diff --git a/spec/command_line.md b/spec/command_line.md
index 34f2de9..25cf793 100644
--- a/spec/command_line.md
+++ b/spec/command_line.md
@@ -192,7 +192,7 @@ When `HERDR_ENV=1` and `HERDR_PANE_ID` are present, the script includes a guarde
 | Variable | Description |
 |----------|-------------|
 | `HOME` | Used to resolve default tries path (`$HOME/src/tries`) |
-| `SHELL` | Used by `init` to detect shell type |
+| `SHELL` | Fallback used by `init` when the invoking shell cannot be detected |
 | `NO_COLOR` | If set, disables colors (equivalent to `--no-colors`) |
 | `HERDR_ENV`, `HERDR_PANE_ID`, `HERDR_WORKSPACE_ID` | Identify the current Herdr panel and workspace for renaming |
 | `CMUX_SOCKET_PATH`, `CMUX_BUNDLE_ID` | Identify cmux for tab renaming |
diff --git a/spec/init_spec.md b/spec/init_spec.md
index 6e61448..3ca8439 100644
--- a/spec/init_spec.md
+++ b/spec/init_spec.md
@@ -11,7 +11,7 @@ The shell function wrapper is necessary because:
 
 ## Shell Detection
 
-The init command should detect the user's shell via the `$SHELL` environment variable and output the appropriate function syntax.
+The init command should detect the shell that invokes it from the parent process and output the appropriate function syntax. If the parent process cannot be determined or is not a shell, it should fall back to the user's configured shell in `$SHELL`.
 
 Supported shells:
 - **Bash/Zsh**: POSIX-compatible function syntax
@@ -96,8 +96,8 @@ Test that init produces valid shell syntax:
 # Test Bash syntax
 bash -n <(try init)
 
