diff --git a/server/src/crdt_wire.rs b/server/src/crdt_wire.rs
new file mode 100644
index 00000000..19aeb242
--- /dev/null
+++ b/server/src/crdt_wire.rs
@@ -0,0 +1,318 @@
+/// Wire codec for `SignedOp` CRDT sync between nodes.
+///
+/// # Wire Format
+///
+/// **Chosen format: versioned JSON envelope (v1)**
+///
+/// Rationale: JSON is already used throughout the codebase for `SignedOp`
+/// serialisation (`serde_json` + `serde_with::Bytes` annotations).  Switching
+/// to a binary format (CBOR / bincode / postcard) would require adding a new
+/// dependency and a custom derive macro for the `Bytes` fields.  JSON keeps
+/// the codec zero-dependency, human-readable in logs, and consistent with the
+/// existing `crdt_sync.rs` protocol.  A version tag in the envelope ensures
+/// we can migrate to a binary format in a future version without breaking
+/// peers that are still on v1.
+///
+/// Each encoded message is a JSON object:
+///
+/// ```json
+/// {"v": 1, "op": { ...SignedOp fields... }}
+/// ```
+///
+/// - `v` — u32 version tag.  Currently `1`.  Unknown values are rejected with
+///   [`WireError::UnknownVersion`] so peers can fail fast instead of
+///   silently misinterpreting data.
+/// - `op` — the full `SignedOp` serialised via its existing `serde` impl.
+///   Binary fields (`signed_digest`, `depends_on`, `id`, …) use
+///   `serde_with::Bytes` so they appear as base64 strings in JSON.
+///
+/// # Upgrading the format
+///
+/// Bump `WIRE_VERSION` and add a new arm to the `match envelope.v` block in
+/// `decode`.  Old decoders will return `WireError::UnknownVersion(N)` for
+/// the new version, which is the intended failure mode.
+use bft_json_crdt::json_crdt::SignedOp;
+use serde::{Deserialize, Serialize};
+
+/// The current wire format version.
+pub const WIRE_VERSION: u32 = 1;
+
+// ── Error type ──────────────────────────────────────────────────────────────
+
+/// Errors returned by [`decode`].
+#[derive(Debug, PartialEq)]
+pub enum WireError {
+    /// The message carries a version tag that this codec does not recognise.
+    /// The inner value is the version found in the message.
+    UnknownVersion(u32),
+    /// The bytes could not be parsed as a valid wire message (malformed JSON,
+    /// wrong structure, or the embedded `SignedOp` failed to deserialise).
+    Malformed(String),
+}
+
+impl std::fmt::Display for WireError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            WireError::UnknownVersion(v) => write!(f, "unknown wire version {v}"),
+            WireError::Malformed(msg) => write!(f, "malformed wire message: {msg}"),
+        }
+    }
+}
+
+impl std::error::Error for WireError {}
+
+// ── Internal envelope ────────────────────────────────────────────────────────
+
+#[derive(Serialize, Deserialize)]
+struct WireEnvelope {
+    v: u32,
+    op: serde_json::Value,
+}
+
+// ── Public API ───────────────────────────────────────────────────────────────
+
+/// Encode a `SignedOp` into wire bytes.
+///
+/// Returns a JSON-encoded versioned envelope.  Panics only if `SignedOp`'s
+/// serde impl is broken (which would be a programming error, not a runtime
+/// condition).
+pub fn encode(op: &SignedOp) -> Vec<u8> {
+    let op_value = serde_json::to_value(op).expect("SignedOp serialisation must not fail");
+    let envelope = WireEnvelope {
+        v: WIRE_VERSION,
+        op: op_value,
+    };
+    serde_json::to_vec(&envelope).expect("WireEnvelope serialisation must not fail")
+}
+
+/// Decode wire bytes into a `SignedOp`.
+///
+/// Returns [`WireError::UnknownVersion`] if the version tag is not recognised,
+/// or [`WireError::Malformed`] if the bytes are not valid JSON / the envelope
+/// structure is wrong / the embedded op fails to deserialise.
