Fixes 13 issues found in PR #20774 review:
1. Wiring: engine selection moved from run_agent.py to agent/agent_init.py
(where init_agent lives on current main). Transform hook moved from
run_agent.py to agent/conversation_loop.py (where run_conversation lives).
2. Prompt caching: replace copy.deepcopy with copy-on-write (shallow list
copy + clone only messages that are mutated). Use last_prompt_tokens
from update_from_response instead of re-estimating tokens every call.
System extension injection is idempotent (one-time cache break).
3. Signature mismatch: _message_signature renamed to _content_signature
and now excludes tool_calls/tool_call_id from the hash. This prevents
mismatches when _canonicalize_api_tool_calls re-serializes argument
JSON with sort_keys=True on the API copy.
4. update_model: accepts api_mode parameter (required by agent_init.py).
5. Reconciled with select_context: transform_api_messages is a separate
hook that runs AFTER select_context and sanitization, before
prompt-cache marker placement. Both hooks coexist with clear ordering.
6. Dedup/purge: kept as DCP-specific strategies (different semantics from
ContextCompressor._prune_old_tool_results — DCP deduplicates by
tool+args signature, not by content hash).
7. Removed copy.deepcopy: replaced with shallow list copy + copy-on-write
via _clone_if_needed. Only messages that are actually mutated get
cloned.
8. Removed redundant _ensure_refs call: _match_api_messages_to_refs no
longer calls _ensure_refs (the caller already called it).
9. _message_key still uses index (needed for positional ref assignment),
but _content_signature is cached per id(msg) to avoid re-hashing.
10. _inject_nudge: only injects into user messages, never falls back to
non-user messages (prevents role semantics violations).
11. Memory: _evict_inactive_blocks bounds blocks_by_id to
_MAX_INACTIVE_BLOCKS (50) deactivated blocks.
12. Merged _range_tool_schema and _message_tool_schema into a single
_compress_tool_schema. Merged _handle_range_compress and
_handle_message_compress into _handle_compress.
13. Dropped DCP_CONTEXT_ENGINE_PR_SPEC.md (temporary file, not for tree).
Config defaults kept minimal in hermes_cli/config_defaults.py (only
the keys the engine actually reads, not the full DCP-compatible surface).
Closes#20717
kshitij
9841a6c651