Bug Report: Xcode 26.6 Codex Agent Fails with Response Decoding and Model Compatibility Errors

Severity: High — Core agent functionality unavailable
Component: Xcode Intelligence / Codex Agent Integration
Environment: macOS, Xcode 26.6 (17F113)
Date reported: November 1, 2026

The built-in Codex coding agent in Xcode consistently fails to process requests, despite successful ChatGPT Pro authentication and a functioning standalone Codex CLI.

The problem occurs even with trivial prompts and prevents use of Codex's autonomous development functionality, including the Pursue Goal workflow.

The standard ChatGPT integration inside Xcode works correctly. The failure is isolated to the Codex agent integration.

When initiating a conversation with the built-in Codex agent in Xcode, the request fails with the following messages:

Error 1: Response decoding failure

The data couldn’t be read because it isn’t in the correct format.

Accompanied by:

Your request couldn't be completed.

Error 2: Unsupported model

With the original model selection:

Codex encountered an error:
The 'gpt-5.5' model is not supported when
using Codex with a ChatGPT account.

After explicitly configuring gpt-6.1-sol:

Codex encountered an error:
The 'gpt-6.1-sol' model is not supported when
using Codex with a ChatGPT account.

Error 3: ACP communication failure

When testing a separately registered Codex ACP adapter:

JSON-RPC request with ID [...] failed to produce a response.

This was observed in a separate integration experiment and should not automatically be assumed to share the same root cause as the built-in Codex failure.

The built-in Codex agent should:

1. Authenticate with an eligible ChatGPT Pro account.
2. Select a model supported by the account and installed agent version.
3. Correctly initialize the agent and decode its protocol responses.
4. Execute basic prompts without errors.
5. Support Xcode's native agentic development workflows, including Pursue Goal.
6. Present actionable diagnostics when authentication, model selection, or protocol initialization fails.

1. Open Xcode 26.6.

2. Open an existing SwiftUI project.

3. Open the Coding Assistant.

4. Start a new conversation.

5. Select Codex under Agents, rather than OpenAI under Chat.

6. Submit a trivial prompt, such as:

   Reply with exactly OK.

7. Observe the request failing with a response-format error or an unsupported-model error, depending on configuration.

The issue is reproducible across multiple attempts and is not specific to a complex coding task.

The following troubleshooting actions have already been attempted:

• Reauthenticated using the ChatGPT account.
• Tested configuration-file-based authentication.
• Reset Codex authentication.
• Reinstalled Xcode.
• Reinstalled the Xcode Codex component.
• Restarted Xcode and created new conversations.
• Tested explicit model selection through config.toml.
• Tested standalone Codex with the same ChatGPT Pro account.
• Tested a separate ACP adapter integration.

The built-in Codex agent remains unusable.

Standalone Codex CLI 0.166.0 successfully executed a request using GPT-6.1 Sol.

Command:

codex exec --model gpt-6.1-sol --skip-git-repo-check \
"Reply with exactly OK. Do not edit files or use tools."

Observed output:

OpenAI Codex v0.166.0
model: gpt-6.1-sol
provider: openai
reasoning effort: high

codex
OK

The request completed without an API error.

Additionally, Xcode's separate ChatGPT integration displays the ChatGPT Pro account as authenticated, with usage status Under Limit.

These observations strongly suggest that the problem is specific to the Xcode Codex integration or its runtime environment, rather than a general account authentication failure.

The following are hypotheses requiring investigation, not confirmed causes:

1. Bundled agent version incompatibility

Xcode was observed using Codex 0.140.0, while standalone Codex 0.166.0 successfully accessed GPT-6.1 Sol.

Please verify whether the bundled Codex agent is compatible with currently available ChatGPT-authenticated models.

2. Model discovery or selection mismatch

The bundled agent's model catalog included models that were subsequently rejected by the service.

Please investigate whether Xcode refreshes the model catalog, respects model configuration overrides, and selects models based on account eligibility.

3. Agent protocol response decoding

The generic data-format error suggests a possible failure to decode an agent response, handle an unsupported response schema, or propagate an underlying API error.

Please inspect the agent initialization, response parsing, and error-handling paths.

4. Configuration and authentication isolation

The normal ChatGPT integration authenticates successfully, while the Codex agent fails.

Please verify how Xcode manages distinct authentication contexts, configuration locations, agent versions, and model capabilities.

Please investigate and address the following:

• Ensure the Xcode Codex agent is compatible with supported ChatGPT-authenticated models.
• Verify that the bundled agent version is current and that component updates are correctly delivered.
• Validate model availability before dispatching requests.
• Ensure the model selected in Xcode matches the actual model submitted by the agent.
• Correct any incompatible protocol or JSON response handling.
• Expose the underlying error details rather than only reporting a generic data-format failure.
• Confirm that reinstalling or updating the Xcode Codex component restores a functional agent without requiring manual configuration changes.
• Add regression coverage for ChatGPT Pro authentication, model selection, agent initialization, and basic prompt execution.

The issue prevents me from using the Codex agent directly inside Xcode, despite having a ChatGPT Pro subscription and a fully functional standalone Codex CLI.

As a result, I cannot use the integrated agentic coding experience or the Pursue Goal workflow that previously worked.

Please investigate the bundled Codex runtime, model selection, and protocol compatibility. The successful standalone Codex 0.166.0 test provides a control case for reproducing and isolating the failure.