[BUG] Model provider switch resets conversation context and memory state

Preflight Checklist

• [x] I have searched existing issues and this has not been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

Switching between model providers (e.g., from default provider to a custom ANTHROPIC_BASE_URL endpoint, or switching between two different configured models) causes complete loss of conversation context, memory state, and in-progress understanding.

This happens even when:

• The session is within normal context window limits
• No /clear or explicit reset command was issued
• Both models are configured with the same API credentials

Reproduction

1. Start a conversation with Model A (default provider or configured endpoint)
2. Establish context, load project memory files, develop an understanding of the codebase
3. Switch to Model B (different provider or different model on the same provider)
4. All context is lost — the model starts "fresh" with no memory of prior conversation

Observed Behavior

The following patterns are lost across a model switch:

• Memory files: .claude/learnings/ and MEMORY.md entries from the session are not recalled
• Conversation history: Previous messages are not visible in the new model's context
• In-progress understanding: Key findings discovered earlier in the session are forgotten
• Tool use memory: The new model does not know what tools have been used or their results

This forces users to write critical findings to memory files before any model switch — which is a fragile workaround, not a solution.

Expected Behavior

Switching models within the same session should preserve:

• Access to memory files (they were already loaded by the system)
• Conversation context (same session ID, same conversation history)
• In-progress work (same state, just a different model processing it)

Root Cause Hypothesis

Model providers maintain isolated conversation states. Each new model session starts fresh unless explicitly handed off. When Claude Code switches providers mid-session, the context handoff does not occur.

Workaround

Users must:

1. Write all critical findings to MEMORY.md before any model switch
2. Use /clear + explicit command before starting new work in a different model context
3. Reference files rather than conversation history for continuity

Impact

• Broken multi-provider workflows: Teams using different providers for cost/performance optimization lose all context when routing between them
• Forced write-before-switch pattern: Requires discipline that breaks natural conversation flow
• Context loss on model rotation: Even rotating between the same model family on the same provider causes context loss

Suggested Labels

area:providers, area:core, bug