diff --git a/src/domain/analysisSnapshot.test.ts b/src/domain/analysisSnapshot.test.ts
index b79f38704..ee3920be5 100644
--- a/src/domain/analysisSnapshot.test.ts
+++ b/src/domain/analysisSnapshot.test.ts
@@ -127,14 +127,25 @@ describe("analysis snapshot contract", () => {
   ] as const)(
     "preserves %s target and subject formats",
     (format, architecture) => {
-      const target = {
-        path: ANALYSIS_SNAPSHOT_TARGET.path,
-        sha256: ANALYSIS_SNAPSHOT_TARGET.sha256,
-        kind: "executable" as const,
-        format,
-        architecture,
-        availableArchitectures: [architecture],
-      };
+      const target: BinaryTarget =
+        format === "mach-o-universal"
+          ? {
+              path: ANALYSIS_SNAPSHOT_TARGET.path,
+              sha256: ANALYSIS_SNAPSHOT_TARGET.sha256,
+              kind: "executable",
+              format,
+              architecture,
+              availableArchitectures: [architecture],
+              universalSlice: { offset: 0, size: 32 },
+            }
+          : {
+              path: ANALYSIS_SNAPSHOT_TARGET.path,
+              sha256: ANALYSIS_SNAPSHOT_TARGET.sha256,
+              kind: "executable",
+              format,
+              architecture,
+              availableArchitectures: [architecture],
+            };
       const evidence = createEvidence(target, ANALYSIS_SNAPSHOT_PROVIDER, {
         operation: "analyze_function",
         parameters: {},
diff --git a/src/domain/binaryTarget.ts b/src/domain/binaryTarget.ts
index 66edd2590..79a9daefa 100644
--- a/src/domain/binaryTarget.ts
+++ b/src/domain/binaryTarget.ts
@@ -1,6 +1,7 @@
 import type {
   BinaryArchitecture,
   MipsElfMetadata,
+  UniversalMachOSlice,
 } from "./binaryTargetTypes.js";
 import { err, ok, type Result } from "./result.js";
 import { mzWindowsHeaderOffset, parseDosMzHeader } from "./dosMz.js";
@@ -15,17 +16,22 @@ export type ExecutableMetadata =
       readonly mips: MipsElfMetadata;
     }
   | {
-      readonly format:
-        | "mach-o"
-        | "mach-o-universal"
-        | "elf"
-        | "dos-mz"
-        | "dos-com";
+      readonly format: "mach-o" | "elf" | "dos-mz" | "dos-com";
       readonly architecture: Exclude<BinaryArchitecture, "mips">;
       readonly availableArchitectures: readonly Exclude<
         BinaryArchitecture,
         "mips"
       >[];
+      readonly universalSlice?: never;
+    }
+  | {
+      readonly format: "mach-o-universal";
+      readonly architecture: Exclude<BinaryArchitecture, "mips">;
+      readonly availableArchitectures: readonly Exclude<
+        BinaryArchitecture,
+        "mips"
+      >[];
+      readonly universalSlice: UniversalMachOSlice;
     }
   | {
       readonly format: "pe";
@@ -157,10 +163,15 @@ const parseFatMachO = (
     return err(
       `truncated FAT slice: the ${preferred} slice ends at byte ${sliceEnd}; the file has ${fileSize}`,
     );
+  const sliceOffset = Number(slice.offset);
+  const sliceSize = Number(slice.size);
+  if (!Number.isSafeInteger(sliceOffset) || !Number.isSafeInteger(sliceSize))
+    return err(`FAT slice for ${preferred} exceeds addressable byte bounds`);
   return ok({
     format: "mach-o-universal",
     architecture: preferred,
     availableArchitectures: architectures,
+    universalSlice: { offset: sliceOffset, size: sliceSize },
   });
 };
 
diff --git a/src/domain/binaryTargetTypes.ts b/src/domain/binaryTargetTypes.ts
index 3bd79144f..e73d86cec 100644
--- a/src/domain/binaryTargetTypes.ts
+++ b/src/domain/binaryTargetTypes.ts
@@ -44,6 +44,12 @@ export interface MipsAbiFlags {
   readonly flags2: number;
 }
 
+/** Byte range of the selected slice inside its universal Mach-O container. */
+export interface UniversalMachOSlice {
+  readonly offset: number;
+  readonly size: number;
+}
+
 interface BinaryTargetIdentity {
   readonly path: string;
   readonly sourcePath?: string;
@@ -83,12 +89,15 @@ export type BinaryTarget =
       readonly managed?: never;
     })
   | (ExecutableTarget & {
-      readonly format:
-        | "mach-o"
-        | "mach-o-universal"
-        | "elf"
-        | "dos-mz"
