Preserve unread session state fields when marshaling after a read

[PratikDhanave (PratikDhanave)]

Problem

stateValue.MarshalJSON (agent/value.go) preferred the cached typed value over the original raw JSON:

if v.hasCached {
    return json.Marshal(v.cached)
}
if v.raw != nil {
    return slices.Clone(v.raw), nil
}

readInto caches a typed copy the first time a key is read — and that copy can be a narrower/partial view of the stored object (a forward/backward-compat partial read, or a different component's view). So this sequence loses data:

1. A Session is deserialized from persisted JSON where key k holds {"A":1,"B":2}.
2. Some component calls session.Get("k", &struct{ A int }{}) — caching the typed value {A:1}.
3. The session is re-serialized to persist it.

Result: {"k":{"A":1}} — field B is silently dropped. Without the intervening Get, marshaling returns the raw {"A":1,"B":2} intact. So a read corrupts the persisted state on the next save, contrary to Session's documented contract that it "can be serialized and deserialized directly with encoding/json … saved in a persistent store."

Fix

Prefer the raw JSON when present — it is the authoritative serialized form for a deserialized value and preserves unread fields. Values created via Set have raw == nil and still marshal losslessly from cached.

Test

TestSession_MarshalAfterGet_PreservesUnreadFields deserializes a session, reads a key with a partial struct, re-marshals, and asserts the output equals the original. Fails before the fix (B dropped), passes after.

[Copilot]

🟢 Approval recommended

The fix includes regression coverage and no blocking issues were identified.

Pull request overview

Preserves unread session state fields when values are read and re-marshaled.

Changes:

• Prefer original raw JSON when available.
• Add regression coverage for partial reads.

File summaries

File	Summary
agent/value.go	Preserves original raw state JSON during marshaling.
agent/session_test.go	Tests unread-field preservation after Get.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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[github-actions]

Scope: internal-only

Changed Go contract: None. stateValue (agent/value.go) and its MarshalJSON/readInto methods are unexported; the public Session.Get/Session.Set/json.Marshal(*Session) contract is unchanged. Behavior fixed: Session round-trips through encoding/json without silently dropping unread fields after a partial Get, matching the type's documented "can be serialized and deserialized directly with encoding/json" contract.

Upstream evidence reviewed:

• .NET dotnet/src/Microsoft.Agents.AI.Abstractions/AgentSessionStateBagValue.cs — AgentSessionStateBagValue stores the original JsonElement (_jsonValue) and only overwrites it when the caller explicitly sets a new value (JsonValue setter, SetDeserialized). TryReadDeserializedValue/ReadDeserializedValue populate a _cache for fast subsequent reads of the same type but never touch _jsonValue, so serialization (JsonValue getter) re-encodes only from _cache when the value was set (not merely read/deserialized) — reads alone never cause data loss on re-serialization.
• .NET dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentSessionStateBagTests.cs (JsonSerializerRoundtrip_WithComplexObject_PreservesData, etc.) — confirms round-trip fidelity is an explicit, tested contract.
• Python python/packages/core/agent_framework/_sessions.py (AgentSession.to_dict/from_dict, _serialize_state/_deserialize_state) — session state is serialized directly from the stored state dict via explicit codecs; there is no lazy "read caches a narrower view that silently overwrites the original on save" pathway analogous to the bug being fixed here.

Result: aligned. The fix (preferring raw over hasCached in MarshalJSON) makes the Go stateValue behave like .NET's AgentSessionStateBagValue, where a read-only access never causes information loss on the next serialization. The change is confined to unexported implementation code, is well-tested (TestSession_MarshalAfterGet_PreservesUnreadFields), and does not diverge from upstream semantics. No parity concerns.

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