Session resume fails with HTTP 400 when crash leaves orphaned tool call followed by tool result

[ckrabach617]

Summary

When a session crashes mid-tool-execution, resuming it fails with an Anthropic HTTP 400 error due to consecutive user-role messages in the API request. This creates an unresumable session that requires manual transcript surgery to fix.

Affected component: amplifier-core (session resume logic)

Reproduction

1. Start a session with tool use
2. Session crashes mid-tool-execution (network timeout, process kill, etc.) — leaving an assistant message with tool_calls but no corresponding tool_result in the transcript
3. Attempt to resume the session with amplifier session resume <id>
4. HTTP 400 from Anthropic

Root Cause

The resume logic strips orphaned assistant tool_use messages (correctly — can't send tool_use without tool_result). However, this leaves the preceding tool_result as the effective last message, which maps to role: "user" in the Anthropic API. The resume prompt is then appended as another role: "user" message, creating consecutive same-role messages that Anthropic rejects.

The sequence that fails:

[N-1] user (tool_result)     ← was paired with the now-stripped tool_use
[N]   user (resume prompt)   ← injected by resume logic
→ HTTP 400: consecutive user messages

A similar collision occurs when hooks (e.g., hooks-status-context) inject user-role content between a tool_result and the next API call.

Observed Impact

• Session ba107234-91eb-403f-a671-a668a3f614de required four manual transcript repairs due to this bug
• Each resume attempt hit HTTP 400, crashed again, and left additional corrupt state
• The only workaround is manual transcript truncation back to a safe assistant text message with no pending tool calls

Suggested Fix

When constructing the API request for a resumed session:

1. After stripping orphaned tool calls, check if the effective last message is role: "user"
2. If so, either:
   • Merge the resume prompt into the existing user message, OR
   • Insert a synthetic assistant acknowledgment (e.g., "Resuming session.") before the resume prompt to maintain role alternation
3. Apply the same merge logic when hooks inject user-role content after a tool_result

More generally, the message construction pipeline should enforce role alternation as a post-processing step before sending to any provider that requires it.

Environment

• Platform: WSL2 (Linux 6.6.87.2-microsoft-standard-WSL2)
• Provider: Anthropic (claude-opus-4-6)
• Session: ba107234-91eb-403f-a671-a668a3f614de