diff --git a/.agents/skills/secondmate-provisioning/SKILL.md b/.agents/skills/secondmate-provisioning/SKILL.md
index ecc364939ac..f9e68937ab9 100644
--- a/.agents/skills/secondmate-provisioning/SKILL.md
+++ b/.agents/skills/secondmate-provisioning/SKILL.md
@@ -78,10 +78,13 @@ This section is the single owner of the secondmate sync and inherited-local-mate
 Before launch, `fm-spawn.sh --secondmate` locally fast-forwards the home to the primary firstmate checkout's current default-branch commit when it is safe; dirty, diverged, or in-flight homes launch unchanged with a warning.
 The locked session-start bootstrap sweep runs the same guarded fast-forward for every live secondmate home, discovered from `state/<id>.meta` records with `kind=secondmate` (`data/secondmates.md` only backfills `home=` for older records).
 That no-fetch path is a purely local fast-forward of tracked files, never an origin fetch, and it never touches the gitignored operational dirs, so a secondmate's backlog, projects, and in-flight work are never disturbed; a linked worktree advances immediately, while a standalone clone that lacks the target receives firstmate updates through `/updatefirstmate`'s origin refresh.
-The same launch and the same locked bootstrap sweep also propagate the primary's declared inherited local material: `config/crew-dispatch.json`, `config/crew-harness`, `config/backlog-backend`, `config/herdr-presentation-spaces`, and the one shared captain-preference file `data/captain-shared.md`.
+The same launch and the same locked bootstrap sweep also propagate the primary's declared inherited local material: `config/crew-dispatch.json`, `config/crew-harness`, `config/backlog-backend`, `config/backend`, `config/herdr-presentation-spaces`, and the one shared captain-preference file `data/captain-shared.md`.
 Because these paths are gitignored, that propagation is a separate, primary-authoritative copy independent of the tracked-files fast-forward: it re-converges every live home whether or not its tracked files advanced, and it touches only the declared items.
 Propagation failures warn without blocking secondmate launch or session-start continuation, and the destination keeps whatever safely validated state the helper left behind.
 Inheritance copies the literal `config/crew-harness` file, so a secondmate's own crewmates use the primary's crewmate harness only when it names a concrete adapter such as `codex`; an unset or `default` value has nothing concrete to inherit, and the secondmate's own crewmates fall back to the secondmate's own or detected harness instead.
+Inherited `config/backend` becomes that secondmate home's local runtime-backend default for future spawns only; it never retargets, rewrites, migrates, stops, or restarts an already-live worker endpoint.
+A present primary value always converges byte-exact into validated secondmate homes, and primary absence removes the destination so those homes keep runtime auto-detection.
+Explicit per-spawn `--backend` and `FM_BACKEND` remain stronger than every home's local `config/backend`, including an inherited default.
 `config/secondmate-harness` is not inherited because it is only the primary's knob for launching secondmate agents.
 `data/captain-shared.md` is main-authoritative in the primary home and read-only in secondmate homes.
 Its primary file header must state that the file is main-authoritative, read-only in secondmate homes, must not be edited there, and that new captain-preference discoveries are routed to the main firstmate through marked status or a document pointer.
@@ -96,7 +99,7 @@ Keep every `data/learnings.md` fully local by captain decision; route fleet-gene
 No AGENTS.md reread nudge is needed at spawn or respawn because the agent reads instructions fresh on launch; only the bootstrap sweep's running-home instruction-surface advance needs that AGENTS.md re-read.
 Bootstrap reports successful AGENTS.md re-read sends as `BOOTSTRAP_INFO:` and only emits `NUDGE_SECONDMATES:` when that send fails and needs retry.
 A separate, literal-content config reread is required whenever inherited `config/*` material changes under an already-running secondmate.
-After each successful allowlisted config write, both the locked bootstrap convergence path and mid-session `bin/fm-config-push.sh` use the shared propagation report to build one per-home generation-specific private instruction file from the validated destination post-write bytes for only the allowlisted config items that actually changed for that home (`config/crew-dispatch.json`, `config/crew-harness`, `config/backlog-backend`, `config/herdr-presentation-spaces`), in deterministic allowlist order.
+After each successful allowlisted config write, both the locked bootstrap convergence path and mid-session `bin/fm-config-push.sh` use the shared propagation report to build one per-home generation-specific private instruction file from the validated destination post-write bytes for only the allowlisted config items that actually changed for that home (`config/crew-dispatch.json`, `config/crew-harness`, `config/backlog-backend`, `config/backend`, `config/herdr-presentation-spaces`), in deterministic allowlist order.
 Each changed path is printed with clear begin/end delimiters and the destination file's full exact new bytes unparsed, or the explicit token `ABSENT` when propagation removed the destination copy.
 The instruction uses only minimal framing that these are defaults/rules and do not remove judgment; it never includes SHA values, selected profiles, parsed summaries, or any other generated interpretation.
 `data/captain-shared.md` is not a config file and is never inlined into this instruction file or message.
diff --git a/AGENTS.md b/AGENTS.md
index 4991118cb4b..82bcbe8e55a 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -67,7 +67,7 @@ config/crew-harness  crewmate harness override; LOCAL, gitignored; absent or "de
 config/crew-dispatch.json  optional crewmate dispatch profiles; LOCAL, gitignored; firstmate-maintained but human-editable natural-language rules that choose a per-task harness/model/effort profile (section 4). Inherited by secondmate homes
 config/secondmate-harness  harness the PRIMARY uses to launch SECONDMATE agents, optionally followed by a model and effort token on the same line ("<harness> [<model>] [<effort>]"; section 4); LOCAL, gitignored; absent or "default" harness falls back to config/crew-harness then firstmate's own. The primary's own setting; NOT inherited into secondmate homes (secondmates do not spawn secondmates)
 config/backlog-backend  backlog backend override; LOCAL, gitignored; absent or "tasks-axi" = default tasks-axi backend, "manual" = force routine backlog updates to hand-editing; inherited by secondmate homes (section 10)
-config/backend  runtime session-provider backend override for new tasks; LOCAL, gitignored; absent = falls through to runtime auto-detection (the runtime firstmate itself is executing inside), then tmux; tmux is the verified reference backend (docs/tmux-backend.md), while herdr, zellij, orca, and cmux are experimental spawn backends (docs/herdr-backend.md, docs/zellij-backend.md, docs/orca-backend.md, docs/cmux-backend.md) - herdr and cmux can also be selected by runtime auto-detection, zellij and orca never are (always explicit), and codex-app is not accepted; see docs/codex-app-backend.md; not inherited into secondmate homes
+config/backend  runtime session-provider backend override for new tasks; LOCAL, gitignored; absent = falls through to runtime auto-detection (the runtime firstmate itself is executing inside), then tmux; tmux is the verified reference backend (docs/tmux-backend.md), while herdr, zellij, orca, and cmux are experimental spawn backends (docs/herdr-backend.md, docs/zellij-backend.md, docs/orca-backend.md, docs/cmux-backend.md) - herdr and cmux can also be selected by runtime auto-detection, zellij and orca never are (always explicit), and codex-app is not accepted; see docs/codex-app-backend.md; inherited by secondmate homes under the primary-authoritative contract in secondmate-provisioning
 config/calm     Pi Calm presentation preference; LOCAL, gitignored, and not inherited; see docs/configuration.md "Pi Calm preference"
 config/herdr-presentation-spaces  optional presence flag for Herdr's default-off disposable single-task visual projection; LOCAL, gitignored; inherited by secondmate homes; see docs/herdr-backend.md "Optional presentation spaces"
 config/cmux-socket-password  optional cmux control-socket password; LOCAL, gitignored; read fresh on every cmux CLI call and passed through without ever overriding an operator's own ambient CMUX_SOCKET_PASSWORD when absent (docs/cmux-backend.md "Setup")
diff --git a/bin/fm-config-inherit-lib.sh b/bin/fm-config-inherit-lib.sh
index 95abba2439a..22109aa87aa 100644
--- a/bin/fm-config-inherit-lib.sh
+++ b/bin/fm-config-inherit-lib.sh
@@ -5,10 +5,12 @@
 # (e.g. primary config/crew-dispatch.json makes a secondmate use the same dispatch
 # profile rules, primary config/crew-harness=codex makes a secondmate's crewmates
 # spawn on codex too, primary config/backlog-backend=manual makes that home
-# hand-edit backlog files too, and primary config/herdr-presentation-spaces
-# enables the same default-off Herdr presentation projection). It also pushes
-# the one primary-authoritative shared captain-preference file,
-# data/captain-shared.md, into each secondmate home's data/ as a read-only copy.
+# hand-edit backlog files too, primary config/backend pins that home's local
+# runtime-backend default for future spawns, and primary
+# config/herdr-presentation-spaces enables the same default-off Herdr presentation
+# projection). It also pushes the one primary-authoritative shared
+# captain-preference file, data/captain-shared.md, into each secondmate home's
+# data/ as a read-only copy.
 #
 # Usage: . bin/fm-config-inherit-lib.sh   (no FM_* setup required)
 #
@@ -40,7 +42,7 @@ FM_SHARED_CAPTAIN_MODE="444"
 # The declared inheritable set (space-separated, config-dir-relative item paths).
 # Extend here to inherit more of the primary's local config; override via the
 # environment only in tests. Items must not contain whitespace.
-FM_INHERITABLE_CONFIG="${FM_INHERITABLE_CONFIG:-crew-dispatch.json crew-harness backlog-backend herdr-presentation-spaces}"
+FM_INHERITABLE_CONFIG="${FM_INHERITABLE_CONFIG:-crew-dispatch.json crew-harness backlog-backend backend herdr-presentation-spaces}"
 
