#187 test: reproduce large→small model-switch overflow (GLM-5.2 512k → GPT-5.5 272k)

Bug

When switching from a large-context model to a smaller one mid-session (e.g. GLM-5.2 512k → GPT-5.5 272k), the historian does not reduce the context to fit the new model before sending the prompt, resulting in Input exceeds context window errors.

Root cause

The model-change branch in transform.ts:502-547 detects the switch and clears lastContextPercentage / lastInputTokens to 0. This suppresses every reduction path on the same pass:

• Historian trigger (checkCompartmentTrigger): 0% < proactive floor → shouldFire=false
• 95% emergency block (transform-compartment-phase.ts:328): 0% < 95% → no block
• Overflow recovery bump (transform.ts:616-628): needsEmergencyRecovery was just cleared → no bump

The oversized prompt — sized for the old model's window — is sent to the new smaller model and rejected. Recovery only arms on the second pass, after the real overflow error fires the event handler's recordOverflowDetected. The user sees the error on the first request.

Fix direction

In the model-change branch (transform.ts:502), before clearing, compare sessionMeta.lastInputTokens (the old model's last measured input tokens) against the new model's context limit (resolveTrustedContextLimit). When oldInputTokens > newContextLimit, call recordOverflowDetected(db, sessionId, newContextLimit, newModelKey) to arm recovery — which makes the existing :616-628 bump-to-95% path fire on the same pass, so historian + emergency drops run before the prompt is sent.

Reproduction

This PR adds a failing test (transform.test.ts) reproducing the scenario:

• Session at 300k input tokens on GLM-5.2 (58% of 512k — no trigger fired)
• Switch to GPT-5.5 (272k) — 300k is ~110% of the new window
• Asserts needsEmergencyRecovery === true and the oversized tool output is dropped

The test currently fails: needsEmergencyRecovery is false and the tool output stays active.

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Summary by cubic

Adds a failing test in packages/plugin/src/hooks/magic-context/transform.test.ts that reproduces an overflow when switching mid-session from a large-context model (GLM-5.2 512k) to a smaller one (GPT-5.5 272k).
The test expects emergency recovery to arm immediately and drop oversized tool output before send; it currently fails, confirming the model-change branch clears usage and allows an oversized prompt through.

Confidence Score: 2/5

The test is safe to merge in isolation — no production code is touched — but as written it will not turn green when the corresponding fix lands, leaving CI in a permanently failing state for this case.

The test correctly identifies the bug and the right observable outcomes, but cannot fulfill its role as a reproducer: without a models.json fixture carrying GPT-5.5's context limit, the fix mechanism gets undefined for the new model and skips arming recovery entirely. A separate OpenCode DB / live-messages ID mismatch could cause the tool-tag assertion to fail for an unrelated reason even if recovery were somehow armed. Both issues need to be resolved before the test can serve as a reliable regression guard.

packages/plugin/src/hooks/magic-context/transform.test.ts — specifically the new describe block starting at line 2551.

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant TF as transform.ts
    participant MC as model-change branch (502-547)
    participant OVF as overflow state (DB)
    participant LCU as loadContextUsage
    participant BUMP as 95% bump path (616-628)
    participant ED as emergency-drop

    TF->>MC: "liveModel=GPT-5.5, lastAssistant=GLM-5.2 → mismatch"
    MC->>OVF: clearEmergencyRecovery()
    MC->>MC: contextUsageMap.delete(sessionId)
    Note over MC: (BUG) no recordOverflowDetected called
    Note over MC: (FIX) resolveTrustedContextLimit(GPT-5.5)<br/>needs models.json mock — returns undefined without it
    MC->>LCU: loadContextUsage → 0%
    LCU-->>TF: "percentage=0%"
    TF->>OVF: "getOverflowState → needsEmergencyRecovery=false"
    Note over BUMP: 0% < 95% but needsEmergencyRecovery=false → no bump
    TF->>ED: skipped — no 95% bump
    Note over ED: tool output stays active → prompt sent oversized → overflow error

%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
sequenceDiagram
    participant TF as transform.ts
    participant MC as model-change branch (502-547)
    participant OVF as overflow state (DB)
    participant LCU as loadContextUsage
    participant BUMP as 95% bump path (616-628)
    participant ED as emergency-drop

    TF->>MC: "liveModel=GPT-5.5, lastAssistant=GLM-5.2 → mismatch"
    MC->>OVF: clearEmergencyRecovery()
    MC->>MC: contextUsageMap.delete(sessionId)
    Note over MC: (BUG) no recordOverflowDetected called
    Note over MC: (FIX) resolveTrustedContextLimit(GPT-5.5)<br/>needs models.json mock — returns undefined without it
    MC->>LCU: loadContextUsage → 0%
    LCU-->>TF: "percentage=0%"
    TF->>OVF: "getOverflowState → needsEmergencyRecovery=false"
    Note over BUMP: 0% < 95% but needsEmergencyRecovery=false → no bump
    TF->>ED: skipped — no 95% bump
    Note over ED: tool output stays active → prompt sent oversized → overflow error

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