From a2cd2c57cac5e58474aee1a7c9707efd228350a9 Mon Sep 17 00:00:00 2001
From: Logan Cusano <logan@simplestepsolutions.com>
Date: Sun, 16 Aug 2026 16:26:41 -0400
Subject: [PATCH] Serve call audio through c2-core instead of GCS signed URLs
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

upload_audio() could only sign a URL when GCP_CREDENTIALS_PATH pointed at a
service-account key file. The deployed VM runs on Application Default
Credentials with no key file, so every upload silently took the fallback
branch and returned a bare gs:// URI. That broke two things at once:

  * Browsers cannot fetch a gs:// URI, so no recording was ever playable.
  * _public_url_to_gcs_uri() only matched https://storage.googleapis.com/ and
    returned None for it, so `if gcs_uri:` in the upload path was always false
    and transcription never ran. Nothing was logged, which is why this looked
    like an OpenAI credits problem rather than a storage one.

The fallback also interpolated the client-supplied filename instead of the
call_id-derived safe name, so the URI did not even name the object written.

Calls now store only the canonical gs:// location. A short-lived playback link
is minted per read as an HMAC over (call_id, expiry) keyed by SERVICE_KEY, and
audio is served from the private bucket by the new /media route. An <audio src>
cannot carry an Authorization header, so the link has to be the credential;
that router is therefore public with the check done inline, as enrollment.py
already does. Signing GCS URLs from the VM would have needed a
serviceAccountTokenCreator grant on its own service account — this avoids the
IAM change entirely and keeps the bucket private.

gcs_uri_for_call() reconstructs the object name from call_id, so recordings
made before this fix are reachable again without a data migration.

Frontend rows come straight from Firestore via onSnapshot and never see a
server-minted field, so CallRow fetches the link lazily on expand.

Also removes the last long-lived (1 year) signed URL and the log line that
printed it.
---
 drb-c2-core/app/config.py                     |  10 +
 drb-c2-core/app/internal/storage.py           | 178 +++++++++++++++---
 drb-c2-core/app/main.py                       |   5 +-
 drb-c2-core/app/models.py                     |   3 +-
 drb-c2-core/app/routers/calls.py              |  12 +-
 drb-c2-core/app/routers/media.py              |  55 ++++++
 drb-c2-core/app/routers/upload.py             |  29 +--
 drb-frontend/components/CallRow.tsx           |  46 ++++-
 drb-frontend/lib/types.ts                     |   7 +
 .../roles/deploy/templates/c2-core.env.j2     |   8 +
 10 files changed, 291 insertions(+), 62 deletions(-)
 create mode 100644 drb-c2-core/app/routers/media.py

diff --git a/drb-c2-core/app/config.py b/drb-c2-core/app/config.py
index bd9795b..3b187c8 100644
--- a/drb-c2-core/app/config.py
+++ b/drb-c2-core/app/config.py
@@ -68,6 +68,16 @@ class Settings(BaseSettings):
     # Upload size limit — reject audio files larger than this (bytes). Default 100 MB.
     upload_max_bytes: int = 100 * 1024 * 1024
 
