diff --git a/packages/server/src/server/agent/providers/codex-mcp-agent.test.ts b/packages/server/src/server/agent/providers/codex-mcp-agent.test.ts
index 8c44ed2b4..3e5f4970c 100644
--- a/packages/server/src/server/agent/providers/codex-mcp-agent.test.ts
+++ b/packages/server/src/server/agent/providers/codex-mcp-agent.test.ts
@@ -504,7 +504,6 @@ describe("CodexMcpAgentClient (MCP integration)", () => {
         provider: "codex-mcp",
         cwd,
         modeId: "full-access",
-        model: "gpt-5.1-codex",
       } as AgentSessionConfig;
 
       let session: AgentSession | null = null;
diff --git a/packages/server/src/server/agent/providers/codex-mcp-agent.ts b/packages/server/src/server/agent/providers/codex-mcp-agent.ts
index 89da6acbd..664691558 100644
--- a/packages/server/src/server/agent/providers/codex-mcp-agent.ts
+++ b/packages/server/src/server/agent/providers/codex-mcp-agent.ts
@@ -87,7 +87,6 @@ const CODEX_MODES: AgentMode[] = [
 ];
 
 const DEFAULT_CODEX_MODE_ID = "auto";
-const DEFAULT_CODEX_MODEL_ID = "gpt-5.1-codex";
 
 const MODE_PRESETS: Record<
   string,
@@ -219,14 +218,17 @@ function buildCodexMcpConfig(
   const sandbox = config.sandboxMode ?? preset.sandbox;
   const extra = config.extra?.codex ?? undefined;
 
-  return {
+  const configPayload: Record<string, unknown> = {
     prompt,
     cwd: config.cwd,
-    model: config.model ?? DEFAULT_CODEX_MODEL_ID,
     "approval-policy": approvalPolicy,
     sandbox,
     config: extra,
   };
+  if (typeof config.model === "string" && config.model.length > 0) {
+    configPayload.model = config.model;
+  }
+  return configPayload;
 }
 
 function isUnsupportedChatGptModelError(error: unknown): boolean {
@@ -715,10 +717,6 @@ class CodexMcpAgentSession implements AgentSession {
     }
 
     this.updateIdentifiersFromResponse(response);
-    if (this.modelRejected && !this.runtimeModel) {
-      this.runtimeModel = DEFAULT_CODEX_MODEL_ID;
-    }
-
     if (!turnState.sawAssistant) {
       const text = extractTextContent(response);
       if (text) {
diff --git a/plan.md b/plan.md
index ed81745f7..10b18d52e 100644
--- a/plan.md
+++ b/plan.md
@@ -74,12 +74,13 @@ Build a new Codex MCP provider side‑by‑side with the existing Codex SDK prov
   - Update tests and provider to use a valid default model.
   - **Done (2025-12-24 18:58)**: Updated Codex MCP default model to gpt-5.1-codex and switched runtime info test to use the valid model id.
 
-- [ ] **Fix**: Remove hardcoded default model - passthrough user choice.
+- [x] **Fix**: Remove hardcoded default model - passthrough user choice.
 
   - Pass `config.model` if user specifies one.
   - If user doesn't specify, omit `model` field - let Codex CLI pick its default.
   - Do NOT hardcode any fallback model in the provider.
   - Tests should not specify a model unless testing model passthrough.
+  - **Done (2025-12-24 19:14)**: Dropped the default model constant, omitted `model` from MCP config when unset, and removed the hardcoded fallback in runtime info; adjusted Codex MCP runtime test to avoid specifying a model.
 
 - [ ] **Review**: Flag ALL workarounds/hacks in `codex-mcp-agent.ts` - they are NOT acceptable.