 fm_inherit_file_mode() {
   if [ "$(uname)" = Darwin ]; then
diff --git a/bin/fm-config-push.sh b/bin/fm-config-push.sh
index b4056744bc7..b760666dd1b 100755
--- a/bin/fm-config-push.sh
+++ b/bin/fm-config-push.sh
@@ -3,8 +3,8 @@
 # Usage: fm-config-push.sh [--help]
 #
 # Mid-session convergence for inherited local material such as
-# config/crew-dispatch.json edits or data/captain-shared.md updates. This
-# discovers live secondmate homes from state/*.meta, backfills
+# config/crew-dispatch.json, config/backend, or data/captain-shared.md updates.
+# This discovers live secondmate homes from state/*.meta, backfills
 # home= from data/secondmates.md for older meta records, and reuses the same
 # propagation machinery as bootstrap, but deliberately does not
 # fast-forward tracked files.
diff --git a/docs/configuration.md b/docs/configuration.md
index d9a06bf4cad..1b8ea3bb592 100644
--- a/docs/configuration.md
+++ b/docs/configuration.md
@@ -93,7 +93,7 @@ Use the guarded cleanup path described in [`docs/zellij-backend.md`](zellij-back
 cmux has no session layer at all - one workspace per task, in whatever cmux window is open - and its socket password (when configured) is read from local, gitignored `config/cmux-socket-password` under the effective config directory, never committed.
 The caller-facing label remains `fm-<id>`, but the actual cmux workspace title is scoped by the active `FM_HOME` readable label plus a short hash of the resolved `FM_ROOT` path as `fm-<home-label>-<id>`.
 Test cleanup must use the guarded path in [`docs/cmux-backend.md`](cmux-backend.md#current-operation-and-safety), never enumerate-and-close every workspace.
-The `config/backend` file is not inherited by secondmate homes.
+`config/backend` is inherited into secondmate homes under the primary-authoritative contract owned by [`secondmate-provisioning`](../.agents/skills/secondmate-provisioning/SKILL.md).
 