+    # Public origin this API is reachable on, e.g. "https://api.drb.example.com".
+    # Only used to build absolute call-audio playback links: an <audio src> is
+    # fetched by the browser directly, so a relative path would resolve against
+    # the frontend origin, not this one.
+    public_api_url: Optional[str] = None
+
+    # How long a minted call-audio playback link stays valid. Long enough for a
+    # browsing session, short enough that a copied link isn't durable access.
+    audio_link_ttl_seconds: int = 6 * 60 * 60
+
     # CORS — set to your frontend origin(s) in production, e.g. ["https://app.example.com"]
     # Defaults to "*" for local development only.
     cors_origins: list[str] = ["*"]
diff --git a/drb-c2-core/app/internal/storage.py b/drb-c2-core/app/internal/storage.py
index 384b1ee..db64040 100644
--- a/drb-c2-core/app/internal/storage.py
+++ b/drb-c2-core/app/internal/storage.py
@@ -1,9 +1,40 @@
+"""
+Call-audio storage and playback links.
+
+TWO THINGS THIS MODULE DELIBERATELY DOES NOT DO ANY MORE:
+
+1. It does not return a GCS *signed* URL from the upload path. Signing needs a
+   service-account private key, and the deployed VM runs on Application Default
+   Credentials with no key file (see ansible c2-core.env.j2). The old code
+   silently fell back to returning a bare ``gs://`` URI, which broke two things
+   at once: browsers can't fetch a gs:// URI, so no recording was ever
+   playable, and ``_public_url_to_gcs_uri`` in upload.py returned None for it,
+   so the transcription step was skipped without logging anything at all.
+
+2. It does not store a long-lived URL on the call document. What gets persisted
+   is the canonical ``gs://`` object location; a short-lived playback link is
+   minted on read instead. Nothing durable and nothing loggable is a credential.
+
+Playback goes through c2-core's own /media route rather than GCS directly,
+because an <audio src> cannot carry an Authorization header — so the link
+itself has to be the credential. It is a plain HMAC over (call_id, expiry)
+keyed by SERVICE_KEY, which costs no network round-trip, keeps the bucket
+fully private, and needs no IAM change on the VM's service account.
+"""
 import asyncio
-import datetime
-from typing import Optional
+import hashlib
+import hmac
+import os
+import time
+from typing import Optional, Tuple
 from app.config import settings
 from app.internal.logger import logger
 
+# Domain separation: the audio-link key is derived from SERVICE_KEY rather than
+# being SERVICE_KEY itself, so a leaked playback link can never be replayed as
+# a service-key bearer token against the rest of the API.
+_KEY_CONTEXT = b"drb-audio-link-v1"
+
 
 def _safe_audio_filename(filename: str, call_id: str) -> str:
     """Return a safe GCS object name derived from the call_id.
@@ -12,7 +43,6 @@ def _safe_audio_filename(filename: str, call_id: str) -> str:
     call_id (which we control) to prevent path traversal via crafted filenames.
     The original extension is preserved only if it's a known audio type.
     """
-    import os
     ext = os.path.splitext(filename)[-1].lower() if filename else ""
     if ext not in (".mp3", ".wav", ".ogg", ".m4a", ".aac", ".flac"):
         ext = ".mp3"
@@ -20,38 +50,140 @@ def _safe_audio_filename(filename: str, call_id: str) -> str:
 
 
 async def upload_audio(data: bytes, filename: str, call_id: str = "") -> Optional[str]:
-    """Upload audio bytes to GCS and return a signed URL, or None if disabled."""
+    """Upload audio bytes to GCS and return the canonical gs:// URI, or None if disabled."""
     if not settings.gcs_bucket:
         logger.info("GCS_BUCKET not configured — skipping audio upload.")
         return None
 
-    def _upload() -> str:
+    safe_name = _safe_audio_filename(filename, call_id)
+    blob_path = f"calls/{safe_name}"
+
+    def _upload() -> None:
         from google.cloud import storage
-        from google.oauth2 import service_account as sa
         if settings.gcp_credentials_path:
             client = storage.Client.from_service_account_json(settings.gcp_credentials_path)
-            signing_creds = sa.Credentials.from_service_account_file(settings.gcp_credentials_path)
         else:
             client = storage.Client()
-            signing_creds = None
-        bucket = client.bucket(settings.gcs_bucket)
-        safe_name = _safe_audio_filename(filename, call_id)
-        blob = bucket.blob(f"calls/{safe_name}")
+        blob = client.bucket(settings.gcs_bucket).blob(blob_path)
         blob.upload_from_string(data, content_type="audio/mpeg")
-        if signing_creds:
-            return blob.generate_signed_url(
-                version="v2",
-                expiration=datetime.timedelta(days=365),
-                method="GET",
-                credentials=signing_creds,
-            )
-        # Fallback: return the gs:// URI (no public access)
-        return f"gs://{settings.gcs_bucket}/calls/{filename}"
 
