Exact error
Match the message before using the fix
MCP startup failed: handshaking with MCP server failed: JSON-RPC error: -32603Internal error: The data couldn’t be read because it isn’t in the correct format.MCP startup incomplete (failed: computer-history, messages)Quick answer
Start here
First confirm that both bundled plugins fail with the same `-32603` message while the desktop-bundled CLI still connects. The source report isolates the trigger to a non-empty `capabilities.experimental` object sent by CLI 0.154.0; disabling auth elicitation did not remove it. Update Codex first. If the regression remains, use the known-working desktop-bundled client for those plugins rather than editing their installed files.
Diagnosis
Why it happens
- The report reproduced failure with both `codex/auth-change` and an unrelated experimental capability entry.
- The same helpers initialized when the experimental object was empty or omitted.
- The exact parser defect inside the bundled helper was not inspected, so the evidence establishes a compatibility boundary rather than the internal implementation cause.
Safest first
Fixes, in order
Confirm this exact compatibility branch
Applies when: Messages and Computer History both fail before any tool call
Generic -32603 errors can have unrelated causes.
- Record CLI, Desktop, and bundled plugin versions.
- Run `/mcp` with the current CLI and note both failed plugin names.
- Run the same check through the desktop-bundled CLI if available.
- Do not send any real message/history content during the test.
Expected: The current CLI fails both bundled helpers while the bundled client connects with the same installed plugins.
Update the client and bundled plugins together
Applies when: The version pair matches or predates the reported regression
A client/helper capability mismatch should be corrected by compatible official builds.
- Update Codex Desktop and CLI through their official channels.
- Fully quit old client and helper processes.
- Start the updated client and recheck `/mcp` before changing configuration.
Expected: Messages and Computer History initialize, or the mismatch reproduces with a current version pair.
Use the known-working official client path temporarily
Applies when: The standalone CLI still fails but the Desktop-bundled CLI works
The report verified the older bundled client without modifying plugin files or protocol payloads.
- Use the Desktop surface for tasks requiring those bundled plugins.
- Leave the bundled helper installation and signatures unchanged.
- Report the full version matrix and exact nested error upstream.
Expected: The bundled plugins remain usable through the compatible official client while the regression is open.
Verification
Prove the fix worked
- Open `/mcp` and confirm both bundled servers show connected.
- Run one harmless list/read action only after initialization succeeds.
- Confirm the logs no longer contain the data-format -32603 during initialize.
Escalation
If it still fails
- Do not patch the installed helper or strip capabilities with an unsupported wrapper.
- Do not assume every MCP -32603 is this bug; match the two plugin names and data-format message.
- Do not expose message content, history, tokens, or local usernames in diagnostics.
Scope
Environment and version notes
- The primary report used CLI 0.154.0, Desktop 26.903.61454, and bundled plugins 1.0.1000968.
- Desktop's bundled CLI 0.153.4 connected to the same helpers in that report.
- The issue remained open when rechecked September 23, 2026.
Evidence
Sources
Source labels describe the evidence available on the checked date. A closed issue is not automatically a shipped fix.