diff --git a/bin/fm-spawn.sh b/bin/fm-spawn.sh
index ed030a08a8..6cdf9879ac 100755
--- a/bin/fm-spawn.sh
+++ b/bin/fm-spawn.sh
@@ -450,7 +450,15 @@ launch_template() {
     # does NOT suppress the interactive ghost text (verified empirically), so the env
     # var is the correct control. The dim-aware composer reader in fm-tmux-lib.sh is
     # the defense-in-depth backstop for any pane this flag cannot reach.
-    claude) printf '%s' 'CLAUDE_CODE_ENABLE_PROMPT_SUGGESTION=false claude --dangerously-skip-permissions __MODELFLAG____EFFORTFLAG__"$(__OPINPUT__ encode launch-brief < __BRIEF__)"' ;;
+    # env -u ANTHROPIC_API_KEY scopes out any ambient key (e.g. one set on the
+    # herdr server process this pane inherits from) so claude's own auth-source
+    # check always prefers the claude.ai OAuth login over API-key billing.
+    # Verified empirically (claude doctor) that an assignment to empty
+    # (ANTHROPIC_API_KEY=) is also treated as absent by claude's own check, but
+    # env -u is used here as the unambiguous true-unset idiom. Scoped to this
+    # one launch command; never touches the captain's own environment or
+    # dotfiles.
+    claude) printf '%s' 'env -u ANTHROPIC_API_KEY CLAUDE_CODE_ENABLE_PROMPT_SUGGESTION=false claude --dangerously-skip-permissions __MODELFLAG____EFFORTFLAG__"$(__OPINPUT__ encode launch-brief < __BRIEF__)"' ;;
     codex)
       if [ "$kind" = secondmate ]; then
         printf '%s' 'codex __MODELFLAG____EFFORTFLAG__--dangerously-bypass-approvals-and-sandbox "$(__OPINPUT__ encode launch-brief < __BRIEF__)"'
diff --git a/tests/fm-spawn-dispatch-profile.test.sh b/tests/fm-spawn-dispatch-profile.test.sh
index 7e57655218..d099fe782f 100755
--- a/tests/fm-spawn-dispatch-profile.test.sh
+++ b/tests/fm-spawn-dispatch-profile.test.sh
@@ -123,7 +123,7 @@ test_no_profile_keeps_claude_profile_defaults() {
   assert_meta_profile "$HOME_DIR/state/$id.meta" claude default default
 
   launch=$(cat "$LAUNCH_LOG")
-  expected="FM_HOME_BINDING= CLAUDE_CODE_ENABLE_PROMPT_SUGGESTION=false claude --dangerously-skip-permissions \"\$('${ROOT}/bin/fm-operational-input.sh' encode launch-brief < '$HOME_DIR/data/$id/brief.md')\""
+  expected="FM_HOME_BINDING= env -u ANTHROPIC_API_KEY CLAUDE_CODE_ENABLE_PROMPT_SUGGESTION=false claude --dangerously-skip-permissions \"\$('${ROOT}/bin/fm-operational-input.sh' encode launch-brief < '$HOME_DIR/data/$id/brief.md')\""
   [ "$launch" = "$expected" ] || fail "no-profile claude launch did not use the canonical launch kind"$'\n'"expected: $expected"$'\n'"actual:   $launch"
   pass "no --model/--effort records defaults and types the claude launch instructions"
 }
@@ -599,7 +599,7 @@ test_claude_forwards_firstmate_config_dir_when_set() {
   status=$?
   expect_code 0 "$status" "claude spawn with CLAUDE_CONFIG_DIR set should succeed"
   launch=$(cat "$LAUNCH_LOG")
-  assert_contains "$launch" "CLAUDE_CONFIG_DIR='/opt/test/claude-work' CLAUDE_CODE_ENABLE_PROMPT_SUGGESTION=false claude" \
+  assert_contains "$launch" "CLAUDE_CONFIG_DIR='/opt/test/claude-work' env -u ANTHROPIC_API_KEY CLAUDE_CODE_ENABLE_PROMPT_SUGGESTION=false claude" \
     "claude launch did not forward firstmate's CLAUDE_CONFIG_DIR to the crewmate pane"
   pass "claude forwards firstmate's CLAUDE_CONFIG_DIR so the crewmate uses the same credential store"
 }
@@ -637,6 +637,36 @@ test_non_claude_harness_ignores_config_dir() {
   pass "non-claude harnesses do not receive the claude CLAUDE_CONFIG_DIR prefix"
 }
 
+test_claude_scopes_out_ambient_anthropic_api_key() {
+  local rec id out status launch
+  id=profile-claude-authkey-z20
+  rec=$(make_spawn_case profile-claude-authkey claude "$id")
+  read_case_record "$rec"
+
+  out=$(run_spawn "$HOME_DIR" "$WT_DIR" "$FAKEBIN_DIR" "$LAUNCH_LOG" "$id" "$PROJ_DIR")
+  status=$?
+  expect_code 0 "$status" "claude spawn should succeed"
+  launch=$(cat "$LAUNCH_LOG")
+  assert_contains "$launch" "env -u ANTHROPIC_API_KEY CLAUDE_CODE_ENABLE_PROMPT_SUGGESTION=false claude" \
+    "claude launch must scope out any ambient ANTHROPIC_API_KEY so OAuth login wins over API-key billing"
+  pass "claude launch unsets ANTHROPIC_API_KEY ahead of the claude binary"
+}
+
+test_non_claude_harness_does_not_scope_anthropic_api_key() {
+  local rec id out status launch
+  id=profile-codex-authkey-z21
+  rec=$(make_spawn_case profile-codex-authkey codex "$id")
+  read_case_record "$rec"
+
+  out=$(run_spawn "$HOME_DIR" "$WT_DIR" "$FAKEBIN_DIR" "$LAUNCH_LOG" "$id" "$PROJ_DIR")
+  status=$?
+  expect_code 0 "$status" "codex spawn should succeed"
+  launch=$(cat "$LAUNCH_LOG")
+  assert_not_contains "$launch" "ANTHROPIC_API_KEY" \
+    "non-claude harness launch must not be touched by the claude-specific ANTHROPIC_API_KEY scoping"
+  pass "non-claude harnesses are unaffected by the claude ANTHROPIC_API_KEY fix"
+}
+
 test_active_dispatch_profile_does_not_block_secondmate_launch() {
   local rec id sm out status
   id=profile-secondmate-z16
@@ -680,6 +710,8 @@ test_batch_forwards_shared_profile_flags
 test_claude_forwards_firstmate_config_dir_when_set
 test_claude_omits_config_dir_prefix_when_unset
 test_non_claude_harness_ignores_config_dir
+test_claude_scopes_out_ambient_anthropic_api_key
+test_non_claude_harness_does_not_scope_anthropic_api_key
 test_active_dispatch_profile_does_not_block_secondmate_launch
 
 echo "# all fm-spawn-dispatch-profile tests passed"