diff --git a/changelog.d/fixes/2540-gpt5-tools-reasoning-effort.md b/changelog.d/fixes/2540-gpt5-tools-reasoning-effort.md
new file mode 100644
index 0000000000..bdd460cfcb
--- /dev/null
+++ b/changelog.d/fixes/2540-gpt5-tools-reasoning-effort.md
@@ -0,0 +1 @@
+- **fix(openai):** strip `reasoning_effort`/`reasoning` for GPT-5.x models on the raw `openai` Chat Completions surface when the request carries function `tools` — upstream rejects that combination with HTTP 400 ("Function tools with reasoning_effort are not supported ... Please use /v1/responses instead"), and the dashboard has no `reasoning_effort:"none"` override to work around it client-side — thanks @techsolutionmta
diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index e134a744f8..ec016439c7 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -110,7 +110,10 @@ import { normalizeMimoThinking } from "../services/mimoThinking.ts";
 import { normalizeClaudeAdaptiveThinking } from "../services/claudeAdaptiveThinking.ts";
 import { normalizeClaudeHaikuConstraints } from "../services/claudeHaikuConstraints.ts";
 import { echoModelInObject } from "../services/responseModelEcho.ts";
-import { stripGpt5SamplingWhenReasoning } from "../services/gpt5SamplingGuard.ts";
+import {
+  stripGpt5SamplingWhenReasoning,
+  stripGpt5ReasoningWhenTools,
+} from "../services/gpt5SamplingGuard.ts";
 import { getUnsupportedParams, REGISTRY } from "../config/providerRegistry.ts";
 import { supportsMaxTokens } from "@/lib/modelCapabilities.ts";
 import { normalizeThinkingForModel } from "@/shared/constants/modelSpecs.ts";
@@ -2090,6 +2093,19 @@ export async function handleChatCore({
     log
   );
 
+  // GPT-5.x reasoning models (raw openai Chat Completions) also reject function `tools`
+  // combined with an active `reasoning_effort`: HTTP 400 "Function tools with
+  // reasoning_effort are not supported ... Please use /v1/responses instead." Unlike the
+  // openai-compatible-* MCP/tool_search shape (forceResponsesUpstream.ts), the plain
+  // `openai` provider always stays on /chat/completions, so strip the reasoning fields
+  // here instead of rerouting. Port of 9router#2540.
+  translatedBody = stripGpt5ReasoningWhenTools(
+    translatedBody,
+    provider,
+    finalModelToUpstream,
+    log
+  );
+
   // Rename max_tokens to max_completion_tokens if not supported (#1961)
   if (!supportsMaxTokens({ provider, model })) {
     if (translatedBody.max_tokens !== undefined) {
diff --git a/open-sse/services/gpt5SamplingGuard.ts b/open-sse/services/gpt5SamplingGuard.ts
index b5b35863fc..e96703ae2d 100644
--- a/open-sse/services/gpt5SamplingGuard.ts
+++ b/open-sse/services/gpt5SamplingGuard.ts
@@ -79,3 +79,64 @@ export function stripGpt5SamplingWhenReasoning<T extends Record<string, unknown>
   );
   return next as T;
 }
+
+const REASONING_FIELDS = ["reasoning_effort", "reasoning"] as const;
+
+/**
+ * True when the request carries a non-empty `tools` array holding at least one
+ * function-shaped tool entry (`{type:"function", ...}` or a bare `{name, ...}`
+ * without a `type`, the OpenAI Chat Completions convention).
+ */
+function hasFunctionTools(record: JsonRecord): boolean {
+  if (!Array.isArray(record.tools) || record.tools.length === 0) return false;
+  return record.tools.some((toolValue) => {
+    const tool = asRecord(toolValue);
+    if (!tool) return false;
+    const toolType = typeof tool.type === "string" ? tool.type : "";
+    return toolType === "" || toolType === "function";
+  });
+}
+
+/**
+ * Raw api.openai.com Chat Completions rejects GPT-5.x reasoning models that
+ * carry BOTH function `tools` and an active `reasoning_effort` with HTTP 400:
+ * "Function tools with reasoning_effort are not supported for <model> in
+ * /v1/chat/completions. Please use /v1/responses instead." OmniRoute's
+ * `shouldForceResponsesUpstream` guard only re-routes `openai-compatible-*`
+ * connections carrying MCP/tool_search tool shapes to `/responses` — the
+ * plain `openai` provider always stays on `/chat/completions`, so this
+ * combination still reaches the upstream 400 today. Strip the reasoning
+ * fields instead so the request succeeds on `/chat/completions` (the
+ * dashboard offers no `reasoning_effort:"none"` override, so this cannot be
+ * worked around client-side). Port of 9router#2540.
