diff --git a/.github/actions/launchplane-request/action.yml b/.github/actions/launchplane-request/action.yml
index 95e2a27d..d036f375 100644
--- a/.github/actions/launchplane-request/action.yml
+++ b/.github/actions/launchplane-request/action.yml
@@ -27,6 +27,14 @@ inputs:
       are valid JSON literals or objects. Example: request.reason=manual.
     required: false
     default: ""
+  payload-json-files:
+    description: >-
+      Newline-separated JSON path assignments to overlay onto the payload after
+      payload-fields. Assignment values are local file paths whose parsed JSON
+      contents are written at the target path. Example:
+      publish.manifest=artifact.json.
+    required: false
+    default: ""
   method:
     description: HTTP method to use.
     required: false
diff --git a/.github/actions/launchplane-request/dist/index.js b/.github/actions/launchplane-request/dist/index.js
index 5ae1de3e..9a33fafb 100644
--- a/.github/actions/launchplane-request/dist/index.js
+++ b/.github/actions/launchplane-request/dist/index.js
@@ -195,6 +195,34 @@ function applyPayloadFields(payload) {
   return payload;
 }
 
+function applyPayloadJsonFiles(payload) {
+  const payloadJsonFiles = getInput("payload-json-files");
+  if (!payloadJsonFiles) {
+    return payload;
+  }
+  if (payload === null || typeof payload !== "object" || Array.isArray(payload)) {
+    throw new Error("payload-json-files requires payload or payload-file to contain a JSON object.");
+  }
+  for (const line of payloadJsonFiles.split(/\r?\n/)) {
+    const trimmedLine = line.trim();
+    if (!trimmedLine || trimmedLine.startsWith("#")) {
+      continue;
+    }
+    const separatorIndex = trimmedLine.indexOf("=");
+    if (separatorIndex <= 0) {
+      throw new Error(`Invalid payload JSON file '${trimmedLine}'. Use json.path=file-path.`);
+    }
+    const path = trimmedLine.slice(0, separatorIndex).trim();
+    const filePath = line.slice(line.indexOf("=") + 1).trim();
+    if (!filePath) {
+      throw new Error(`Payload JSON file '${path}' requires a file path.`);
+    }
+    const fileValue = JSON.parse(fs.readFileSync(filePath, "utf8"));
+    writeJsonPath(payload, path, fileValue);
+  }
+  return payload;
+}
+
 function splitCommaSeparated(value) {
   return String(value ?? "")
     .split(",")
@@ -354,7 +382,7 @@ async function main() {
   const requestUrl = new URL(routePath, launchplaneUrl).toString();
   const audience = getInput("audience") || launchplaneUrl.host;
   const idempotencyKey = getInput("idempotency-key");
-  const payload = applyPayloadFields(readPayload());
+  const payload = applyPayloadJsonFiles(applyPayloadFields(readPayload()));
   const options = getActionOptions();
   const token = await requestGitHubOidcToken(audience, options);
   const headers = {
diff --git a/docs/product-repo-contract.md b/docs/product-repo-contract.md
index 01e4d1a2..a6a7af0b 100644
--- a/docs/product-repo-contract.md
+++ b/docs/product-repo-contract.md
@@ -189,9 +189,21 @@ payload-fields: |-
 
 Each `payload-fields` line is `json.path=value`. Values are strings unless they
 parse as JSON literals or objects, so `false`, `300`, and `{}` keep their JSON
-types. Product repos may still need small payload-builder scripts for large
-manifest-derived payloads until Launchplane owns the next layer of
-product-specific request assembly.
+types. Use `payload-json-files` when a workflow already has a JSON artifact file
+and only needs to splice it into a static Launchplane request:
+
+```yaml
+payload: >-
+  {"schema_version":1,"product":"odoo","publish":{"schema_version":1}}
+payload-fields: |-
+  publish.context=cm
+  publish.instance=${{ github.event.inputs.instance }}
+payload-json-files: |-
+  publish.manifest=${{ steps.publish.outputs.manifest_file }}
+```
+
+Each `payload-json-files` line is `json.path=file-path`; the action parses the
+file as JSON and writes that value into the request before sending it.
 
 For asynchronous Launchplane routes that report a temporary status, configure
 polling instead of reimplementing OIDC and retry logic in the product repo:
diff --git a/tests/test_launchplane_request_action.py b/tests/test_launchplane_request_action.py
index b4a23e9e..f564d7dd 100644
--- a/tests/test_launchplane_request_action.py
+++ b/tests/test_launchplane_request_action.py
@@ -161,6 +161,35 @@ def test_overlays_payload_fields_before_request(self) -> None:
         self.assertIs(payload["rollback"]["wait_for_deploy"], False)
         self.assertEqual(payload["rollback"]["timeout_seconds"], 300)
 
+    def test_overlays_payload_json_files_before_request(self) -> None:
+        with TemporaryDirectory() as temporary_directory:
+            manifest_path = Path(temporary_directory) / "artifact.json"
+            manifest_path.write_text(
+                json.dumps(
+                    {
+                        "artifact_id": "artifact-cm-123",
+                        "source_commit": "abc123",
+                    }
+                ),
+                encoding="utf-8",
+            )
+            result = self.run_action(
+                inputs={
+                    "launchplane-url": "https://launchplane.example",
+                    "route-path": "/v1/drivers/odoo/artifact-publish",
+                    "payload": '{"schema_version":1,"product":"odoo","publish":{"schema_version":1,"context":"cm"}}',
+                    "payload-fields": "publish.instance=testing",
+                    "payload-json-files": f"publish.manifest={manifest_path}",
+                },
+            )
+
+        self.assertEqual(result.returncode, 0, result.stderr)
+        calls = json.loads(result.stderr.strip().splitlines()[-1])
+        payload = json.loads(calls[1]["body"])
+        self.assertEqual(payload["publish"]["instance"], "testing")
+        self.assertEqual(payload["publish"]["manifest"]["artifact_id"], "artifact-cm-123")
+        self.assertEqual(payload["publish"]["manifest"]["source_commit"], "abc123")
+
     def test_rejects_payload_fields_without_object_payload(self) -> None:
         result = self.run_action(
             inputs={
@@ -173,6 +202,18 @@ def test_rejects_payload_fields_without_object_payload(self) -> None:
         self.assertNotEqual(result.returncode, 0)
         self.assertIn("payload-fields requires payload or payload-file", result.stderr)
 
+    def test_rejects_payload_json_files_without_object_payload(self) -> None:
+        result = self.run_action(
+            inputs={
+                "launchplane-url": "https://launchplane.example",
+                "route-path": "/v1/drivers/odoo/artifact-publish",
+                "payload-json-files": "publish.manifest=artifact.json",
+            },
+        )
+
+        self.assertNotEqual(result.returncode, 0)
+        self.assertIn("payload-json-files requires payload or payload-file", result.stderr)
+
     def test_fails_when_driver_result_status_is_configured_as_failure(self) -> None:
         result = self.run_action(
             inputs={