Resume banner shows saved session model while routing matrix silently routes to a different provider

[danshapiro]

Summary

amplifier resume <session-id> displays the saved session's model/provider in the banner (e.g., "Model: gpt-5.6-sol"), but the actual routing decision is made at resume time by the routing matrix, silently overriding the saved provider without warning. This creates a UX mismatch and masked a cross-provider incompatibility (see companion issue).

Repro

1. Create a session with OpenAI provider (e.g., gpt-5.6-sol)
2. Resume it while routing-matrix "balanced" is active
3. Banner displays: Model: gpt-5.6-sol (from saved metadata)
4. Actual request routing: anthropic/claude-fable-5 (from routing matrix decision)
5. User is misled about which provider will run

Real example:

• Session ID: 3aaf5712-7e3c-4d70-9b6e-a1743b84e745 (project: iago)
• Saved model: gpt-5.6-sol (OpenAI)
• Routing decision at resume: anthropic/claude-fable-5
• Banner showed: "Model: gpt-5.6-sol" (misleading)
• In this case, the silent cross-provider switch also triggered a hard failure in provider-anthropic (see companion issue: Cross-provider resume: thinking blocks with null signature passed to Anthropic API cause invalid_request_error)

Impact

• User confusion: Users expect the model shown in the banner to be what actually runs
• Silent provider substitution: No warning when routing overrides the saved session's provider, hiding potential incompatibilities
• Debugging friction: When errors occur, the user's mental model doesn't match reality (especially painful if the error is provider-specific, like the thinking-block validation error)

Ask

Either:

1. The banner should reflect the model that will actually be used after routing resolution, OR
2. Resume should honor/pin the saved provider-model pair, OR
3. At minimum, warn the user that routing is overriding their saved session's provider

Triage note: This may belong in amplifier-bundle-routing-matrix if the intended design is that resume should pin to the saved provider.

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