greybeard: 53600,
+  neckbeard: 72300,
+  bruckheimer: 23200,
+  gaasbot: 31700,
+  draper: 15100,
+  emil: 16600,
+  rand: 15000,
+  shakespeare: 54700,
+  testsmith: 16200,
+  tester: 13900,
+};
+
+const BYTE_BUDGET: Record<DirectorId, number> = {
+  skywalker: 28100,
+  builder: 50000,
+  explorer: 15200,
+  counsel: 53900,
+  intern: 17800,
+  critic: 55500,
+  greybeard: 54800,
+  neckbeard: 73400,
+  bruckheimer: 24300,
+  gaasbot: 32800,
+  draper: 16200,
+  emil: 17700,
+  rand: 16100,
+  shakespeare: 55900,
+  testsmith: 17200,
+  tester: 15000,
+};
+
+function budgetMessage(
+  directorId: DirectorId,
+  family: PromptSizeFamily,
+  chars: number,
+  bytes: number,
+): string {
+  return (
+    `Director "${directorId}" [${family}]: ${chars} chars / ${bytes} bytes ` +
+    `exceeds budget (${CHAR_BUDGET[directorId]} chars / ` +
+    `${BYTE_BUDGET[directorId]} bytes). Trim the prompt (preferred) or ` +
+    `consciously raise the budget here with justification. ` +
+    `Repro: bun -e 'import { directorPromptSizeTable, ` +
+    `formatPromptSizeTable } from "./src/agent/prompt-sizes.ts"; ` +
+    `console.log(formatPromptSizeTable(directorPromptSizeTable()))'.`
+  );
+}
+
+describe("director prompt size budget", () => {
+  const rows = directorPromptSizeTable();
+
+  test("covers every director in both families", () => {
+    expect(rows.length).toBe(DIRECTOR_IDS.length * 2);
+    for (const directorId of DIRECTOR_IDS) {
+      for (const family of ["default", "grok"] as const) {
+        expect(
+          rows.some((r) => r.directorId === directorId && r.family === family),
+        ).toBe(true);
+      }
+    }
+  });
+
+  test("every assembled prompt stays within budget", () => {
+    for (const row of rows) {
+      const overChars = row.chars > CHAR_BUDGET[row.directorId];
+      const overBytes = row.bytes > BYTE_BUDGET[row.directorId];
+      expect(
+        overChars || overBytes,
+        budgetMessage(row.directorId, row.family, row.chars, row.bytes),
+      ).toBe(false);
+    }
+  });
+
+  test("every assembled prompt is a real prompt, not an empty assembly", () => {
+    for (const row of rows) {
+      expect(row.chars).toBeGreaterThan(5000);
+      expect(row.bytes).toBeGreaterThanOrEqual(row.chars);
+    }
+  });
+
+  test("grok family never shrinks a prompt; only leaves grow", () => {
+    for (const directorId of DIRECTOR_IDS) {
+      const base = rows.find(
+        (r) => r.directorId === directorId && r.family === "default",
+      );
+      const grok = rows.find(
+        (r) => r.directorId === directorId && r.family === "grok",
+      );
+      expect(grok?.chars ?? 0).toBeGreaterThanOrEqual(base?.chars ?? 0);
+      if (DIRECTOR_REGISTRY[directorId].spawn.maySpawn) {
+        expect(grok?.chars).toBe(base?.chars);
+      } else {
+        expect(grok?.chars ?? 0).toBeGreaterThan(base?.chars ?? 0);
+      }
+    }
+  });
+
+  test("measurement is deterministic", () => {
+    const again = directorPromptSizeTable();
+    expect(again.map((r) => r.chars)).toEqual(rows.map((r) => r.chars));
+    expect(again.map((r) => r.bytes)).toEqual(rows.map((r) => r.bytes));
+  });
+
+  test("tool names match the production mount: no dupes, no phantoms", () => {
+    for (const directorId of DIRECTOR_IDS) {
+      for (const family of ["default", "grok"] as const) {
+        const names = canonicalToolNamesForDirector(
+          DIRECTOR_REGISTRY[directorId],
+          family,
+        );
+        expect(new Set(names).size, `${directorId} [${family}]`).toBe(
+          names.length,
+        );
+        // Neither fixture family is Codex, so the Codex proxies
+        // (createCodexToolProxies returns [] when !isCodex) must be absent,
+        // as must list_dir, which no subagent mount installs.
