diff --git a/src/__tests__/drift/bedrock-misbehavior-live.drift.ts b/src/__tests__/drift/bedrock-misbehavior-live.drift.ts
index 223f52a4..9f769bc0 100644
--- a/src/__tests__/drift/bedrock-misbehavior-live.drift.ts
+++ b/src/__tests__/drift/bedrock-misbehavior-live.drift.ts
@@ -20,6 +20,7 @@ import { Readable, Transform } from "node:stream";
 import { afterAll, beforeAll, describe, expect, it } from "vitest";
 import { LLMock } from "../../llmock.js";
 import type { Fixture } from "../../types.js";
+import { boundedDiagnosticLine } from "./bounded-diagnostic.js";
 import { withClaudeFault } from "../helpers/misbehavior-claude-stage3.js";
 
 const cells = [
@@ -419,36 +420,36 @@ async function runAwsCell(
       networkEnded,
       decoded,
     };
+    const { rawWire, ...observed } = observation;
     console.log(
-      JSON.stringify({
-        cell,
-        request: cell.wire === "invoke" ? invokeInput : converseInput,
-        ...observation,
-        rawWire: { encoding: "base64", bytes: observation.rawWire.toString("base64") },
-      }),
+      boundedDiagnosticLine(
+        { cell, request: cell.wire === "invoke" ? invokeInput : converseInput, ...observed },
+        rawWire,
+      ),
     );
     return observation;
   } catch (error) {
     const errorMetadata =
       error instanceof Error && "$metadata" in error ? error.$metadata : undefined;
+    const diagnostic = {
+      cell,
+      request: cell.wire === "invoke" ? invokeInput : converseInput,
+      requestCount,
+      http,
+      metadata: errorMetadata ?? metadata,
+      sdkResponse,
+      sdkEvents,
+      decodedEvents: cell.wire === "invoke" ? events : converseEvents,
+      networkEnded,
+      error: error instanceof Error ? { name: error.name, message: error.message } : String(error),
+    };
     console.log(
-      JSON.stringify({
-        cell,
-        request: cell.wire === "invoke" ? invokeInput : converseInput,
-        requestCount,
-        http,
-        metadata: errorMetadata ?? metadata,
-        sdkResponse,
-        sdkEvents,
-        decodedEvents: cell.wire === "invoke" ? events : converseEvents,
-        networkEnded,
-        rawWire:
-          !live && limits.diagnosticWire === "summary"
-            ? { omitted: true, byteLength: Buffer.concat(chunks).length }
-            : { encoding: "base64", bytes: Buffer.concat(chunks).toString("base64") },
-        error:
-          error instanceof Error ? { name: error.name, message: error.message } : String(error),
-      }),
+      !live && limits.diagnosticWire === "summary"
+        ? JSON.stringify({
+            ...diagnostic,
+            rawWire: { omitted: true, byteLength: Buffer.concat(chunks).length },
+          })
+        : boundedDiagnosticLine(diagnostic, Buffer.concat(chunks)),
     );
     throw failure ?? error;
   } finally {
diff --git a/src/__tests__/drift/bounded-diagnostic.test.ts b/src/__tests__/drift/bounded-diagnostic.test.ts
new file mode 100644
index 00000000..3ac3be51
--- /dev/null
+++ b/src/__tests__/drift/bounded-diagnostic.test.ts
@@ -0,0 +1,146 @@
+import { createHash } from "node:crypto";
+import { describe, expect, it } from "vitest";
+import { MAX_EVIDENCE_RECORD_BYTES } from "../../../scripts/drift-evidence-reporter.js";
+import { boundedDiagnosticLine, fitsEvidenceRecord } from "./bounded-diagnostic.js";
+
+const sha256 = (bytes: Uint8Array) => createHash("sha256").update(bytes).digest("hex");
+
+// Shape of the live Bedrock invoke K9 stream=true record observed on the
+// scheduled main drift run: ~318 SDK chunk events plus ~90KiB of raw wire.
