diff --git a/changelog.d/fixes/6718-reasoningcontrols-case-collision.md b/changelog.d/fixes/6718-reasoningcontrols-case-collision.md
new file mode 100644
index 0000000000..4c4449755f
--- /dev/null
+++ b/changelog.d/fixes/6718-reasoningcontrols-case-collision.md
@@ -0,0 +1 @@
+- **fix(startup):** webpack build broke on case-insensitive filesystems (macOS APFS default, Windows) with a casing-collision warning plus "not exported" errors in `StudioConfigPane.tsx`/`ChatTab.tsx` (#6584) — `src/app/(dashboard)/dashboard/playground/components/ReasoningControls.tsx` (the component) and `reasoningControls.ts` (the utils module) shared the same lower-cased stem in the same directory, and two importers used the extensionless form `from "./reasoningControls"`, the exact resolution path that becomes ambiguous once casing is folded. Renamed the utils module to `reasoningControlUtils.ts` (no collision) and updated the 3 import sites. Regression guard: `tests/unit/case-collision-6584.test.ts` (scans `src/`/`open-sse/` for any same-directory, case-only filename collision). (#6584)
diff --git a/src/app/(dashboard)/dashboard/playground/components/ReasoningControls.tsx b/src/app/(dashboard)/dashboard/playground/components/ReasoningControls.tsx
index 85777f71c1..d77beeb16c 100644
--- a/src/app/(dashboard)/dashboard/playground/components/ReasoningControls.tsx
+++ b/src/app/(dashboard)/dashboard/playground/components/ReasoningControls.tsx
@@ -7,7 +7,7 @@
 // (fallback to the canonical vocabulary), resolved by `resolveReasoningControls`.
 
 import type { PlaygroundParams } from "./ParamSliders";
-import type { ReasoningControlSpec } from "./reasoningControls";
+import type { ReasoningControlSpec } from "./reasoningControlUtils";
 
 interface ReasoningControlsProps {
   spec: ReasoningControlSpec;
diff --git a/src/app/(dashboard)/dashboard/playground/components/StudioConfigPane.tsx b/src/app/(dashboard)/dashboard/playground/components/StudioConfigPane.tsx
index 0efa69f6e0..ecf79741d2 100644
--- a/src/app/(dashboard)/dashboard/playground/components/StudioConfigPane.tsx
+++ b/src/app/(dashboard)/dashboard/playground/components/StudioConfigPane.tsx
@@ -20,7 +20,7 @@ import ReasoningControls from "./ReasoningControls";
 import {
   resolveReasoningControls,
   type ReasoningControlSpec,
-} from "./reasoningControls";
+} from "./reasoningControlUtils";
 
 export interface ConfigState {
   endpoint: PlaygroundEndpoint;
diff --git a/src/app/(dashboard)/dashboard/playground/components/reasoningControls.ts b/src/app/(dashboard)/dashboard/playground/components/reasoningControlUtils.ts
similarity index 100%
rename from src/app/(dashboard)/dashboard/playground/components/reasoningControls.ts
rename to src/app/(dashboard)/dashboard/playground/components/reasoningControlUtils.ts
diff --git a/src/app/(dashboard)/dashboard/playground/components/tabs/ChatTab.tsx b/src/app/(dashboard)/dashboard/playground/components/tabs/ChatTab.tsx
index 8a0c3512f5..7e31060ee4 100644
--- a/src/app/(dashboard)/dashboard/playground/components/tabs/ChatTab.tsx
+++ b/src/app/(dashboard)/dashboard/playground/components/tabs/ChatTab.tsx
@@ -9,7 +9,7 @@ import { useStreamMetrics } from "../../hooks/useStreamMetrics";
 import { getModelPricing } from "@/lib/playground/types";
 import type { ConfigState } from "../StudioConfigPane";
 import type { StreamMetrics } from "@/shared/schemas/playground";
-import { buildReasoningRequestFields } from "../reasoningControls";
+import { buildReasoningRequestFields } from "../reasoningControlUtils";
 
 interface Message {
   role: "system" | "user" | "assistant";
diff --git a/src/app/(dashboard)/dashboard/translator/hooks/useAvailableModels.tsx b/src/app/(dashboard)/dashboard/translator/hooks/useAvailableModels.tsx
index 339c716466..0a74df0386 100644
--- a/src/app/(dashboard)/dashboard/translator/hooks/useAvailableModels.tsx
+++ b/src/app/(dashboard)/dashboard/translator/hooks/useAvailableModels.tsx
@@ -2,7 +2,7 @@
 
 import { useState, useEffect, useCallback, useMemo } from "react";
 import { compareTr } from "@/shared/utils/turkishText";
-import type { ModelReasoningCapabilities } from "@/app/(dashboard)/dashboard/playground/components/reasoningControls";
+import type { ModelReasoningCapabilities } from "@/app/(dashboard)/dashboard/playground/components/reasoningControlUtils";
 
