fix(codex): strip regex lookaround from tool schema patterns (port from 9router#1556)

Codex/OpenAI's Responses API rejects JSON Schema pattern fields using regex lookaround (e.g. ^(?=.*@).+$) with a 400 'regex lookaround is not supported' error. The existing numeric-field sanitizer (coerceSchemaNumericFields) was only wired into the translated-request path (openai-to-claude.ts), not the native codex/openai passthrough path (normalizeCodexTools in open-sse/executors/codex/tools.ts), so lookahead/lookbehind patterns reached upstream unmodified and broke tool calls for clients that emit them (e.g. IDE agent harnesses validating an email field).

Reported-by: evin (@evinjohnn) (https://github.com/decolua/9router/issues/1556)

CHANGELOG.md

@@ -6,6 +6,10 @@
 
 ## [3.8.49] — TBD
 
+### 🐛 Bug Fixes
+
+- **fix(codex):** strip regex `pattern` lookaround (lookahead/lookbehind) from tool JSON Schemas on the Codex/OpenAI native passthrough path — previously only the translated-request path coerced tool schemas, so a `pattern` like `^(?=.*@).+$` reached OpenAI unmodified and was rejected with `regex lookaround is not supported`. (thanks @evinjohnn)
+
 ---
 
 ## [3.8.48] — 2026-07-13

open-sse/executors/codex/tools.ts

@@ -1,6 +1,8 @@
 // Codex Responses-API tool normalization (hosted-tool passthrough + free-plan gating).
 // Extracted verbatim from codex.ts. Self-contained (console.debug only).
 
+import { stripUnsupportedRegexPatterns } from "../../translator/helpers/schemaCoercion.ts";
+
 // Responses-API hosted tool types that OpenAI/Codex executes server-side.
 // These arrive shaped as `{ type, ...params }` with no `function` object and no `name` —
 // e.g. Codex CLI injects `{ type: "image_generation", output_format: "png" }` or
@@ -133,6 +135,11 @@ export function normalizeCodexTools(
           ? functionObject.strict
           : undefined;
 
+    // Codex/OpenAI Responses API rejects `pattern` fields using regex lookaround
+    // (e.g. `^(?=.*@).+$`) with a 400 "regex lookaround is not supported" error.
+    // Strip those before the schema reaches upstream (9router#1556).
+    const sanitizedParameters = stripUnsupportedRegexPatterns(parameters);
+
     // Rewrite in-place to Responses format
     for (const key of Object.keys(tool)) {
       delete tool[key];
@@ -140,7 +147,7 @@ export function normalizeCodexTools(
     tool.type = "function";
     tool.name = name.slice(0, 128);
     if (description) tool.description = description;
-    tool.parameters = parameters;
+    tool.parameters = sanitizedParameters;
     if (strict !== undefined) tool.strict = strict;
 
     validToolNames.add(name);

open-sse/translator/helpers/schemaCoercion.ts

@@ -24,6 +24,18 @@ const NUMERIC_SCHEMA_FIELDS = [
   "multipleOf",
 ] as const;
 
+// Fix (9router#1556): OpenAI/Codex's Responses API rejects JSON Schema `pattern`
+// values that use regex lookaround (lookahead/lookbehind) with
+// "Invalid JSON schema: regex lookaround is not supported.". IDE/SDK agent
+// harnesses commonly emit lookahead patterns (e.g. `^(?=.*@).+$`), so any
+// `pattern` field containing `(?=`, `(?!`, `(?<=`, or `(?<!` must be dropped
+// before the schema reaches the Codex/OpenAI upstream.
+const REGEX_LOOKAROUND_PATTERN = /\(\?<?[=!]/;
+
+function hasUnsupportedRegexLookaround(pattern: unknown): boolean {
+  return typeof pattern === "string" && REGEX_LOOKAROUND_PATTERN.test(pattern);
+}
+
 function isPlainObject(value: unknown): value is JsonRecord {
   return Boolean(value) && typeof value === "object" && !Array.isArray(value);
 }
@@ -81,6 +93,11 @@ export function coerceSchemaNumericFields(schema: unknown): unknown {
     delete result.default;
   }
 
+  // Fix (9router#1556): drop unsupported regex lookaround from `pattern`.
+  if (hasUnsupportedRegexLookaround(result.pattern)) {
+    delete result.pattern;
+  }
+
   for (const field of NUMERIC_SCHEMA_FIELDS) {
     if (field in result) {
       result[field] = coerceNumericString(result[field]);
@@ -142,6 +159,82 @@ export function coerceSchemaNumericFields(schema: unknown): unknown {
   return result;
 }
 