-# Test Fish syntax (if fish is available)
-fish -n <(SHELL=/usr/bin/fish try init)
+# Test Fish syntax, even when the login shell is different
+env SHELL=/bin/zsh fish -c 'try init | fish --no-execute'
 ```
 
 Test that the wrapper works correctly:
diff --git a/spec/tests/test_14_init_shells.sh b/spec/tests/test_14_init_shells.sh
index 24d1504..51f44ae 100644
--- a/spec/tests/test_14_init_shells.sh
+++ b/spec/tests/test_14_init_shells.sh
@@ -18,13 +18,17 @@ else
     fail "bash function should include --path with specified path" "--path '$TEST_TRIES'" "$output" "init_spec.md"
 fi
 
-# Test: init with fish shell emits fish function
-output=$(SHELL=/usr/bin/fish try_run init "$TEST_TRIES" 2>&1)
+# Test: init invoked by fish emits fish function
+FAKE_PS_DIR=$(mktemp -d)
+printf '#!/bin/sh\nprintf "fish\\n"\n' > "$FAKE_PS_DIR/ps"
+chmod +x "$FAKE_PS_DIR/ps"
+output=$(PATH="$FAKE_PS_DIR:$PATH" SHELL=/bin/zsh try_run init "$TEST_TRIES" 2>&1)
 if echo "$output" | grep -q "function try"; then
     pass
 else
     fail "init with fish should emit fish function" "function try" "$output" "init_spec.md"
 fi
+rm -rf "$FAKE_PS_DIR"
 
 # Test: init output contains the real, full path to try binary
 output=$(SHELL=/bin/bash try_run init "$TEST_TRIES" 2>&1)
diff --git a/spec/tests/test_34_shell_init.sh b/spec/tests/test_34_shell_init.sh
index 127bd8c..544de67 100644
--- a/spec/tests/test_34_shell_init.sh
+++ b/spec/tests/test_34_shell_init.sh
@@ -3,14 +3,45 @@
 
 section "shell-init"
 
-# Test: SHELL=fish emits fish function
-output=$(SHELL=/usr/local/bin/fish try_run init "$TEST_TRIES" 2>&1)
+# Test: the invoking shell takes precedence over the login shell in $SHELL
+FAKE_PS_DIR=$(mktemp -d)
+printf '#!/bin/sh\nprintf "fish\\n"\n' > "$FAKE_PS_DIR/ps"
+chmod +x "$FAKE_PS_DIR/ps"
+output=$(PATH="$FAKE_PS_DIR:$PATH" SHELL=/bin/zsh try_run init "$TEST_TRIES" 2>&1)
 if echo "$output" | grep -q "function try"; then
     pass
 else
-    fail "SHELL=fish should emit fish function" "function try" "$output" "shell_init"
+    fail "fish parent should override SHELL=zsh" "function try" "$output" "shell_init"
 fi
 
+# Test: a non-fish parent also takes precedence over SHELL=fish
+printf '#!/bin/sh\nprintf "zsh\\n"\n' > "$FAKE_PS_DIR/ps"
+output=$(PATH="$FAKE_PS_DIR:$PATH" SHELL=/usr/local/bin/fish try_run init "$TEST_TRIES" 2>&1)
+if echo "$output" | grep -q "try() {"; then
+    pass
+else
+    fail "zsh parent should override SHELL=fish" "try() {" "$output" "shell_init"
+fi
+
+# Test: SHELL is used when the parent shell cannot be detected
+printf '#!/bin/sh\n' > "$FAKE_PS_DIR/ps"
+output=$(PATH="$FAKE_PS_DIR:$PATH" SHELL=/usr/local/bin/fish try_run init "$TEST_TRIES" 2>&1)
+if echo "$output" | grep -q "function try"; then
+    pass
+else
+    fail "SHELL=fish should be the fallback" "function try" "$output" "shell_init"
+fi
+
+# Test: a non-shell parent (e.g. a script runner) is ignored in favor of SHELL
+printf '#!/bin/sh\nprintf "ruby\\n"\n' > "$FAKE_PS_DIR/ps"
+output=$(PATH="$FAKE_PS_DIR:$PATH" SHELL=/usr/local/bin/fish try_run init "$TEST_TRIES" 2>&1)
+if echo "$output" | grep -q "function try"; then
+    pass
+else
+    fail "non-shell parent should fall back to SHELL=fish" "function try" "$output" "shell_init"
+fi
+rm -rf "$FAKE_PS_DIR"
+
 # Test: SHELL=zsh emits bash/zsh function
 output=$(SHELL=/bin/zsh try_run init "$TEST_TRIES" 2>&1)
 if echo "$output" | grep -q "try() {"; then
diff --git a/spec/tests/test_36_shell_eval.sh b/spec/tests/test_36_shell_eval.sh
index e19bc3c..28c16a6 100644
--- a/spec/tests/test_36_shell_eval.sh
+++ b/spec/tests/test_36_shell_eval.sh
@@ -65,7 +65,7 @@ fi
 
 if command -v nix-shell >/dev/null 2>&1; then
     # Test: fish can eval the init output and defines try function
-    fish_out=$(nix-shell -p fish --run "SHELL=fish fish -c 'eval ($TRY_BIN_PATH init --path $EVAL_DIR | string collect); type try'" 2>&1)
+    fish_out=$(nix-shell -p fish --run "SHELL=/bin/zsh fish -c 'eval ($TRY_BIN_PATH init --path $EVAL_DIR | string collect); type try'" 2>&1)
     if echo "$fish_out" | grep -qi "try is a function\|function try"; then
         pass
     else
@@ -81,7 +81,7 @@ if command -v nix-shell >/dev/null 2>&1; then
     fi
 
     # Test: fish init output is valid fish syntax (no parse errors)
-    fish_syntax=$(SHELL=fish "$TRY_BIN_PATH" init --path "$EVAL_DIR" 2>&1)
+    fish_syntax=$(nix-shell -p fish --run "SHELL=/bin/zsh fish -c '$TRY_BIN_PATH init --path $EVAL_DIR'" 2>&1)
     fish_parse=$(echo "$fish_syntax" | nix-shell -p fish --run "fish --no-execute" 2>&1)
     if [ $? -eq 0 ]; then
         pass
diff --git a/try.rb b/try.rb
index c3b3c0a..07ad351 100755
--- a/try.rb
+++ b/try.rb
@@ -1642,12 +1642,10 @@ def resolve_unique_name_with_versioning(tries_path, date_prefix, base)
   end
 
   # shell detection for init wrapper
-  # Check $SHELL first (user's configured shell), then parent process as fallback
+  # Check the invoking shell first; fall back to $SHELL when the parent isn't a shell
   def fish?
-    shell = ENV["SHELL"].to_s
-    if shell.empty?
-      shell = (`ps c -p #{Process.ppid} -o 'ucomm='`.strip rescue "").to_s
-    end
+    shell = (`ps c -p #{Process.ppid} -o 'ucomm=' 2>/dev/null`.strip rescue "").to_s
+    shell = ENV["SHELL"].to_s unless shell.match?(/\A-?\w*sh\z/)
     shell.include?('fish')
   end