Python: Channels

[eavanvalkenburg]

Tracking issue for the Python hosting core + pluggable channels work.

The implementation is being split into small PRs. The v1 boundary is intentionally minimal: one host owns app composition and target invocation; channels own protocol parsing/rendering; linking, multicast, durable delivery, active-channel routing, and cross-channel delivery are follow-up design topics tracked by ADR-0028.

Merged

• Hosting: ADRs and implementation spec #6584 — ADRs and implementation spec — PR Python: Add hosting channel ADRs and spec #6578 ✅
• Hosting: core host and Responses channel #6585 — Core host and Responses channel — PR Python: Add hosting core and Responses channel #6580 ✅
• Hosting: Foundry Hosted Agent History Provider #6586 — Minimal Foundry Hosted Agent helper — PR Python: Add Foundry Hosted Agent history provider #6593 ✅

In Review (Sprint 4)

• Hosting: Telegram channel and Telegram samples #6588 — Telegram channel and samples — PR Python: Add Telegram channel for agent-framework-hosting #6698 🔍
• Hosting: A2A channel #6591 — A2A channel and sample — PR Python: Add A2A channel for agent-framework-hosting #6699 🔍
• Hosting: MCP channel #6592 — MCP channel and sample — PR Python: Add MCP channel for agent-framework-hosting #6700 🔍

Remaining v1 slices

• Hosting: Invocations channel and Foundry hosted agent sample #6587 — Invocations channel and Foundry hosted agent sample
• Hosting: Activity Protocol channel #6589 — Activity Protocol channel
• Hosting: Discord channel #6590 — Discord channel

Split rules

• Samples ship with the channel they use, or with the last not-yet-landed channel when a sample uses multiple channels.
• Each channel PR should restore only its relevant files from feature/python-hosting and reconcile them against the simplified host API.
• New non-core package tests must use tests/<unique_name>/ and must not include tests/__init__.py.
• New packages should follow the current typing setup: Pyright for source, no package-local source mypy task.
• Do not reintroduce the old linking/multicast/cross-channel-delivery implementation into v1 channel PRs.

Follow-up / proposed work

Tracked by ADR-0028 rather than v1 implementation PRs:

• Identity linking and unlink flows
• Multicast and cross-channel delivery
• Active-channel routing
• Background continuations and durable delivery
• Retry/replay and delivery observability
• Payload serialization for durable cross-channel delivery