+pub fn decode(bytes: &[u8]) -> Result<SignedOp, WireError> {
+    let envelope: WireEnvelope =
+        serde_json::from_slice(bytes).map_err(|e| WireError::Malformed(e.to_string()))?;
+
+    match envelope.v {
+        1 => {
+            let op: SignedOp = serde_json::from_value(envelope.op)
+                .map_err(|e| WireError::Malformed(e.to_string()))?;
+            Ok(op)
+        }
+        unknown => Err(WireError::UnknownVersion(unknown)),
+    }
+}
+
+// ── Tests ────────────────────────────────────────────────────────────────────
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use bft_json_crdt::json_crdt::{BaseCrdt, JsonValue, OpState};
+    use bft_json_crdt::keypair::make_keypair;
+    use bft_json_crdt::op::ROOT_ID;
+    use serde_json::json;
+
+    use crate::crdt_state::PipelineDoc;
+
+    // ── helpers ──────────────────────────────────────────────────────────────
+
+    /// Build a fresh CRDT and return its keypair along with a signed insert op.
+    fn make_insert_op() -> (BaseCrdt<PipelineDoc>, bft_json_crdt::keypair::Ed25519KeyPair, SignedOp) {
+        let kp = make_keypair();
+        let mut crdt = BaseCrdt::<PipelineDoc>::new(&kp);
+        let item: JsonValue = json!({
+            "story_id": "505_story_wire_test",
+            "stage": "1_backlog",
+            "name": "Wire Test",
+            "agent": "",
+            "retry_count": 0.0,
+            "blocked": false,
+            "depends_on": "",
+        })
+        .into();
+        let op = crdt.doc.items.insert(ROOT_ID, item).sign(&kp);
+        crdt.apply(op.clone());
+        (crdt, kp, op)
+    }
+
+    // ── round-trip tests (one per op variant) ────────────────────────────────
+
+    /// Insert op: round-trip through encode → decode.
+    #[test]
+    fn roundtrip_insert_op() {
+        let (_crdt, _kp, op) = make_insert_op();
+        let bytes = encode(&op);
+        let decoded = decode(&bytes).expect("decode must succeed");
+        assert_eq!(op, decoded);
+    }
+
+    /// Update op (LWW register set): round-trip.
+    #[test]
+    fn roundtrip_update_op() {
+        let (mut crdt, kp, _insert) = make_insert_op();
+        let update_op = crdt.doc.items[0]
+            .stage
+            .set("2_current".to_string())
+            .sign(&kp);
+        crdt.apply(update_op.clone());
+
+        let bytes = encode(&update_op);
+        let decoded = decode(&bytes).expect("decode must succeed");
+        assert_eq!(update_op, decoded);
+    }
+
+    /// Delete op: round-trip.
+    #[test]
+    fn roundtrip_delete_op() {
+        let (mut crdt, kp, insert_op) = make_insert_op();
+        // Delete the inserted item.
+        let delete_op = crdt
+            .doc
+            .items
+            .delete(insert_op.inner.id)
+            .sign(&kp);
+        crdt.apply(delete_op.clone());
+
+        let bytes = encode(&delete_op);
+        let decoded = decode(&bytes).expect("decode must succeed");
+        assert_eq!(delete_op, decoded);
+    }
+
+    /// Op with multiple causal dependencies: round-trip.
+    #[test]
+    fn roundtrip_op_with_causal_deps() {
+        let (mut crdt, kp, _op1) = make_insert_op();
+        let op2 = crdt.doc.items[0]
+            .stage
+            .set("2_current".to_string())
+            .sign(&kp);
+        crdt.apply(op2.clone());
+
+        // op3 depends on both _op1 and op2 causally.
+        let op3 = crdt.doc.items[0]
+            .name
+            .set("Updated Name".to_string())
+            .sign(&kp);
+        crdt.apply(op3.clone());
+
+        let bytes = encode(&op3);
+        let decoded = decode(&bytes).expect("decode must succeed");
+        assert_eq!(op3, decoded);
+    }
+
+    /// Signed digest is preserved exactly (no lossy base64 round-trip).
+    #[test]
+    fn signed_digest_preserved() {
+        let (_crdt, _kp, op) = make_insert_op();
+        let bytes = encode(&op);
+        let decoded = decode(&bytes).expect("decode must succeed");
+        assert_eq!(op.signed_digest, decoded.signed_digest);
+    }
+
+    // ── version tag tests ─────────────────────────────────────────────────────
+
+    /// Encoded bytes must carry the current version tag.
