diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3a1012b09f7..1b4842dfa6b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -103,7 +103,7 @@ jobs:
         run: |
           node --input-type=module <<'EOF'
           import { appendFileSync } from "node:fs";
-          import { listChangedExtensionIds } from "./scripts/test-extension.mjs";
+          import { listChangedExtensionIds } from "./scripts/lib/changed-extensions.mjs";
 
           const extensionIds = listChangedExtensionIds({
             base: process.env.BASE_SHA,
diff --git a/scripts/lib/changed-extensions.mjs b/scripts/lib/changed-extensions.mjs
new file mode 100644
index 00000000000..2e6763dd8c4
--- /dev/null
+++ b/scripts/lib/changed-extensions.mjs
@@ -0,0 +1,136 @@
+import { execFileSync } from "node:child_process";
+import fs from "node:fs";
+import path from "node:path";
+import { BUNDLED_PLUGIN_PATH_PREFIX, BUNDLED_PLUGIN_ROOT_DIR } from "./bundled-plugin-paths.mjs";
+
+const repoRoot = path.resolve(import.meta.dirname, "..", "..");
+
+function runGit(args, options = {}) {
+  return execFileSync("git", args, {
+    cwd: repoRoot,
+    stdio: ["ignore", "pipe", "pipe"],
+    encoding: "utf8",
+    ...options,
+  });
+}
+
+function normalizeRelative(inputPath) {
+  return inputPath.split(path.sep).join("/");
+}
+
+function hasGitCommit(ref) {
+  if (!ref || /^0+$/.test(ref)) {
+    return false;
+  }
+
+  try {
+    runGit(["rev-parse", "--verify", `${ref}^{commit}`]);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+function resolveChangedPathsBase(params = {}) {
+  const base = params.base;
+  const head = params.head ?? "HEAD";
+  const fallbackBaseRef = params.fallbackBaseRef;
+
+  if (hasGitCommit(base)) {
+    return base;
+  }
+
+  if (fallbackBaseRef) {
+    const remoteBaseRef = fallbackBaseRef.startsWith("origin/")
+      ? fallbackBaseRef
+      : `origin/${fallbackBaseRef}`;
+    if (hasGitCommit(remoteBaseRef)) {
+      const mergeBase = runGit(["merge-base", remoteBaseRef, head]).trim();
+      if (hasGitCommit(mergeBase)) {
+        return mergeBase;
+      }
+    }
+  }
+
+  if (!base) {
+    throw new Error("A git base revision is required to list changed extensions.");
+  }
+
+  throw new Error(`Git base revision is unavailable locally: ${base}`);
+}
+
+function listChangedPaths(base, head = "HEAD") {
+  if (!base) {
+    throw new Error("A git base revision is required to list changed extensions.");
+  }
+
+  return runGit(["diff", "--name-only", base, head])
+    .split("\n")
+    .map((line) => line.trim())
+    .filter((line) => line.length > 0);
+}
+
+function hasExtensionPackage(extensionId) {
+  return fs.existsSync(path.join(repoRoot, BUNDLED_PLUGIN_ROOT_DIR, extensionId, "package.json"));
+}
+
+export function listAvailableExtensionIds() {
+  const extensionsDir = path.join(repoRoot, BUNDLED_PLUGIN_ROOT_DIR);
+  if (!fs.existsSync(extensionsDir)) {
+    return [];
+  }
+
+  return fs
+    .readdirSync(extensionsDir, { withFileTypes: true })
+    .filter((entry) => entry.isDirectory())
+    .map((entry) => entry.name)
+    .filter((extensionId) => hasExtensionPackage(extensionId))
+    .toSorted((left, right) => left.localeCompare(right));
+}
+
+export function detectChangedExtensionIds(changedPaths) {
+  const extensionIds = new Set();
+
+  for (const rawPath of changedPaths) {
