diff --git a/src/resources/extensions/gsd/auto-start.ts b/src/resources/extensions/gsd/auto-start.ts
index 34b5467ab..50f137c2b 100644
--- a/src/resources/extensions/gsd/auto-start.ts
+++ b/src/resources/extensions/gsd/auto-start.ts
@@ -161,7 +161,7 @@ export async function bootstrapAutoSession(
   // (#3517).  The session model (ctx.model) comes from findInitialModel() which
   // reads defaultProvider/defaultModel from ~/.gsd/agent/settings.json.  When
   // the user has explicit model preferences in PREFERENCES.md, those should win.
-  const preferredModel = resolveDefaultSessionModel();
+  const preferredModel = resolveDefaultSessionModel(ctx.model?.provider);
   const startModelSnapshot = preferredModel
     ?? (ctx.model
       ? { provider: ctx.model.provider, id: ctx.model.id }
diff --git a/src/resources/extensions/gsd/preferences-models.ts b/src/resources/extensions/gsd/preferences-models.ts
index 40b41d04c..2e4171687 100644
--- a/src/resources/extensions/gsd/preferences-models.ts
+++ b/src/resources/extensions/gsd/preferences-models.ts
@@ -116,10 +116,19 @@ export function resolveModelWithFallbacksForUnit(unitType: string): ResolvedMode
  * we treat that as the session default.  Falls back through execution →
  * planning → first configured model.
  *
- * Returns `{ provider, id }` parsed from the `provider/model` format,
- * or `undefined` if no model preference is configured.
+ * Accepts an optional `sessionProvider` for bare model IDs that don't
+ * include an explicit provider prefix (e.g. `gpt-5.4` instead of
+ * `openai-codex/gpt-5.4`).  When a bare ID is found and sessionProvider
+ * is available, the session provider is used.  Without sessionProvider,
+ * bare IDs are still returned with provider set to the bare ID itself
+ * so downstream resolution (resolveModelId) can match it.
+ *
+ * Returns `{ provider, id }` or `undefined` if no model preference is
+ * configured.
  */
-export function resolveDefaultSessionModel(): { provider: string; id: string } | undefined {
+export function resolveDefaultSessionModel(
+  sessionProvider?: string,
+): { provider: string; id: string } | undefined {
   const prefs = loadEffectiveGSDPreferences();
   if (!prefs?.preferences.models) return undefined;
 
@@ -138,15 +147,38 @@ export function resolveDefaultSessionModel(): { provider: string; id: string } |
 
   for (const cfg of candidates) {
     if (!cfg) continue;
-    const modelStr = typeof cfg === "string"
-      ? cfg
-      : cfg.provider && !cfg.model.includes("/")
-        ? `${cfg.provider}/${cfg.model}`
-        : cfg.model;
 
-    if (modelStr.includes("/")) {
-      const slashIdx = modelStr.indexOf("/");
-      return { provider: modelStr.slice(0, slashIdx), id: modelStr.slice(slashIdx + 1) };
+    // Normalize to provider + id from the various config shapes
+    let provider: string | undefined;
+    let id: string;
+
+    if (typeof cfg === "string") {
+      const slashIdx = cfg.indexOf("/");
+      if (slashIdx !== -1) {
+        provider = cfg.slice(0, slashIdx);
+        id = cfg.slice(slashIdx + 1);
+      } else {
+        // Bare model ID (e.g. "gpt-5.4") — use session provider as context
+        provider = sessionProvider;
+        id = cfg;
+      }
+    } else {
+      // Object config: { model, provider?, fallbacks? }
+      if (cfg.provider) {
+        provider = cfg.provider;
+      } else if (cfg.model.includes("/")) {
+        const slashIdx = cfg.model.indexOf("/");
+        provider = cfg.model.slice(0, slashIdx);
+        id = cfg.model.slice(slashIdx + 1);
+        return { provider, id };
+      } else {
+        provider = sessionProvider;
+      }
+      id = cfg.model;
+    }
+
+    if (provider && id) {
+      return { provider, id };
     }
   }
 
diff --git a/src/resources/extensions/gsd/tests/auto-start-model-capture.test.ts b/src/resources/extensions/gsd/tests/auto-start-model-capture.test.ts
index d18a1ada0..2ffb5bf96 100644
--- a/src/resources/extensions/gsd/tests/auto-start-model-capture.test.ts
+++ b/src/resources/extensions/gsd/tests/auto-start-model-capture.test.ts
@@ -31,9 +31,13 @@ test("bootstrapAutoSession restores autoModeStartModel from the early snapshot (
 
 test("bootstrapAutoSession prefers GSD PREFERENCES.md over settings.json for start model (#3517)", () => {
   // resolveDefaultSessionModel() should be called before the snapshot is built
-  const preferredIdx = source.indexOf("const preferredModel = resolveDefaultSessionModel()");
+  const preferredIdx = source.indexOf("const preferredModel = resolveDefaultSessionModel(");
   assert.ok(preferredIdx > -1, "auto-start.ts should call resolveDefaultSessionModel()");
 
+  // Session provider should be passed for bare model ID resolution
+  const withProviderIdx = source.indexOf("resolveDefaultSessionModel(ctx.model?.provider)");
+  assert.ok(withProviderIdx > -1, "auto-start.ts should pass ctx.model?.provider for bare ID resolution");
+
   const snapshotIdx = source.indexOf("const startModelSnapshot = preferredModel");
   assert.ok(snapshotIdx > -1, "startModelSnapshot should use preferredModel when available");
 
diff --git a/src/resources/extensions/gsd/tests/model-isolation.test.ts b/src/resources/extensions/gsd/tests/model-isolation.test.ts
index 27177b327..6dd107b12 100644
--- a/src/resources/extensions/gsd/tests/model-isolation.test.ts
+++ b/src/resources/extensions/gsd/tests/model-isolation.test.ts
@@ -200,6 +200,19 @@ describe("GSD preferences override settings.json for session model (#3517)", ()
       "should be null when neither preferences nor ctx.model exist");
   });
 
+  it("bare model ID uses session provider when available", () => {
+    // Simulates: PREFERENCES.md has "gpt-5.4" (no provider), session is openai-codex
+    const preferredModel = { provider: "openai-codex", id: "gpt-5.4" }; // from resolveDefaultSessionModel("openai-codex")
+    const ctxModel = { provider: "openai-codex", id: "claude-sonnet-4-6" };
+
+    const startModelSnapshot = preferredModel
+      ?? { provider: ctxModel.provider, id: ctxModel.id };
+
+    assert.equal(startModelSnapshot.provider, "openai-codex");
+    assert.equal(startModelSnapshot.id, "gpt-5.4",
+      "bare model ID from preferences should still override ctx.model");
+  });
+
   it("stale settings.json does not leak when preferences are set", () => {
     // Scenario: settings.json has claude-code, PREFERENCES.md has openai-codex
     const settingsJsonDefault = { provider: "claude-code", id: "claude-sonnet-4-6" };