Structured Input Active-Turn Validation Rejects Sideband User Messages Mid-Turn

Structured Input Active-Turn Validation Rejects Sideband User Messages Mid-Turn

Summary

When Codex TUI is launched with --xml-input-fd, externally injected XML user messages appear to work only when Codex is already idle and waiting for input.

If a structured XML user message is injected while Codex is in the middle of a turn, the TUI reports a structured-input controller validation error instead of cleanly queueing the message for later delivery.

Observed Symptom

Example observed error:

Structured input controller validation error for thread 019dafa6-caef-7e50-bfad-323af643e3ce:
turn/started turn id `019dbdcd-09a9-70d1-83e9-1c54a7270fba`
did not match active turn id `019dbdcc-2d2a-76d3-b476-ae1b1682ea55`

The same session later shows:

Queued structured input
↳ [AfterToolCall] <c2c event="message" ...>...</c2c>

So the fork clearly has a queueing model for structured input, but the injected message path is still able to trip active-turn validation instead of being consistently deferred.

Practical User Impact

This breaks auto-delivery of inbound c2c messages into Codex when Codex is busy.

Behavior seen from the c2c side:

• inbound message arrives cleanly when it is already the agent's turn
• inbound message triggers active-turn validation noise when Codex is mid-turn

That makes sideband delivery unreliable precisely when external messaging matters most.

Repro Shape

1. Launch normal Codex TUI with --xml-input-fd <fd>.
2. Start a non-trivial turn so Codex has an active turn in progress.
3. While that turn is still active, write a structured XML user message through the XML sideband:

<message type="user"><c2c event="message" from="peer" alias="peer">hello</c2c></message>

1. Observe the structured-input controller validation error instead of the message being cleanly queued.

Expected Behavior

Sideband XML user messages delivered during an active turn should be queued for later execution using the same structured-input queue semantics the fork already documents, rather than causing active-turn validation errors.

At minimum, externally injected non-steer messages should have a well-defined queued behavior while another turn is active.

Why This Looks Like A Codex-Fork Issue

The fork docs and code already point to queued structured input as a supported concept:

• docs/x-client-changes.md documents:
• <message type="user" queue="AfterToolCall">...
• <message type="user">...
• the docs also mention a visible "Queued structured input" section when --xml-input-fd is in use
• turn-start-bridge-core/src/controller.rs contains explicit active-turn validation logic and emits the same mismatch text seen live
• turn-start-bridge/src/main.rs and the controller code both reference active-turn-not-steerable and pending queue windows

Relevant files:

• docs/x-client-changes.md
• codex-rs/turn-start-bridge/src/main.rs
• codex-rs/turn-start-bridge-core/src/controller.rs
• codex-rs/turn-start-bridge-core/src/message.rs

Current Suspicion

The likely issue is that externally injected sideband user messages are being handled as immediate turn-start requests even while another turn is active, instead of being normalized into queued structured input semantics.

Concretely, the path may require:

• automatic queueing for sideband user messages when a turn is already active, or
• explicit treatment of plain <message type="user">...</message> as queued/non-immediate under active-turn conditions, or
• a documented requirement that callers must send queue="AfterToolCall" whenever the session is busy

Right now the behavior appears inconsistent enough that an external harness cannot rely on it safely.

Suggested Direction

One of these needs to become true and stable:

1. Plain sideband user messages are automatically queued if Codex is already in an active turn.
2. The fork explicitly requires a queue attribute for mid-turn structured input, and rejects only non-queued input with a clearer error.
3. The XML sideband path itself rewrites or routes non-steer mid-turn messages into the queued-input path.

Notes From c2c Dogfooding

There was also a separate c2c-side bug where message text was over-escaped and rendered " literally. That is independent and already identifiable on the c2c side.

This report is specifically about the active-turn structured-input validation failure that occurs when Codex receives inbound sideband XML while it is busy.