dispatch_food_truck per-dispatch backend override (heterogeneous routing R1)

[Trecek]

Summary

Add an optional backend parameter to the dispatch_food_truck MCP tool that selects which coding-agent backend executes the dispatched child session. When omitted, behavior is byte-for-byte identical to today (uses ctx.backend). When supplied, the per-dispatch backend is used for: capability gating (food_truck_capable), command building (build_food_truck_cmd), pre-launch validation (ensure_pre_launch), recipe admission control (capability overrides, backend-incompatible-skill rule, effective_backend_map), session-log resolution, and record persistence. The DispatchRecord gains a backend_name field so downstream consumers (reset, diagnostics, session-log lookup) resolve against the correct backend. The field is added to _RETRY_IDENTITY_FIELDS so retried dispatches preserve the backend override.

The implementation follows the existing run_headless_core call-local override pattern: ctx.backend is never mutated; a resolved CodingAgentBackend instance is threaded through the dispatch path as a function argument.

Requirements

REQ-HBR-001: The dispatch_food_truck MCP tool (server/tools/tools_fleet_dispatch.py) must accept an optional backend parameter whose accepted values are the names registered in BACKEND_REGISTRY ("claude-code", "codex").

REQ-HBR-002: When the backend parameter is omitted, dispatch behavior must be byte-for-byte identical to the current ctx.backend path — zero behavior change for existing single-backend flows.

REQ-HBR-003: The child session command must be built via the per-dispatch backend's build_food_truck_cmd, with backend-correct env injection (AUTOSKILLIT_AGENT_BACKEND__BACKEND, AUTOSKILLIT_AGENT_BACKEND, AUTOSKILLIT_MCP_CLIENT_BACKEND) so the child process resolves the per-dispatch backend at boot.

REQ-HBR-004: ensure_pre_launch() must be invoked on the per-dispatch backend before spawn (e.g., Codex MCP registration in config.toml).

REQ-HBR-005: The food_truck_capable capability gate must be evaluated against the per-dispatch backend, not the kitchen backend.

REQ-HBR-006: Recipe admission control and capability overrides must be computed against the per-dispatch backend: _provider_aware_capability_overrides, the backend_supports_git_write ingredient, _compute_effective_backend_map, and the backend-incompatible-skill validation rule (server/tools/_auto_overrides.py, recipe/rules/rules_backend_compat.py).

REQ-HBR-007: Per-dispatch backend selection must compose with existing provider profiles (e.g., a MiniMax implement-step provider override via ANTHROPIC_BASE_URL) and with the R0 capability-driven auto-route (#4176) inside the child session — existing provider recipe overrides must behave identically.

REQ-HBR-008: The DispatchRecord must persist which backend the dispatch ran on, so that reset_dispatch, diagnostics, and session-log lookup (Channel B for Claude Code vs CodexSessionLocator for Codex) resolve against the correct backend.

REQ-HBR-009: Any BackendCapabilities field addition required by this work must be forward-declared with a tracking entry in _FORWARD_DECLARED in test_capability_consumption.py (frozen-dataclass constraint noted in the forward-obligations doc).

Closes #4179

Implementation Plan

Plan file: .autoskillit/temp/make-plan/dispatch_food_truck_per_dispatch_backend_override_plan_2026-07-04_152400.md

🤖 Generated with Claude Code via AutoSkillit

Token Usage Summary

Step	Model	count	uncached	output	cache_read	peak_ctx	turns	cache_write	time
plan*	opus[1m]	1	2.2k	44.9k	4.1M	168.5k	87	149.6k	22m 37s
review_approach*	opus	1	9.2k	5.3k	199.4k	49.0k	12	61.8k	6m 1s
verify*	fable	1	31.1k	41.3k	2.0M	169.7k	44	170.1k	14m 49s
implement*	MiniMax-M3	1	96.2k	18.9k	7.0M	0	156	0	18m 6s
fix*	opus	1	77	14.4k	4.4M	112.5k	97	93.3k	12m 11s
Total			138.7k	124.8k	17.6M	169.7k		474.7k	1h 13m

* Step used a non-Anthropic provider; caching behavior may differ.

Token Efficiency

Step	LoC Changed	cache_read/LoC	cache_write/LoC	output/LoC
plan	0	—	—	—
review_approach	0	—	—	—
verify	0	—	—	—
implement	577	12052.9	0.0	32.8
fix	37	119182.5	2520.4	388.2
Total	614	28647.1	773.2	203.2

Model Usage Breakdown

Model	steps	uncached	output	cache_read	cache_write	time
opus[1m]	1	2.2k	44.9k	4.1M	149.6k	22m 37s
opus	2	9.3k	19.7k	4.6M	155.0k	18m 13s
fable	1	31.1k	41.3k	2.0M	170.1k	14m 49s
MiniMax-M3	1	96.2k	18.9k	7.0M	0	18m 6s