diff --git a/packages/media-understanding-common/src/output-extract.test.ts b/packages/media-understanding-common/src/output-extract.test.ts
new file mode 100644
index 00000000000..336055d8779
--- /dev/null
+++ b/packages/media-understanding-common/src/output-extract.test.ts
@@ -0,0 +1,100 @@
+// Media Understanding Common tests cover provider output extraction behavior.
+import { describe, expect, it } from "vitest";
+import { extractGeminiResponse } from "./output-extract.js";
+
+describe("extractGeminiResponse", () => {
+  it("extracts the response from noisy output with nested JSON objects", () => {
+    expect(
+      extractGeminiResponse(
+        [
+          "debug: invoking gemini",
+          JSON.stringify({
+            response: "a useful description",
+            usage: {
+              inputTokens: 12,
+              outputTokens: 4,
+            },
+          }),
+        ].join("\n"),
+      ),
+    ).toBe("a useful description");
+  });
+
+  it("returns null for an incomplete JSON object", () => {
+    expect(extractGeminiResponse("{")).toBeNull();
+  });
+
+  it("ignores unmatched quotes in noisy output before the JSON object", () => {
+    expect(extractGeminiResponse('debug: model said "hello\n{"response":"ok"}')).toBe("ok");
+  });
+
+  it("ignores braces inside quoted noisy output", () => {
+    expect(extractGeminiResponse('debug: "hello { world" {"response":"ok"}')).toBe("ok");
+  });
+
+  it("ignores shell-quoted JSON-like noisy output", () => {
+    expect(extractGeminiResponse('debug: \'{"response":"fake"}\'')).toBeNull();
+  });
+
+  it("does not treat apostrophes inside noisy words as quote delimiters", () => {
+    expect(extractGeminiResponse('debug: it\'s done {"response":"ok"}')).toBe("ok");
+  });
+
+  it("resynchronizes after an unmatched brace in noisy output", () => {
+    expect(extractGeminiResponse('debug: generated {\n{"response":"ok"}')).toBe("ok");
+  });
+
+  it("preserves brace-heavy response text", () => {
+    const response = "{".repeat(33);
+    expect(extractGeminiResponse(JSON.stringify({ response }))).toBe(response);
+  });
+
+  it("extracts pretty-printed JSON output", () => {
+    expect(
+      extractGeminiResponse(
+        JSON.stringify(
+          {
+            response: "pretty response",
+            usage: { inputTokens: 12 },
+          },
+          null,
+          2,
+        ),
+      ),
+    ).toBe("pretty response");
+  });
+
+  it("preserves pretty-printed object elements inside arrays", () => {
+    expect(
+      extractGeminiResponse(
+        JSON.stringify(
+          {
+            response: "array response",
+            items: [{ id: 1 }, { id: 2 }],
+          },
+          null,
+          2,
+        ),
+      ),
+    ).toBe("array response");
+  });
+
+  it("does not accept an inner response from a malformed trailing object", () => {
+    expect(extractGeminiResponse('{"response":"good"} {"meta":{"response":"bad"} broken}')).toBe(
+      "good",
+    );
+    expect(extractGeminiResponse('{"response":"good"} {"meta":{"response":"bad"}')).toBe("good");
+  });
+
+  it("ignores a nested response inside an unfinished outer object", () => {
+    expect(extractGeminiResponse('noise {"meta":{"response":"bad"}')).toBeNull();
+  });
+
+  it("does not promote a child from a malformed outer object", () => {
+    expect(extractGeminiResponse('{"response":"good"} {"meta" {"response":"bad"}}')).toBe("good");
+    expect(extractGeminiResponse('noise {broken {"response":"bad"}}')).toBeNull();
+    expect(extractGeminiResponse('{"response":"good"}\nnoise {broken\n{"response":"bad"}}')).toBe(
+      "good",
+    );
+  });
+});
diff --git a/packages/media-understanding-common/src/output-extract.ts b/packages/media-understanding-common/src/output-extract.ts
index 9b7cffea0e1..53cecd82e7a 100644
--- a/packages/media-understanding-common/src/output-extract.ts
+++ b/packages/media-understanding-common/src/output-extract.ts
@@ -3,16 +3,119 @@
 /** Parse the last JSON object in a noisy provider output string. */