+    #[test]
+    fn encoded_bytes_contain_version_tag() {
+        let (_crdt, _kp, op) = make_insert_op();
+        let bytes = encode(&op);
+        let text = std::str::from_utf8(&bytes).expect("output is UTF-8 JSON");
+        // The envelope must include `"v":1`.
+        assert!(
+            text.contains("\"v\":1"),
+            "expected version tag in wire bytes, got: {text}"
+        );
+    }
+
+    /// Version 0 is rejected with UnknownVersion.
+    #[test]
+    fn unknown_version_zero_is_rejected() {
+        let payload = br#"{"v":0,"op":{}}"#;
+        assert_eq!(decode(payload), Err(WireError::UnknownVersion(0)));
+    }
+
+    /// A future version (e.g. 99) is rejected with UnknownVersion.
+    #[test]
+    fn unknown_version_future_is_rejected() {
+        let payload = br#"{"v":99,"op":{}}"#;
+        assert_eq!(decode(payload), Err(WireError::UnknownVersion(99)));
+    }
+
+    // ── malformed-bytes tests ─────────────────────────────────────────────────
+
+    /// Completely invalid UTF-8 / non-JSON input is rejected with Malformed.
+    #[test]
+    fn malformed_random_bytes_is_rejected() {
+        let payload = b"\x00\x01\x02\x03garbage";
+        match decode(payload) {
+            Err(WireError::Malformed(_)) => {}
+            other => panic!("expected Malformed, got {other:?}"),
+        }
+    }
+
+    /// Valid JSON but missing the `v` field.
+    #[test]
+    fn malformed_missing_version_field() {
+        let payload = br#"{"op":{}}"#;
+        match decode(payload) {
+            Err(WireError::Malformed(_)) => {}
+            other => panic!("expected Malformed, got {other:?}"),
+        }
+    }
+
+    /// Valid envelope version but `op` field is not a valid `SignedOp`.
+    #[test]
+    fn malformed_invalid_op_payload() {
+        let payload = br#"{"v":1,"op":{"totally":"wrong"}}"#;
+        match decode(payload) {
+            Err(WireError::Malformed(_)) => {}
+            other => panic!("expected Malformed, got {other:?}"),
+        }
+    }
+
+    /// Empty bytes.
+    #[test]
+    fn malformed_empty_bytes() {
+        match decode(b"") {
+            Err(WireError::Malformed(_)) => {}
+            other => panic!("expected Malformed, got {other:?}"),
+        }
+    }
+
+    // ── cross-node round-trip ─────────────────────────────────────────────────
+
+    /// Encode on node A, decode on node B, apply to node B's CRDT.
+    #[test]
+    fn cross_node_apply_after_decode() {
+        let (_crdt_a, kp_a, op) = make_insert_op();
+
+        let wire = encode(&op);
+
+        // Node B decodes and applies.
+        let kp_b = make_keypair();
+        let mut crdt_b = BaseCrdt::<PipelineDoc>::new(&kp_b);
+        let decoded = decode(&wire).expect("decode must succeed");
+        let result = crdt_b.apply(decoded);
+        assert_eq!(result, OpState::Ok);
+        assert_eq!(crdt_b.doc.items.view().len(), 1);
+
+        let _ = kp_a;
+    }
+
+    // ── WireError display ─────────────────────────────────────────────────────
+
+    #[test]
+    fn wire_error_display_unknown_version() {
+        let e = WireError::UnknownVersion(42);
+        assert_eq!(e.to_string(), "unknown wire version 42");
+    }
+
+    #[test]
+    fn wire_error_display_malformed() {
+        let e = WireError::Malformed("bad json".to_string());
+        assert!(e.to_string().contains("malformed wire message"));
+    }
+}
diff --git a/server/src/main.rs b/server/src/main.rs
index 09231fe3..b3ba071f 100644
--- a/server/src/main.rs
+++ b/server/src/main.rs
@@ -8,6 +8,7 @@ mod chat;
 mod config;
 pub mod crdt_state;
 pub mod crdt_sync;
+pub mod crdt_wire;
 mod db;
 mod http;
 mod io;