+ */
+export function stripGpt5ReasoningWhenTools<T extends Record<string, unknown>>(
+  body: T,
+  provider: string | null | undefined,
+  model: string | null | undefined,
+  log?: { warn?: (tag: string, message: string) => void } | null
+): T {
+  if (provider !== "openai") return body;
+  if (typeof model !== "string" || !/^gpt-5/i.test(model)) return body;
+
+  const record = asRecord(body);
+  if (!record) return body;
+  if (!hasFunctionTools(record)) return body;
+  if (!hasActiveReasoning(record, model)) return body;
+
+  const stripped: string[] = [];
+  for (const field of REASONING_FIELDS) {
+    if (Object.hasOwn(record, field)) stripped.push(field);
+  }
+  if (stripped.length === 0) return body;
+
+  const next: JsonRecord = { ...record };
+  for (const field of stripped) delete next[field];
+
+  log?.warn?.(
+    "PARAMS",
+    `Stripped ${stripped.join(", ")} for ${model} (function tools + reasoning_effort ` +
+      `are rejected on /v1/chat/completions; use /v1/responses instead)`
+  );
+  return next as T;
+}
diff --git a/tests/unit/gpt5-tools-reasoning-guard.test.ts b/tests/unit/gpt5-tools-reasoning-guard.test.ts
new file mode 100644
index 0000000000..410b2d734c
--- /dev/null
+++ b/tests/unit/gpt5-tools-reasoning-guard.test.ts
@@ -0,0 +1,103 @@
+/**
+ * GPT-5 tools+reasoning guard — `stripGpt5ReasoningWhenTools`.
+ *
+ * On the raw `openai` Chat Completions surface, GPT-5.x reasoning models reject a
+ * request that carries BOTH function tools and an active `reasoning_effort` with
+ * HTTP 400: "Function tools with reasoning_effort are not supported for
+ * <model> in /v1/chat/completions. Please use /v1/responses instead."
+ * (port of 9router#2540). OmniRoute's `forceResponsesUpstream` guard only fires
+ * for `openai-compatible-*` connections carrying MCP/tool_search tool shapes —
+ * the plain `openai` provider has no equivalent guard, so this scenario still
+ * reaches the upstream 400 today. Strip `reasoning_effort`/`reasoning` when
+ * function tools are present so the request succeeds on /v1/chat/completions.
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+import { stripGpt5ReasoningWhenTools } from "../../open-sse/services/gpt5SamplingGuard.ts";
+
+test("strips reasoning_effort for openai gpt-5.x when function tools are present", () => {
+  const body = {
+    model: "gpt-5.6-sol",
+    reasoning_effort: "high",
+    tools: [{ type: "function", function: { name: "read_file" } }],
+    messages: [],
+  };
+  const result = stripGpt5ReasoningWhenTools(body, "openai", "gpt-5.6-sol");
+  assert.equal(result.reasoning_effort, undefined);
+});
+
+test("strips nested reasoning.effort for openai gpt-5.x when function tools are present", () => {
+  const body = {
+    model: "gpt-5.6-sol",
+    reasoning: { effort: "medium" },
+    tools: [{ type: "function", function: { name: "read_file" } }],
+  };
+  const result = stripGpt5ReasoningWhenTools(body, "openai", "gpt-5.6-sol");
+  assert.equal(result.reasoning, undefined);
+});
+
+test("keeps reasoning_effort=none untouched (already non-reasoning mode)", () => {
+  const body = {
+    model: "gpt-5.6-sol",
+    reasoning_effort: "none",
+    tools: [{ type: "function", function: { name: "read_file" } }],
+  };
+  const result = stripGpt5ReasoningWhenTools(body, "openai", "gpt-5.6-sol");
+  assert.equal(result.reasoning_effort, "none");
+});
+
+test("keeps reasoning_effort when there are no tools", () => {
+  const body = { model: "gpt-5.6-sol", reasoning_effort: "high", messages: [] };
+  const result = stripGpt5ReasoningWhenTools(body, "openai", "gpt-5.6-sol");
+  assert.equal(result.reasoning_effort, "high");
+});
+
+test("keeps reasoning_effort when tools array is empty", () => {
+  const body = { model: "gpt-5.6-sol", reasoning_effort: "high", tools: [] };
+  const result = stripGpt5ReasoningWhenTools(body, "openai", "gpt-5.6-sol");
+  assert.equal(result.reasoning_effort, "high");
+});
+
+test("non-openai provider is untouched", () => {
+  const body = {
+    model: "gpt-5.6-sol",
+    reasoning_effort: "high",
+    tools: [{ type: "function", function: { name: "x" } }],
+  };
+  const result = stripGpt5ReasoningWhenTools(body, "codex", "gpt-5.6-sol");
+  assert.equal(result.reasoning_effort, "high");
+});
+
+test("non-gpt-5 openai model is untouched", () => {
+  const body = {
+    model: "gpt-4o",
+    reasoning_effort: "high",
+    tools: [{ type: "function", function: { name: "x" } }],
+  };
+  const result = stripGpt5ReasoningWhenTools(body, "openai", "gpt-4o");
+  assert.equal(result.reasoning_effort, "high");
+});
+
+test("returns the same reference when nothing to strip", () => {
+  const body = { model: "gpt-5.6-sol", tools: [{ type: "function" }], messages: [] };
+  const result = stripGpt5ReasoningWhenTools(body, "openai", "gpt-5.6-sol");
+  assert.equal(result, body);
+});
+
+test("logs the stripped fields when a logger is provided", () => {
+  const calls: Array<[string, string]> = [];
+  const log = { warn: (tag: string, message: string) => calls.push([tag, message]) };
+  stripGpt5ReasoningWhenTools(
+    {
+      model: "gpt-5.6-sol",
+      reasoning_effort: "high",
+      tools: [{ type: "function", function: { name: "x" } }],
+    },
+    "openai",
+    "gpt-5.6-sol",
+    log
+  );
+  assert.equal(calls.length, 1);
+  assert.equal(calls[0][0], "PARAMS");
+  assert.match(calls[0][1], /reasoning_effort/);
+});