+function liveK9StreamRecord(chunkCount = 318, chunkBytes = 280) {
+  const sdkEvents = Array.from({ length: chunkCount }, (_, index) => ({
+    chunk: { bytes: new Uint8Array(chunkBytes).fill(97 + (index % 26)) },
+  }));
+  const decoded = { wire: "invoke", stream: true, events: [{ type: "message_stop" }] };
+  const record = { cell: { wire: "invoke", fault: "K9", stream: true }, sdkEvents, decoded };
+  const wire = Buffer.alloc(chunkCount * chunkBytes, 0x62);
+  return { record, wire };
+}
+
+describe("boundedDiagnosticLine", () => {
+  it("keeps a small record complete, with byte arrays as base64", () => {
+    const wire = Buffer.from("raw-wire");
+    const line = boundedDiagnosticLine(
+      { cell: "x", sdkEvents: [{ chunk: { bytes: new Uint8Array([123, 125]) } }] },
+      wire,
+    );
+    expect(JSON.parse(line)).toEqual({
+      cell: "x",
+      sdkEvents: [{ chunk: { bytes: { encoding: "base64", bytes: "e30=" } } }],
+      rawWire: { encoding: "base64", bytes: wire.toString("base64") },
+    });
+  });
+
+  it("bounds the live K9 stream record that the reporter used to drop", () => {
+    const { record, wire } = liveK9StreamRecord();
+    // The pre-fix line: JSON.stringify with the raw wire inline.
+    const unbounded = JSON.stringify({
+      ...record,
+      rawWire: { encoding: "base64", bytes: wire.toString("base64") },
+    });
+    expect(fitsEvidenceRecord(unbounded)).toBe(false);
+
+    const line = boundedDiagnosticLine(record, wire);
+    expect(fitsEvidenceRecord(line)).toBe(true);
+    const parsed = JSON.parse(line);
+    expect(parsed.decoded).toEqual(record.decoded);
+    expect(parsed.sdkEvents).toHaveLength(record.sdkEvents.length);
+    expect(parsed.sdkEvents[0].chunk.bytes).toEqual({
+      encoding: "base64",
+      bytes: Buffer.from(record.sdkEvents[0].chunk.bytes).toString("base64"),
+    });
+    expect(parsed.rawWire).toEqual({ encoding: "base64", bytes: wire.toString("base64") });
+  });
+
+  it("digests the raw wire when the base64 events and wire together exceed the limit", () => {
+    const { record, wire } = liveK9StreamRecord(318, 400);
+    const line = boundedDiagnosticLine(record, wire);
+    expect(fitsEvidenceRecord(line)).toBe(true);
+    const parsed = JSON.parse(line);
+    expect(parsed.decoded).toEqual(record.decoded);
+    expect(parsed.sdkEvents).toHaveLength(318);
+    expect(parsed.rawWire).toEqual({
+      omitted: true,
+      byteLength: wire.length,
+      sha256: sha256(wire),
+    });
+  });
+
+  it("digests the SDK events too when they alone exceed the record limit", () => {
+    const { record, wire } = liveK9StreamRecord(400, 1024);
+    const line = boundedDiagnosticLine(record, wire);
+    expect(fitsEvidenceRecord(line)).toBe(true);
+    const parsed = JSON.parse(line);
+    expect(parsed.decoded).toEqual(record.decoded);
+    expect(parsed.sdkEvents).toMatchObject({ omitted: true, count: 400 });
+    expect(parsed.sdkEvents.sha256).toMatch(/^[0-9a-f]{64}$/);
+    expect(parsed.rawWire).toMatchObject({ omitted: true, byteLength: wire.length });
+  });
+});
+
+describe("boundedDiagnosticLine never returns an oversized line", () => {
+  const deltas = (count: number) =>
