fix(sessions): keep a session's model through recovery, refuse it off claude

SessionState now carries the model a session launched with, and both
recovery constructors (mux recovery and reboot restore) pass it back, so a
recovered session relaunches on the same --model rather than the account
default. A top-level `model` sent with any other CLI is refused, since
those take their model in their own config object, and an empty string
means no per-session model, as it does for modelOverride.

CLAUDE.md now describes both routes for a Claude model. The tests pin
which of `model` and `modelOverride` reaches the launch and which the
case file, and that a model opening with a dash renders as --model's value.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/session.ts

@@ -1827,6 +1827,7 @@ export class Session extends EventEmitter {
       ompConfig: this._ompConfig,
       resumeSessionId: this._resumeSessionId,
       effort: this._effort,
+      model: this._model,
       customModel: this.customModel,
       // COD-118: runtime-only — surfaced so the frontend can require explicit user
       // intent before restarting a crash-looped session. Deliberately NOT restored

src/types/session.ts

@@ -795,6 +795,13 @@ export interface SessionState {
   resumeSessionId?: string;
   /** Claude CLI effort level (soft default via --settings, switchable in-session via /effort) */
   effort?: EffortLevel;
+  /**
+   * The model the session was LAUNCHED with (`--model`): the caller's per-session `model`, or
+   * the app-wide default when there was none. Persisted so a recovered session relaunches on
+   * the same model rather than whatever the default is by then. Not `cliModel`, which is what
+   * the CLI's banner reports.
+   */
+  model?: string;
   /**
    * Custom Model Endpoint Profiles (docs/custom-model-endpoints-plan.md): the custom
    * OpenAI-compatible endpoint (local or cloud) this session's CLI is currently pointed

src/web/routes/reboot-restore-routes.ts

@@ -195,6 +195,7 @@ export function registerRebootRestoreRoutes(app: FastifyInstance, ctx: RebootRes
               (saved as { __envOverrides?: Record<string, string> }).__envOverrides
             ),
             effort: saved.effort,
+            model: saved.model,
             attachmentHistory:
               (saved as { __attachmentHistory?: SessionAttachmentHistoryItem[] }).__attachmentHistory ??
               saved.attachmentHistory,

src/web/routes/session-routes.ts

@@ -888,6 +888,15 @@ export function registerSessionRoutes(
     if (capMsg) return createErrorResponse(ApiErrorCode.OPERATION_FAILED, capMsg);
 
     const body = parseBody(CreateSessionSchema, req.body);
+    // The top-level `model` is Claude's per-session `--model`. Every other CLI takes its model
+    // in its own config object (`codexConfig.model` and so on), so a `model` here would be
+    // dropped without a word; refuse it before anything is written for the session.
+    if (body.model && getCli(body.mode ?? 'claude')?.capabilities.model.source !== 'claude-settings-file') {
+      return createErrorResponse(
+        ApiErrorCode.INVALID_INPUT,
+        'model applies to claude sessions only; other CLIs take their model in their own config object, such as codexConfig.model'
+      );
+    }
     let workingDir = body.workingDir || process.cwd();
     let remote = undefined;
 

src/web/schemas.ts

@@ -529,12 +529,15 @@ export const CreateSessionSchema = z.object({
   /**
    * Claude model for THIS session only, passed as `claude --model <id>`; nothing is written to
    * disk. Wins over the app-wide default model. Same character set as the registry's
-   * `model-claude` pattern, so a value accepted here is never rejected at launch.
+   * `model-claude` pattern, so a value accepted here is never rejected at launch. An empty
+   * string means no per-session model, as it does for `modelOverride`. Claude only: the route
+   * refuses it for any other CLI.
    */
   model: z
     .string()
     .max(100)
     .regex(/^[a-zA-Z0-9._\-[\]]+$/)
+    .or(z.literal(''))
     .optional(),
   openCodeConfig: OpenCodeConfigSchema,
   codexConfig: CodexConfigSchema,

src/web/server.ts

@@ -3489,6 +3489,7 @@ export class WebServer extends EventEmitter {
               ompConfig: muxSession.mode === 'omp' ? savedState?.ompConfig : undefined,
               envOverrides: savedEnvOverrides,
               effort: savedState?.effort,
+              model: savedState?.model,
               attachmentHistory: savedAttachmentHistory,
               // The pane's last Enter. Without it the response viewer would show
               // the launch conversation until the user types again, even though