diff --git a/README.md b/README.md
index 9ac8840..f4861f8 100644
--- a/README.md
+++ b/README.md
@@ -60,6 +60,8 @@ cargo run -p ec-node -- wt-publish \
   --control-endpoint-addr-out /tmp/la-nbc-control-endpoint.json
 ```
 
+`wt-publish` defaults to `--passthrough=true` so relay and archive retain exact CMAF fragment bytes.
+
 Watch (web):
 
 ```txt
diff --git a/crates/ec-node/src/main.rs b/crates/ec-node/src/main.rs
index 37a7eaa..b4b68b3 100644
--- a/crates/ec-node/src/main.rs
+++ b/crates/ec-node/src/main.rs
@@ -435,7 +435,7 @@ struct WtPublishArgs {
     transcode: bool,
     /// Transmit fMP4 fragments directly (passthrough mode).
     /// When false, the importer may reframe into CMAF fragments.
-    #[arg(long, default_value_t = false, action = clap::ArgAction::Set)]
+    #[arg(long, default_value_t = true, action = clap::ArgAction::Set)]
     passthrough: bool,
     /// Danger: disable TLS verification for the relay.
     #[arg(long, default_value_t = false)]
diff --git a/evolution/proposals/ECP-0072-cmaf-seedbox-invariant.md b/evolution/proposals/ECP-0072-cmaf-seedbox-invariant.md
new file mode 100644
index 0000000..d33a44d
--- /dev/null
+++ b/evolution/proposals/ECP-0072-cmaf-seedbox-invariant.md
@@ -0,0 +1,26 @@
+# ECP-0072: CMAF Seedbox Invariant For Relay Archive
+
+## Context
+
+Archive replay currently stores and serves relay groups exactly as received, but many existing broadcasts were published in `legacy` container mode. Those bytes are not browser-HLS compatible, so archive playback fails despite a valid timeline and object store.
+
+## Decision
+
+Make `wt-publish` default to CMAF passthrough (`--passthrough=true`) so:
+
+- relay subscribers and archive workers observe the same CMAF fragment bytes (`moof+mdat`);
+- archive replay can serve those exact bytes without re-encoding;
+- the archive acts as a history seedbox of previously played chunks.
+
+Update the NixOS module default `services.every-channel.ec-node.passthrough = true` to keep host deployments aligned with the seedbox invariant.
+
+## Rationale
+
+- Zero-transform archive path is simpler and more auditable.
+- Exact-byte retention avoids drift between live and replay.
+- Browsers can play CMAF fragments via standard HLS tooling; no custom legacy converter is required for new streams.
+
+## Reversibility
+
+- Operators can explicitly set `passthrough = false` per host to revert to legacy framing.
+- Existing archived legacy data remains readable for timeline/index use, but may require separate migration tooling for browser playback.
diff --git a/nix/modules/ec-node.nix b/nix/modules/ec-node.nix
index 0771cf1..c69eecc 100644
--- a/nix/modules/ec-node.nix
+++ b/nix/modules/ec-node.nix
@@ -60,8 +60,8 @@ in
 
     passthrough = lib.mkOption {
       type = lib.types.bool;
-      default = false;
-      description = "Whether to transmit fMP4 fragments directly (moq-mux passthrough).";
+      default = true;
+      description = "Whether to transmit CMAF fMP4 fragments directly (seedbox-compatible passthrough).";
     };
 
     tlsDisableVerify = lib.mkOption {