diff --git a/src/services/ai-review.ts b/src/services/ai-review.ts
index 6b164bb80d..0cd4faa72c 100644
--- a/src/services/ai-review.ts
+++ b/src/services/ai-review.ts
@@ -1007,6 +1007,11 @@ function isSubscriptionCliTimeout(error: unknown): boolean {
   return error instanceof Error && error.message === "subscription_cli_timeout";
 }
 
+/** Cap on the diagnostic prefix logged for an unparseable model response (#observability-unparseable) -- long
+ *  enough to tell a markdown-fenced/truncated-mid-JSON/plain-prose response apart, short enough to never dump
+ *  a large chunk of model output into Sentry/audit context. */
+const UNPARSEABLE_RESPONSE_SNIPPET_MAX_CHARS = 400;
+
 /** One reviewer opinion (whichever provider `env.AI` resolves to — self-host Codex/Claude Code/etc, or the
  *  legacy Workers-AI pair) with a per-slot reliable fallback and a 3× retry on the primary. */
 async function runWorkersOpinion(
@@ -1037,7 +1042,7 @@ async function runWorkersOpinion(
   // logs are warn (noisy retries, skipped by the central Sentry forwarder); the exhausted summary is error (#26).
   let lastError: unknown;
   let lastUnparseable:
-    | { model: string; attempt: number; responseChars: number; hasJsonObject: boolean }
+    | { model: string; attempt: number; responseChars: number; hasJsonObject: boolean; responseSnippet: string }
     | undefined;
   const models = fallback && fallback !== primary ? [primary, fallback] : [primary];
   for (const [modelIndex, model] of models.entries()) {
@@ -1085,10 +1090,15 @@ async function runWorkersOpinion(
           return { review: parsed };
         }
         const hasJsonObject = Boolean(extractLastJsonObject(text));
-        const status = text.trim() ? "unparseable_output" : "empty_output";
+        const trimmedText = text.trim();
+        const status = trimmedText ? "unparseable_output" : "empty_output";
         diagnostics.push({ model, attempt, status, responseChars: text.length, hasJsonObject, ...usageFields });
-        if (text.trim()) {
-          lastUnparseable = { model, attempt, responseChars: text.length, hasJsonObject };
+        if (trimmedText) {
+          // NOT added to the diagnostics entry above: reviewDiagnostics flows into result/Sentry context that
+          // must never carry raw provider text (see the "withholds unsafe provider and reviewer fallback text"
+          // test) -- logged here instead, which reaches only the structured-log Sentry forwarder, never `result`.
+          const responseSnippet = trimmedText.slice(0, UNPARSEABLE_RESPONSE_SNIPPET_MAX_CHARS);
+          lastUnparseable = { model, attempt, responseChars: text.length, hasJsonObject, responseSnippet };
           console.warn(
             JSON.stringify({
               level: "warn",
@@ -1097,6 +1107,7 @@ async function runWorkersOpinion(
               attempt,
               responseChars: text.length,
               hasJsonObject,
+              responseSnippet,
             }),
           );
         }
@@ -1149,6 +1160,7 @@ async function runWorkersOpinion(
         attempt: lastUnparseable.attempt,
         responseChars: lastUnparseable.responseChars,
         hasJsonObject: lastUnparseable.hasJsonObject,
+        responseSnippet: lastUnparseable.responseSnippet,
       }),
     );
   }
diff --git a/test/unit/ai-review.test.ts b/test/unit/ai-review.test.ts
index 00603577c0..bb7c02fa65 100644
--- a/test/unit/ai-review.test.ts
+++ b/test/unit/ai-review.test.ts
@@ -8,6 +8,7 @@ import {
   resolveEffectiveAiReviewPlan,
   runGittensoryAiReview,
   type AiContentBlock,
+  type AiReviewDiagnostic,
   type GittensoryAiReviewInput,
 } from "../../src/services/ai-review";
 import { createTestEnv } from "../helpers/d1";
@@ -3081,7 +3082,7 @@ describe("pure helpers", () => {
     warnSpy.mockRestore();
   });
 
-  it("logs unparseable exhaustion separately when the model runs but returns unparseable output", async () => {
+  it("logs unparseable exhaustion separately when the model runs but returns unparseable output, including a response snippet for diagnosis (#observability-unparseable)", async () => {
     const logSpy = vi.spyOn(console, "log").mockImplementation(() => {});
     const run = vi.fn(async () => ({ response: "not json at all" }));
     const env = createTestEnv({ AI: { run } as unknown as Ai });
@@ -3092,14 +3093,35 @@ describe("pure helpers", () => {
         .map((c) => c[0])
         .some((l) => typeof l === "string" && l.includes("ai_review_provider_exhausted")),
     ).toBe(false);
-    expect(
-      logSpy.mock.calls
-        .map((c) => c[0])
-        .some((l) => typeof l === "string" && l.includes("ai_review_provider_unparseable_exhausted")),
-    ).toBe(true);
+    const exhausted = logSpy.mock.calls
+      .map((c) => c[0])
+      .find((l) => typeof l === "string" && l.includes("ai_review_provider_unparseable_exhausted"));
+    expect(exhausted).toBeDefined();
+    expect(JSON.parse(exhausted as string)).toMatchObject({
+      event: "ai_review_provider_unparseable_exhausted",
+      responseSnippet: "not json at all",
+    });
     logSpy.mockRestore();
   });
 
+  it("truncates the unparseable-output response snippet to 400 chars instead of logging the full response (#observability-unparseable), and never puts it on the returned diagnostics (#4111-style public/private boundary)", async () => {
+    const warnSpy = vi.spyOn(console, "warn").mockImplementation(() => {});
+    const longResponse = "not json, ".repeat(60); // 600 chars, well over the 400-char cap
+    const run = vi.fn(async () => ({ response: longResponse }));
+    const env = createTestEnv({ AI: { run } as unknown as Ai });
+    const diagnostics: AiReviewDiagnostic[] = [];
+    await runWorkersOpinion(env, "primary-model", "primary-model", "sys", "user", 256, diagnostics);
+    // reviewDiagnostics flows into result/Sentry context that must never carry raw provider text (see the
+    // "withholds unsafe provider and reviewer fallback text" test) -- the snippet only ever reaches the log.
+    expect(diagnostics[0]).not.toHaveProperty("responseSnippet");
+    const firstWarn = warnSpy.mock.calls
+      .map((c) => c[0])
+      .find((l) => typeof l === "string" && l.includes("ai_review_provider_unparseable_output"));
+    expect(JSON.parse(firstWarn as string).responseSnippet).toBe(longResponse.slice(0, 400));
+    expect(JSON.parse(firstWarn as string).responseSnippet.length).toBe(400);
+    warnSpy.mockRestore();
+  });
+
   it("applies the default daily neuron budget when none is configured", async () => {
     const run = vi.fn(async (_model: string) => ({ response: reviewJson() }));
     const env = createTestEnv({