-        | "dos-com";
+      readonly format: "mach-o" | "elf" | "dos-mz" | "dos-com";
+      readonly universalSlice?: never;
+      readonly executableRole?: never;
+      readonly managed?: never;
+    })
+  | (ExecutableTarget & {
+      readonly format: "mach-o-universal";
+      /** Selected slice range a provider must analyze instead of the container. */
+      readonly universalSlice: UniversalMachOSlice;
       readonly executableRole?: never;
       readonly managed?: never;
     })
diff --git a/src/ghidra/GhidraProvider.behavior.test.ts b/src/ghidra/GhidraProvider.behavior.test.ts
index eb3ca1171..a3b2ccf2a 100644
--- a/src/ghidra/GhidraProvider.behavior.test.ts
+++ b/src/ghidra/GhidraProvider.behavior.test.ts
@@ -242,6 +242,7 @@ describe("Ghidra Mach-O slice support", () => {
       format: "mach-o-universal",
       architecture: "arm64",
       availableArchitectures: ["x86_64", "arm64"],
+      universalSlice: { offset: 0, size: 32 },
     };
 
     expect(ghidra.inspectTargetSupport(universal)).toMatchObject({
diff --git a/src/hopper/HopperAnalysisProfile.test.ts b/src/hopper/HopperAnalysisProfile.test.ts
index b71e0d09e..7c3c715f2 100644
--- a/src/hopper/HopperAnalysisProfile.test.ts
+++ b/src/hopper/HopperAnalysisProfile.test.ts
@@ -11,6 +11,7 @@ it("routes physical universal Mach-O containers through Hopper's FAT loader", ()
       format: "mach-o-universal",
       architecture: "arm64",
       availableArchitectures: ["x86_64", "arm64"],
+      universalSlice: { offset: 0, size: 32 },
     },
     "fat64",
   );
diff --git a/src/ida/IdaSessionClient.ts b/src/ida/IdaSessionClient.ts
index f2a624569..b7e0eaf2a 100644
--- a/src/ida/IdaSessionClient.ts
+++ b/src/ida/IdaSessionClient.ts
@@ -506,7 +506,8 @@ export class IdaSessionClient implements AnalysisClient {
       onObserved?.(raw);
       if (
         !sameIdaHostPath(metadata.input_path, this.#workspace.inputPath) ||
-        (await idaFileDigest(this.#workspace.inputPath)) !== this.target.sha256
+        (await idaFileDigest(this.#workspace.inputPath)) !==
+          this.#workspace.inputSha256
       )
         throw new AnalysisProtocolError(
           "IDA headless input identity changed after target admission.",
diff --git a/src/ida/IdaWorkspace.ts b/src/ida/IdaWorkspace.ts
index 009754d35..0b3c36231 100644
--- a/src/ida/IdaWorkspace.ts
+++ b/src/ida/IdaWorkspace.ts
@@ -1,6 +1,6 @@
 import { createHash, randomUUID } from "node:crypto";
-import { createReadStream, constants } from "node:fs";
-import { copyFile, realpath } from "node:fs/promises";
+import { createReadStream } from "node:fs";
+import { open, realpath } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { basename, join, normalize } from "node:path";
 import type { BinaryTarget } from "../domain/binaryTargetTypes.js";
@@ -23,6 +23,7 @@ export const sameIdaHostPath = (left: string, right: string): boolean =>
 /** Private input copy isolates upstream database and sidecar writes from the original. */
 export class IdaWorkspace {
   readonly sessionId = `rea-${randomUUID()}`;
+  #inputSha256: string | undefined;
   private constructor(
     private readonly runtime: PrivateRuntimeRoot,
     readonly inputPath: string,
@@ -34,6 +35,11 @@ export class IdaWorkspace {
     return this.runtime.path;
   }
 
+  /** SHA-256 of the private input bytes copyInput admitted; undefined until prepared. */
+  get inputSha256(): string | undefined {
+    return this.#inputSha256;
+  }
+
   /** Allocate a protected workspace so its owner can retain it before input preparation. */
   static async allocate(
     target: BinaryTarget,
@@ -49,13 +55,54 @@ export class IdaWorkspace {
     return new IdaWorkspace(runtime, inputPath, target);
   }
 
-  /** Copy and verify admitted bytes after the session owns this workspace's cleanup. */
+  /**
+   * Copy and verify admitted bytes after the session owns this workspace's
+   * cleanup. Universal Mach-O containers are thinned to the selected slice so
+   * the upstream engine analyzes the architecture admission reported; the
+   * whole-source digest still anchors the copy against admission-time changes.