+    Array.from({ length: count }, (_, index) => ({
+      type: "content_block_delta",
+      index: 0,
+      delta: { type: "text_delta", text: `delta ${index} `.repeat(8) },
+    }));
+
+  it("digests decoded events when they alone exceed the record limit", () => {
+    const decoded = { wire: "invoke", stream: true, events: deltas(3000) };
+    const record = { cell: { wire: "invoke", fault: "K9", stream: true }, sdkEvents: [], decoded };
+    expect(fitsEvidenceRecord(JSON.stringify(record))).toBe(false);
+
+    const line = boundedDiagnosticLine(record, Buffer.from("wire"));
+    expect(fitsEvidenceRecord(line)).toBe(true);
+    const parsed = JSON.parse(line);
+    expect(parsed.cell).toEqual(record.cell);
+    const bytes = Buffer.from(JSON.stringify(decoded));
+    expect(parsed.decoded).toEqual({
+      omitted: true,
+      byteLength: bytes.length,
+      sha256: sha256(bytes),
+    });
+  });
+
+  it("digests decodedEvents on the error path the same way", () => {
+    const record = {
+      cell: { wire: "invoke", fault: "K9", stream: true },
+      sdkEvents: [],
+      decodedEvents: deltas(3000),
+      error: { name: "Error", message: "boom" },
+    };
+    const line = boundedDiagnosticLine(record, Buffer.from("wire"));
+    expect(fitsEvidenceRecord(line)).toBe(true);
+    const parsed = JSON.parse(line);
+    expect(parsed.decodedEvents).toMatchObject({ omitted: true });
+    expect(parsed.error).toEqual(record.error);
+  });
+
+  it("falls back to a minimal line when other fields still exceed the limit", () => {
+    const record = {
+      cell: { wire: "converse", fault: "K5", stream: false },
+      sdkEvents: [],
+      sdkResponse: { text: "y".repeat(MAX_EVIDENCE_RECORD_BYTES) },
+      error: { name: "Error", message: "z".repeat(MAX_EVIDENCE_RECORD_BYTES) },
+    };
+    const line = boundedDiagnosticLine(record, Buffer.from("wire"));
+    expect(fitsEvidenceRecord(line)).toBe(true);
+    const parsed = JSON.parse(line);
+    expect(parsed.cell).toEqual(record.cell);
+    expect(parsed.omitted).toEqual(expect.arrayContaining(["sdkResponse", "error"]));
+    expect(parsed.byteLength).toBeGreaterThan(MAX_EVIDENCE_RECORD_BYTES);
+    expect(parsed.sha256).toMatch(/^[0-9a-f]{64}$/);
+  });
+});
+
+describe("fitsEvidenceRecord", () => {
+  it("applies the reporter limit to the JSON-encoded console record", () => {
+    // Quotes are escaped once more inside the record, so a line under the
+    // limit by raw length can still exceed it once encoded.
+    const quoted = '"'.repeat(MAX_EVIDENCE_RECORD_BYTES / 2);
+    expect(Buffer.byteLength(quoted)).toBeLessThan(MAX_EVIDENCE_RECORD_BYTES);
+    expect(fitsEvidenceRecord(quoted)).toBe(false);
+    expect(fitsEvidenceRecord("x".repeat(1024))).toBe(true);
+  });
+});
diff --git a/src/__tests__/drift/bounded-diagnostic.ts b/src/__tests__/drift/bounded-diagnostic.ts
new file mode 100644
index 00000000..f7701541
--- /dev/null
+++ b/src/__tests__/drift/bounded-diagnostic.ts
@@ -0,0 +1,87 @@
+import { createHash } from "node:crypto";
+import { MAX_EVIDENCE_RECORD_BYTES } from "../../../scripts/drift-evidence-reporter.js";
+
+// Vitest task ids are short; 64 characters leaves a margin for the envelope.