     try:
-        url = await asyncio.to_thread(_upload)
-        logger.info(f"Audio uploaded: {url}")
-        return url
+        await asyncio.to_thread(_upload)
     except Exception as e:
         logger.error(f"GCS upload failed: {e}")
         return None
+
+    gcs_uri = f"gs://{settings.gcs_bucket}/{blob_path}"
+    logger.info(f"Audio uploaded: {gcs_uri}")
+    return gcs_uri
+
+
+async def download_audio(gcs_uri: str) -> Optional[bytes]:
+    """Fetch an object back out of GCS. Server-side read — no signing involved."""
+    bucket_name, blob_path = split_gcs_uri(gcs_uri)
+    if not bucket_name:
+        return None
+
+    def _download() -> bytes:
+        from google.cloud import storage
+        if settings.gcp_credentials_path:
+            client = storage.Client.from_service_account_json(settings.gcp_credentials_path)
+        else:
+            client = storage.Client()
+        return client.bucket(bucket_name).blob(blob_path).download_as_bytes()
+
+    try:
+        return await asyncio.to_thread(_download)
+    except Exception as e:
+        logger.warning(f"GCS download failed for {gcs_uri}: {e}")
+        return None
+
+
+def split_gcs_uri(gcs_uri: str) -> Tuple[Optional[str], Optional[str]]:
+    """``gs://bucket/path/to.mp3`` → ``("bucket", "path/to.mp3")``."""
+    if not gcs_uri or not gcs_uri.startswith("gs://"):
+        return None, None
+    without_scheme = gcs_uri[len("gs://"):]
+    if "/" not in without_scheme:
+        return None, None
+    bucket_name, blob_path = without_scheme.split("/", 1)
+    return bucket_name, blob_path
+
+
+def gcs_uri_for_call(call: dict) -> Optional[str]:
+    """Resolve the audio object for a call document.
+
+    Prefers the canonical ``audio_gcs_uri`` written by /upload. Falls back to
+    reconstructing the object name from the call_id for documents written
+    before this module was fixed: those stored a gs:// URI built from the
+    *client-supplied* filename, which never matched the object actually
+    written (always ``calls/{call_id}.mp3``). Reconstructing rather than
+    trusting the stored value is what makes every pre-existing recording
+    playable again without a data migration.
+    """
+    uri = call.get("audio_gcs_uri")
+    if uri:
+        return uri
+    call_id = call.get("call_id")
+    if call.get("audio_url") and call_id and settings.gcs_bucket:
+        return f"gs://{settings.gcs_bucket}/calls/{call_id}.mp3"
+    return None
+
+
+def _link_key() -> Optional[bytes]:
+    if not settings.service_key:
+        return None
+    return hmac.new(settings.service_key.encode("utf-8"), _KEY_CONTEXT, hashlib.sha256).digest()
+
+
+def sign_audio_link(call_id: str, expires_at: int) -> Optional[str]:
+    key = _link_key()
+    if not key:
+        return None
+    msg = f"{call_id}:{expires_at}".encode("utf-8")
+    return hmac.new(key, msg, hashlib.sha256).hexdigest()
+
+
+def verify_audio_link(call_id: str, expires_at: int, signature: str) -> bool:
+    if expires_at < int(time.time()):
+        return False
+    expected = sign_audio_link(call_id, expires_at)
+    if not expected:
+        return False
+    return hmac.compare_digest(expected, signature)
+
+
+_warned_no_service_key = False
+_warned_no_public_url = False
+
+
+def playback_url(call: dict) -> Optional[str]:
+    """Mint a short-lived playback URL for a call, or None if it has no audio."""
+    global _warned_no_service_key, _warned_no_public_url
+
+    call_id = call.get("call_id")
+    if not call_id or not gcs_uri_for_call(call):
+        return None
+
+    expires_at = int(time.time()) + settings.audio_link_ttl_seconds
+    signature = sign_audio_link(call_id, expires_at)
+    if not signature:
+        if not _warned_no_service_key:
+            logger.error("SERVICE_KEY not set — call audio cannot be served.")
+            _warned_no_service_key = True
+        return None
+
+    # Loud rather than silent: a relative link here would 404 against the
+    # frontend origin, which is the exact failure mode this module exists to
+    # stop repeating. Deploy via ansible so c2-core.env.j2 sets PUBLIC_API_URL.
+    if not settings.public_api_url and not _warned_no_public_url:
+        logger.error("PUBLIC_API_URL not set — call audio links will be relative and will not resolve.")
+        _warned_no_public_url = True
+
+    base = (settings.public_api_url or "").rstrip("/")
+    return f"{base}/media/calls/{call_id}/audio?exp={expires_at}&sig={signature}"
+
+
+def with_playback_url(call: dict) -> dict:
+    """Return the call dict with a freshly minted ``audio_url``."""
+    return {**call, "audio_url": playback_url(call)}
diff --git a/drb-c2-core/app/main.py b/drb-c2-core/app/main.py
index f2e7ba7..d6b1e5e 100644
--- a/drb-c2-core/app/main.py
+++ b/drb-c2-core/app/main.py
@@ -15,7 +15,7 @@ from app.internal.auth import (
     require_node_service_or_firebase_token,
 )
 from app.routers import nodes, systems, calls, upload, tokens, incidents, alerts, admin, trips, places, links, users
-from app.routers import enrollment
+from app.routers import enrollment, media
 from app.internal import dynsec
 from app.internal import firestore as fstore
 
