diff --git a/src/browser/history/protobuf/ProtobufBody.ts b/src/browser/history/protobuf/ProtobufBody.ts
index 7d040fd9c1..eb3d91777b 100644
--- a/src/browser/history/protobuf/ProtobufBody.ts
+++ b/src/browser/history/protobuf/ProtobufBody.ts
@@ -3,7 +3,7 @@ import {
   prepareProtobufBody,
   type PreparedProtobufBody,
 } from "./ProtobufFraming.js";
-import { CaptureFormatError } from "../CaptureFormatError.js";
+import { parseGrpcWebTrailers } from "./ProtobufTrailers.js";
 import type {
   ProtobufDecoding,
   ProtobufFrame,
@@ -155,33 +155,22 @@ export const decodeProtobufBody = async (
         };
       } else {
         budget.charge(length * 8);
-        try {
-          const text = new TextDecoder("utf-8", {
-            fatal: true,
-            ignoreBOM: true,
-          }).decode(data.subarray(at + 5, at + 5 + length));
-          frame.trailers = [];
-          for (const line of text.split("\r\n")) {
-            if (line === "") continue;
-            const colon = line.indexOf(":");
-            if (
-              colon <= 0 ||
-              !/^[!#$%&'*+.^_`|~0-9A-Za-z-]+$/.test(line.slice(0, colon))
-            )
-              throw new Error("Malformed trailer header");
-            budget.charge(512);
-            frame.trailers.push({
-              name: redact(line.slice(0, colon)),
-              value: redact(line.slice(colon + 1).replace(/^[ \t]+/, "")),
-            });
-          }
-          frame.examined_bytes = length;
-        } catch (cause) {
-          if (cause instanceof CaptureFormatError) throw cause;
+        const parsed = parseGrpcWebTrailers(
+          data.subarray(at + 5, at + 5 + length),
+          budget,
+        );
+        const facts = parsed.ok ? parsed.value : parsed.error;
+        frame.examined_bytes = facts.examined_bytes;
+        if (parsed.ok || parsed.error.kind !== "invalid-utf8")
+          frame.trailers = facts.trailers.map(({ name, value }) => ({
+            name: redact(name),
+            value: redact(value),
+          }));
+        if (!parsed.ok) {
           frame.status = "partial";
           frame.error = {
             offset: at + 5,
-            reason: "Invalid gRPC-web trailer header block",
+            reason: parsed.error.reason,
           };
         }
       }
diff --git a/src/browser/history/protobuf/ProtobufBodyTrailer.test.ts b/src/browser/history/protobuf/ProtobufBodyTrailer.test.ts
new file mode 100644
index 0000000000..71dbb30306
--- /dev/null
+++ b/src/browser/history/protobuf/ProtobufBodyTrailer.test.ts
@@ -0,0 +1,111 @@
+import { expect, it } from "vitest";
+import { decodeProtobufBody } from "./ProtobufBody.js";
+import { ProtobufBudget } from "./ProtobufWire.js";
+
+const source = {
+  pointer: "/response/content/text",
+  content_type: "application/grpc-web+proto",
+  body_sha256: "a".repeat(64),
+};
+const same = (value: string) => value;
+const trailerFrame = (text: string) => {
+  const payload = Buffer.from(text, "utf8");
+  const frame = Buffer.alloc(5 + payload.length);
+  frame[0] = 128;
+  frame.writeUInt32BE(payload.length, 1);
+  payload.copy(frame, 5);
+  return frame;
+};
+
+it("rejects forbidden trailer controls while retaining valid earlier trailers", async () => {
+  for (const code of [
+    ...Array.from({ length: 32 }, (_, index) => index),
+    127,
+  ]) {
+    if (code === 9) continue;
+    const control = String.fromCharCode(code);
+    const text = `grpc-message: kept\r\ngrpc-status: 0${control}bad\r\n`;
+    const input = trailerFrame(text);
+    const result = await decodeProtobufBody(
+      input,
+      "grpc-web",
+      source,
+      same,
+      new ProtobufBudget(),
+    );
+    expect(result.status, JSON.stringify(control)).toBe("partial");
+    expect(result.frames[0]?.error).toMatchObject({ offset: 5 });
+    expect(result.examined_bytes).toBe(5 + Buffer.byteLength(text));
+    expect(result.frames[0]).toMatchObject({
+      status: "partial",
+      examined_bytes: Buffer.byteLength(text),
+    });
+    expect(result.frames[0]?.trailers).toEqual([
+      { name: "grpc-message", value: "kept" },
+    ]);
+    expect(input.subarray(5)).toEqual(Buffer.from(text, "utf8"));