+   */
   async copyInput(): Promise<void> {
-    await copyFile(this.target.path, this.inputPath, constants.COPYFILE_EXCL);
-    if ((await idaFileDigest(this.inputPath)) !== this.target.sha256)
-      throw new AnalysisProtocolError(
-        "IDA target changed between admission and the private input copy; reopen the target.",
-      );
+    const slice =
+      this.target.format === "mach-o-universal"
+        ? this.target.universalSlice
+        : undefined;
+    const source = await open(this.target.path, "r");
+    try {
+      const input = await open(this.inputPath, "wx");
+      try {
+        const sourceHash = createHash("sha256");
+        const inputHash = createHash("sha256");
+        const chunk = Buffer.allocUnsafe(64 * 1024);
+        let position = 0;
+        for (;;) {
+          const observed = await source.read(chunk, 0, chunk.length, position);
+          if (observed.bytesRead === 0) break;
+          const bytes = chunk.subarray(0, observed.bytesRead);
+          sourceHash.update(bytes);
+          const start =
+            slice === undefined ? position : Math.max(position, slice.offset);
+          const end =
+            slice === undefined
+              ? position + bytes.length
+              : Math.min(position + bytes.length, slice.offset + slice.size);
+          const admitted = bytes.subarray(start - position, end - position);
+          if (admitted.length > 0) {
+            inputHash.update(admitted);
+            await input.write(admitted);
+          }
+          position += observed.bytesRead;
+        }
+        if (sourceHash.digest("hex") !== this.target.sha256)
+          throw new AnalysisProtocolError(
+            "IDA target changed between admission and the private input copy; reopen the target.",
+          );
+        this.#inputSha256 = inputHash.digest("hex");
+      } finally {
+        await input.close();
+      }
+    } finally {
+      await source.close();
+    }
   }
 
   /** Delete only this object's privately allocated directory after worker cleanup. */
diff --git a/src/ida/IdaWorkspaceThinning.test.ts b/src/ida/IdaWorkspaceThinning.test.ts
new file mode 100644
index 000000000..7feca3ddc
--- /dev/null
+++ b/src/ida/IdaWorkspaceThinning.test.ts
@@ -0,0 +1,131 @@
+import { createHash } from "node:crypto";
+import { readFile, rm, writeFile } from "node:fs/promises";
+import { join } from "node:path";
+import { afterEach, describe, expect, it } from "vitest";
+
+import { parseBinaryTarget } from "../application/BinaryTargetResolver.js";
+import { parseExecutableHeader } from "../domain/binaryTarget.js";
+import { fat } from "../domain/binaryTarget.fixture.js";
+import {
+  createIdaTarget,
+  RecordingIdaMcp,
+} from "../../tests/fixtures/idaMcp.js";
+import { createTestTempDirectory } from "../../tests/fixtures/temporaryDirectory.js";
+import { IdaSessionClient } from "./IdaSessionClient.js";
+import { IdaWorkspace } from "./IdaWorkspace.js";
+
+const roots: string[] = [];
+afterEach(async () => {
+  for (const root of roots.splice(0)) {
+    await rm(root, { recursive: true, force: true });
+  }
+});
+
+/** Write a two-slice universal fixture and resolve it for the given host. */
+const universalTarget = async (hostArchitecture: "arm64" | "x64") => {
+  const directory = await createTestTempDirectory("rea-ida-thin-");
+  roots.push(directory);
+  const path = join(directory, "universal");
+  const bytes = fat([0x01000007, 0x0100000c]);
+  await writeFile(path, bytes);
+  const resolved = await parseBinaryTarget(path, { hostArchitecture });
+  if (!resolved.ok) throw resolved.error;
+  if (resolved.value.format !== "mach-o-universal")
+    throw new Error("the universal fixture did not resolve as a FAT container");
+  return { bytes, target: resolved.value };
+};
+
+describe("IDA workspace input thinning", () => {
+  it.skipIf(process.platform === "win32")(
+    "thins the selected arm64 slice into the private headless input",