 ## Away-mode supervisor backend (FM_SUPERVISOR_BACKEND / FM_SUPERVISOR_TARGET)
 
@@ -283,7 +283,7 @@ When a running home advances and its loaded instruction surface (`AGENTS.md`, `b
 If that send fails, bootstrap keeps an idempotent retry marker and emits `NUDGE_SECONDMATES:` with the failure reason.
 The same bootstrap run emits `SECONDMATE_LIVENESS:` only when a registered secondmate is skipped or its relaunch fails; already-live and successfully relaunched secondmates are handled silently.
 For a mid-session inherited local-material edit where tracked-file sync is not needed, run `bin/fm-config-push.sh`.
-It uses the same live secondmate discovery and propagation helper as bootstrap, prints each live home's `crew-dispatch.json`, `crew-harness`, `backlog-backend`, `herdr-presentation-spaces`, and `data/captain-shared.md` result as `pushed`, `unchanged`, `skipped`, or `error`, and exits non-zero for real propagation errors or config-reread send failures.
+It uses the same live secondmate discovery and propagation helper as bootstrap, prints each live home's `crew-dispatch.json`, `crew-harness`, `backlog-backend`, `backend`, `herdr-presentation-spaces`, and `data/captain-shared.md` result as `pushed`, `unchanged`, `skipped`, or `error`, and exits non-zero for real propagation errors or config-reread send failures.
 When an allowlisted config item changes for an already-running home, it sends the literal-content reread pointer described in [`secondmate-provisioning`](../.agents/skills/secondmate-provisioning/SKILL.md); unchanged allowlisted config sends no pointer unless a previous delivery is pending.
 The locked bootstrap inheritance pass uses the same per-home changed-set and reread path for already-running homes; see `secondmate-provisioning` for the single contract owner.
 That live discovery starts from `state/*.meta` records with `kind=secondmate`; `data/secondmates.md` only backfills `home=` for older or incomplete meta records.
diff --git a/tests/fm-backend-herdr-presentation-e2e.test.sh b/tests/fm-backend-herdr-presentation-e2e.test.sh
index 194d2053ce9..158e943211c 100755
--- a/tests/fm-backend-herdr-presentation-e2e.test.sh
+++ b/tests/fm-backend-herdr-presentation-e2e.test.sh
@@ -866,7 +866,7 @@ touch "$SECOND_HOME_A/state/.last-watcher-beat" "$SECOND_HOME_B/state/.last-watc
 # may write config/herdr-presentation-spaces.
 git -C "$SECOND_HOME_A" init -q
 git -C "$SECOND_HOME_B" init -q
-printf 'config/herdr-presentation-spaces\nconfig/crew-harness\nconfig/crew-dispatch.json\nconfig/backlog-backend\n' \
+printf 'config/herdr-presentation-spaces\nconfig/crew-harness\nconfig/crew-dispatch.json\nconfig/backlog-backend\nconfig/backend\n' \
   > "$SECOND_HOME_A/.gitignore"
 cp "$SECOND_HOME_A/.gitignore" "$SECOND_HOME_B/.gitignore"
 git -C "$SECOND_HOME_A" add .gitignore
diff --git a/tests/fm-secondmate-harness.test.sh b/tests/fm-secondmate-harness.test.sh
index 39ca2021bf1..b9efe54033d 100755
--- a/tests/fm-secondmate-harness.test.sh
+++ b/tests/fm-secondmate-harness.test.sh
@@ -14,11 +14,12 @@
 #      explicit per-spawn harness arg still wins.
 #   B) Inheritance. The primary pushes a declared, extensible set of LOCAL
 #      (gitignored) config items - config/crew-dispatch.json, config/crew-harness,
-#      config/backlog-backend, and config/herdr-presentation-spaces - down into
-#      each secondmate home's config/, so the secondmate's OWN crewmates,
-#      dispatch profiles, backlog backend, and Herdr presentation opt-in inherit
-#      the primary's settings. It is primary-authoritative (re-pushed at
-#      secondmate spawn, on the bootstrap secondmate sweep, and by config push).
+#      config/backlog-backend, config/backend, and config/herdr-presentation-spaces -
+#      down into each secondmate home's config/, so the secondmate's OWN crewmates,
+#      dispatch profiles, backlog backend, runtime-backend default, and Herdr
+#      presentation opt-in inherit the primary's settings. It is primary-authoritative
+#      (re-pushed at secondmate spawn, on the bootstrap secondmate sweep, and by
+#      config push).
 #      config/secondmate-harness is deliberately NOT inherited (secondmates do
 #      not spawn secondmates). After a successful push that changes allowlisted
 #      config under an already-running home, a literal-content reread instruction
@@ -191,16 +192,18 @@ SH
 # B) propagate_inheritable_config unit behavior
 # ===========================================================================
 test_propagate_lib() {
-  local d src dest m1 m2 outside stdout stderr guard_repo err_text
+  local d src dest home m1 m2 outside stdout stderr guard_repo err_text
   d="$TMP_ROOT/prop-lib"
   src="$d/src"
-  dest="$d/dest"
-  mkdir -p "$src" "$dest"
+  home="$d/home1"
+  dest="$home/config"
+  mkdir -p "$src" "$dest" "$home/state"
 