+  }
+});
+
+it("retains allowed whitespace, visible text, and valid UTF-8 obs-text", async () => {
+  const text = "grpc-message: alpha\tbeta \u00e9\r\n";
+  const result = await decodeProtobufBody(
+    trailerFrame(text),
+    "grpc-web",
+    source,
+    same,
+    new ProtobufBudget(),
+  );
+  expect(result.status).toBe("complete");
+  expect(result.frames[0]?.trailers).toEqual([
+    { name: "grpc-message", value: "alpha\tbeta \u00e9" },
+  ]);
+  expect(result.examined_bytes).toBe(5 + Buffer.byteLength(text));
+});
+
+it.each(["Grpc-Status", "GRPC-STATUS"])(
+  "retains a valid prefix when the trailer name %s violates gRPC-Web casing",
+  async (name) => {
+    const text = `grpc-message: kept\r\n${name}: 0\r\n`;
+    const result = await decodeProtobufBody(
+      trailerFrame(text),
+      "grpc-web",
+      source,
+      same,
+      new ProtobufBudget(),
+    );
+    expect(result.status).toBe("partial");
+    expect(result.examined_bytes).toBe(5 + Buffer.byteLength(text));
+    expect(result.frames[0]?.trailers).toEqual([
+      { name: "grpc-message", value: "kept" },
+    ]);
+  },
+);
+
+it("leaves undecodable trailer fields unknown and preserves unexpected failures", async () => {
+  const input = trailerFrame("grpc-status: 0\r\n");
+  input[5] = 255;
+  const malformed = await decodeProtobufBody(
+    input,
+    "grpc-web",
+    source,
+    same,
+    new ProtobufBudget(),
+  );
+  expect(malformed.status).toBe("partial");
+  expect(malformed.frames[0]?.examined_bytes).toBe(0);
+  expect(malformed.frames[0]?.trailers).toBeUndefined();
+
+  const failure = { kind: "controlled-redaction-failure" };
+  await expect(
+    decodeProtobufBody(
+      trailerFrame("grpc-status: 0\r\n"),
+      "grpc-web",
+      source,
+      () => {
+        throw failure;
+      },
+      new ProtobufBudget(),
+    ),
+  ).rejects.toBe(failure);
+});
diff --git a/src/browser/history/protobuf/ProtobufCapture.test.ts b/src/browser/history/protobuf/ProtobufCapture.test.ts
index 7e711471be..a0d7e6d609 100644
--- a/src/browser/history/protobuf/ProtobufCapture.test.ts
+++ b/src/browser/history/protobuf/ProtobufCapture.test.ts
@@ -10,47 +10,78 @@ import {
   decodeCaptureProtobuf,
 } from "./ProtobufCapture.js";
 
-it("restricts framing overrides to native body facts while preserving header bytes", async () => {
-  const binary = (pointer: string, bytes: Buffer) => ({
-    pointer,
-    representation: "producer-bytes" as const,
-    state: "retained" as const,
-    bytes: bytes.length,
-    sha256: createHash("sha256").update(bytes).digest("hex"),
-    content_base64: bytes.toString("base64"),
-  });
-  const headers = [
-    binary("/response/headers/0/0", Buffer.from("content-type")),
-    binary("/response/headers/0/1", Buffer.from("application/grpc")),
-  ];
-  const report: WebNetworkCapture = {
-    artifact: { path: "/fixture.mitm", bytes: 100, sha256: "a".repeat(64) },
-    decoder: { id: "fixture", name: "Native producer facts", version: "1" },
-    container: {
-      reported: null,
+const binary = (pointer: string, bytes: Buffer) => ({
+  pointer,
+  representation: "producer-bytes" as const,
+  state: "retained" as const,
+  bytes: bytes.length,
+  sha256: createHash("sha256").update(bytes).digest("hex"),
+  content_base64: bytes.toString("base64"),
+});
+
+const captureWithBody = (
+  contentType: string,
+  body: Buffer,
+): WebNetworkCapture => ({
+  artifact: { path: "/fixture.mitm", bytes: 100, sha256: "a".repeat(64) },
+  decoder: { id: "fixture", name: "Native producer facts", version: "1" },
+  container: {
+    reported: null,
+    numeric_literals: [],
+    redactions: [],
+    records_pointer: "/flows",
+  },
+  format: "mitmproxy",
+  total_records: 1,
+  runtime_attribution: "unknown",
+  limitations: [],
+  records: [
+    {
+      ordinal: 0,
+      location: { kind: "byte-range", offset: 0, bytes: 100 },
+      reported: { response: { headers: [[null, null]], content: null } },
+      binary_fields: [
+        binary("/response/headers/0/0", Buffer.from("content-type")),
+        binary("/response/headers/0/1", Buffer.from(contentType)),
+        binary("/response/content", body),