 /**
  * Prefix-based format→model matching, used to pick a smart default
diff --git a/tests/unit/case-collision-6584.test.ts b/tests/unit/case-collision-6584.test.ts
new file mode 100644
index 0000000000..b3c3931785
--- /dev/null
+++ b/tests/unit/case-collision-6584.test.ts
@@ -0,0 +1,78 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+// Regression guard for issue #6584: webpack (OMNIROUTE_USE_TURBOPACK=0) build breaks on
+// case-insensitive filesystems (macOS APFS default, Windows) because two source
+// files in the same directory differ only by casing:
+//   src/app/(dashboard)/dashboard/playground/components/ReasoningControls.tsx
+//   src/app/(dashboard)/dashboard/playground/components/reasoningControls.ts
+// On a case-sensitive FS (Linux, this sandbox) both files resolve fine, so this
+// test does not literally reproduce the webpack warning/build error. Instead it
+// encodes the same-directory case-collision root cause as a filesystem-level
+// invariant check that fails deterministically on ANY OS: two distinct files
+// whose relative path is identical after lower-casing (stem, ignoring extension,
+// since extensionless imports are what collide) must not coexist in the same
+// directory tree.
+
+const ROOT = path.resolve(__dirname, "..", "..");
+const SCAN_DIRS = ["src", "open-sse"];
+const EXTENSIONS = new Set([".ts", ".tsx", ".js", ".jsx"]);
+const IGNORE_DIR_NAMES = new Set(["node_modules", ".next", "dist", "build", ".git"]);
+
+function walk(dir: string, out: string[]): void {
+  const entries = fs.readdirSync(dir, { withFileTypes: true });
+  for (const entry of entries) {
+    if (IGNORE_DIR_NAMES.has(entry.name)) continue;
+    const full = path.join(dir, entry.name);
+    if (entry.isDirectory()) {
+      walk(full, out);
+    } else if (EXTENSIONS.has(path.extname(entry.name))) {
+      out.push(full);
+    }
+  }
+}
+
+test("#6584: no two source files in the same directory differ only by casing", () => {
+  const files: string[] = [];
+  for (const d of SCAN_DIRS) {
+    const abs = path.join(ROOT, d);
+    if (fs.existsSync(abs)) walk(abs, files);
+  }
+
+  // Group by (directory, lower-cased basename WITHOUT extension). Extensionless
+  // imports (e.g. `from "./reasoningControls"`) are resolved by webpack/Node's
+  // module resolver trying several extensions; on a case-insensitive filesystem
+  // that resolution collapses "ReasoningControls.tsx" and "reasoningControls.ts"
+  // into the same candidate, so the stem — not the full filename — is what must
+  // stay unique per directory regardless of extension.
+  const byDirLowerStem = new Map<string, string[]>();
+  for (const f of files) {
+    const dir = path.dirname(f);
+    const ext = path.extname(f);
+    const stem = path.basename(f, ext);
+    const lowerStem = stem.toLowerCase();
+    const key = `${dir}::${lowerStem}`;
+    const arr = byDirLowerStem.get(key) ?? [];
+    arr.push(f);
+    byDirLowerStem.set(key, arr);
+  }
+
+  const collisions: string[][] = [];
+  for (const arr of byDirLowerStem.values()) {
+    const distinctStems = new Set(arr.map((f) => path.basename(f, path.extname(f))));
+    if (distinctStems.size > 1) {
+      collisions.push(arr);
+    }
+  }
+
+  assert.deepEqual(
+    collisions,
+    [],
+    `Found case-only filename collisions (breaks webpack builds on case-insensitive filesystems): ${JSON.stringify(collisions, null, 2)}`
+  );
+});
diff --git a/tests/unit/playground-reasoning-controls-6241.test.ts b/tests/unit/playground-reasoning-controls-6241.test.ts
index 318eb1e2b2..3dda9effce 100644
--- a/tests/unit/playground-reasoning-controls-6241.test.ts
+++ b/tests/unit/playground-reasoning-controls-6241.test.ts
@@ -4,7 +4,7 @@ import {
   resolveReasoningControls,
   buildReasoningRequestFields,
   type ReasoningControlSpec,
-} from "../../src/app/(dashboard)/dashboard/playground/components/reasoningControls.ts";
+} from "../../src/app/(dashboard)/dashboard/playground/components/reasoningControlUtils.ts";
 import { CANONICAL_EFFORT_VALUES } from "../../src/shared/reasoning/effortStandardization.ts";
 
 // #6241: the Playground effort selector + thinking toggle read a model's `supportsThinking` /