+    const relativePath = normalizeRelative(String(rawPath).trim());
+    if (!relativePath) {
+      continue;
+    }
+
+    const extensionMatch = relativePath.match(
+      new RegExp(`^${BUNDLED_PLUGIN_PATH_PREFIX.replace("/", "\\/")}([^/]+)(?:/|$)`),
+    );
+    if (extensionMatch) {
+      const extensionId = extensionMatch[1];
+      if (hasExtensionPackage(extensionId)) {
+        extensionIds.add(extensionId);
+      }
+      continue;
+    }
+
+    const pairedCoreMatch = relativePath.match(/^src\/([^/]+)(?:\/|$)/);
+    if (pairedCoreMatch && hasExtensionPackage(pairedCoreMatch[1])) {
+      extensionIds.add(pairedCoreMatch[1]);
+    }
+  }
+
+  return [...extensionIds].toSorted((left, right) => left.localeCompare(right));
+}
+
+export function listChangedExtensionIds(params = {}) {
+  const head = params.head ?? "HEAD";
+  const unavailableBaseBehavior = params.unavailableBaseBehavior ?? "error";
+
+  try {
+    const base = resolveChangedPathsBase(params);
+    return detectChangedExtensionIds(listChangedPaths(base, head));
+  } catch (error) {
+    if (unavailableBaseBehavior === "all") {
+      return listAvailableExtensionIds();
+    }
+    if (unavailableBaseBehavior === "empty") {
+      return [];
+    }
+    throw error;
+  }
+}
diff --git a/scripts/lib/extension-test-plan.mjs b/scripts/lib/extension-test-plan.mjs
new file mode 100644
index 00000000000..8f350db10da
--- /dev/null
+++ b/scripts/lib/extension-test-plan.mjs
@@ -0,0 +1,104 @@
+import fs from "node:fs";
+import path from "node:path";
+import { channelTestRoots } from "../../vitest.channel-paths.mjs";
+import { BUNDLED_PLUGIN_PATH_PREFIX, BUNDLED_PLUGIN_ROOT_DIR } from "./bundled-plugin-paths.mjs";
+
+const repoRoot = path.resolve(import.meta.dirname, "..", "..");
+
+function normalizeRelative(inputPath) {
+  return inputPath.split(path.sep).join("/");
+}
+
+function countTestFiles(rootPath) {
+  let total = 0;
+  const stack = [rootPath];
+
+  while (stack.length > 0) {
+    const current = stack.pop();
+    if (!current || !fs.existsSync(current)) {
+      continue;
+    }
+    for (const entry of fs.readdirSync(current, { withFileTypes: true })) {
+      const fullPath = path.join(current, entry.name);
+      if (entry.isDirectory()) {
+        if (entry.name === "node_modules" || entry.name === "dist") {
+          continue;
+        }
+        stack.push(fullPath);
+        continue;
+      }
+      if (entry.isFile() && (fullPath.endsWith(".test.ts") || fullPath.endsWith(".test.tsx"))) {
+        total += 1;
+      }
+    }
+  }
+
+  return total;
+}
+
+function resolveExtensionDirectory(targetArg, cwd = process.cwd()) {
+  if (targetArg) {
+    const asGiven = path.resolve(cwd, targetArg);
+    if (fs.existsSync(path.join(asGiven, "package.json"))) {
+      return asGiven;
+    }
+
+    const byName = path.join(repoRoot, BUNDLED_PLUGIN_ROOT_DIR, targetArg);
+    if (fs.existsSync(path.join(byName, "package.json"))) {
+      return byName;
+    }
+
+    throw new Error(
+      `Unknown extension target "${targetArg}". Use a plugin name like "slack" or a path inside the bundled plugin workspace tree.`,
+    );
+  }
+
+  let current = cwd;
+  while (true) {
+    if (
+      normalizeRelative(path.relative(repoRoot, current)).startsWith(BUNDLED_PLUGIN_PATH_PREFIX) &&
+      fs.existsSync(path.join(current, "package.json"))