+/**
+ * Strip regex `pattern` constraints that use lookaround (lookahead/lookbehind),
+ * which OpenAI/Codex's Responses API rejects outright with a 400
+ * ("Invalid JSON schema: regex lookaround is not supported."). Walks the same
+ * JSON Schema shape as `coerceSchemaNumericFields` (properties, items,
+ * anyOf/oneOf/allOf, $defs/definitions, etc). See 9router#1556.
+ */
+export function stripUnsupportedRegexPatterns(schema: unknown): unknown {
+  if (Array.isArray(schema)) {
+    return schema.map((entry) => stripUnsupportedRegexPatterns(entry));
+  }
+  if (!isPlainObject(schema)) return schema;
+
+  const result: JsonRecord = { ...schema };
+
+  if (hasUnsupportedRegexLookaround(result.pattern)) {
+    delete result.pattern;
+  }
+
+  if (isPlainObject(result.properties)) {
+    result.properties = Object.fromEntries(
+      Object.entries(result.properties).map(([key, value]) => [
+        key,
+        stripUnsupportedRegexPatterns(value),
+      ])
+    );
+  }
+  if (isPlainObject(result.patternProperties)) {
+    result.patternProperties = Object.fromEntries(
+      Object.entries(result.patternProperties).map(([key, value]) => [
+        key,
+        stripUnsupportedRegexPatterns(value),
+      ])
+    );
+  }
+  if (isPlainObject(result.definitions)) {
+    result.definitions = Object.fromEntries(
+      Object.entries(result.definitions).map(([key, value]) => [
+        key,
+        stripUnsupportedRegexPatterns(value),
+      ])
+    );
+  }
+  if (isPlainObject(result.$defs)) {
+    result.$defs = Object.fromEntries(
+      Object.entries(result.$defs).map(([key, value]) => [
+        key,
+        stripUnsupportedRegexPatterns(value),
+      ])
+    );
+  }
+  if (result.items !== undefined) {
+    result.items = stripUnsupportedRegexPatterns(result.items);
+  }
+  if (result.additionalProperties && typeof result.additionalProperties === "object") {
+    result.additionalProperties = stripUnsupportedRegexPatterns(result.additionalProperties);
+  }
+  if (Array.isArray(result.prefixItems)) {
+    result.prefixItems = result.prefixItems.map((entry) => stripUnsupportedRegexPatterns(entry));
+  }
+  if (Array.isArray(result.anyOf)) {
+    result.anyOf = result.anyOf.map((entry) => stripUnsupportedRegexPatterns(entry));
+  }
+  if (Array.isArray(result.oneOf)) {
+    result.oneOf = result.oneOf.map((entry) => stripUnsupportedRegexPatterns(entry));
+  }
+  if (Array.isArray(result.allOf)) {
+    result.allOf = result.allOf.map((entry) => stripUnsupportedRegexPatterns(entry));
+  }
+  if (isPlainObject(result.not)) {
+    result.not = stripUnsupportedRegexPatterns(result.not);
+  }
+
+  return result;
+}
+
 export function sanitizeToolDescription(tool: unknown): unknown {
   if (!isPlainObject(tool)) return tool;
 

tests/unit/codex-tools-regex-lookaround.test.ts

@@ -0,0 +1,74 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+import { normalizeCodexTools } from "../../open-sse/executors/codex/tools.ts";
+
+// Port of 9router#1556: OpenAI/Codex Responses API rejects JSON Schema `pattern`
+// fields containing regex lookaround (lookahead/lookbehind) with:
+//   "Invalid JSON schema: regex lookaround is not supported. Found at $.properties.email.pattern."
+// Clients (e.g. IDE agent harnesses) commonly emit lookahead patterns such as
+// `^(?=.*@).+$` for "must contain an @". These must be stripped before the
+// tool schema reaches the Codex/OpenAI Responses API.
+test("normalizeCodexTools strips regex lookaround from function tool parameter patterns", () => {
+  const body: Record<string, unknown> = {
+    tools: [
+      {
+        type: "function",
+        function: {
+          name: "send_email",
+          description: "Send an email",
+          parameters: {
+            type: "object",
+            properties: {
+              email: {
+                type: "string",
+                pattern: "^(?=.*@).+$",
+              },
+            },
+          },
+        },
+      },
+    ],
+  };
+
+  normalizeCodexTools(body);
+
+  const tools = body.tools as Array<Record<string, unknown>>;
+  const parameters = tools[0].parameters as Record<string, unknown>;
+  const properties = parameters.properties as Record<string, unknown>;
+  const emailSchema = properties.email as Record<string, unknown>;
+
+  assert.equal(
+    emailSchema.pattern,
+    undefined,
+    "lookaround pattern must be stripped, not forwarded upstream"
+  );
+});
+
+test("normalizeCodexTools preserves plain (non-lookaround) regex patterns", () => {
+  const body: Record<string, unknown> = {
+    tools: [
+      {
+        type: "function",
+        function: {
+          name: "send_email",
+          parameters: {
+            type: "object",
+            properties: {
+              zip: { type: "string", pattern: "^[0-9]{5}$" },
+            },
+          },
+        },
+      },
+    ],
+  };
+
+  normalizeCodexTools(body);
+
+  const tools = body.tools as Array<Record<string, unknown>>;
+  const parameters = tools[0].parameters as Record<string, unknown>;
+  const properties = parameters.properties as Record<string, unknown>;
+  const zipSchema = properties.zip as Record<string, unknown>;
+
+  assert.equal(zipSchema.pattern, "^[0-9]{5}$");
+});