Coordination & Resilience¶

This page covers system-level features that span multiple agents and protect against failure: crash recovery with checkpoint resume, graceful shutdown strategies, concurrent workspace isolation (Git worktrees / virtual filesystem / per-branch), and multi-agent coordination topology (centralized, decentralized, context-dependent dispatchers).

Agent Crash Recovery¶

When an agent execution fails unexpectedly (unhandled exception, OOM, process kill), the framework applies a recovery mechanism. Recovery strategies are implemented behind a RecoveryStrategy protocol, making the system pluggable.

RecoveryStrategy Protocol¶

RecoveryResult Model¶

failure_category is inferred from the error message via infer_failure_category() (keyword-based heuristic). UNKNOWN is the deliberate default when no keyword rule matches -- an honest classification is more useful than a silent TOOL_FAILURE lie that would masquerade unknown causes in dashboards, reports, and reconciliation prompts. Checkpoint reconciliation messages include the category and any unmet criteria (both passed through sanitize_message to strip paths, URLs, and prompt-injection markers) so the resumed agent has structured context about what failed without carrying leaked secrets.

Cross-field invariants. RecoveryResult enforces two cross-field rules at construction:

• stagnation_evidence is set iff failure_category is STAGNATION (and the evidence verdict must not be NO_STAGNATION -- evidence that the detector ruled out stagnation cannot back a STAGNATION result).
• criteria_failed must be non-empty when failure_category is QUALITY_GATE_FAILED.

Strategies that only have an error string (FailAndReassignStrategy, CheckpointRecoveryStrategy._build_resume_result) use infer_failure_category_without_evidence(), which clamps STAGNATION / QUALITY_GATE_FAILED to UNKNOWN -- the unclamped helper would crash construction on any error message containing the keywords "stagnation", "quality", or "criteria" because those strategies cannot supply the required sidecar data.

Transition-reason wire format. After a recovery, the post-execution pipeline embeds failure_category (and a sanitized summary of criteria_failed when present) into the task-status transition reason as "Post-recovery status: <status> (failure_category=<value>[, unmet_criteria=<summary>])". The (failure_category=<value>) suffix is a hook for downstream consumers (e.g. routing / reassignment components) to read category metadata from status history without re-parsing the raw error message. The key name (failure_category) and value format are a stable contract: future consumers will depend on it, so changes require a coordinated rollout.

Recovery Strategies¶

crash_recovery:
  strategy: "fail_reassign"            # fail_reassign, checkpoint

crash_recovery:
  strategy: "checkpoint"
  checkpoint:
    persist_every_n_turns: 1           # checkpoint frequency
    # Storage backend determined by the injected CheckpointRepository
    heartbeat_interval_seconds: 30     # detect unresponsive agents
    max_resume_attempts: 2             # retry limit before falling back to fail_reassign

Graceful Shutdown Protocol¶

When the process receives SIGTERM/SIGINT (user Ctrl+C, Docker stop, systemd shutdown), the framework stops cleanly without losing work or leaking costs. Shutdown strategies are implemented behind a ShutdownStrategy protocol.

Strategy 1: Cooperative with Timeout (Default / MVP)¶

The engine sets a shutdown event, stops accepting new tasks, and gives in-flight agents a grace period to finish their current turn. Agents check the shutdown event at turn boundaries (between LLM calls, before tool invocations) and exit cooperatively. After the grace period, remaining agents are force-cancelled. All tasks terminated by this strategy -- whether they exited cooperatively or were force-cancelled -- are marked INTERRUPTED by the engine layer. (Strategy 4 uses SUSPENDED for successfully checkpointed tasks instead; see Strategy 4.)

graceful_shutdown:
  strategy: "cooperative_timeout"    # cooperative_timeout, immediate, finish_tool, checkpoint
  grace_seconds: 30                  # time for agents to finish cooperatively
  cleanup_seconds: 5                 # time for final cleanup (persist cost records, close connections)

On shutdown signal:

1. Set shutdown_event (asyncio.Event) -- agents check this at turn boundaries
2. Stop accepting new tasks (drain gate closes)
3. Wait up to grace_seconds for agents to exit cooperatively
4. Force-cancel remaining agents (task.cancel()) -- tasks transition to INTERRUPTED
5. Cleanup phase (cleanup_seconds): persist cost records, close provider connections, flush logs

Strategy 2: Immediate Cancel¶

All agent tasks are cancelled immediately via task.cancel() with no grace period. Fastest shutdown but highest data loss -- partial tool side effects, billed-but-lost LLM responses. Tasks are marked INTERRUPTED.

graceful_shutdown:
  strategy: "immediate"
  cleanup_seconds: 5

Strategy 3: Finish Current Tool¶

Like cooperative timeout, but uses a per-tool timeout (default 60s) to allow the current tool invocation to complete. The execution loop finishes the current tool before checking shutdown at turn boundaries; this strategy gives a longer window for that. Tasks that exceed the tool timeout are force-cancelled and marked INTERRUPTED.