@@ -101,6 +101,9 @@ app.include_router(admin.router)     # auth is per-endpoint (read: firebase, wri
 app.include_router(users.router)     # auth: admin only
 app.include_router(links.router)     # auth is per-endpoint (generate: firebase, resolve: service key)
 app.include_router(enrollment.router)  # public; auth is the enrollment/pickup-secret tokens, checked inline
+# public by necessity — an <audio src> can't send a bearer token, so the
+# short-lived HMAC in the URL is the credential. Checked inline in media.py.
+app.include_router(media.router)
 # NOTE: there used to be an app.routers.mqtt_auth router here (an HTTP
 # backend for the mosquitto-go-auth plugin). That plugin's upstream project
 # is archived (no CVE patches) and was rejected for an internet-facing
diff --git a/drb-c2-core/app/models.py b/drb-c2-core/app/models.py
index e237008..10e8b3d 100644
--- a/drb-c2-core/app/models.py
+++ b/drb-c2-core/app/models.py
@@ -62,7 +62,8 @@ class CallRecord(BaseModel):
     srcaddr: Optional[str] = None
     started_at: datetime
     ended_at: Optional[datetime] = None
-    audio_url: Optional[str] = None
+    audio_gcs_uri: Optional[str] = None   # canonical gs:// object location
+    audio_url: Optional[str] = None       # NOT stored — minted per read, see internal/storage.py
     transcript: Optional[str] = None      # populated later by STT
     incident_ids: List[str] = []          # one per scene detected in the recording
     location: Optional[Dict[str, float]] = None  # {lat, lng}
diff --git a/drb-c2-core/app/routers/calls.py b/drb-c2-core/app/routers/calls.py
index 9e5ab39..e7d0f64 100644
--- a/drb-c2-core/app/routers/calls.py
+++ b/drb-c2-core/app/routers/calls.py
@@ -4,6 +4,7 @@ from pydantic import BaseModel
 from typing import Optional
 from app.internal import firestore as fstore
 from app.internal.auth import require_admin_token
+from app.internal.storage import gcs_uri_for_call, with_playback_url
 
 
 class TranscriptUpdate(BaseModel):
@@ -25,7 +26,9 @@ async def list_calls(
         filters["status"] = status
     if system_id:
         filters["system_id"] = system_id
-    return await fstore.collection_list("calls", **filters)
+    calls = await fstore.collection_list("calls", **filters)
+    # audio_url is not stored — it's a short-lived signed link minted per read.
+    return [with_playback_url(c) for c in calls]
 
