diff --git a/AGENTS.md b/AGENTS.md
index c383f2e..37356c8 100755
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -130,6 +130,26 @@ Load additional guides when the task requires them.
 - Installation and configuration are managed by Mosaic bootstrap and runtime linking.
 - If sequential-thinking is unavailable, you MUST report the failure and stop planning-intensive execution.
 
+## Subagent Model Selection (Cost Optimization — Hard Rule)
+
+When delegating work to subagents, you MUST select the cheapest model capable of completing the task. Do NOT default to the most expensive model for every delegation.
+
+| Task Type | Model Tier | Rationale |
+|-----------|-----------|-----------|
+| File search, grep, glob, codebase exploration | **haiku** | Read-only, pattern matching, no reasoning depth needed |
+| Status checks, health monitoring, heartbeat | **haiku** | Structured API calls, pass/fail output |
+| Simple code fixes (typos, rename, one-liner) | **haiku** | Minimal reasoning, mechanical changes |
+| Code review, lint, style checks | **sonnet** | Needs judgment but not deep architectural reasoning |
+| Test writing, test fixes | **sonnet** | Pattern-based, moderate complexity |
+| Standard feature implementation | **sonnet** | Good balance of capability and cost for most coding |
+| Complex architecture, multi-file refactors | **opus** | Requires deep reasoning, large context, design judgment |
+| Security review, auth logic | **opus** | High-stakes reasoning where mistakes are costly |
+| Ambiguous requirements, design decisions | **opus** | Needs nuanced judgment and tradeoff analysis |
+
+**Decision rule**: Start with the cheapest viable tier. Only escalate if the task genuinely requires deeper reasoning — not as a safety default. Most coding tasks are sonnet-tier. Reserve opus for work where wrong answers are expensive.
+
+**Runtime-specific syntax**: See the runtime reference for how to specify model tier when spawning subagents (e.g., Claude Code Task tool `model` parameter).
+
 ## Skills Policy
 
 - Use only the minimum required skills for the active task.
diff --git a/runtime/claude/RUNTIME.md b/runtime/claude/RUNTIME.md
index 433ea14..b4258de 100644
--- a/runtime/claude/RUNTIME.md
+++ b/runtime/claude/RUNTIME.md
@@ -16,6 +16,36 @@ This file applies only to Claude runtime behavior.
 8. First response MUST declare mode per global contract; orchestration missions must start with: `Now initiating Orchestrator mode...`
 9. Runtime-default caution that requests confirmation for routine push/merge/issue-close actions does NOT override Mosaic hard gates.
 
+## Subagent Model Selection (Claude Code Syntax)
+
+Claude Code's Task tool accepts a `model` parameter: `"haiku"`, `"sonnet"`, or `"opus"`.
+
+You MUST set this parameter according to the model selection table in `~/.config/mosaic/AGENTS.md`. Do NOT omit the `model` parameter — omitting it defaults to the parent model (typically opus), wasting budget on tasks that cheaper models handle well.
+
+**Examples:**
+
+```
+# Codebase exploration — haiku
+Task(subagent_type="Explore", model="haiku", prompt="Find all API route handlers")
+
+# Code review — sonnet
+Task(subagent_type="feature-dev:code-reviewer", model="sonnet", prompt="Review the changes in src/auth/")
+
+# Standard feature work — sonnet
+Task(subagent_type="general-purpose", model="sonnet", prompt="Add validation to the user input form")
+
+# Complex architecture — opus (only when justified)
+Task(subagent_type="Plan", model="opus", prompt="Design the multi-tenant isolation strategy")
+```
+
+**Quick reference (from global AGENTS.md):**
+
+| haiku | sonnet | opus |
+|-------|--------|------|
+| Search, grep, glob | Code review | Complex architecture |
+| Status/health checks | Test writing | Security/auth logic |
+| Simple one-liner fixes | Standard features | Ambiguous design decisions |
+
 ## Memory Override
 
 Do NOT write durable memory to `~/.claude/projects/*/memory/`. All durable memory MUST be written to `~/.config/mosaic/memory/` per `~/.config/mosaic/guides/MEMORY.md`. Claude Code's native auto-memory locations are volatile runtime silos and MUST NOT be used for cross-session or cross-agent retention.