graceful_shutdown:
  strategy: "finish_tool"
  tool_timeout_seconds: 60
  cleanup_seconds: 5

Strategy 4: Checkpoint and Stop¶

On shutdown signal, agents checkpoint cooperatively during the grace period. Stragglers are checkpointed via a checkpoint_saver callback, then cancelled. Successfully checkpointed tasks transition to SUSPENDED (not INTERRUPTED); failed checkpoints fall back to INTERRUPTED. On restart, the engine loads checkpoints and resumes execution from the exact point of interruption. This naturally extends Checkpoint Recovery -- the only difference is whether the checkpoint was written proactively (graceful shutdown) or loaded from the last turn (crash recovery).

graceful_shutdown:
  strategy: "checkpoint"
  grace_seconds: 30
  cleanup_seconds: 5

Concurrent Workspace Isolation¶

When multiple agents work on the same codebase concurrently, they may need to edit overlapping files. The framework provides a pluggable WorkspaceIsolationStrategy protocol for managing concurrent file access.

Strategy 1: Planner + Git Worktrees (Default)¶

The task planner decomposes work to minimize file overlap across agents. Each agent operates in its own git worktree (shared .git object database, independent working tree). On completion, branches are merged sequentially.

This is the dominant industry pattern (used by major coding agent products and IDE background agents).

flowchart TD
    P[Planner decomposes task]
    P --> A[Agent A: src/auth/ worktree-A]
    P --> B[Agent B: src/api/ worktree-B]
    P --> C[Agent C: tests/ worktree-C]
    A --> M[Sequential merge]
    B --> M
    C --> M
    M --> T[Textual conflicts: escalate to human or review agent]
    M --> S[Semantic conflicts: review agent evaluates merged result]

workspace_isolation:
  strategy: "planner_worktrees"      # planner_worktrees, sequential, file_locking
  planner_worktrees:
    max_concurrent_worktrees: 8
    merge_order: "completion"        # completion (first done merges first), priority, manual
    conflict_escalation: "human"     # human, review_agent
    cleanup_on_merge: true
    semantic_analysis:
      enabled: false
      file_extensions: [".py"]
      max_files: 50
      max_file_bytes: 524288
      git_concurrency: 10
      llm_model: null
      llm_temperature: 0.1
      llm_max_tokens: 4096
      llm_max_retries: 2

• True filesystem isolation -- agents cannot overwrite each other's work
• Maximum parallelism during execution; conflicts deferred to merge time
• Leverages mature git infrastructure for merge, diff, and history

Semantic Conflict Detection¶

Git merges catch textual conflicts (overlapping edits to the same lines), but many real-world integration bugs are semantic - the merge succeeds textually yet the combined code is broken. The semantic conflict detection subsystem analyzes merged results to catch these issues before they reach main.

SemanticAnalyzer protocol and composite pattern. The SemanticAnalyzer protocol defines a single analyze(workspace, changed_files, repo_root, base_sources) method. The default CompositeSemanticAnalyzer dispatches all configured analyzers concurrently via asyncio.TaskGroup and deduplicates their combined results, allowing AST-based checks and optional LLM-based analysis to compose transparently. Analyzer failures are logged and skipped without aborting the remaining analyzers.

AST-based checks. Four pure-function checks run against the merged source without external dependencies:

1. Removed references - detects calls or imports referencing names that no longer exist in the merged code (e.g., Agent A renames a function, Agent B calls the old name).
2. Signature mismatches - detects functions whose signatures changed between base and merged versions in ways that break existing call sites.
3. Duplicate definitions - detects multiple top-level definitions of the same name in a single file (e.g., two agents independently add a process() function that git merges into disjoint hunks).
4. Import conflicts - detects conflicting imports of the same name from different modules.

Optional LLM-based analysis. When llm_model is configured in SemanticAnalysisConfig, a provider-backed analyzer sends the diff to an LLM for deeper reasoning about subtle semantic issues that AST checks cannot catch.

SemanticAnalysisConfig. A frozen Pydantic model controlling the analysis pipeline: enabled toggle, file_extensions filter, max_files and max_file_bytes limits to bound analysis cost, git_concurrency to cap concurrent git show subprocess fan-out, and LLM-specific settings (llm_model, llm_temperature, llm_max_tokens, llm_max_retries).

Flow through MergeResult and MergeOrchestrator. After a textually successful merge, the MergeOrchestrator invokes the configured SemanticAnalyzer. Any detected issues are attached to the MergeResult as semantic_conflicts (tuple of MergeConflict with conflict_type=SEMANTIC). The calling code can then decide whether to accept, revert, or escalate based on the severity and count of semantic conflicts.