+        for (const phantom of [
+          "list_dir",
+          "apply_patch",
+          "shell",
+          "update_plan",
+        ]) {
+          expect(names, `${directorId} [${family}]`).not.toContain(phantom);
+        }
+        if (DIRECTOR_REGISTRY[directorId].spawn.maySpawn) {
+          for (const verb of ["spawn_agent", "send_input"]) {
+            expect(
+              names.filter((n) => n === verb).length,
+              `${directorId} [${family}] mounts ${verb} once`,
+            ).toBe(1);
+          }
+        }
+      }
+    }
+  });
+
+  test("formatPromptSizeTable renders one row per director", () => {
+    const table = formatPromptSizeTable(rows);
+    expect(table).toContain(
+      "| director | default chars (bytes) | grok chars (bytes) |",
+    );
+    for (const directorId of DIRECTOR_IDS) {
+      const base = rows.find(
+        (r) => r.directorId === directorId && r.family === "default",
+      );
+      const grok = rows.find(
+        (r) => r.directorId === directorId && r.family === "grok",
+      );
+      expect(table).toContain(
+        `| ${directorId} | ${base?.chars} (${base?.bytes}) | ${grok?.chars} (${grok?.bytes}) |`,
+      );
+    }
+  });
+});
diff --git a/src/agent/prompt-sizes.ts b/src/agent/prompt-sizes.ts
new file mode 100644
index 000000000..c0eaad44a
--- /dev/null
+++ b/src/agent/prompt-sizes.ts
@@ -0,0 +1,208 @@
+import { TOOL_NAMES } from "@intx/tools-posix";
+import { LSP_TOOL_DEFINITION } from "@intx/tools-lsp";
+import type { EnvironmentInfo } from "./environment.js";
+import {
+  DIRECTOR_REGISTRY,
+  packageToCapabilities,
+} from "./directors/registry.js";
+import { formatDirectorSystemPrompt } from "./directors/identity.js";
+import {
+  DIRECTOR_IDS,
+  type DirectorId,
+  type DirectorPackage,
+} from "./directors/types.js";
+import { buildSubAgentSystemPrompt } from "./prompts.js";
+import { shouldApplyGrokAntiThrash } from "../subagent/provider-family.js";
+import { isCodexProviderName } from "../config/codex-providers.js";
+import { shellCollectDefinition } from "./background-shell-tool.js";
+import {
+  applyPatchDefinition,
+  shellDefinition,
+  updatePlanDefinition,
+} from "./codex-tool-proxies.js";
+import { manageTasksDefinition } from "./tasks.js";
+import { DELETE_FILE_DEFINITION } from "../plugins/delete-file-plugin.js";
+import { webFetchDefinition } from "../tools/web-fetch.js";
+import { webSearchDefinition } from "../tools/web-search.js";
+
+/**
+ * Canonical prompt-size fixture (CL-7664).
+ *
+ * Assembles each director prompt exactly as src/subagent/run.ts does:
+ * extensions=[director systemPromptRole] + environment + tools +
+ * appendix, with the Grok finish-bias note gated by
+ * shouldApplyGrokAntiThrash (leaves on Grok-family providers only).
+ *
+ * The env and provider inputs are pinned here so sizes never drift with the
+ * machine, date, or checkout — only real prompt changes move the numbers.
+ */
+export const CANONICAL_PROMPT_ENV: EnvironmentInfo = {
+  cwd: "/repo",
+  platform: "Darwin 25.0.0",
+  arch: "arm64",
+  runtime: "Bun 1.2.0",
+  date: new Date("2026-01-15T12:00:00Z"),
+  isGitRepo: true,
+  gitBranch: "main",
+  gitDirtyCount: 0,
+  topLevel: "AGENTS.md  CONTRIBUTING.md  src/  tests/  docs/  plugins/",
+};
+
+const GROK_PROVIDER = { providerName: "xai/default", model: "grok-4.6" };
+const DEFAULT_PROVIDER = {
+  providerName: "anthropic",
+  model: "claude-sonnet-4",
+};
+
+/** Families in the size table: default assembly vs Grok (+finish-bias note). */
+export type PromptSizeFamily = "default" | "grok";
+
+/**
+ * Pre-filter mount names in run.ts install order: posix base (TOOL_NAMES,
+ * shared with createPosixTools) + delete_file / lsp plugin tools
+ * (buildCorePosixToolPlugins) + core web tools (coreSubAgentWebTools) +
+ * shell_collect (run.ts:678-694). Codex proxies (apply_patch, shell,
+ * update_plan) join only when isCodex — createCodexToolProxies returns []
+ * otherwise (run.ts:708-718, codex-tool-proxies.ts:163-166).
+ */
+function preFilterMountNames(isCodex: boolean): readonly string[] {
+  return [
+    ...Object.values(TOOL_NAMES),
+    DELETE_FILE_DEFINITION.name,
+    LSP_TOOL_DEFINITION.name,
+    webFetchDefinition.name,
+    webSearchDefinition.name,
+    shellCollectDefinition.name,
+    ...(isCodex
+      ? [
+          applyPatchDefinition.name,
+          shellDefinition.name,
+          updatePlanDefinition.name,
+        ]
+      : []),
+  ];
+}
+
+/**
+ * Canonical tool names per director, assembled exactly as run.ts mounts them:
+ * the pre-filter set above narrowed by the package capability filter (the
+ * same packageToCapabilities agent-fleet dispatches with; allow keeps only
+ * mounted names, exclude drops denials — run.ts:720-722), then manage_tasks
+ * (run.ts:727-736), leaf-only submit_result + ask_director (run.ts:740-785),
+ * then orchestrator fleet tools with Tier-1-only search_agents
+ * (run.ts:791-918, tier gate at 796-797). Allowlist entries that name no
+ * mounted tool (list_dir, fleet verbs, off-family Codex proxies) fall out at
+ * the filter instead of inflating the prompt.