   # 1. present source is copied
   printf '{"default":{"harness":"codex"}}\n' > "$src/crew-dispatch.json"
   printf 'codex\n' > "$src/crew-harness"
   printf 'manual\n' > "$src/backlog-backend"
+  printf 'tmux\n' > "$src/backend"
   : > "$src/herdr-presentation-spaces"
   stdout="$d/clean-copy.out"
   stderr="$d/clean-copy.err"
@@ -210,7 +213,11 @@ test_propagate_lib() {
   [ "$(cat "$dest/crew-dispatch.json")" = '{"default":{"harness":"codex"}}' ] || fail "crew-dispatch.json not propagated"
   [ "$(cat "$dest/crew-harness")" = codex ] || fail "crew-harness not propagated"
   [ "$(cat "$dest/backlog-backend")" = manual ] || fail "backlog-backend not propagated"
+  [ "$(cat "$dest/backend")" = tmux ] || fail "backend not propagated"
   [ -f "$dest/herdr-presentation-spaces" ] || fail "herdr-presentation-spaces not propagated"
+  printf 'herdr\n' > "$dest/backend"
+  propagate_inheritable_config "$src" "$dest"
+  [ "$(cat "$dest/backend")" = tmux ] || fail "primary backend did not overwrite a divergent destination"
 
   # 2. idempotent: an unchanged re-run does not churn the mtime
   m1=$(date -r "$dest/crew-harness" +%s 2>/dev/null || stat -c %Y "$dest/crew-harness")
@@ -227,10 +234,12 @@ test_propagate_lib() {
   printf '{"default":{"harness":"claude"}}\n' > "$src/crew-dispatch.json"
   printf 'claude\n' > "$src/crew-harness"
   printf 'tasks-axi\n' > "$src/backlog-backend"
+  printf 'zellij\n' > "$src/backend"
   propagate_inheritable_config "$src" "$dest"
   [ "$(cat "$dest/crew-dispatch.json")" = '{"default":{"harness":"claude"}}' ] || fail "changed dispatch profile did not converge"
   [ "$(cat "$dest/crew-harness")" = claude ] || fail "changed value did not converge"
   [ "$(cat "$dest/backlog-backend")" = tasks-axi ] || fail "changed backlog backend did not converge"
+  [ "$(cat "$dest/backend")" = zellij ] || fail "changed backend did not converge"
 
   outside="$d/outside-target"
   rm -f "$dest/crew-harness" "$outside"
@@ -243,11 +252,14 @@ test_propagate_lib() {
   [ "$(cat "$outside")" = outside ] || fail "destination symlink target was overwritten"
 
   # 4. removing the source mirrors absence downstream (primary-authoritative)
-  rm -f "$src/crew-dispatch.json" "$src/crew-harness" "$src/backlog-backend" "$src/herdr-presentation-spaces"
+  printf 'herdr\n' > "$dest/backend"
+  rm -f "$src/crew-dispatch.json" "$src/crew-harness" "$src/backlog-backend" \
+    "$src/backend" "$src/herdr-presentation-spaces"
   propagate_inheritable_config "$src" "$dest"
   [ -e "$dest/crew-dispatch.json" ] && fail "dispatch profile absence not mirrored downstream"
   [ -e "$dest/crew-harness" ] && fail "absence not mirrored downstream"
   [ -e "$dest/backlog-backend" ] && fail "backlog-backend absence not mirrored downstream"
+  [ -e "$dest/backend" ] && fail "backend absence not mirrored downstream"
   [ -e "$dest/herdr-presentation-spaces" ] && fail "herdr-presentation-spaces absence not mirrored downstream"
 
   rm -f "$dest/crew-harness"
@@ -265,22 +277,25 @@ test_propagate_lib() {
   [ -d "$dest/crew-harness" ] || fail "failed absence mirror removed the wrong path"
   rm -rf "$dest/crew-harness"
 
-  # 5. secondmate-harness is never inherited
+  # 5. secondmate-harness is never inherited; backend still is
   printf 'grok\n' > "$src/secondmate-harness"
   printf '{"default":{"harness":"codex"}}\n' > "$src/crew-dispatch.json"
   printf 'codex\n' > "$src/crew-harness"
   printf 'manual\n' > "$src/backlog-backend"
-  rm -rf "$d/dest2"
-  mkdir -p "$d/dest2"
-  propagate_inheritable_config "$src" "$d/dest2"
-  [ -e "$d/dest2/secondmate-harness" ] && fail "secondmate-harness was inherited (must not be)"
-  [ "$(cat "$d/dest2/crew-dispatch.json")" = '{"default":{"harness":"codex"}}' ] || fail "crew-dispatch.json not propagated alongside"
-  [ "$(cat "$d/dest2/crew-harness")" = codex ] || fail "crew-harness not propagated alongside"
-  [ "$(cat "$d/dest2/backlog-backend")" = manual ] || fail "backlog-backend not propagated alongside"
+  printf 'herdr\n' > "$src/backend"
+  rm -rf "$d/home2"
+  mkdir -p "$d/home2/config" "$d/home2/state"
+  propagate_inheritable_config "$src" "$d/home2/config"
+  [ -e "$d/home2/config/secondmate-harness" ] && fail "secondmate-harness was inherited (must not be)"
+  [ "$(cat "$d/home2/config/crew-dispatch.json")" = '{"default":{"harness":"codex"}}' ] || fail "crew-dispatch.json not propagated alongside"
+  [ "$(cat "$d/home2/config/crew-harness")" = codex ] || fail "crew-harness not propagated alongside"
+  [ "$(cat "$d/home2/config/backlog-backend")" = manual ] || fail "backlog-backend not propagated alongside"
+  [ "$(cat "$d/home2/config/backend")" = herdr ] || fail "backend not propagated alongside"
 