Future Strategies¶

Strategy 2: Sequential Dependencies

Tasks with overlapping file scopes are ordered sequentially via a dependency graph. Prevents conflicts by construction but limits parallelism. Requires upfront knowledge of which files a task will touch.

Strategy 3: File-Level Locking

Files are locked at task assignment time. Eliminates conflicts at the source but requires predicting file access -- difficult for LLM agents that discover what to edit as they go. Risk of deadlock if multiple agents need overlapping file sets.

State Coordination vs Workspace Isolation¶

These are complementary systems handling different types of shared state:

Worktree Disk Quota¶

Per-worktree disk usage limits with a background watcher that emits warning and exceeded events when thresholds are crossed.

Configuration (on PlannerWorktreesConfig):

Watcher (DiskQuotaWatcher): checks worktree disk usage via recursive directory size computation. Emits WORKSPACE_DISK_WARNING at the warning threshold and WORKSPACE_DISK_EXCEEDED at the limit. Does not delete worktrees directly -- signals the WorkspaceManager to act.

Module: src/synthorg/engine/workspace/disk_quota.py

Task Decomposability & Coordination Topology¶

Empirical research on agent scaling (Kim et al., 2025 -- 180 controlled experiments across 3 LLM families and 4 benchmarks) demonstrates that task decomposability is the strongest predictor of multi-agent effectiveness -- stronger than team size, model capability, or coordination architecture.

Task Structure Classification¶

Each task carries a task_structure field classifying its decomposability:

Classification can be:

• Explicit -- set in task config by the task creator or manager agent
• Inferred -- derived from task properties (tool count, dependency graph, acceptance criteria structure) by the task router

Per-Task Coordination Topology¶

The communication pattern is configured at the company level, but coordination topology can be selected per-task based on task structure and properties.

Auto Topology Selector¶

When topology is set to "auto", the engine selects coordination topology based on measurable task properties:

coordination:
  topology: "auto"                    # auto, sas, centralized, decentralized, context_dependent
  auto_topology_rules:
    sequential_override: "sas"
    parallel_default: "centralized"
    mixed_default: "context_dependent"
  max_concurrency_per_wave: null        # None = unlimited
  max_delegation_rounds: 3             # soft cap; hard abort at 2x (6)
  fail_fast: false
  enable_workspace_isolation: true
  base_branch: main

The auto-selector uses task structure, artifact count, and (when available from the memory subsystem) historical single-agent success rate as inputs. Kim et al. achieved 87% accuracy predicting optimal architecture from task properties across held-out configurations.

Coordination Group Size Bounds¶

Per-task coordination-group size is not the same as per-company size. An Enterprise Org template with 20-50 agents does not run 20-50-agent coordination waves -- it runs small coordination groups drawn from the roster.

Multi-Agent Coordination Pipeline¶

The MultiAgentCoordinator orchestrates the end-to-end pipeline that transforms a parent task into parallel agent work:

decompose -> route -> resolve topology -> validate -> dispatch -> rollup -> update parent

Pipeline phases:

1. Decompose -- DecompositionService breaks the parent task into subtasks with a dependency DAG
2. Route -- TaskRoutingService assigns each subtask to an agent based on skills, workload, and topology
3. Resolve topology -- reads topology from routing decisions; falls back to CENTRALIZED if AUTO was not resolved upstream
4. Validate -- fails the pipeline if all subtasks are unroutable
5. Dispatch -- a TopologyDispatcher executes waves (workspace setup -> parallel execution -> merge -> teardown)
6. Rollup -- aggregates subtask statuses into a SubtaskStatusRollup
7. Update parent -- transitions the parent task via TaskEngine (if provided)

Each phase produces a CoordinationPhaseResult (success/failure + duration).

Topology dispatchers:

The select_dispatcher factory maps a resolved CoordinationTopology to the appropriate dispatcher; AUTO must be resolved before dispatch.

Per-Agent Attribution¶

After the pipeline completes, build_agent_contributions() in coordination/attribution.py produces a tuple[AgentContribution, ...] from routing decisions and wave outcomes:

• AgentContribution -- frozen Pydantic model recording each agent's contribution_score (0.0--1.0), failure_attribution classification, and optional evidence excerpt.
• Failure attribution categories -- "direct" (agent's own failure), "upstream_contamination" (bad input from another agent), "coordination_overhead" (system-initiated: budget, shutdown, parking), "quality_gate" (failed quality check).
• Integration -- contributions are fed into PerformanceTracker .record_coordination_contributions() for trend analysis.

See Also¶

• Task & Workflow Engine -- task dispatch, state coordination
• Agent Execution -- per-agent execution loop, prompt profiles
• Verification & Quality -- review pipeline, verification stage
• Design Overview -- full index