+    async () => {
+      const { bytes, target } = await universalTarget("arm64");
+      const workspace = await IdaWorkspace.allocate(target);
+      roots.push(workspace.directory);
+      await workspace.copyInput();
+
+      const input = await readFile(workspace.inputPath);
+      const { offset, size } = target.universalSlice;
+      expect(offset).toBeGreaterThan(0);
+      expect(input.equals(bytes.subarray(offset, offset + size))).toBe(true);
+      expect(input.length).toBe(size);
+      expect(input.readUInt32BE(0)).toBe(0xfeedfacf);
+      expect(input.readUInt32BE(4)).toBe(0x0100000c);
+      expect(parseExecutableHeader(input, "arm64")).toMatchObject({
+        ok: true,
+        value: { format: "mach-o", architecture: "arm64" },
+      });
+      expect(createHash("sha256").update(input).digest("hex")).toBe(
+        workspace.inputSha256,
+      );
+      await workspace.remove();
+    },
+  );
+
+  it.skipIf(process.platform === "win32")(
+    "thins the x86_64 slice when the host is intel",
+    async () => {
+      const { bytes, target } = await universalTarget("x64");
+      const workspace = await IdaWorkspace.allocate(target);
+      roots.push(workspace.directory);
+      await workspace.copyInput();
+
+      const input = await readFile(workspace.inputPath);
+      const { offset, size } = target.universalSlice;
+      expect(input.equals(bytes.subarray(offset, offset + size))).toBe(true);
+      expect(input.readUInt32BE(4)).toBe(0x01000007);
+      await workspace.remove();
+    },
+  );
+
+  it("keeps a whole-file copy for thin targets", async () => {
+    const { root, target } = await createIdaTarget("arm64");
+    roots.push(root);
+    const workspace = await IdaWorkspace.allocate(target, root);
+    roots.push(workspace.directory);
+    await workspace.copyInput();
+
+    const input = await readFile(workspace.inputPath);
+    const source = await readFile(target.path);
+    expect(input.equals(source)).toBe(true);
+    expect(workspace.inputSha256).toBe(target.sha256);
+    await workspace.remove();
+  });
+
+  it.skipIf(process.platform === "win32")(
+    "rejects a universal source that changed after admission",
+    async () => {
+      const { target } = await universalTarget("arm64");
+      const workspace = await IdaWorkspace.allocate({
+        ...target,
+        sha256: "a".repeat(64),
+      });
+      roots.push(workspace.directory);
+      await expect(workspace.copyInput()).rejects.toThrow("target changed");
+      await workspace.remove();
+    },
+  );
+
+  it("binds a headless session to the thinned input identity", async () => {
+    const { target } = await universalTarget("arm64");
+    const root = await createTestTempDirectory("rea-ida-thin-root-");
+    roots.push(root);
+    const producer = new RecordingIdaMcp(target, "headless");
+    const client = new IdaSessionClient(
+      {
+        command: "fixture",
+        args: [],
+        env: {},
+        mode: "headless",
+        timeoutMs: 1000,
+        workspaceRoot: root,
+      },
+      target,
+      producer,
+    );
+    expect((await client.execute("health", {})).ok).toBe(true);
+    const open = producer.calls.find(({ name }) => name === "idb_open");
+    expect(String(open?.args.input_path)).toContain("universal");
+    expect((await client.close()).ok).toBe(true);
+  });
+});
diff --git a/tests/boundary/providers/native/artifactProvider.universalExtraction.test.ts b/tests/boundary/providers/native/artifactProvider.universalExtraction.test.ts
index f451c3863..c89cc9dfc 100644
--- a/tests/boundary/providers/native/artifactProvider.universalExtraction.test.ts
+++ b/tests/boundary/providers/native/artifactProvider.universalExtraction.test.ts
@@ -70,4 +70,5 @@ const universalTarget = (path: string): BinaryTarget => ({
   format: "mach-o-universal",
   architecture: "arm64",
   availableArchitectures: ["arm64"],
+  universalSlice: { offset: 0, size: 32 },
 });