+      ],
       numeric_literals: [],
       redactions: [],
-      records_pointer: "/flows",
+      limitations: [],
     },
-    format: "mitmproxy",
-    total_records: 1,
-    runtime_attribution: "unknown",
-    limitations: [],
-    records: [
-      {
-        ordinal: 0,
-        location: { kind: "byte-range", offset: 0, bytes: 100 },
-        reported: { response: { headers: [[null, null]], content: null } },
-        binary_fields: [
-          ...headers,
-          binary("/response/content", Buffer.from([0, 0, 0, 0, 2, 8, 1])),
-        ],
-        numeric_literals: [],
-        redactions: [],
-        limitations: [],
-      },
-    ],
-  };
+  ],
+});
+
+const decode = async (capture: WebNetworkCapture) => {
+  const input = inspectWebNetworkCaptureInputSchema.parse({
+    capture_path: capture.artifact.path,
+    format: capture.format,
+    record_ordinals: [0],
+  });
+  const resources = new ArtifactResourceScope();
+  try {
+    const prepared = await prepareCaptureProtobuf(capture, input);
+    await decodeCaptureProtobuf(capture, input, resources, prepared);
+  } finally {
+    await resources.close();
+  }
+};
+
+const grpcWebTrailerFrame = (text: Buffer) => {
+  const frame = Buffer.alloc(5 + text.length);
+  frame[0] = 128;
+  frame.writeUInt32BE(text.length, 1);
+  text.copy(frame, 5);
+  return frame;
+};
+
+it("restricts framing overrides to native body facts while preserving header bytes", async () => {
+  const payload = Buffer.from([8, 1]);
+  const body = Buffer.alloc(5 + payload.length);
+  body.writeUInt32BE(payload.length, 1);
+  payload.copy(body, 5);
+  const report = captureWithBody("application/grpc", body);
+  const headers = report.records[0]!.binary_fields.slice(0, 2);
   const input = inspectWebNetworkCaptureInputSchema.parse({
     capture_path: report.artifact.path,
     format: report.format,
@@ -75,3 +106,20 @@ it("restricts framing overrides to native body facts while preserving header byt
   expect(report.records[0]?.binary_fields.slice(0, 2)).toEqual(headers);
   expect(report.protobuf_coverage?.status).toBe("complete");
 });
+
+it("keeps capture coverage partial for a malformed gRPC-web trailer", async () => {
+  const trailerText = Buffer.from("grpc-status: 0\u0000bad\r\n");
+  const body = grpcWebTrailerFrame(trailerText);
+  const report = captureWithBody("application/grpc-web", body);
+  const retainedBody = report.records[0]!.binary_fields[2];
+  await decode(report);
+
+  expect(report.records[0]?.protobuf?.[0]).toMatchObject({
+    framing: "grpc-web",
+    status: "partial",
+    examined_bytes: body.length,
+    frames: [{ status: "partial", examined_bytes: trailerText.length }],
+  });
+  expect(report.protobuf_coverage?.status).toBe("partial");
+  expect(report.records[0]?.binary_fields[2]).toEqual(retainedBody);
+});
diff --git a/src/browser/history/protobuf/ProtobufTrailers.ts b/src/browser/history/protobuf/ProtobufTrailers.ts
new file mode 100644
index 0000000000..040923202d
--- /dev/null
+++ b/src/browser/history/protobuf/ProtobufTrailers.ts
@@ -0,0 +1,60 @@
+import { err, ok, type Result } from "../../../domain/result.js";
+import type { ProtobufBudget } from "./ProtobufWire.js";
+
+interface TrailerFacts {
+  readonly trailers: { name: string; value: string }[];
+  readonly examined_bytes: number;
+}
+
+type TrailerFailure = TrailerFacts & {
+  readonly kind: "invalid-utf8" | "invalid-name" | "invalid-value";
+  readonly reason: string;
+};
+
+/** Parse the gRPC-Web HTTP/1 field block without discarding its valid prefix. */
+export const parseGrpcWebTrailers = (
+  bytes: Uint8Array,
+  budget: ProtobufBudget,
+): Result<TrailerFacts, TrailerFailure> => {
+  let text: string;
+  try {
+    text = new TextDecoder("utf-8", {
+      fatal: true,
+      ignoreBOM: true,
+    }).decode(bytes);
+  } catch (cause) {
+    if (!(cause instanceof TypeError)) throw cause;
+    return err({
+      kind: "invalid-utf8",
+      reason: "gRPC-web trailer header block is not valid UTF-8",
+      trailers: [],
+      examined_bytes: 0,
+    });
+  }
+  // The whole UTF-8 block has been examined even when field validation stops.