   # 6. nothing to propagate -> destination dir is never created (a true no-op)
   rm -rf "$d/src3" "$d/dest3"
   mkdir -p "$d/src3"
+  # Keep backend out of the empty-source case by clearing it from src3 only.
   propagate_inheritable_config "$d/src3" "$d/dest3/config"
   [ -e "$d/dest3/config" ] && fail "empty-source propagation created a destination dir"
 
@@ -371,6 +386,7 @@ test_spawn_split_and_inherit() {
   printf 'claude\n' > "$w/home/config/crew-harness"
   printf 'codex\n' > "$w/home/config/secondmate-harness"
   printf 'manual\n' > "$w/home/config/backlog-backend"
+  printf 'zellij\n' > "$w/home/config/backend"
   make_seeded_home "$sm" sm
 
   spawn_secondmate "$w" sm "$sm"
@@ -385,6 +401,8 @@ test_spawn_split_and_inherit() {
     || fail "split: home crew-dispatch.json not inherited"
   [ "$(cat "$sm/config/backlog-backend" 2>/dev/null)" = manual ] \
     || fail "split: home backlog-backend not inherited as manual"
+  [ "$(cat "$sm/config/backend" 2>/dev/null)" = zellij ] \
+    || fail "split: home backend not inherited as zellij"
   [ -e "$sm/config/secondmate-harness" ] \
     && fail "split: secondmate-harness leaked into the secondmate home"
   pass "B2 spawn: secondmate runs the secondmate harness; its home inherits declared config"
@@ -537,6 +555,50 @@ spawn_secondmate_capture() {
     "$ROOT/bin/fm-spawn.sh" "$id" "$home" "$@" --secondmate
 }
 
+test_spawn_backend_precedence_over_inherited_config() {
+  local w sm meta launchlog out status
+  w="$TMP_ROOT/spawn-backend-env-precedence"
+  sm="$w/sm"
+  launchlog="$w/launch.log"
+  mkdir -p "$w/home/config"
+  printf 'herdr\n' > "$w/home/config/backend"
+  make_seeded_home "$sm" sm
+
+  out=$(FM_BACKEND=tmux spawn_secondmate_capture \
+    "$w" sm "$sm" "$launchlog" 2>&1); status=$?
+  expect_code 0 "$status" \
+    "FM_BACKEND=tmux should beat inherited config/backend=herdr"$'\n'"$out"
+
+  meta="$w/home/state/sm.meta"
+  [ "$(cat "$sm/config/backend")" = herdr ] \
+    || fail "backend precedence fixture did not inherit config/backend=herdr"
+  assert_no_grep '^backend=' "$meta" \
+    "FM_BACKEND=tmux did not beat inherited config/backend=herdr"
+  pass "B5b spawn: FM_BACKEND wins over inherited config/backend"
+}
+
+test_spawn_explicit_backend_precedence_over_env_and_inherited_config() {
+  local w sm meta launchlog out status
+  w="$TMP_ROOT/spawn-backend-flag-precedence"
+  sm="$w/sm"
+  launchlog="$w/launch.log"
+  mkdir -p "$w/home/config"
+  printf 'herdr\n' > "$w/home/config/backend"
+  make_seeded_home "$sm" sm
+
+  out=$(FM_BACKEND=zellij spawn_secondmate_capture \
+    "$w" sm "$sm" "$launchlog" --backend tmux 2>&1); status=$?
+  expect_code 0 "$status" \
+    "explicit --backend tmux should beat FM_BACKEND=zellij and inherited config/backend=herdr"$'\n'"$out"
+
+  meta="$w/home/state/sm.meta"
+  [ "$(cat "$sm/config/backend")" = herdr ] \
+    || fail "explicit backend precedence fixture did not inherit config/backend=herdr"
+  assert_no_grep '^backend=' "$meta" \
+    "explicit --backend tmux did not beat FM_BACKEND=zellij and inherited config/backend=herdr"
+  pass "B5c spawn: explicit --backend wins over FM_BACKEND and inherited config/backend"
+}
+
 # A bare "<harness>" secondmate-harness file (today's format) must launch with
 # NO --model/--effort flag at all, and meta must keep recording model=default,
 # effort=default - the core backward-compat requirement of the new format.
@@ -770,6 +832,7 @@ new_world() {
     printf 'projects/\nstate/\ndata/\n.no-mistakes/\n'
     [ "$dispatch_ignore" = no ] || printf 'config/crew-dispatch.json\n'
     printf 'config/crew-harness\nconfig/secondmate-harness\nconfig/backlog-backend\n'
+    printf 'config/backend\nconfig/herdr-presentation-spaces\n'
   } > "$w/main/.gitignore"
   printf 'v1\n' > "$w/main/AGENTS.md"
   printf 'r1\n' > "$w/main/README.md"
@@ -965,6 +1028,7 @@ test_bootstrap_sweep_propagates_and_reconverges() {
   printf '{"default":{"harness":"codex"}}\n' > "$w/home/config/crew-dispatch.json"
   printf 'codex\n' > "$w/home/config/crew-harness"
   printf 'manual\n' > "$w/home/config/backlog-backend"
+  printf 'tmux\n' > "$w/home/config/backend"
   printf 'grok\n' > "$w/home/config/secondmate-harness"
   run_bootstrap "$w" >/dev/null
   [ "$(cat "$w/sm/config/crew-harness" 2>/dev/null)" = codex ] \
@@ -973,6 +1037,8 @@ test_bootstrap_sweep_propagates_and_reconverges() {
     || fail "sweep: crew-dispatch.json not pushed into the live home"
   [ "$(cat "$w/sm/config/backlog-backend" 2>/dev/null)" = manual ] \
     || fail "sweep: backlog-backend not pushed into the live home"
+  [ "$(cat "$w/sm/config/backend" 2>/dev/null)" = tmux ] \
+    || fail "sweep: backend not pushed into the live home"
   [ -e "$w/sm/config/secondmate-harness" ] \
     && fail "sweep: secondmate-harness was inherited (must not be)"
 
