diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index daf0d1c79..104a1ba4c 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed OpenAI Responses/Codex tool schema normalization stripping provider-rejected regex lookaround patterns from MCP tool parameter schemas. ([#2784](https://github.com/can1357/oh-my-pi/issues/2784))
+
 ## [16.0.2] - 2026-06-16
 
 ### Added
diff --git a/packages/ai/src/utils/schema/normalize.ts b/packages/ai/src/utils/schema/normalize.ts
index 7d16b7ebd..32e3791f3 100644
--- a/packages/ai/src/utils/schema/normalize.ts
+++ b/packages/ai/src/utils/schema/normalize.ts
@@ -936,6 +936,22 @@ export function sanitizeSchemaForOpenAIResponses(schema: JsonObject): JsonObject
  * `normalizeSchemaFor*` dispatcher naming used elsewhere in this module.
  */
 export const normalizeSchemaForOpenAIResponses: (schema: JsonObject) => JsonObject = sanitizeSchemaForOpenAIResponses;
+const OPENAI_UNSUPPORTED_REGEX_LOOKAROUNDS = new Set(["=", "!", "<=", "<!"]);
+
+function hasOpenAIUnsupportedRegexLookaround(pattern: string): boolean {
+	let groupStart = pattern.indexOf("(?");
+	while (groupStart !== -1) {
+		let escapes = 0;
+		for (let i = groupStart - 1; i >= 0 && pattern[i] === "\\"; i--) escapes++;
+		if (escapes % 2 === 0) {
+			const operator =
+				pattern[groupStart + 2] === "<" ? pattern.slice(groupStart + 2, groupStart + 4) : pattern[groupStart + 2];
+			if (OPENAI_UNSUPPORTED_REGEX_LOOKAROUNDS.has(operator)) return true;
+		}
+		groupStart = pattern.indexOf("(?", groupStart + 2);
+	}
+	return false;
+}
 
 function normalizeOpenAIResponsesSchemaNode(value: unknown, cache: WeakMap<JsonObject, JsonObject>): unknown {
 	if (!isJsonObject(value)) return value;
@@ -973,6 +989,14 @@ function normalizeOpenAIResponsesSchemaNode(value: unknown, cache: WeakMap<JsonO
 			changed = true;
 			continue;
 		}
+		if (
+			key === "pattern" &&
+			typeof value.pattern === "string" &&
+			hasOpenAIUnsupportedRegexLookaround(value.pattern)
+		) {
+			changed = true;
+			continue;
+		}
 
 		const child = value[key];
 		let next: unknown = child;
diff --git a/packages/ai/test/openai-codex.test.ts b/packages/ai/test/openai-codex.test.ts
index 2dde67365..76a1b708b 100644
--- a/packages/ai/test/openai-codex.test.ts
+++ b/packages/ai/test/openai-codex.test.ts
@@ -41,6 +41,34 @@ describe("openai-codex tool schemas", () => {
 			parameters: { type: "object", properties: {} },
 		});
 	});
+	it("strips MCP regex lookaround patterns from function parameters", () => {
+		const tools: Tool[] = [
+			{
+				name: "get_design_context",
+				description: "Get Figma design context",
+				parameters: {
+					type: "object",
+					properties: {
+						fileKey: { type: "string", pattern: "^(?!undefined$|null$)" },
+					},
+				},
+			},
+		];
+
+		const converted = convertOpenAICodexResponsesTools(tools, createCodexModel("gpt-5.5"));
+
+		expect(converted[0]).toEqual({
+			type: "function",
+			name: "get_design_context",
+			description: "Get Figma design context",
+			parameters: {
+				type: "object",
+				properties: {
+					fileKey: { type: "string" },
+				},
+			},
+		});
+	});
 });
 
 describe("openai-codex request transformer", () => {
diff --git a/packages/ai/test/schema-normalization.test.ts b/packages/ai/test/schema-normalization.test.ts
index caa79a80d..3093f9e5d 100644
--- a/packages/ai/test/schema-normalization.test.ts
+++ b/packages/ai/test/schema-normalization.test.ts
@@ -519,6 +519,28 @@ describe("sanitizeSchemaForOpenAIResponses", () => {
 		});
 	});
 
+	it("strips regex lookaround patterns unsupported by OpenAI Responses", () => {
+		const schema = {
+			type: "object",
+			properties: {
+				fileKey: { type: "string", pattern: "^(?!undefined$|null$)" },
+				ending: { type: "string", pattern: "(?<=/)node$" },
+				slug: { type: "string", pattern: "^[a-z0-9_-]+$" },
+				literal: { type: "string", pattern: "\\(?!literal" },
+			},
+		};
+
+		expect(sanitizeSchemaForOpenAIResponses(schema)).toEqual({
+			type: "object",
+			properties: {
+				fileKey: { type: "string" },
+				ending: { type: "string" },
+				slug: { type: "string", pattern: "^[a-z0-9_-]+$" },
+				literal: { type: "string", pattern: "\\(?!literal" },
+			},
+		});
+	});
+
 	it("preserves non-array oneOf payloads verbatim instead of dropping them", () => {
 		const malformed = { type: "object", oneOf: { type: "object" } } as unknown as Record<string, unknown>;