Session resume fails when tool.execution_complete writes raw multiline content into events.jsonl

[Itzalive]

Bug

Resuming a session can fail with:

Failed to resume session: Error: Session file is corrupted
(line 1086: SyntaxError: Unterminated string in JSON at position 8038 (line 1 column 8039))

Environment

• Copilot CLI version: 1.0.24
• OS: Linux
• Session ID affected: 46cb26fd-2fc9-4737-954a-1419bea933ff

Root cause

This was not random file corruption. The session's events.jsonl contained tool.execution_complete events that were written with literal newline characters inside result.content, so a single JSON object was split across multiple physical JSONL lines.

Two corrupted spans were present:

• lines 1086-1088
• lines 1577-1579

Both spans reconstructed cleanly into valid single tool.execution_complete events when the intermediate line breaks were replaced with escaped \\n and the combined string was parsed as JSON.

The affected events contained fetched page content beginning with:

• Contents of https://www.anthropic.com/pricing:

So the failure mode appears to be tied to persisted tool output that contains raw multiline content.

Why this breaks resume

events.jsonl expects one complete JSON object per line. Once a writer emits literal newlines inside a JSON string value, the JSONL reader later tries to parse the first physical line as a complete JSON object and fails with Unterminated string in JSON.

Evidence

The Copilot CLI log reported:

Failed to parse session 46cb26fd-2fc9-4737-954a-1419bea933ff:
Failed to read JSONL from ~/.copilot/session-state/46cb26fd-2fc9-4737-954a-1419bea933ff/events.jsonl:
Invalid event at line 1086: SyntaxError: Unterminated string in JSON at position 8038
Event: {"type":"tool.execution_complete", ...}

External validation showed:

• line 1086 alone is invalid JSON
• line 1086 + "\\n" + 1087 + "\\n" + 1088 parses successfully as a single tool.execution_complete event
• same pattern for lines 1577-1579

Expected behavior

Session event writing should always escape embedded newlines inside string values so each event stays on one physical JSONL line.

Suggested fix

1. Fix on write: ensure event serialization for tool.execution_complete always goes through normal JSON string escaping before appending to events.jsonl
2. Defense on read: consider a repair path for malformed JSONL where a line looks like the start of an event and subsequent lines can be safely rejoined
3. Better error message: include the event type and a hint that multiline tool output may have split a JSONL record

Related issues

This looks related to the broader session-corruption family, but it appears distinct from the existing U+2028/U+2029 reports because the bad file here is caused by raw literal newlines in tool output, not line-separator Unicode characters.