diff --git a/CHANGELOG.md b/CHANGELOG.md
index f0329a0..7673b3e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## 0.3.1 - Unreleased
 
+- Improved OpenCode malformed JSON diagnostics with output length, event kinds, and a bounded preview, thanks @rohitjavvadi.
 - Fixed Express route mapping for aliased Router imports that follow block comment banners, thanks @rohitjavvadi.
 - Fixed Bun package-manager detection to recognize the text `bun.lock` lockfile, thanks @austinm911.
 
diff --git a/src/provider.test.ts b/src/provider.test.ts
index 3c9b82e..239185e 100644
--- a/src/provider.test.ts
+++ b/src/provider.test.ts
@@ -1,6 +1,7 @@
 import { afterEach, describe, expect, it } from "vitest";
 import { ClawpatchError } from "./errors.js";
 import { __testing, extractJson, providerByName } from "./provider.js";
+import { safeProviderPreview } from "./provider-json.js";
 import { revalidateOutputSchema, reviewOutputSchema } from "./types.js";
 
 // eslint-disable-next-line no-underscore-dangle
@@ -55,6 +56,10 @@ function expectMalformed(fn: () => unknown, message: RegExp): void {
   throw new Error("expected malformed-output");
 }
 
+function escapeRegExp(value: string): string {
+  return value.replace(/[.*+?^${}()|[\]\\]/gu, "\\$&");
+}
+
 describe("extractJson", () => {
   it("parses strict JSON directly", () => {
     const input = '{"findings":[],"inspected":{"files":[],"symbols":[],"notes":[]}}';
@@ -484,6 +489,46 @@ describe("extractOpencodeJson", () => {
     expectMalformed(() => extractOpencodeJson(stdout), /no extractable text.*step_finish/u);
   });
 
+  it("treats whitespace-only opencode text as no extractable text", () => {
+    const stdout = [
+      JSON.stringify({ type: "text", part: { text: " \n\t " } }),
+      JSON.stringify({ type: "step_finish", part: { reason: "stop" } }),
+    ].join("\n");
+
+    expectMalformed(() => extractOpencodeJson(stdout), /no extractable text.*text, step_finish/u);
+  });
+
+  it("throws malformed-output with a preview when opencode text is unparsable", () => {
+    const stdout = [
+      JSON.stringify({
+        type: "text",
+        part: { text: '{"findings": [' },
+      }),
+      JSON.stringify({ type: "step_finish", part: { reason: "stop" } }),
+    ].join("\n");
+
+    expectMalformed(
+      () => extractOpencodeJson(stdout),
+      /unparsable JSON.*text chars=14.*observed event kinds: \[text, step_finish\].*output preview: \{"findings": \[/u,
+    );
+  });
+
+  it("bounds the opencode unparsable text preview", () => {
+    const text = `{"findings":["${"x".repeat(300)}`;
+    const stdout = JSON.stringify({
+      type: "text",
+      part: { text },
+    });
+    const preview = safeProviderPreview(text);
+
+    expect(preview.length).toBe(200);
+
+    expectMalformed(
+      () => extractOpencodeJson(stdout),
+      new RegExp(`output preview: ${escapeRegExp(preview)}\\)`, "u"),
+    );
+  });
+
   it("throws provider-failure for opencode error events", () => {
     const stdout = JSON.stringify({
       type: "error",
diff --git a/src/provider.ts b/src/provider.ts
index 1efacf1..72628e6 100644
--- a/src/provider.ts
+++ b/src/provider.ts
@@ -630,7 +630,13 @@ export function extractOpencodeJson(stdout: string): unknown {
   }
   const parsed = extractJson(combined);
   if (parsed === null) {
-    throw new ClawpatchError("opencode provider produced unparsable JSON", 8, "malformed-output");
+    throw new ClawpatchError(
+      `opencode provider produced unparsable JSON ` +
+        `(text chars=${combined.length}, observed event kinds: ` +
+        `[${[...observedKinds].join(", ")}], output preview: ${safeProviderPreview(combined)})`,
+      8,
+      "malformed-output",
+    );
   }
   return parsed;
 }