diff --git a/__tests__/resolve-payload-fields.test.ts b/__tests__/resolve-payload-fields.test.ts
index d302f6bf..0db6a3c6 100644
--- a/__tests__/resolve-payload-fields.test.ts
+++ b/__tests__/resolve-payload-fields.test.ts
@@ -1,7 +1,11 @@
 import { gzipSync } from 'zlib'
 
 /* eslint-disable @typescript-eslint/no-require-imports, @typescript-eslint/no-var-requires */
-const { run, toStepOutputs } = require('../scripts/resolve-payload-fields.js')
+const {
+  run,
+  toStepOutputs,
+  normalizeForEngine
+} = require('../scripts/resolve-payload-fields.js')
 /* eslint-enable @typescript-eslint/no-require-imports, @typescript-eslint/no-var-requires */
 
 const RESOLVER_URL = 'https://resolver.example.com/api'
@@ -315,3 +319,54 @@ describe('run with an oversized-payload reference', () => {
     expect(outputsOf(core).cm_repo_ref).toBe('oversized-payload-reference')
   })
 })
+
+describe('normalizeForEngine', () => {
+  const blob = gzipSync(Buffer.from(JSON.stringify(payload))).toString('base64')
+  const reference = {
+    type: 'oversized-payload-reference',
+    payloadUrl: 'https://resolver.example.com/api/v1/gitstream/payload/key',
+    resolverToken: 'token',
+    pullRequestNumber: 7
+  }
+
+  it.each([
+    ['plain, double-encoded', JSON.stringify(JSON.stringify(payload))],
+    ['plain, single-encoded', JSON.stringify(payload)],
+    ['bare base64(gzip)', blob]
+  ])('passes %s through byte-identical', (_label, raw) => {
+    expect(normalizeForEngine(raw)).toBe(raw)
+  })
+
+  it('hands core the bare blob out of a compressed-payload envelope', () => {
+    const wrapped = JSON.stringify(
+      JSON.stringify({
+        type: 'compressed-payload',
+        data: blob,
+        pullRequestNumber: 7
+      })
+    )
+
+    expect(normalizeForEngine(wrapped)).toBe(blob)
+  })
+
+  it('hands core a single-encoded reference, which is the depth it parses', () => {
+    const wrapped = JSON.stringify(JSON.stringify(reference))
+    const normalized = normalizeForEngine(wrapped)
+
+    expect(JSON.parse(normalized)).toEqual(reference)
+  })
+
+  it('leaves a payload that merely carries a type field alone', () => {
+    const raw = JSON.stringify(JSON.stringify({ ...payload, type: 'push' }))
+
+    expect(normalizeForEngine(raw)).toBe(raw)
+  })
+
+  it('emits the normalized payload as a step output', async () => {
+    const core = await runWith(
+      JSON.stringify(JSON.stringify({ type: 'compressed-payload', data: blob }))
+    )
+
+    expect(outputsOf(core).client_payload).toBe(blob)
+  })
+})
diff --git a/action.yml b/action.yml
index b13ca13a..a15150f6 100644
--- a/action.yml
+++ b/action.yml
@@ -73,7 +73,7 @@ runs:
       env:
         BASE_REF_ARG: ${{ inputs.base_ref }}
         HEAD_REF_ARG: ${{ inputs.head_ref }}
-        PAYLOAD_ARG: ${{ inputs.client_payload }}
+        PAYLOAD_ARG: ${{ steps.payload-fields.outputs.client_payload }}
         URL_ARG: ${{ steps.payload-fields.outputs.url }}
       with:
         script: |
diff --git a/scripts/resolve-payload-fields.js b/scripts/resolve-payload-fields.js
index b6686938..717c7ff5 100644
--- a/scripts/resolve-payload-fields.js
+++ b/scripts/resolve-payload-fields.js
@@ -130,6 +130,25 @@ async function resolvePayload(raw, resolverUrl, core) {
   return { mode: 'plain', payload: parsePayload(raw) }
 }
 
+/**
+ * @param {string} raw the `client_payload` input, verbatim
+ * @returns {string} the form gitstream-core already understands
+ */
+function normalizeForEngine(raw) {
+  const envelope = tryParsePayload(raw)
+  if (!envelope) {
+    // Bare base64(gzip), which core inflates on its own. Bitbucket sends this shape today.
+    return raw
+  }
+  if (envelope.type === COMPRESSED_PAYLOAD && envelope.data) {
+    return envelope.data
+  }
+  if (envelope.type === OVERSIZED_PAYLOAD_REFERENCE) {
+    return JSON.stringify(envelope)
+  }
+  return raw
+}
+
 /**
  * Maps a resolved payload to the step outputs. Output values are strings, so
  * booleans are stringified to be compared as `== 'true'` in step conditions.
@@ -156,7 +175,10 @@ async function run({ core, clientPayload, resolverUrl }) {
     )
     core.info(`client_payload mode=${mode}`)
 
-    const outputs = toStepOutputs(payload)
+    const outputs = {
+      ...toStepOutputs(payload),
+      client_payload: normalizeForEngine(clientPayload || '')
+    }
 
     if (outputs.github_token) {
       core.setSecret(outputs.github_token)
@@ -171,5 +193,6 @@ async function run({ core, clientPayload, resolverUrl }) {
 
 module.exports = {
   run,
-  toStepOutputs
+  toStepOutputs,
+  normalizeForEngine
 }