diff --git a/nix/checked-candidate.nix b/nix/checked-candidate.nix
index 4ccb7a6..9446844 100644
--- a/nix/checked-candidate.nix
+++ b/nix/checked-candidate.nix
@@ -7,6 +7,7 @@
   generator,
   contractOnly ? false,
   workspaceId ? null,
+  platformSourceJSON ? null,
   system ? builtins.currentSystem,
 }:
 let
@@ -71,9 +72,16 @@ let
     else
       { };
   execution = (configuration.backend or "typescript") == "execution";
-  runtime = builtins.getFlake (
-    "git+${root.lock.quixos.repository}?rev=${root.lock.quixos.commit}&ref=refs/tags/quixos-reachability/${root.lock.quixos.commit}"
-  );
+  platformSource = if platformSourceJSON == null then null else builtins.fromJSON platformSourceJSON;
+  runtime =
+    if platformSource == null then
+      builtins.getFlake (
+        "git+${root.lock.quixos.repository}?rev=${root.lock.quixos.commit}&ref=refs/tags/quixos-reachability/${root.lock.quixos.commit}"
+      )
+    else
+      assert platformSource.repository == root.lock.quixos.repository;
+      assert platformSource.commit == root.lock.quixos.commit;
+      builtins.getFlake "path:${builtins.storePath platformSource.directory}";
   runtimePackages = runtime.packages.${system};
   resourceClosure =
     includeBuild: node:
diff --git a/src/capability-language/checked-build.ts b/src/capability-language/checked-build.ts
index e7d21a0..922079d 100644
--- a/src/capability-language/checked-build.ts
+++ b/src/capability-language/checked-build.ts
@@ -113,6 +113,7 @@ export async function buildImmutableCandidate(
   contractOnly = false,
   observe?: (phase: "nixEvaluationMs" | "nixBuildMs", ms: number) => void,
   workspaceId?: string,
+  platformSource?: { repository: string; commit: string; directory: string },
 ): Promise<string> {
   if (workspaceId !== undefined && (kind !== "workspace" || !/^[a-zA-Z0-9][a-zA-Z0-9:._-]{0,255}$/.test(workspaceId)))
     throw new Error("Instance binding requires a valid Workspace identity and a Workspace root");
@@ -121,6 +122,13 @@ export async function buildImmutableCandidate(
   const url = new URL(source.repository);
   if (url.protocol !== "https:" || url.username || url.password || url.search || url.hash)
     throw new Error("Candidate origin must be credential-free HTTPS");
+  if (
+    platformSource &&
+    (Object.keys(platformSource).sort().join(",") !== "commit,directory,repository" ||
+      !/^[a-f0-9]{40}$/.test(platformSource.commit) ||
+      !/^\/nix\/store\/[a-z0-9]{32}-[^/\s]+$/.test(platformSource.directory))
+  )
+    throw Error("Invalid immutable platform source input");
   const generator =
     process.env.QUIXOS_CHECK_GENERATOR ?? path.resolve(path.dirname(fileURLToPath(import.meta.url)), "../..");
   if (!/^\/nix\/store\/[^/]+$/.test(generator)) throw new Error("Use the installed Quixos checker");
@@ -204,6 +212,7 @@ export async function buildImmutableCandidate(
         "contractOnly",
         contractOnly ? "true" : "false",
         ...(workspaceId === undefined ? [] : ["--argstr", "workspaceId", workspaceId]),
+        ...(platformSource ? ["--argstr", "platformSourceJSON", JSON.stringify(platformSource)] : []),
         "--add-root",
         gcRoot,
         "--indirect",
diff --git a/src/capability-language/source-check-driver.ts b/src/capability-language/source-check-driver.ts
index 4a4169a..e627287 100644
--- a/src/capability-language/source-check-driver.ts
+++ b/src/capability-language/source-check-driver.ts
@@ -6,5 +6,13 @@ const [repository, commit, log, workspaceId] = process.argv.slice(2);
 if (!repository || !commit || !log || !workspaceId || process.argv.length !== 6)
   throw Error("source-check-driver REPOSITORY COMMIT LOG WORKSPACE_ID");
 process.stdout.write(
-  (await buildImmutableCandidate({ repository, commit }, "workspace", log, false, undefined, workspaceId)) + "\n",
+  (await buildImmutableCandidate(
+    { repository, commit },
+    "workspace",
+    log,
+    false,
+    undefined,
+    workspaceId,
+    process.env.QUIXOS_CHECK_PLATFORM_SOURCE ? JSON.parse(process.env.QUIXOS_CHECK_PLATFORM_SOURCE) : undefined,
+  )) + "\n",
 );
diff --git a/src/resource-lock/cli.ts b/src/resource-lock/cli.ts
index 3562ed8..2d220ff 100644
--- a/src/resource-lock/cli.ts
+++ b/src/resource-lock/cli.ts
@@ -1,5 +1,20 @@
 #!/usr/bin/env node
 import { loadQuixosLock } from "./loader.js";
+import { parseQuixosLockDocument } from "./parser.js";
+import { readFile, stat } from "node:fs/promises";
+
+// A trusted fetcher can inspect the root's declared platform pin without
+// following authored import paths or evaluating Nix. The input is a fetched
+// Git blob, not a filesystem tree provided by an author.
+if (process.argv[2] === "--platform-only") {
+  const filename = process.argv[3];
+  if (process.argv.length !== 4 || !filename || (await stat(filename)).size > 1024 * 1024)
+    throw Error("INVALID_PLATFORM_SOURCE_LOCK");
+  const result = parseQuixosLockDocument(await readFile(filename, "utf8"));
+  if (!result.ok || result.document.kind !== "root") throw Error("INVALID_PLATFORM_SOURCE_LOCK");
+  console.log(JSON.stringify({ repository: result.document.quixos.repository, commit: result.document.quixos.commit }));
+  process.exit(0);
+}
 
 const fileName = process.argv[2];
 if (!fileName || process.argv.length !== 3) {