@@ -980,6 +1046,7 @@ test_bootstrap_sweep_propagates_and_reconverges() {
   printf '{"default":{"harness":"claude"}}\n' > "$w/home/config/crew-dispatch.json"
   printf 'claude\n' > "$w/home/config/crew-harness"
   printf 'tasks-axi\n' > "$w/home/config/backlog-backend"
+  printf 'zellij\n' > "$w/home/config/backend"
   run_bootstrap "$w" >/dev/null
   [ "$(cat "$w/sm/config/crew-harness" 2>/dev/null)" = claude ] \
     || fail "sweep: home did not re-converge to the primary's new crew-harness"
@@ -987,9 +1054,12 @@ test_bootstrap_sweep_propagates_and_reconverges() {
     || fail "sweep: home did not re-converge to the primary's new crew-dispatch.json"
   [ "$(cat "$w/sm/config/backlog-backend" 2>/dev/null)" = tasks-axi ] \
     || fail "sweep: home did not re-converge to the primary's new backlog-backend"
+  [ "$(cat "$w/sm/config/backend" 2>/dev/null)" = zellij ] \
+    || fail "sweep: home did not re-converge to the primary's new backend"
 
   # Mirror absence: primary clears inherited config; the home's copies are removed.
-  rm -f "$w/home/config/crew-dispatch.json" "$w/home/config/crew-harness" "$w/home/config/backlog-backend"
+  rm -f "$w/home/config/crew-dispatch.json" "$w/home/config/crew-harness" \
+    "$w/home/config/backlog-backend" "$w/home/config/backend"
   run_bootstrap "$w" >/dev/null
   [ -e "$w/sm/config/crew-dispatch.json" ] \
     && fail "sweep: home crew-dispatch.json not removed after the primary cleared it"
@@ -997,6 +1067,8 @@ test_bootstrap_sweep_propagates_and_reconverges() {
     && fail "sweep: home crew-harness not removed after the primary cleared it"
   [ -e "$w/sm/config/backlog-backend" ] \
     && fail "sweep: home backlog-backend not removed after the primary cleared it"
+  [ -e "$w/sm/config/backend" ] \
+    && fail "sweep: home backend not removed after the primary cleared it"
   pass "B7 bootstrap sweep pushes, re-converges, and mirrors absence; never inherits secondmate-harness"
 }
 
@@ -1011,6 +1083,7 @@ test_bootstrap_sweep_propagates_when_tracked_current() {
   printf '{"default":{"harness":"codex"}}\n' > "$w/home/config/crew-dispatch.json"
   printf 'codex\n' > "$w/home/config/crew-harness"
   printf 'manual\n' > "$w/home/config/backlog-backend"
+  printf 'tmux\n' > "$w/home/config/backend"
   run_bootstrap "$w" >/dev/null
   [ "$(cat "$w/sm/config/crew-dispatch.json" 2>/dev/null)" = '{"default":{"harness":"codex"}}' ] \
     || fail "crew-dispatch.json did not propagate to a tracked-current home"
@@ -1018,6 +1091,8 @@ test_bootstrap_sweep_propagates_when_tracked_current() {
     || fail "config did not propagate to a tracked-current home"
   [ "$(cat "$w/sm/config/backlog-backend" 2>/dev/null)" = manual ] \
     || fail "backlog-backend did not propagate to a tracked-current home"
+  [ "$(cat "$w/sm/config/backend" 2>/dev/null)" = tmux ] \
+    || fail "backend did not propagate to a tracked-current home"
   pass "B8 bootstrap sweep propagates config even when the home's tracked files are already current"
 }
 
