diff --git a/aimdb-core/Cargo.toml b/aimdb-core/Cargo.toml
index 6b2da2c6..c664f375 100644
--- a/aimdb-core/Cargo.toml
+++ b/aimdb-core/Cargo.toml
@@ -52,7 +52,7 @@ connector-session = ["alloc"]
 
 # Observability features (available on both std/no_std)
 tracing = ["dep:tracing"] # Works in both std and no_std environments
-defmt = ["dep:defmt"]     # Embedded logging via probe (no_std)
+defmt = ["dep:defmt", "defmt/alloc"]     # Embedded logging via probe (no_std)
 # A second facade destination, for hosts that cannot be handed a global
 # `tracing` subscriber (design 050). Not in `default`: 66 call sites on an MCU
 # should not each grow a level check because an FFI layer wanted one.
diff --git a/aimdb-core/src/error.rs b/aimdb-core/src/error.rs
index c6d23747..6d2bdf9b 100644
--- a/aimdb-core/src/error.rs
+++ b/aimdb-core/src/error.rs
@@ -223,6 +223,16 @@ pub enum DbError {
     },
 }
 
+// Through `Display` (`thiserror`), not a derive: the `std`-only variants carry
+// `io::Error` / `serde_json::Error`, which do not implement `defmt::Format`,
+// and docs.rs builds with every feature on, `std` and `defmt` together.
+#[cfg(feature = "defmt")]
+impl defmt::Format for DbError {
+    fn format(&self, f: defmt::Formatter) {
+        defmt::write!(f, "{}", defmt::Display2Format(self))
+    }
+}
+
 // ===== DbError Implementation =====
 impl DbError {
     /// Builds a [`RuntimeError`](DbError::RuntimeError).
diff --git a/aimdb-core/src/lib.rs b/aimdb-core/src/lib.rs
index acab7040..b770c5fe 100644
--- a/aimdb-core/src/lib.rs
+++ b/aimdb-core/src/lib.rs
@@ -75,6 +75,22 @@ pub mod __private {
     pub use ::tracing;
 }
 
+// Every type the `log_*!` macros format must implement `defmt::Format` when
+// `defmt` is on (#352); this fails the build if one stops doing so.
+#[cfg(all(feature = "defmt", feature = "connector-session"))]
+const _: () = {
+    fn format<T: defmt::Format + ?Sized>() {}
+    let _ = format::<DbError>;
+    let _ = format::<outbound::SerializeFailure>;
+    let _ = format::<session::TransportError>;
+    let _ = format::<session::CodecError>;
+    let _ = format::<session::AuthError>;
+    let _ = format::<session::FrameFault>;
+    let _ = format::<alloc::string::String>;
+    let _ = format::<alloc::vec::Vec<alloc::string::String>>;
+    let _ = format::<alloc::sync::Arc<str>>;
+};
+
 pub mod buffer;
 pub mod builder;
 #[cfg(feature = "remote")]
diff --git a/aimdb-core/src/outbound/routes.rs b/aimdb-core/src/outbound/routes.rs
index 2d2644e3..d598cd35 100644
--- a/aimdb-core/src/outbound/routes.rs
+++ b/aimdb-core/src/outbound/routes.rs
@@ -126,6 +126,15 @@ pub(crate) enum SerializeFailure {
     InvalidLength { len: usize, capacity: usize },
 }
 
+// Through `Display`: the variants carry a `SerializeError` and lengths, and
+// one rendering keeps the defmt line identical to the tracing/log one.
+#[cfg(feature = "defmt")]
+impl defmt::Format for SerializeFailure {
+    fn format(&self, f: defmt::Formatter) {
+        defmt::write!(f, "{}", defmt::Display2Format(self))
+    }
+}
+
 impl core::fmt::Display for SerializeFailure {
     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         match self {
diff --git a/aimdb-core/src/session/io.rs b/aimdb-core/src/session/io.rs
index c7344c80..0fd5b4f1 100644
--- a/aimdb-core/src/session/io.rs
+++ b/aimdb-core/src/session/io.rs
@@ -196,6 +196,7 @@ pub trait Delay {
 /// none, so nothing tells payload bytes from the next header. Only the framer
 /// knows which case it is in, so it says, rather than the connection guessing.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
 pub enum FrameFault {
     /// Bad frame, good link: skip it and keep reading.
     Recoverable,
diff --git a/aimdb-core/src/session/mod.rs b/aimdb-core/src/session/mod.rs
index 79a8caac..eaa25c0a 100644
--- a/aimdb-core/src/session/mod.rs
+++ b/aimdb-core/src/session/mod.rs
@@ -224,6 +224,7 @@ impl Default for SessionLimits {
 
 /// Transport-layer failure (Layer 1).
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
 #[non_exhaustive]
 pub enum TransportError {
     /// The connection was closed or reset by the peer.
@@ -239,6 +240,7 @@ pub enum TransportError {
 
 /// Envelope-codec failure — a frame could not be decoded/encoded.
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
 #[non_exhaustive]
 pub enum CodecError {
     /// The frame was not valid for this envelope format.
@@ -262,6 +264,7 @@ pub enum RpcError {
 
 /// Authentication failure raised by [`Dispatch::authenticate`].
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
 #[non_exhaustive]
 pub enum AuthError {
     /// Credentials were missing or rejected.