+const TASK_ID_ALLOWANCE = "x".repeat(64);
+
+const sha256 = (bytes: Uint8Array) => createHash("sha256").update(bytes).digest("hex");
+
+// Without this, JSON.stringify writes a Uint8Array as {"0":123,"1":34,...},
+// which is about ten times the size of its base64 form.
+const bytesAsBase64 = (_key: string, value: unknown) =>
+  value instanceof Uint8Array && !Buffer.isBuffer(value)
+    ? { encoding: "base64", bytes: Buffer.from(value).toString("base64") }
+    : value;
+
+/** Mirrors the evidence reporter's console record encoding and size check. */
+export function fitsEvidenceRecord(line: string, taskId = TASK_ID_ALLOWANCE): boolean {
+  const encoded = JSON.stringify({
+    kind: "console",
+    taskId,
+    stream: "stdout",
+    content: line + "\n",
+  });
+  return Buffer.byteLength(encoded) <= MAX_EVIDENCE_RECORD_BYTES;
+}
+
+const digest = (value: unknown) => {
+  const bytes = Buffer.from(JSON.stringify(value, bytesAsBase64) ?? "null");
+  return { omitted: true, byteLength: bytes.length, sha256: sha256(bytes) };
+};
+
+/**
+ * Serialize one AWS drift diagnostic console line so the evidence reporter can
+ * still bind it to its test.
+ *
+ * The reporter replaces any console record over MAX_EVIDENCE_RECORD_BYTES with
+ * `console-unavailable`. That taints the owning test, and one tainted live
+ * assertion makes the collector withhold the whole observation artifact, so
+ * the native drift gate cannot grade the run.
+ *
+ * SDK event bytes are written as base64. While the line is still too large,
+ * the raw wire bytes, then the SDK events, then the decoded events are
+ * replaced by their size and SHA-256. If it still does not fit, the line keeps
+ * only the cell, the names of the omitted fields, and the size and SHA-256 of
+ * the complete line. Every returned line fits the reporter limit.
+ */
+export function boundedDiagnosticLine(
+  record: Record<string, unknown> & { sdkEvents: unknown[] },
+  wire: Buffer,
+): string {
+  const rawWire = { encoding: "base64", bytes: wire.toString("base64") };
+  const wireDigest = { omitted: true, byteLength: wire.length, sha256: sha256(wire) };
+  const eventsDigest = { count: record.sdkEvents.length, ...digest(record.sdkEvents) };
+  const decodedDigests = Object.fromEntries(
+    (["decoded", "decodedEvents"] as const)
+      .filter((key) => key in record)
+      .map((key) => [key, digest(record[key])]),
+  );
+  const full = JSON.stringify({ ...record, rawWire }, bytesAsBase64);
+  const candidates = [
+    () => full,
+    () => ({ ...record, rawWire: wireDigest }),
+    () => ({ ...record, sdkEvents: eventsDigest, rawWire: wireDigest }),
+    () => ({
+      ...record,
+      sdkEvents: eventsDigest,
+      ...decodedDigests,
+      rawWire: wireDigest,
+    }),
+  ];
+  for (const candidate of candidates) {
+    const value = candidate();
+    const line = typeof value === "string" ? value : JSON.stringify(value, bytesAsBase64);
+    if (fitsEvidenceRecord(line)) return line;
+  }
+  const fullBytes = Buffer.from(full);
+  const summary = {
+    omitted: [...Object.keys(record).filter((key) => key !== "cell"), "rawWire"],
+    byteLength: fullBytes.length,
+    sha256: sha256(fullBytes),
+  };
+  const minimal = JSON.stringify({ cell: record.cell, ...summary }, bytesAsBase64);
+  // `cell` is a short fixed tuple; drop it too rather than ever exceed the limit.
+  return fitsEvidenceRecord(minimal)
+    ? minimal
+    : JSON.stringify({ ...summary, omitted: [...summary.omitted, "cell"] });
+}