 
 @router.get("/{call_id}")
@@ -33,7 +36,7 @@ async def get_call(call_id: str):
     call = await fstore.doc_get("calls", call_id)
     if not call:
         raise HTTPException(404, f"Call '{call_id}' not found.")
-    return call
+    return with_playback_url(call)
 
 
 @router.post("/{call_id}/reprocess")
@@ -43,10 +46,9 @@ async def reprocess_call(call_id: str, background_tasks: BackgroundTasks):
     if not call:
         raise HTTPException(404, f"Call '{call_id}' not found.")
 
-    from app.routers.upload import _run_intelligence_pipeline, _public_url_to_gcs_uri
+    from app.routers.upload import _run_intelligence_pipeline
 
-    audio_url = call.get("audio_url")
-    gcs_uri = _public_url_to_gcs_uri(audio_url) if audio_url else None
+    gcs_uri = gcs_uri_for_call(call)
 
     background_tasks.add_task(
         _run_intelligence_pipeline,
diff --git a/drb-c2-core/app/routers/media.py b/drb-c2-core/app/routers/media.py
new file mode 100644
index 0000000..09de51a
--- /dev/null
+++ b/drb-c2-core/app/routers/media.py
@@ -0,0 +1,55 @@
+"""
+Call-audio playback.
+
+Public router by necessity: a browser's <audio src="..."> cannot attach an
+Authorization header, so the link itself carries the credential — a short-lived
+HMAC over (call_id, expiry) minted by app/internal/storage.py. That is why this
+router is included in main.py WITHOUT a router-level auth dependency; the check
+happens inline below, in the same spirit as routers/enrollment.py.
+
+The bucket stays fully private and c2-core reads the object server-side with
+Application Default Credentials, so no GCS signed URL — and therefore no
+service-account private key on the VM — is involved anywhere in this path.
+"""
+from fastapi import APIRouter, HTTPException, Query, Response
+from app.internal import firestore as fstore
+from app.internal.storage import verify_audio_link, gcs_uri_for_call, download_audio
+
+router = APIRouter(prefix="/media", tags=["media"])
+
+
+@router.get("/calls/{call_id}/audio")
+async def get_call_audio(
+    call_id: str,
+    exp: int = Query(..., description="Link expiry, unix seconds."),
+    sig: str = Query(..., description="HMAC over call_id and expiry."),
+):
+    # Verify before touching Firestore so an invalid link costs nothing.
+    if not verify_audio_link(call_id, exp, sig):
+        raise HTTPException(403, "Invalid or expired audio link")
+
+    call = await fstore.doc_get("calls", call_id)
+    if not call:
+        raise HTTPException(404, f"Call '{call_id}' not found.")
+
+    gcs_uri = gcs_uri_for_call(call)
+    if not gcs_uri:
+        raise HTTPException(404, "No audio for this call.")
+
+    data = await download_audio(gcs_uri)
+    if not data:
+        raise HTTPException(404, "Audio object missing from storage.")
+
+    return Response(
+        content=data,
+        media_type="audio/mpeg",
+        headers={
+            "Content-Length": str(len(data)),
+            # Recordings are small (16 kbps mono — a 30s call is ~60 KB), so the
+            # whole body is sent at once and the browser seeks within its own
+            # buffer. Range support would only matter for long files.
+            "Accept-Ranges": "none",
+            # Immutable content, but the URL expires — cache privately only.
+            "Cache-Control": "private, max-age=3600",
+        },
+    )
diff --git a/drb-c2-core/app/routers/upload.py b/drb-c2-core/app/routers/upload.py
index ad44f51..dca6ff3 100644
--- a/drb-c2-core/app/routers/upload.py
+++ b/drb-c2-core/app/routers/upload.py
@@ -47,16 +47,15 @@ async def upload_call_audio(
     if len(data) > settings.upload_max_bytes:
         raise HTTPException(413, f"File too large (max {settings.upload_max_bytes // (1024*1024)} MB).")
 