+    ) {
+      return current;
+    }
+    const parent = path.dirname(current);
+    if (parent === current) {
+      break;
+    }
+    current = parent;
+  }
+
+  throw new Error(
+    "No extension target provided, and current working directory is not inside the bundled plugin workspace tree.",
+  );
+}
+
+export function resolveExtensionTestPlan(params = {}) {
+  const cwd = params.cwd ?? process.cwd();
+  const targetArg = params.targetArg;
+  const extensionDir = resolveExtensionDirectory(targetArg, cwd);
+  const extensionId = path.basename(extensionDir);
+  const relativeExtensionDir = normalizeRelative(path.relative(repoRoot, extensionDir));
+
+  const roots = [relativeExtensionDir];
+  const pairedCoreRoot = path.join(repoRoot, "src", extensionId);
+  if (fs.existsSync(pairedCoreRoot)) {
+    roots.push(normalizeRelative(path.relative(repoRoot, pairedCoreRoot)));
+  }
+
+  const usesChannelConfig = roots.some((root) => channelTestRoots.includes(root));
+  const config = usesChannelConfig ? "vitest.channels.config.ts" : "vitest.extensions.config.ts";
+  const testFileCount = roots.reduce(
+    (sum, root) => sum + countTestFiles(path.join(repoRoot, root)),
+    0,
+  );
+
+  return {
+    config,
+    extensionDir: relativeExtensionDir,
+    extensionId,
+    hasTests: testFileCount > 0,
+    roots,
+    testFileCount,
+  };
+}
diff --git a/scripts/test-extension.mjs b/scripts/test-extension.mjs
index 34258374289..5239be1f219 100644
--- a/scripts/test-extension.mjs
+++ b/scripts/test-extension.mjs
@@ -1,244 +1,16 @@
 #!/usr/bin/env node
 
-import { execFileSync, spawn } from "node:child_process";
-import fs from "node:fs";
+import { spawn } from "node:child_process";
 import path from "node:path";
 import { fileURLToPath, pathToFileURL } from "node:url";
-import { channelTestRoots } from "../vitest.channel-paths.mjs";
-import {
-  BUNDLED_PLUGIN_PATH_PREFIX,
-  BUNDLED_PLUGIN_ROOT_DIR,
-} from "./lib/bundled-plugin-paths.mjs";
+import { listAvailableExtensionIds, listChangedExtensionIds } from "./lib/changed-extensions.mjs";
+import { resolveExtensionTestPlan } from "./lib/extension-test-plan.mjs";
 
 const __filename = fileURLToPath(import.meta.url);
 const __dirname = path.dirname(__filename);
 const repoRoot = path.resolve(__dirname, "..");
 const pnpm = "pnpm";
 
-function runGit(args, options = {}) {
-  return execFileSync("git", args, {
-    cwd: repoRoot,
-    stdio: ["ignore", "pipe", "pipe"],
-    encoding: "utf8",
-    ...options,
-  });
-}
-
-function normalizeRelative(inputPath) {
-  return inputPath.split(path.sep).join("/");
-}
-
-function countTestFiles(rootPath) {
-  let total = 0;
-  const stack = [rootPath];
-
-  while (stack.length > 0) {
-    const current = stack.pop();
-    if (!current || !fs.existsSync(current)) {
-      continue;
-    }
-    for (const entry of fs.readdirSync(current, { withFileTypes: true })) {
-      const fullPath = path.join(current, entry.name);
-      if (entry.isDirectory()) {
-        if (entry.name === "node_modules" || entry.name === "dist") {
-          continue;
-        }
-        stack.push(fullPath);
-        continue;
-      }
-      if (entry.isFile() && (fullPath.endsWith(".test.ts") || fullPath.endsWith(".test.tsx"))) {
-        total += 1;
-      }
-    }
-  }
-
-  return total;
-}
-
-function hasGitCommit(ref) {