+  const facts: TrailerFacts = { trailers: [], examined_bytes: bytes.length };
+  for (const line of text.split("\r\n")) {
+    if (line === "") continue;
+    const colon = line.indexOf(":");
+    // Unlike ordinary HTTP/1 headers, in-body gRPC-Web names must be lower-case.
+    if (colon <= 0 || !/^[!#$%&'*+.^_`|~0-9a-z-]+$/.test(line.slice(0, colon)))
+      return err({
+        ...facts,
+        kind: "invalid-name",
+        reason:
+          "gRPC-web trailer header names must be lower-case HTTP field names",
+      });
+    const value = line.slice(colon + 1).replace(/^[ \t]+/, "");
+    // RFC 9110 permits HTAB but excludes other C0 controls and DEL.
+    if (/[\u0000-\u0008\u000a-\u001f\u007f]/u.test(value))
+      return err({
+        ...facts,
+        kind: "invalid-value",
+        reason:
+          "gRPC-web trailer field value contains a forbidden control character",
+      });
+    budget.charge(512);
+    facts.trailers.push({ name: line.slice(0, colon), value });
+  }
+  return ok(facts);
+};
diff --git a/tests/acceptance/tools/protobufCapture.test.ts b/tests/acceptance/tools/protobufCapture.test.ts
index 921c1b13d3..10e6b821c8 100644
--- a/tests/acceptance/tools/protobufCapture.test.ts
+++ b/tests/acceptance/tools/protobufCapture.test.ts
@@ -556,3 +556,71 @@ captureTest(
     });
   },
 );
+
+captureTest(
+  "keeps malformed gRPC-web trailers partial through CLI and MCP without changing capture bytes",
+  async ({ cli, mcp }) => {
+    const root = await createTestTempDirectory("rea-grpc-web-trailers-");
+    for (const framing of ["grpc-web", "grpc-web-text"] as const)
+      for (const [name, headerName, value, status] of [
+        ["valid", "grpc-status", "0", "complete"],
+        ["uppercase", "Grpc-Status", "0", "partial"],
+        ["nul", "grpc-status", "0\u0000bad", "partial"],
+        ["lf", "grpc-status", "0\nbad", "partial"],
+        ["cr", "grpc-status", "0\rbad", "partial"],
+        ["vt", "grpc-status", "0\u000bbad", "partial"],
+        ["del", "grpc-status", "0\u007fbad", "partial"],
+      ] as const) {
+        const path = join(root, `${framing}-${name}.har`);
+        const trailer = Buffer.from(`${headerName}: ${value}\r\n`);
+        const body = Buffer.concat([
+          protobufFrame(Buffer.from([8, 1])),
+          protobufFrame(trailer, 128),
+        ]);
+        const original = protobufHar(
+          framing === "grpc-web-text"
+            ? Buffer.from(body.toString("base64"))
+            : body,
+          `application/${framing}`,
+        );
+        await writeFile(path, original);
+        const response = await cli.run({
+          arguments: [
+            "inspect-web-network-capture",
+            path,
+            "har",
+            "--record",
+            "0",
+            "--protobuf-framing",
+            "auto",
+            "--json",
+          ],
+        });
+        expect(response.exitCode, response.stdout + response.stderr).toBe(0);
+        const evidence = parseEvidence(response.json);
+        const report = webNetworkCaptureSchema.parse(
+          evidence.normalized_result,
+        );
+        expect(report.protobuf_coverage?.status, name).toBe(status);
+        expect(report.records[0]?.protobuf?.[0]).toMatchObject({
+          status,
+          framing,
+          examined_bytes: body.length,
+          frames: [
+            { kind: "data", status: "complete", examined_bytes: 2 },
+            { kind: "trailer", status, examined_bytes: trailer.length },
+          ],
+          fields: [{ number: 1, wire_type: "varint", value: "1" }],
+        });
+        const reply = await mcp.call("inspect_web_network_capture", {
+          capture_path: path,
+          format: "har",
+          record_ordinals: [0],
+          protobuf: { framing: "auto" },
+        });
+        expect(reply.isError, JSON.stringify(reply)).not.toBe(true);
+        expect(reply.structuredContent).toEqual(response.json);
+        expect(await readFile(path, "utf8")).toBe(original);
+      }
+  },
+);