fix(custom-model): keep the launch model out of the stored config

Quick-start wrote a selection's launchModel into the session's stored
<Mode>Config, which is what a clear falls back to, so a clear relaunched
the CLI with the endpoint's model id on its native backend. Only API
callers can reach that, since no page sends a clear.

The first launch now takes the forced model from the session's selection,
through the same _withCustomModelLaunchModel() the respawn path uses, so
the stored config keeps the model chosen at create. That also carries the
model onto a new tmux session for a selection applied to a running
session, which the create path used to lose.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/session.ts

@@ -2990,13 +2990,13 @@ export class Session extends EventEmitter {
    * launch param. Where that param
    * lives is registry DATA — the entry's `legacyConfigField` (`piConfig`, `grokConfig`,
    * ...) or the top-level `model` for an entry that declares none — so this stays a
-   * generic reader rather than a branch per CLI. Applied on the OPTIONS only: the stored
-   * `<Mode>Config` keeps whatever model the user chose at create, which is exactly what a
-   * later clear must fall back to.
+   * generic reader rather than a branch per CLI. Applied on the OPTIONS only, on both the
+   * respawn and the create path: the stored `<Mode>Config` keeps whatever model the user
+   * chose at create, which is exactly what a later clear must fall back to.
    */
-  private _withCustomModelLaunchModel(
-    options: import('./mux-interface.js').RespawnPaneOptions
-  ): import('./mux-interface.js').RespawnPaneOptions {
+  private _withCustomModelLaunchModel<
+    T extends import('./mux-interface.js').RespawnPaneOptions | import('./mux-interface.js').CreateSessionOptions,
+  >(options: T): T {
     const launchModel = this._customModel?.launchModel;
     if (!launchModel) return options;
     const entry = getCli(this.mode);
@@ -3348,7 +3348,7 @@ export class Session extends EventEmitter {
         const { isRestored, respawnedResumeId, respawnedDeadPane } = await this._setupOrAttachMuxSession({
           // Single source of truth shared with reattachRemote() (COD-108).
           respawnPaneOptions: this._buildRespawnPaneOptions(),
-          createSessionOptions: {
+          createSessionOptions: this._withCustomModelLaunchModel({
             sessionId: this.id,
             workingDir: this.workingDir,
             mode: this.mode,
@@ -3378,7 +3378,7 @@ export class Session extends EventEmitter {
             docker: this._docker,
             adopt: this._adopt,
             owner: this._owner,
-          },
+          }),
           spawnErrLabel: 'mux attachment',
         });
 

src/web/routes/session-routes.ts

@@ -4750,7 +4750,6 @@ export function registerSessionRoutes(
     // would hide them, and giving setCustomModel() that set would put
     // CLAUDE_CODE_EFFORT_LEVEL back after the constructor stripped it.
     let qsCustomModelAppliedEnvOverrides: Record<string, string> | undefined;
-    let qsCustomModelLaunchModel: string | undefined;
     let qsCustomModelSessionId: string | undefined;
     let qsCustomModelSwapInProgress = false;
     let qsCustomModelBookkeeping:
@@ -4866,7 +4865,6 @@ export function registerSessionRoutes(
         return createErrorResponse(ApiErrorCode.OPERATION_FAILED, `${mode} has no known custom-model mechanism`);
       }
       qsCustomModelAppliedEnvOverrides = cmApplied.envOverrides;
-      qsCustomModelLaunchModel = cmApplied.launchModel;
       qsCustomModelBookkeeping = {
         endpointId: cmEndpoint.id,
         modelId: customModel.modelId,
@@ -4916,30 +4914,15 @@ export function registerSessionRoutes(
       tmuxHistoryLimit: qsTerminalHistoryConfig.tmuxHistoryLimit,
       parentSessionId: qsParentSessionId,
     };
-    // Force the custom-model selection's launchModel (pi/omp `custom/<id>`, grok's
-    // `[model.<name>]` block name) onto whichever config field the registry says the
-    // CLI's `model` launch param lives in — mirrors Session._withCustomModelLaunchModel,
-    // which the restart-in-place path already uses, rather than a hardcoded per-CLI
-    // branch here that a CLI landing its injection recipe later would silently miss.
-    if (qsCustomModelLaunchModel !== undefined) {
-      const qsCustomModelField = getCli(mode)?.launch.legacyConfigField;
-      if (qsCustomModelField) {
-        const qsSessionOptionsBag = qsSessionOptions as unknown as Record<string, unknown>;
-        qsSessionOptionsBag[qsCustomModelField] = {
-          ...((qsSessionOptionsBag[qsCustomModelField] as Record<string, unknown>) ?? {}),
-          model: qsCustomModelLaunchModel,
-        };
-      } else {
-        qsSessionOptions.model = qsCustomModelLaunchModel;
-      }
-    }
     const session = new Session(qsSessionOptions);
 
     // Records the selection for session.customModel/getCustomModelForPersist() and future
-    // clear/switch calls, and merges the injected env, which the launch below reads along
-    // with the launch-model config set above (piConfig/grokConfig/ompConfig.model). Nothing
-    // has started yet, so this is never a restart: setCustomModel() is synchronous state,
-    // no tmux IO of its own (see its own doc comment in session.ts).
+    // clear/switch calls, and merges the injected env. The launch below reads both: the
+    // selection's launchModel is forced onto the launch options, never onto the stored
+    // config above, because that config is what a later clear falls back to
+    // (Session._withCustomModelLaunchModel). Nothing has started yet, so this is never a
+    // restart: setCustomModel() is synchronous state, no tmux IO of its own (see its own
+    // doc comment in session.ts).
     if (qsCustomModelBookkeeping) {
       session.setCustomModel(qsCustomModelBookkeeping, qsCustomModelAppliedEnvOverrides);
     }