BUG: ag send fails with 'Invalid request body' — follow-up messages blocked

[OneStepAt4time]

Bug Description

ag send <session-id> "message" fails with 'Invalid request body' error. This blocks the core iterative-development use case for Aegis.

Impact

• P1 — Cannot send follow-up messages to a running CC session from the CLI.
• One-shot ag run works, but real 4h+ dev sessions require ag send to iterate.

Reproduction Steps

1. Start a session via ag run <prompt>
2. Wait for session to become active
3. Run ag send <session-id> "follow-up message"
4. Observe: CLI returns 'Invalid request body'

Expected Behavior

ag send should deliver the message to the CC session via the ACP bridge, same as the dashboard send or API POST.

Root Cause (Hypothesis)

Wrong field format in the request payload sent by the CLI to the Aegis server. Likely a mismatch between CLI payload shape and API schema (e.g., missing sessionId vs id field, or wrong content-type).

Evidence

Found during endurance test #4683 (2026-06-13).

Acceptance Criteria

• ag send <session-id> <message> delivers message without 'Invalid request body' error
• E2E test: start session → send follow-up → message appears in session transcript
• Dashboard 'send' and CLI ag send use the same payload shape (or documented divergence)

Related

• [P1] Endurance test: 4h+ CC development session through Aegis #4683 (endurance test findings)
• FEATURE: ag approve/reject CLI commands for programmatic tool call approval #4685 (ag approve/reject — related CLI surface, already shipped)

[OneStepAt4time]

Triage (Athena) — Priority upgraded P2→P1. This is a core-loop blocker found during the P1 endurance test (#4683). Without , iterative CC development through Aegis is broken. Routing to Hep for backend fix. Acceptance criteria are clear — E2E test included in the issue.