diff --git a/.changeset/614-discuss-phase-shim-resolution.md b/.changeset/614-discuss-phase-shim-resolution.md
new file mode 100644
index 000000000..72c55c139
--- /dev/null
+++ b/.changeset/614-discuss-phase-shim-resolution.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 618
+---
+**`/gsd-discuss-phase` now honors `workflow.discuss_mode: assumptions` on shim-only installs** — mode routing resolves `gsd-tools` via the runtime shim instead of the bare PATH command, so a missing PATH binary no longer silently falls back to standard discuss mode.
diff --git a/commands/gsd/discuss-phase.md b/commands/gsd/discuss-phase.md
index d7aa4ad64..2b21790d1 100644
--- a/commands/gsd/discuss-phase.md
+++ b/commands/gsd/discuss-phase.md
@@ -47,7 +47,8 @@ Context files are resolved in-workflow using `init phase-op` and roadmap/state t
 <process>
 **Mode routing:**
 ```bash
-DISCUSS_MODE=$(gsd-tools query config-get workflow.discuss_mode 2>/dev/null || echo "discuss")
+_GSD_SHIM_NAME="gsd-tools.cjs"; _GSD_RUNTIME_ROOT="${RUNTIME_DIR:-$(git rev-parse --show-toplevel 2>/dev/null || pwd)}"; GSD_TOOLS="${_GSD_RUNTIME_ROOT}/get-shit-done/bin/${_GSD_SHIM_NAME}"; if [ -f "$GSD_TOOLS" ]; then gsd_run() { node "$GSD_TOOLS" "$@"; }; elif [ -f "${_GSD_RUNTIME_ROOT}/.claude/get-shit-done/bin/${_GSD_SHIM_NAME}" ]; then GSD_TOOLS="${_GSD_RUNTIME_ROOT}/.claude/get-shit-done/bin/${_GSD_SHIM_NAME}"; gsd_run() { node "$GSD_TOOLS" "$@"; }; elif command -v gsd-tools >/dev/null 2>&1; then GSD_TOOLS="$(command -v gsd-tools)"; gsd_run() { "$GSD_TOOLS" "$@"; }; elif [ -f "$HOME/.claude/get-shit-done/bin/${_GSD_SHIM_NAME}" ]; then GSD_TOOLS="$HOME/.claude/get-shit-done/bin/${_GSD_SHIM_NAME}"; gsd_run() { node "$GSD_TOOLS" "$@"; }; else echo "ERROR: gsd-tools.cjs not found at $GSD_TOOLS and gsd-tools is not on PATH. Run: npx -y @opengsd/gsd-core@latest --claude --local" >&2; exit 1; fi
+DISCUSS_MODE=$(gsd_run query config-get workflow.discuss_mode 2>/dev/null || echo "discuss")
 ```
 
 If `--assumptions` is in $ARGUMENTS:
diff --git a/tests/discuss-mode.test.cjs b/tests/discuss-mode.test.cjs
index 8f91af26c..727bdaa8c 100644
--- a/tests/discuss-mode.test.cjs
+++ b/tests/discuss-mode.test.cjs
@@ -164,4 +164,25 @@ describe('workflow.discuss_mode config', () => {
     assert.ok(doc.includes('discuss'), 'doc should mention discuss');
     assert.ok(doc.includes('config-set'), 'doc should show how to configure');
   });
+
+  test('discuss-phase command mode-routing uses gsd_run (shim-safe) not bare gsd-tools', () => {
+    const command = fs.readFileSync(
+      path.join(__dirname, '..', 'commands', 'gsd', 'discuss-phase.md'), 'utf8'
+    );
+    // Must contain the canonical shim probe marker
+    assert.ok(
+      command.includes('_GSD_SHIM_NAME'),
+      'discuss-phase.md must define _GSD_SHIM_NAME shim probe before mode routing'
+    );
+    // Must use gsd_run for the config lookup
+    assert.ok(
+      command.includes('gsd_run query config-get workflow.discuss_mode'),
+      'discuss-phase.md must use gsd_run (not bare gsd-tools) for discuss_mode lookup'
+    );
+    // Must NOT contain the bare footgun pattern: gsd-tools immediately before the silent default
+    assert.ok(
+      !command.includes('gsd-tools query config-get workflow.discuss_mode 2>/dev/null || echo'),
+      'discuss-phase.md must NOT use bare gsd-tools binary for discuss_mode lookup (shim-only install footgun)'
+    );
+  });
 });