-  if (!ref || /^0+$/.test(ref)) {
-    return false;
-  }
-
-  try {
-    runGit(["rev-parse", "--verify", `${ref}^{commit}`]);
-    return true;
-  } catch {
-    return false;
-  }
-}
-
-function resolveChangedPathsBase(params = {}) {
-  const base = params.base;
-  const head = params.head ?? "HEAD";
-  const fallbackBaseRef = params.fallbackBaseRef;
-
-  if (hasGitCommit(base)) {
-    return base;
-  }
-
-  if (fallbackBaseRef) {
-    const remoteBaseRef = fallbackBaseRef.startsWith("origin/")
-      ? fallbackBaseRef
-      : `origin/${fallbackBaseRef}`;
-    if (hasGitCommit(remoteBaseRef)) {
-      const mergeBase = runGit(["merge-base", remoteBaseRef, head]).trim();
-      if (hasGitCommit(mergeBase)) {
-        return mergeBase;
-      }
-    }
-  }
-
-  if (!base) {
-    throw new Error("A git base revision is required to list changed extensions.");
-  }
-
-  throw new Error(`Git base revision is unavailable locally: ${base}`);
-}
-
-function listChangedPaths(base, head = "HEAD") {
-  if (!base) {
-    throw new Error("A git base revision is required to list changed extensions.");
-  }
-
-  return runGit(["diff", "--name-only", base, head])
-    .split("\n")
-    .map((line) => line.trim())
-    .filter((line) => line.length > 0);
-}
-
-function hasExtensionPackage(extensionId) {
-  return fs.existsSync(path.join(repoRoot, BUNDLED_PLUGIN_ROOT_DIR, extensionId, "package.json"));
-}
-
-export function listAvailableExtensionIds() {
-  const extensionsDir = path.join(repoRoot, BUNDLED_PLUGIN_ROOT_DIR);
-  if (!fs.existsSync(extensionsDir)) {
-    return [];
-  }
-
-  return fs
-    .readdirSync(extensionsDir, { withFileTypes: true })
-    .filter((entry) => entry.isDirectory())
-    .map((entry) => entry.name)
-    .filter((extensionId) => hasExtensionPackage(extensionId))
-    .toSorted((left, right) => left.localeCompare(right));
-}
-
-export function detectChangedExtensionIds(changedPaths) {
-  const extensionIds = new Set();
-
-  for (const rawPath of changedPaths) {
-    const relativePath = normalizeRelative(String(rawPath).trim());
-    if (!relativePath) {
-      continue;
-    }
-
-    const extensionMatch = relativePath.match(
-      new RegExp(`^${BUNDLED_PLUGIN_PATH_PREFIX.replace("/", "\\/")}([^/]+)(?:/|$)`),
-    );
-    if (extensionMatch) {
-      const extensionId = extensionMatch[1];
-      if (hasExtensionPackage(extensionId)) {
-        extensionIds.add(extensionId);
-      }
-      continue;
-    }
-
-    const pairedCoreMatch = relativePath.match(/^src\/([^/]+)(?:\/|$)/);
-    if (pairedCoreMatch && hasExtensionPackage(pairedCoreMatch[1])) {
-      extensionIds.add(pairedCoreMatch[1]);
-    }
-  }
-
-  return [...extensionIds].toSorted((left, right) => left.localeCompare(right));
-}
-
-export function listChangedExtensionIds(params = {}) {
-  const head = params.head ?? "HEAD";
-  const unavailableBaseBehavior = params.unavailableBaseBehavior ?? "error";
-
-  try {
-    const base = resolveChangedPathsBase(params);
-    return detectChangedExtensionIds(listChangedPaths(base, head));
-  } catch (error) {
-    if (unavailableBaseBehavior === "all") {
-      return listAvailableExtensionIds();
-    }
-    if (unavailableBaseBehavior === "empty") {
-      return [];
-    }
-    throw error;
-  }
-}
-
-function resolveExtensionDirectory(targetArg, cwd = process.cwd()) {
-  if (targetArg) {
