diff --git a/src/application/frida/FridaInstrumentationService.test.ts b/src/application/frida/FridaInstrumentationService.test.ts
index 9f53310ab4..c1251f9a52 100644
--- a/src/application/frida/FridaInstrumentationService.test.ts
+++ b/src/application/frida/FridaInstrumentationService.test.ts
@@ -1,3 +1,6 @@
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
 import { expect, it } from "vitest";
 
 import type { FridaInstrumentationPort } from "./FridaInstrumentationPort.js";
@@ -102,6 +105,66 @@ it.each(["startSession", "instrument"] as const)(
   },
 );
 
+it.each([
+  [join(tmpdir(), "rea-missing-frida-script.js"), "no file exists there"],
+  [tmpdir(), "it is not a regular file"],
+])(
+  "rejects the unreadable script file %s before starting Frida",
+  async (path, reason) => {
+    let starts = 0;
+    const provider = createProvider({
+      async startSession() {
+        starts += 1;
+        return { ok: true, value: createSession() };
+      },
+    });
+    const service = new FridaInstrumentationService(provider);
+
+    const result = await service.instrument({
+      mode: "spawn",
+      deviceId: "local",
+      program: "/tmp/fixture",
+      source: { sourceKind: "file", path },
+      durationMs: 1,
+    });
+
+    expect(result.ok).toBe(false);
+    if (result.ok) return;
+    expect(result.error).toBeInstanceOf(AnalysisInputError);
+    expect((result.error as AnalysisInputError).issues).toEqual([
+      {
+        path: ["path"],
+        reason: "invalid_value",
+        message: `Cannot read Frida script file ${path}: ${reason}.`,
+      },
+    ]);
+    expect(starts).toBe(0);
+  },
+);
+
+it("reports cancellation before checking a file script source", async () => {
+  const service = new FridaInstrumentationService(createProvider());
+  const controller = new AbortController();
+  controller.abort(new Error("cancelled before call"));
+
+  const result = await service.instrument(
+    {
+      mode: "spawn",
+      deviceId: "local",
+      program: "/tmp/fixture",
+      source: {
+        sourceKind: "file",
+        path: join(tmpdir(), "rea-missing-frida-script.js"),
+      },
+      durationMs: 1,
+    },
+    controller.signal,
+  );
+
+  expect(result.ok).toBe(false);
+  if (!result.ok) expect(result.error._tag).toBe("AnalysisCancelledError");
+});
+
 it("does not resume a spawned target when cancellation arrives during script loading", async () => {
   const controller = new AbortController();
   let resumes = 0;
diff --git a/src/application/frida/FridaInstrumentationService.ts b/src/application/frida/FridaInstrumentationService.ts
index 9e9e85931e..c359850d66 100644
--- a/src/application/frida/FridaInstrumentationService.ts
+++ b/src/application/frida/FridaInstrumentationService.ts
@@ -1,11 +1,15 @@
+import { constants } from "node:fs";
+import { access, stat } from "node:fs/promises";
 import { setTimeout as delay } from "node:timers/promises";
 
 import { createEvidence, type Evidence } from "../../domain/evidence.js";
 import { jsonObjectSchema, jsonValueSchema } from "../../domain/jsonValue.js";
 import type { AnalysisError } from "../../domain/analysisErrorBase.js";
 import {
+  AnalysisAccessDeniedError,
   AnalysisCancelledError,
   AnalysisCapabilityUnavailableError,
+  AnalysisInputError,
 } from "../../domain/analysisErrorCore.js";
 import { err, ok, type Result } from "../../domain/result.js";
 import type {
@@ -19,6 +23,64 @@ import type {
   StartFridaSessionInput,
 } from "./FridaInstrumentationPort.js";
 
+const unreadableScriptFile = async (
+  source: FridaScriptSource,
+): Promise<AnalysisError | undefined> => {
+  if (source.sourceKind !== "file") return undefined;
+  let reason: string;
+  let cause: unknown;
+  try {
+    if (!(await stat(source.path)).isFile())
+      reason = "it is not a regular file";
+    else {
+      await access(source.path, constants.R_OK);
+      return undefined;
+    }
+  } catch (failure: unknown) {
+    cause = failure;
+    const code =
+      failure instanceof Error && "code" in failure ? failure.code : undefined;
+    if (code === "EACCES" || code === "EPERM")
+      return new AnalysisAccessDeniedError(
+        "instrument_with_frida",
+        source.path,
+        code,
+        { cause },
+      );
+    reason =
+      code === "ENOENT" ? "no file exists there" : String(code ?? failure);
+  }
+  return new AnalysisInputError(
+    "instrument_with_frida",
+    cause === undefined ? undefined : { cause },
+    [
+      {
+        path: ["path"],
+        reason: "invalid_value",
+        message: `Cannot read Frida script file ${source.path}: ${reason}.`,
+      },
+    ],
+  );
+};
+
+/**
+ * Reject an unreadable script file before any attach or spawn effect;
+ * cancellation before or during the check still wins.
+ */
+const rejectBeforeStart = async (
+  source: FridaScriptSource,
+  signal: AbortSignal | undefined,
+): Promise<AnalysisError | undefined> => {
+  const cancelled = () =>
+    new AnalysisCancelledError(
+      "instrument_with_frida",
+      signal?.reason === undefined ? {} : { cause: signal.reason },
+    );
+  if (signal?.aborted) return cancelled();
+  const unreadable = await unreadableScriptFile(source);
+  return signal?.aborted ? cancelled() : unreadable;
+};
+
 /** Shared CLI/MCP workflows for Frida's device and instrumentation operations. */
 export class FridaInstrumentationService {
   readonly #provider: FridaInstrumentationPort;
@@ -163,6 +225,8 @@ export class FridaInstrumentationService {
       AnalysisError
     >
   > {
+    const rejected = await rejectBeforeStart(input.source, signal);
+    if (rejected !== undefined) return err(rejected);
     const started = await this.#startSession(
       input,
       signal,