+ */
+export function canonicalToolNamesForDirector(
+  pkg: DirectorPackage,
+  family: PromptSizeFamily,
+): readonly string[] {
+  const providerName =
+    family === "grok"
+      ? GROK_PROVIDER.providerName
+      : DEFAULT_PROVIDER.providerName;
+  const filtered = [...preFilterMountNames(isCodexProviderName(providerName))];
+  const capabilities = packageToCapabilities(pkg);
+  const names =
+    capabilities === undefined
+      ? filtered
+      : capabilities.mode === "allow"
+        ? filtered.filter((name) => capabilities.tools.includes(name))
+        : filtered.filter((name) => !capabilities.tools.includes(name));
+  names.push(manageTasksDefinition.name);
+  if (pkg.tier === "leaf") {
+    names.push("submit_result", "ask_director");
+  }
+  if (pkg.spawn.maySpawn) {
+    if (pkg.tier === "orchestrator") names.push("search_agents");
+    names.push(
+      "read_agent_trace",
+      "spawn_agent",
+      "wait_agents",
+      "list_agents",
+      "close_agent",
+      "resume_agent",
+      "interrupt_agent",
+      "send_input",
+    );
+  }
+  const dupe = names.find((name, index) => names.indexOf(name) !== index);
+  if (dupe !== undefined) {
+    throw new Error(
+      `canonicalToolNamesForDirector(${pkg.id}): "${dupe}" mounted twice — the assembly drifted from src/subagent/run.ts`,
+    );
+  }
+  return names;
+}
+
+/** Assemble one director prompt exactly as run.ts does. */
+export function assembleDirectorPrompt(
+  directorId: DirectorId,
+  family: PromptSizeFamily,
+): string {
+  const pkg = DIRECTOR_REGISTRY[directorId];
+  const orchestrator = pkg.spawn.maySpawn;
+  const provider = family === "grok" ? GROK_PROVIDER : DEFAULT_PROVIDER;
+  return buildSubAgentSystemPrompt(
+    [formatDirectorSystemPrompt(pkg)],
+    CANONICAL_PROMPT_ENV,
+    undefined,
+    {
+      orchestrator,
+      toolNames: canonicalToolNamesForDirector(pkg, family),
+      grokAntiThrash: shouldApplyGrokAntiThrash({ ...provider, orchestrator }),
+    },
+  );
+}
+
+export interface DirectorPromptSize {
+  directorId: DirectorId;
+  family: PromptSizeFamily;
+  chars: number;
+  bytes: number;
+}
+
+export function measureDirectorPrompt(
+  directorId: DirectorId,
+  family: PromptSizeFamily,
+): DirectorPromptSize {
+  const prompt = assembleDirectorPrompt(directorId, family);
+  return {
+    directorId,
+    family,
+    chars: prompt.length,
+    bytes: Buffer.byteLength(prompt, "utf8"),
+  };
+}
+
+/** Full per-director x per-family size table. */
+export function directorPromptSizeTable(): DirectorPromptSize[] {
+  const rows: DirectorPromptSize[] = [];
+  for (const directorId of DIRECTOR_IDS) {
+    for (const family of ["default", "grok"] as const) {
+      rows.push(measureDirectorPrompt(directorId, family));
+    }
+  }
+  return rows;
+}
+
+/** Render the size table as markdown (for PR bodies and budget updates). */
+export function formatPromptSizeTable(rows: DirectorPromptSize[]): string {
+  const lines = [
+    "| director | default chars (bytes) | grok chars (bytes) |",
+    "| --- | --- | --- |",
+  ];
+  for (const directorId of DIRECTOR_IDS) {
+    const base = rows.find(
+      (r) => r.directorId === directorId && r.family === "default",
+    );
+    const grok = rows.find(
+      (r) => r.directorId === directorId && r.family === "grok",
+    );
+    lines.push(
+      `| ${directorId} | ${base?.chars} (${base?.bytes}) | ${grok?.chars} (${grok?.bytes}) |`,
+    );
+  }
+  return lines.join("\n");
+}
diff --git a/src/plugins/delete-file-plugin.ts b/src/plugins/delete-file-plugin.ts
index e5e011229..0b7905213 100644
--- a/src/plugins/delete-file-plugin.ts
+++ b/src/plugins/delete-file-plugin.ts
@@ -7,7 +7,7 @@ import { formatChangeDiff } from "./change-diff.js";
 
 const DeleteFileArgs = type({ path: "string>0" });
 
-const DELETE_FILE_DEFINITION = {
+export const DELETE_FILE_DEFINITION = {
   name: "delete_file",
   description:
     "Delete one file. Returns success when the file is deleted or already absent. Refuses directories; use this instead of shell rm for file deletion.",