@@ -1069,12 +1144,49 @@ test_bootstrap_sweep_no_inheritance_is_noop() {
 
   [ -e "$w/sm/config/crew-dispatch.json" ] && fail "no-inheritance sweep created a home crew-dispatch.json"
   [ -e "$w/sm/config/crew-harness" ] && fail "no-inheritance sweep created a home crew-harness"
+  [ -e "$w/sm/config/backend" ] && fail "no-inheritance sweep created a home backend"
   [ -e "$w/sm/config" ] && fail "no-inheritance sweep created a home config/ dir"
   [ "$(git -C "$w/sm" rev-parse HEAD)" = "$head" ] \
     || fail "no-inheritance sweep did not still fast-forward the tracked files"
   pass "B10 bootstrap sweep with no inherited config is a config no-op and still fast-forwards"
 }
 
+# config/backend: present and absent primary state converges exactly.
+test_backend_inheritance_present_and_absent() {
+  local w head out err status instruction
+  w=$(new_world backend-inherit)
+  head=$(git -C "$w/main" rev-parse HEAD)
+  add_sm_worktree "$w" sm "$head"
+
+  printf 'tmux\n' > "$w/home/config/backend"
+  err="$w/backend-inherit.err"
+  out=$(run_config_push "$w" 2>"$err"); status=$?
+  expect_code 0 "$status" "backend present push should succeed"
+  assert_contains "$out" "backend: pushed" "backend present value should report pushed"
+  [ "$(cat "$w/sm/config/backend")" = tmux ] || fail "backend present value not pushed"
+  instruction=$(reread_instruction_path "$w/sm") || fail "backend present reread instruction missing"
+  assert_contains "$(cat "$instruction")" $'-----BEGIN config/backend-----\ntmux\n-----END config/backend-----' \
+    "backend present reread must include exact bytes"
+
+  printf 'herdr\n' > "$w/sm/config/backend"
+  printf 'zellij\n' > "$w/home/config/backend"
+  out=$(run_config_push "$w" 2>"$err"); status=$?
+  expect_code 0 "$status" "backend changed push should succeed"
+  assert_contains "$out" "backend: pushed" "backend changed value should report pushed"
+  [ "$(cat "$w/sm/config/backend")" = zellij ] \
+    || fail "primary backend did not overwrite the divergent destination"
+
+  rm -f "$w/home/config/backend"
+  out=$(run_config_push "$w" 2>"$err"); status=$?
+  expect_code 0 "$status" "backend absence push should succeed"
+  assert_contains "$out" "backend: pushed - mirrored primary absence" "backend should mirror primary absence"
+  [ -e "$w/sm/config/backend" ] && fail "backend not removed on primary absence"
+  instruction=$(reread_instruction_path "$w/sm") || fail "backend absence reread instruction missing"
+  assert_contains "$(cat "$instruction")" $'-----BEGIN config/backend-----\nABSENT\n-----END config/backend-----' \
+    "backend absence reread must use ABSENT token"
+  pass "B12b backend inheritance: present values and primary absence converge exactly"
+}
+
 test_bootstrap_sweep_surfaces_config_propagation_failure() {
   local w c1 out fail_line
   w=$(new_world boot-prop-fail)
@@ -1113,7 +1225,7 @@ test_bootstrap_rereads_after_partial_propagation() {
 }
 
 test_config_push_propagates_reports_without_ff_or_nudge() {
-  local w c1 sm_real old_head out err status out2 tmp log
+  local w c1 sm_real old_head out err status out2 tmp log instruction
   w=$(new_world config-push-basic)
   c1=$(git -C "$w/main" rev-parse HEAD)
   add_sm_worktree "$w" sm "$c1"
@@ -1131,6 +1243,7 @@ test_config_push_propagates_reports_without_ff_or_nudge() {
   printf '{"default":{"harness":"codex"}}\n' > "$w/home/config/crew-dispatch.json"
   printf 'codex\n' > "$w/home/config/crew-harness"
   printf 'manual\n' > "$w/home/config/backlog-backend"
+  printf 'tmux\n' > "$w/home/config/backend"
   err="$w/config-push-basic.err"
   log="$w/config-push-basic.tmux.log"
   out=$(run_config_push "$w" "$log" 2>"$err"); status=$?
@@ -1146,12 +1259,18 @@ test_config_push_propagates_reports_without_ff_or_nudge() {
     "config push did not report crew-harness as pushed"
   assert_contains "$out" "backlog-backend: pushed" \
     "config push did not report backlog-backend as pushed"
+  assert_contains "$out" "backend: pushed" \
+    "config push did not report backend as pushed"
   assert_contains "$out" "config-reread: sent" \
     "config push with changed config must send a literal reread instruction"
   assert_not_contains "$out" "NUDGE_SECONDMATES" \
     "config push must not use the AGENTS.md instruction-surface nudge channel"
   [ "$(git -C "$w/sm" rev-parse HEAD)" = "$old_head" ] \
     || fail "config push fast-forwarded tracked files"
+  [ "$(cat "$w/sm/config/backend")" = tmux ] || fail "config push did not write backend"
+  instruction=$(reread_instruction_path "$w/sm") || fail "config-push reread instruction missing"
+  assert_contains "$(cat "$instruction")" $'-----BEGIN config/backend-----\ntmux\n-----END config/backend-----' \
+    "config-push reread must include exact backend bytes"
   [ ! -s "$err" ] || fail "clean config push wrote unexpected stderr: $(cat "$err")"
   assert_contains "$(cat "$log")" "[fm-from-firstmate]" \
     "config reread must use the marked routed secondmate path"
@@ -1165,6 +1284,8 @@ test_config_push_propagates_reports_without_ff_or_nudge() {
     "idempotent config push did not report crew-harness as unchanged"
   assert_contains "$out2" "backlog-backend: unchanged" \
     "idempotent config push did not report backlog-backend as unchanged"
+  assert_contains "$out2" "backend: unchanged" \
+    "idempotent config push did not report backend as unchanged"
   assert_not_contains "$out2" "config-reread: sent" \
     "unchanged config must not send a reread message"
   [ ! -s "$log" ] || fail "unchanged config push still invoked tmux send: $(cat "$log")"
@@ -1301,6 +1422,7 @@ test_config_reread_per_home_changed_sets_and_exact_bytes() {
   printf '%s' "$multiline_json" > "$w/home/config/crew-dispatch.json"
   printf 'codex\n' > "$w/home/config/crew-harness"
   printf 'manual\n' > "$w/home/config/backlog-backend"
+  printf 'tmux\n' > "$w/home/config/backend"
   {
     shared_captain_header_for_tests
     printf '%s\n' "shared secret preference body that must never appear in a config reread"
@@ -1319,6 +1441,7 @@ test_config_reread_per_home_changed_sets_and_exact_bytes() {
     || fail "beta did not receive multiline dispatch"
   [ "$(cat "$w/alpha/config/crew-harness")" = codex ] || fail "alpha harness not updated"
   [ "$(cat "$w/alpha/config/backlog-backend")" = manual ] || fail "alpha backlog-backend not updated"
+  [ "$(cat "$w/alpha/config/backend")" = tmux ] || fail "alpha backend not updated"
 