-    audio_url = await upload_audio(data, file.filename or "", call_id=call_id)
+    gcs_uri = await upload_audio(data, file.filename or "", call_id=call_id)
 
-    if audio_url:
+    if gcs_uri:
         try:
-            await fstore.doc_set("calls", call_id, {"audio_url": audio_url})
+            # Canonical object location only. The playback link is minted per
+            # read in storage.playback_url() — nothing durable is stored here.
+            await fstore.doc_set("calls", call_id, {"audio_gcs_uri": gcs_uri})
         except Exception as e:
-            logger.warning(f"Could not update call {call_id} with audio_url: {e}")
-
-        # Convert public GCS URL to gs:// URI for Speech-to-Text
-        gcs_uri = _public_url_to_gcs_uri(audio_url)
+            logger.warning(f"Could not update call {call_id} with audio_gcs_uri: {e}")
 
         background_tasks.add_task(
             _run_intelligence_pipeline,
@@ -68,21 +67,7 @@ async def upload_call_audio(
             gcs_uri=gcs_uri,
         )
 
-    return {"url": audio_url}
-
-
-def _public_url_to_gcs_uri(url: str) -> Optional[str]:
-    """
-    Convert a public GCS URL (possibly signed) like
-      https://storage.googleapis.com/bucket/calls/file.mp3?Expires=...
-    to a gs:// URI usable by Speech-to-Text.
-    Returns None if the URL doesn't look like a GCS URL.
-    """
-    prefix = "https://storage.googleapis.com/"
-    if url and url.startswith(prefix):
-        path = url[len(prefix):].split("?")[0]  # strip signed-URL query params
-        return "gs://" + path
-    return None
+    return {"url": gcs_uri}
 
 
 async def _correlate_with_consensus(
diff --git a/drb-frontend/components/CallRow.tsx b/drb-frontend/components/CallRow.tsx
index 67db273..32d8a70 100644
--- a/drb-frontend/components/CallRow.tsx
+++ b/drb-frontend/components/CallRow.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { useState } from "react";
+import { useEffect, useState } from "react";
 import type { CallRecord } from "@/lib/types";
 import { c2api } from "@/lib/c2api";
 
@@ -37,11 +37,31 @@ export function CallRow({ call, systemName, isAdmin }: Props) {
     : call.incident_id ? [call.incident_id] : [];
 
   const isActive = call.status === "active";
-  const hasDetails = call.transcript || call.transcript_corrected || (call.tags && call.tags.length > 0) || incidentIds.length > 0 || call.audio_url;
+  // Rows come straight from Firestore (lib/useCalls.ts), and the doc only holds
+  // the private gs:// object location — never a playable URL. Presence of audio
+  // is known from the doc; the actual link is minted by the API on expand.
+  // audio_url is the legacy field: older docs stored an (unplayable) gs:// URI
+  // there, so it still signals "this call has a recording".
+  const hasAudio = !!(call.audio_gcs_uri || call.audio_url);
+  const hasDetails = call.transcript || call.transcript_corrected || (call.tags && call.tags.length > 0) || incidentIds.length > 0 || hasAudio;
   const displayTranscript = (!showOriginal && call.transcript_corrected) ? call.transcript_corrected : call.transcript;
   const hasBoth = !!(call.transcript && call.transcript_corrected);
   const hasSegments = call.segments && call.segments.length > 1;
 