-    const asGiven = path.resolve(cwd, targetArg);
-    if (fs.existsSync(path.join(asGiven, "package.json"))) {
-      return asGiven;
-    }
-
-    const byName = path.join(repoRoot, BUNDLED_PLUGIN_ROOT_DIR, targetArg);
-    if (fs.existsSync(path.join(byName, "package.json"))) {
-      return byName;
-    }
-
-    throw new Error(
-      `Unknown extension target "${targetArg}". Use a plugin name like "slack" or a path inside the bundled plugin workspace tree.`,
-    );
-  }
-
-  let current = cwd;
-  while (true) {
-    if (
-      normalizeRelative(path.relative(repoRoot, current)).startsWith(BUNDLED_PLUGIN_PATH_PREFIX) &&
-      fs.existsSync(path.join(current, "package.json"))
-    ) {
-      return current;
-    }
-    const parent = path.dirname(current);
-    if (parent === current) {
-      break;
-    }
-    current = parent;
-  }
-
-  throw new Error(
-    "No extension target provided, and current working directory is not inside the bundled plugin workspace tree.",
-  );
-}
-
-export function resolveExtensionTestPlan(params = {}) {
-  const cwd = params.cwd ?? process.cwd();
-  const targetArg = params.targetArg;
-  const extensionDir = resolveExtensionDirectory(targetArg, cwd);
-  const extensionId = path.basename(extensionDir);
-  const relativeExtensionDir = normalizeRelative(path.relative(repoRoot, extensionDir));
-
-  const roots = [relativeExtensionDir];
-  const pairedCoreRoot = path.join(repoRoot, "src", extensionId);
-  if (fs.existsSync(pairedCoreRoot)) {
-    const pairedRelativeRoot = normalizeRelative(path.relative(repoRoot, pairedCoreRoot));
-    roots.push(pairedRelativeRoot);
-  }
-
-  const usesChannelConfig = roots.some((root) => channelTestRoots.includes(root));
-  const config = usesChannelConfig ? "vitest.channels.config.ts" : "vitest.extensions.config.ts";
-  const testFileCount = roots.reduce(
-    (sum, root) => sum + countTestFiles(path.join(repoRoot, root)),
-    0,
-  );
-  return {
-    config,
-    extensionDir: relativeExtensionDir,
-    extensionId,
-    hasTests: testFileCount > 0,
-    roots,
-    testFileCount,
-  };
-}
-
 async function runVitestBatch(params) {
   return await new Promise((resolve, reject) => {
     const child = spawn(
diff --git a/test/scripts/test-extension.test.ts b/test/scripts/test-extension.test.ts
index 136817e112f..c325c8e2a2f 100644
--- a/test/scripts/test-extension.test.ts
+++ b/test/scripts/test-extension.test.ts
@@ -5,8 +5,8 @@ import {
   detectChangedExtensionIds,
   listAvailableExtensionIds,
   listChangedExtensionIds,
-  resolveExtensionTestPlan,
-} from "../../scripts/test-extension.mjs";
+} from "../../scripts/lib/changed-extensions.mjs";
+import { resolveExtensionTestPlan } from "../../scripts/lib/extension-test-plan.mjs";
 import { bundledPluginFile, bundledPluginRoot } from "../helpers/bundled-plugin-paths.js";
 
 const scriptPath = path.join(process.cwd(), "scripts", "test-extension.mjs");
@@ -55,7 +55,7 @@ describe("scripts/test-extension.mjs", () => {
     expect(plan.hasTests).toBe(true);
   });
 
-  it("keeps extension-root plans lean when there is no paired core test root", () => {
+  it("omits src/<extension> when no paired core root exists", () => {
     const plan = resolveExtensionTestPlan({ targetArg: "line", cwd: process.cwd() });
 
     expect(plan.roots).toContain(bundledPluginRoot("line"));