diff --git a/docs/flutter-analysis.md b/docs/flutter-analysis.md
index dab4efad9e..cff9a1d258 100644
--- a/docs/flutter-analysis.md
+++ b/docs/flutter-analysis.md
@@ -108,6 +108,8 @@ server; older servers keep the complete request after schema discovery.
   `build_id_candidates` preserves distinct IDs. `build_id` is populated only
   when all declared note ranges were examined and one ID was found;
   conflicting IDs or malformed/bounded note metadata make coverage partial.
+  Nonempty GNU build-ID descriptors up to 64 bytes are retained as lowercase
+  hexadecimal, including IDs shorter than four bytes.
   Matching bytes outside declared note ranges cannot supply a build ID.
 - Labeled strings must fit their complete grammar. `dart_version_capped`
   and `toolchain_capped` independently report labels omitted by grammar or
diff --git a/src/domain/flutter/flutterBuildAnalysis.ts b/src/domain/flutter/flutterBuildAnalysis.ts
index f50dfdf934..30939f3afa 100644
--- a/src/domain/flutter/flutterBuildAnalysis.ts
+++ b/src/domain/flutter/flutterBuildAnalysis.ts
@@ -5,6 +5,14 @@ import { digestSchema } from "../digests.js";
 /** Full logical ZIP path budget, enforced by Flutter library inventory. */
 export const MAX_FLUTTER_LIBRARY_PATH_BYTES = 1024;
 
+/** Retained GNU build-ID descriptor budget, in bytes. */
+export const MAX_FLUTTER_BUILD_ID_BYTES = 64;
+const gnuBuildIdSchema = z
+  .string()
+  .min(2)
+  .max(MAX_FLUTTER_BUILD_ID_BYTES * 2)
+  .regex(/^(?:[0-9a-f]{2})+$/u);
+
 /**
  * A libapp label can occupy every remaining byte of an admitted path. Its
  * Unicode character count cannot exceed that byte count. The query permits
@@ -118,15 +126,13 @@ export const flutterResultSchemas = {
           present: z.boolean(),
           bytes: z.int().nonnegative().nullable(),
           sha256: digestSchema.nullable(),
-          build_id: z
-            .string()
-            .regex(/^[0-9a-f]{8,128}$/u)
+          build_id: gnuBuildIdSchema
             .nullable()
             .describe(
               "The single GNU build ID when ELF note coverage is complete and unambiguous",
             ),
           build_id_candidates: z
-            .array(z.string().regex(/^[0-9a-f]{8,128}$/u))
+            .array(gnuBuildIdSchema)
             .max(64)
             .describe(
               "Distinct GNU build IDs observed in ELF-owned note ranges; conflicting IDs remain explicit",
diff --git a/src/flutter/ElfBuildId.ts b/src/flutter/ElfBuildId.ts
index 91a6ac1c3c..9d1cccd961 100644
--- a/src/flutter/ElfBuildId.ts
+++ b/src/flutter/ElfBuildId.ts
@@ -1,3 +1,4 @@
+import { MAX_FLUTTER_BUILD_ID_BYTES } from "../domain/flutter/flutterBuildAnalysis.js";
 import type { FileHandle } from "node:fs/promises";
 import { setImmediate as yieldToEventLoop } from "node:timers/promises";
 import {
@@ -167,9 +168,9 @@ export const readElfBuildIds = async (
         type === 3 &&
         notes.subarray(at + 12, at + 16).equals(Buffer.from("GNU\0"))
       ) {
-        if (descsz < 4 || descsz > 64)
+        if (descsz < 1 || descsz > MAX_FLUTTER_BUILD_ID_BYTES)
           return failure(
-            "GNU build-id descriptor exceeds the 4–64 byte projection",
+            `GNU build-id descriptor is outside the 1–${MAX_FLUTTER_BUILD_ID_BYTES} byte projection`,
           );
         const candidate = notes.subarray(descriptor, end).toString("hex");
         if (!candidates.has(candidate) && candidates.size === MAX_BUILD_IDS)
diff --git a/tests/acceptance/flutterAnalysis.test.ts b/tests/acceptance/flutterAnalysis.test.ts
index 5e9a8889b8..7d268a9999 100644
--- a/tests/acceptance/flutterAnalysis.test.ts
+++ b/tests/acceptance/flutterAnalysis.test.ts
@@ -1,3 +1,4 @@
+import { gnuBuildIdNote } from "../../src/native/elf/ElfNotes.fixture.js";
 import { createHash } from "node:crypto";
 import { expect } from "vitest";
 import { toolContract } from "../../src/contracts/toolContracts.js";
@@ -245,3 +246,44 @@ cliTest(
     }
   },
 );
+
+cliTest(
+  "CLI and stdio MCP preserve byte-sized GNU build IDs and descriptor coverage",
+  async ({ cli }) => {
+    const { call } = await connectLocalToolsMcp();
+    for (const bytes of [0, 1, 2, 3, 64, 65]) {
+      const hex = "ab".repeat(bytes);
+      const path = await createFlutterApk(`engine-${bytes}.apk`, [
+        { path: "lib/arm64-v8a/libapp.so", bytes: aotFixtureImage() },
+        {
+          path: "lib/arm64-v8a/libflutter.so",
+          bytes: engineFixtureImage({ notes: gnuBuildIdNote(hex) }),
+        },
+      ]);
+      const output = await cli.run({
+        arguments: ["identify-flutter-build", path, "--json"],
+      });
+      expect(output.exitCode, output.stderr).toBe(0);
+      const cliEvidence = parseEvidence(output.json);
+      const response = await call("identify_flutter_build", { path });
+      expect(response.isError, JSON.stringify(response)).not.toBe(true);
+      const mcpEvidence = parseEvidence(response.structuredContent);
+      expect(mcpEvidence.normalized_result).toEqual(
+        cliEvidence.normalized_result,
+      );
+      const result = flutterResultSchemas.identify_flutter_build.parse(
+        cliEvidence.normalized_result,
+      );
+      const admitted = bytes > 0 && bytes <= 64;
+      expect(result.abis[0]?.libflutter).toMatchObject({
+        build_id: admitted ? hex : null,
+        build_id_candidates: admitted ? [hex] : [],
+      });
+      expect(result.coverage).toBe(admitted ? "complete" : "partial");
+      if (!admitted)
+        expect(cliEvidence.limitations.join("\n")).toContain(
+          "1–64 byte projection",
+        );
+    }
+  },
+);