diff --git a/open-sse/utils/streamErrorPatterns.js b/open-sse/utils/streamErrorPatterns.js
new file mode 100644
index 00000000..084d6815
--- /dev/null
+++ b/open-sse/utils/streamErrorPatterns.js
@@ -0,0 +1,47 @@
+/**
+ * Config-driven in-stream error pattern matching.
+ * See docs/2026-08-04-stream-error-patterns-design.md.
+ *
+ * Settings shape: streamErrorPatterns: { [provider]: string[] }
+ * Pattern syntax per entry:
+ *   - "plain text"   → case-insensitive substring match
+ *   - "/regex/flags" → RegExp match (flags after the last "/")
+ */
+
+export function parsePatterns(patterns) {
+  if (!Array.isArray(patterns)) return [];
+  const out = [];
+  for (const entry of patterns) {
+    if (typeof entry !== "string" || !entry.trim()) continue;
+    const raw = entry.trim();
+    const m = raw.match(/^\/(.*)\/([a-z]*)$/s);
+    if (m) {
+      try {
+        out.push({ regex: new RegExp(m[1], m[2]), raw });
+      } catch {
+        // invalid regex → skip; config errors must never break requests
+      }
+      continue;
+    }
+    out.push({ text: raw.toLowerCase(), raw });
+  }
+  return out;
+}
+
+export function matchStreamErrorPatterns(patterns, text) {
+  if (!Array.isArray(patterns) || patterns.length === 0) return null;
+  if (typeof text !== "string" || !text) return null;
+  const lower = text.toLowerCase();
+  for (const p of parsePatterns(patterns)) {
+    if (p.text && lower.includes(p.text)) return p.raw;
+    if (p.regex) {
+      p.regex.lastIndex = 0; // stateless even with /g
+      if (p.regex.test(text)) return p.raw;
+    }
+  }
+  return null;
+}
+
+export function streamStatusForContent(patterns, content) {
+  return matchStreamErrorPatterns(patterns, content) ? "error" : "success";
+}
diff --git a/tests/unit/streamErrorPatterns.test.js b/tests/unit/streamErrorPatterns.test.js
new file mode 100644
index 00000000..4ecde02c
--- /dev/null
+++ b/tests/unit/streamErrorPatterns.test.js
@@ -0,0 +1,50 @@
+import { describe, it, expect } from "vitest";
+import { parsePatterns, matchStreamErrorPatterns, streamStatusForContent } from "../../open-sse/utils/streamErrorPatterns.js";
+
+describe("streamErrorPatterns util", () => {
+  it("matches plain text as case-insensitive substring", () => {
+    expect(matchStreamErrorPatterns(["Network connection lost"], "network CONNECTION LOST.")).toBe("Network connection lost");
+    expect(matchStreamErrorPatterns(["server_error"], "some normal content")).toBeNull();
+  });
+
+  it("matches /regex/flags", () => {
+    expect(matchStreamErrorPatterns(["/generation failed.*retry/i"], "GENERATION FAILED. please RETRY")).toBe("/generation failed.*retry/i");
+    expect(matchStreamErrorPatterns(["/\\d+ tokens/"], "used 123 tokens")).toBe("/\\d+ tokens/");
+  });
+
+  it("skips invalid regex and empty entries", () => {
+    expect(matchStreamErrorPatterns(["/[unclosed/", "", "   "], "anything")).toBeNull();
+  });
+
+  it("returns null for empty patterns or text", () => {
+    expect(matchStreamErrorPatterns([], "x")).toBeNull();
+    expect(matchStreamErrorPatterns(["x"], "")).toBeNull();
+    expect(matchStreamErrorPatterns(null, "x")).toBeNull();
+    expect(matchStreamErrorPatterns(undefined, "x")).toBeNull();
+  });
+
+  it("regex matching is stateless across calls", () => {
+    const pats = ["/error/i"];
+    expect(matchStreamErrorPatterns(pats, "ERROR")).toBe("/error/i");
+    expect(matchStreamErrorPatterns(pats, "ERROR")).toBe("/error/i");
+  });
+
+  it("parsePatterns normalizes entries", () => {
+    const parsed = parsePatterns(["Plain", "/re/g", "", "/bad["]);
+    expect(parsed.length).toBe(3);
+    expect(parsed[0]).toEqual({ text: "plain", raw: "Plain" });
+    expect(parsed[1].regex).toBeInstanceOf(RegExp);
+    expect(parsed[2]).toEqual({ text: "/bad[", raw: "/bad[" });
+  });
+
+  it("skips entries that look like regex but fail to compile", () => {
+    const parsed = parsePatterns(["/[unclosed/", "/ok/g"]);
+    expect(parsed.length).toBe(1);
+    expect(parsed[0].raw).toBe("/ok/g");
+  });
+
+  it("streamStatusForContent maps match to error status", () => {
+    expect(streamStatusForContent(["boom"], "a boom happened")).toBe("error");
+    expect(streamStatusForContent(["boom"], "all good")).toBe("success");
+  });
+});