+  // Fetched lazily on expand: playback links are short-lived, so minting one
+  // for every row up front would waste most of them and expire the rest.
+  const [audioUrl, setAudioUrl] = useState<string | null>(null);
+  const [audioError, setAudioError] = useState(false);
+
+  useEffect(() => {
+    if (!expanded || !hasAudio || audioUrl || audioError) return;
+    let cancelled = false;
+    c2api.getCall(call.call_id)
+      .then((full) => { if (!cancelled) setAudioUrl(full.audio_url ?? null); })
+      .catch(() => { if (!cancelled) setAudioError(true); });
+    return () => { cancelled = true; };
+  }, [expanded, hasAudio, audioUrl, audioError, call.call_id]);
+
   function startEdit() {
     setEditText(call.transcript_corrected ?? call.transcript ?? "");
     setEditing(true);
@@ -89,7 +109,7 @@ export function CallRow({ call, systemName, isAdmin }: Props) {
           )}
         </td>
         <td className="px-4 py-2 text-xs">
-          {call.audio_url ? (
+          {hasAudio ? (
             <span className="text-blue-400">▶</span>
           ) : (
             <span className="text-gray-700">—</span>
@@ -104,13 +124,19 @@ export function CallRow({ call, systemName, isAdmin }: Props) {
         <tr className="bg-gray-900/60 border-b border-gray-800">
           <td colSpan={7} className="px-6 py-3 space-y-2">
             {/* Audio player */}
-            {call.audio_url && (
-              <audio
-                controls
-                src={call.audio_url}
-                className="w-full max-w-sm h-8"
-                onClick={(e) => e.stopPropagation()}
-              />
+            {hasAudio && (
+              audioError ? (
+                <p className="text-xs text-red-400 font-mono">Could not load audio.</p>
+              ) : audioUrl ? (
+                <audio
+                  controls
+                  src={audioUrl}
+                  className="w-full max-w-sm h-8"
+                  onClick={(e) => e.stopPropagation()}
+                />
+              ) : (
+                <p className="text-xs text-gray-600 font-mono">Loading audio…</p>
+              )
             )}
 
             {/* Tags */}
diff --git a/drb-frontend/lib/types.ts b/drb-frontend/lib/types.ts
index 517ba5a..38eaf3a 100644
--- a/drb-frontend/lib/types.ts
+++ b/drb-frontend/lib/types.ts
@@ -93,6 +93,13 @@ export interface CallRecord {
   freq: number | null;
   started_at: string;
   ended_at: string | null;
+  /** Private gs:// object location. Present on the Firestore doc; not playable. */
+  audio_gcs_uri?: string | null;
+  /**
+   * Short-lived playback link. Minted per read by the API — only populated on
+   * calls fetched via c2api, never on docs read straight from Firestore.
+   * On pre-fix docs this holds a legacy (unplayable) gs:// URI instead.
+   */
   audio_url: string | null;
   transcript: string | null;
   transcript_corrected: string | null;
diff --git a/infra/ansible/roles/deploy/templates/c2-core.env.j2 b/infra/ansible/roles/deploy/templates/c2-core.env.j2
index 1f30633..6ba475a 100644
--- a/infra/ansible/roles/deploy/templates/c2-core.env.j2
+++ b/infra/ansible/roles/deploy/templates/c2-core.env.j2
@@ -12,9 +12,17 @@ MQTT_DYNSEC_ADMIN_PASS={{ vault_mqtt_dynsec_admin_pass }}
 
 # No GCP_CREDENTIALS_PATH — the VM uses Application Default Credentials
 # via the GCE metadata server. The Terraform IAM bindings grant the required roles.
+# NOTE: because there is no service-account key file here, c2-core cannot mint
+# GCS *signed* URLs. Call audio is therefore served through c2-core's own
+# /media route (app/routers/media.py) rather than direct-from-bucket links.
 FIRESTORE_DATABASE={{ vault_firestore_database }}
 GCS_BUCKET={{ vault_gcs_bucket }}
 
+# Absolute origin for call-audio playback links. The browser fetches <audio src>
+# directly, so a relative path would resolve against the frontend origin
+# (https://{{ domain }}) instead of the API's.
+PUBLIC_API_URL=https://api.{{ domain }}
+
 OPENAI_API_KEY={{ vault_openai_api_key }}
 GOOGLE_MAPS_API_KEY={{ vault_google_maps_api_key }}
 GEMINI_API_KEY={{ vault_gemini_api_key }}