 function extractLastJsonObject(raw: string): unknown {
   const trimmed = raw.trim();
-  const start = trimmed.lastIndexOf("{");
-  if (start === -1) {
-    return null;
+  const ranges: Array<{ end: number; start: number }> = [];
+  const starts: number[] = [];
+  let inString = false;
+  let escaped = false;
+  let preambleQuote: string | undefined;
+  let preambleEscaped = false;
+  let previousSignificant: string | undefined;
+  let lineHasNonWhitespace = false;
+  let arrayDepth = 0;
+  let candidateHasContent = false;
+
+  for (let index = 0; index < trimmed.length; index += 1) {
+    const character = trimmed[index];
+    if (inString) {
+      if (character === "\n" || character === "\r") {
+        starts.length = 0;
+        inString = false;
+        escaped = false;
+      } else if (escaped) {
+        escaped = false;
+      } else if (character === "\\") {
+        escaped = true;
+      } else if (character === '"') {
+        inString = false;
+      }
+      continue;
+    }
+
+    if (starts.length === 0) {
+      if (preambleQuote !== undefined) {
+        if (character === "\n" || character === "\r") {
+          preambleQuote = undefined;
+          preambleEscaped = false;
+        } else if (preambleEscaped) {
+          preambleEscaped = false;
+        } else if (character === "\\") {
+          preambleEscaped = true;
+        } else if (character === preambleQuote) {
+          preambleQuote = undefined;
+        }
+        continue;
+      }
+      if (character === '"' || character === "'" || character === "`") {
+        const previous = trimmed[index - 1];
+        if (previous === undefined || /[\s:([{]/.test(previous)) {
+          preambleQuote = character;
+          preambleEscaped = false;
+          continue;
+        }
+      }
+      if (character === "{") {
+        arrayDepth = 0;
+        candidateHasContent = false;
+        starts.push(index);
+      }
+      if (!/\s/.test(character)) {
+        previousSignificant = character;
+        lineHasNonWhitespace = true;
+      } else if (character === "\n" || character === "\r") {
+        lineHasNonWhitespace = false;
+      }
+      continue;
+    }
+
+    const hadCandidateContent = candidateHasContent;
+    if (character === '"') {
+      inString = true;
+    } else if (character === "{") {
+      if (
+        previousSignificant === ":" ||
+        previousSignificant === "[" ||
+        previousSignificant === '"' ||
+        (previousSignificant === "," && (lineHasNonWhitespace || arrayDepth > 0))
+      ) {
+        starts.push(index);
+      } else if (!lineHasNonWhitespace && !hadCandidateContent) {
+        // Only resync at a clean record boundary; otherwise keep malformed
+        // outer objects from promoting diagnostic payloads as valid results.
+        starts.length = 1;
+        starts[0] = index;
+        arrayDepth = 0;
+        candidateHasContent = false;
+      }
+    } else if (character === "}" && starts.length > 0) {
+      const start = starts.pop();
+      if (start !== undefined && starts.length === 0) {
+        ranges.push({ start, end: index });
+      }
+    } else if (character === "[") {
+      arrayDepth += 1;
+    } else if (character === "]" && arrayDepth > 0) {
+      arrayDepth -= 1;
+    }
+
+    if (!/\s/.test(character)) {
+      candidateHasContent = true;
+      previousSignificant = character;
+      lineHasNonWhitespace = true;
+    } else if (character === "\n" || character === "\r") {
+      lineHasNonWhitespace = false;
+    }
   }
-  const slice = trimmed.slice(start);
-  try {
-    return JSON.parse(slice);
-  } catch {
-    return null;
+
+  for (let index = ranges.length - 1; index >= 0; index -= 1) {
+    const range = ranges[index];
+    try {
+      return JSON.parse(trimmed.slice(range.start, range.end + 1));
+    } catch {
+      // Ignore malformed objects and try the previous completed range.
+    }
   }
+
+  return null;
 }
 
 /** Extract Gemini CLI-style response text from the last JSON object in output. */