   instr_a=$(reread_instruction_path "$w/alpha") || fail "alpha instruction missing after config push"
   instr_b=$(reread_instruction_path "$w/beta") || fail "beta instruction missing after config push"
@@ -1328,19 +1451,21 @@ test_config_reread_per_home_changed_sets_and_exact_bytes() {
   [ "$(reread_mode "$instr_b")" = 600 ] || fail "beta instruction is not private"
 
   # Deterministic allowlist path order and exact destination bytes for alpha
-  # (all three config items were missing/stale and therefore pushed).
+  # (allowlisted config items were missing/stale and therefore pushed).
   assert_grep "These inherited config files changed" "$instr_a" "alpha framing missing"
   assert_grep "defaults/rules" "$instr_a" "alpha must preserve agent judgment framing"
   assert_contains "$(cat "$instr_a")" "config/crew-dispatch.json" "alpha missing dispatch path"
   assert_contains "$(cat "$instr_a")" "config/crew-harness" "alpha missing harness path"
   assert_contains "$(cat "$instr_a")" "config/backlog-backend" "alpha missing backlog path"
+  assert_contains "$(cat "$instr_a")" "config/backend" "alpha missing backend path"
   # Path order follows FM_INHERITABLE_CONFIG.
   awk '
     /config\/crew-dispatch\.json/ { d=NR }
     /config\/crew-harness/ { h=NR }
     /config\/backlog-backend/ { b=NR }
+    /config\/backend/ && !/backlog-backend/ { k=NR }
     END {
-      if (!(d && h && b && d < h && h < b)) exit 1
+      if (!(d && h && b && k && d < h && h < b && b < k)) exit 1
     }
   ' "$instr_a" || fail "alpha instruction path order is not deterministic allowlist order"
 
@@ -1351,6 +1476,8 @@ test_config_reread_per_home_changed_sets_and_exact_bytes() {
     "alpha instruction must include exact harness scalar bytes"
   assert_contains "$(cat "$instr_a")" $'-----BEGIN config/backlog-backend-----\nmanual\n-----END config/backlog-backend-----' \
     "alpha instruction must include exact backlog-backend scalar bytes"
+  assert_contains "$(cat "$instr_a")" $'-----BEGIN config/backend-----\ntmux\n-----END config/backend-----' \
+    "alpha instruction must include exact backend scalar bytes"
 
   # No parsed/effective summary, no SHA, no captain-shared dump.
   assert_not_contains "$(cat "$instr_a")" "Default worker" "must not emit parsed worker summary"
@@ -1434,6 +1561,7 @@ test_config_reread_isolation_and_absent_and_send_failure() {
     printf '%s\n' $'crew-dispatch.json\tpushed\tmirrored primary absence'
     printf '%s\n' $'crew-harness\tunchanged\t'
     printf '%s\n' $'backlog-backend\tunchanged\t'
+    printf '%s\n' $'backend\tunchanged\t'
     printf '%s\n' $'data/captain-shared.md\tpushed\t'
   } > "$report"
   rm -f "$w/beta/config/crew-dispatch.json"
@@ -2124,6 +2252,8 @@ test_spawn_backward_compat_crew_fallback
 test_spawn_bare_backward_compat
 test_spawn_explicit_harness_wins
 test_spawn_unverified_secondmate_harness_refused
+test_spawn_backend_precedence_over_inherited_config
+test_spawn_explicit_backend_precedence_over_env_and_inherited_config
 test_spawn_bare_harness_no_model_effort_flag
 test_spawn_secondmate_harness_model_token
 test_spawn_secondmate_harness_model_and_effort_tokens
@@ -2136,6 +2266,7 @@ test_bootstrap_sweep_propagates_and_reconverges
 test_bootstrap_sweep_propagates_when_tracked_current
 test_bootstrap_sweep_defers_dispatch_on_stale_unignored_home
 test_bootstrap_sweep_no_inheritance_is_noop
+test_backend_inheritance_present_and_absent
 test_bootstrap_sweep_surfaces_config_propagation_failure
 test_bootstrap_rereads_after_partial_propagation
 test_config_push_propagates_reports_without_ff_or_nudge