diff --git a/crates/mcp-brain-server/src/routes.rs b/crates/mcp-brain-server/src/routes.rs
index dade87ee3..033bc3002 100644
--- a/crates/mcp-brain-server/src/routes.rs
+++ b/crates/mcp-brain-server/src/routes.rs
@@ -463,10 +463,12 @@ pub fn run_enhanced_training_cycle(state: &AppState) -> EnhancedTrainingResult {
         }
     }
 
-    // 3c. Cache MinCut partition result (avoids recomputing on every /v1/partition request)
+    // 3c. Cache partition result (avoids recomputing on every /v1/partition request)
+    // Only compute exact MinCut for small graphs; large graphs skip to avoid timeout
     {
         let graph = state.graph.read();
-        if graph.node_count() > 0 {
+        let edge_count = graph.edge_count();
+        if graph.node_count() > 0 && edge_count <= 100_000 {
             let (clusters, cut_value, edge_strengths) = graph.partition_full(2);
             let result = crate::types::PartitionResult {
                 total_memories: graph.node_count(),
@@ -478,6 +480,8 @@ pub fn run_enhanced_training_cycle(state: &AppState) -> EnhancedTrainingResult {
             };
             *state.cached_partition.write() = Some(result);
         }
+        // For large graphs, partition cache is populated by the scheduled
+        // rebuild_graph job which runs asynchronously without timeout pressure
     }
 
     // 4. Internal voice reflection (ADR-110)
diff --git a/docs/adr/ADR-124-dynamic-partition-cache.md b/docs/adr/ADR-124-dynamic-partition-cache.md
new file mode 100644
index 000000000..c36212afd
--- /dev/null
+++ b/docs/adr/ADR-124-dynamic-partition-cache.md
@@ -0,0 +1,38 @@
+# ADR-124: Dynamic Partition Cache with Large-Graph Guard
+
+## Status
+Accepted
+
+## Context
+The pi.ruv.io brain server's `/v1/partition` endpoint runs exact Stoer-Wagner MinCut on the knowledge graph. With 2,090+ nodes and 971K+ edges, this computation exceeds Cloud Run's 300s timeout and the MCP SSE transport's 60s tool call timeout.
+
+PR #287 (ADR-117) introduced a canonical source-anchored MinCut with deterministic hashing, but it shares the same O(V*E) complexity for the initial computation.
+
+## Decision
+
+### 1. Partition Caching
+Add `cached_partition: Arc<RwLock<Option<PartitionResult>>>` to `AppState`. The cache is populated during training cycles and served instantly via:
+- **MCP `brain_partition`**: Returns cached compact partition (sub-millisecond)
+- **REST `/v1/partition`**: Returns cache by default; `?force=true` recomputes
+
+### 2. Large-Graph Guard
+During training cycles, only compute exact MinCut if `edge_count <= 100,000`. For larger graphs, the cache remains unpopulated until the scheduled `rebuild_graph` job runs asynchronously without timeout pressure.
+
+### 3. Tier Roadmap (from ADR-117)
+- **Tier 1 (shipped)**: Exact Stoer-Wagner + source-anchored canonical cut
+- **Tier 2 (next)**: Tree packing for O(V^2 log V) fast path
+- **Tier 3 (future)**: Incremental maintenance for evolving graphs (dynamic MinCut)
+
+## Consequences
+- MCP `brain_partition` returns instantly from cache instead of timing out
+- Enhanced training cycle no longer blocks on MinCut for large graphs
+- Fresh partition data depends on scheduled background jobs for graphs >100K edges
+- REST `?force=true` still allows on-demand recomputation (may timeout for large graphs)
+
+## Benchmark Results (Canonical MinCut - ADR-117)
+
+| Graph Type | Nodes | Time |
+|-----------|-------|------|
+| Cycle | 50 | 3.09 us |
+| Complete | 10 | 2.61 us |
+| Hash stability | 100 | 1.39 us |
diff --git a/examples/dragnes/.svelte-kit/ambient.d.ts b/examples/dragnes/.svelte-kit/ambient.d.ts
index ef7722f65..73d21c109 100644
--- a/examples/dragnes/.svelte-kit/ambient.d.ts
+++ b/examples/dragnes/.svelte-kit/ambient.d.ts
@@ -50,6 +50,7 @@ declare module '$env/static/private' {
 	export const NVS_ROOT: string;
 	export const GIT_EDITOR: string;
 	export const RVM_PATH: string;
+	export const BRAIN_URL: string;
 	export const FEATURE_SPARK_POST_COMMIT_CREATE_ITERATION: string;
 	export const HOSTNAME: string;
 	export const GIT_ASKPASS: string;
@@ -156,6 +157,7 @@ declare module '$env/static/private' {
 	export const npm_config_globalconfig: string;
 	export const npm_config_init_module: string;
 	export const JAVA_HOME: string;
+	export const BRAIN_API_KEY: string;
 	export const NVS_USE_XZ: string;
 	export const PWD: string;
 	export const INTERNAL_VSCS_TARGET_URL: string;
@@ -278,6 +280,7 @@ declare module '$env/dynamic/private' {
 		NVS_ROOT: string;
 		GIT_EDITOR: string;
 		RVM_PATH: string;
+		BRAIN_URL: string;
 		FEATURE_SPARK_POST_COMMIT_CREATE_ITERATION: string;
 		HOSTNAME: string;
 		GIT_ASKPASS: string;
@@ -384,6 +387,7 @@ declare module '$env/dynamic/private' {
 		npm_config_globalconfig: string;
 		npm_config_init_module: string;
 		JAVA_HOME: string;
+		BRAIN_API_KEY: string;
 		NVS_USE_XZ: string;
 		PWD: string;
 		INTERNAL_VSCS_TARGET_URL: string;
diff --git a/examples/dragnes/.svelte-kit/generated/server/internal.js b/examples/dragnes/.svelte-kit/generated/server/internal.js
index b23f53788..6fef9988d 100644
--- a/examples/dragnes/.svelte-kit/generated/server/internal.js
+++ b/examples/dragnes/.svelte-kit/generated/server/internal.js
@@ -25,7 +25,7 @@ export const options = {
 		app: ({ head, body, assets, nonce, env }) => "<!DOCTYPE html>\n<html lang=\"en\" class=\"dark\">\n<head>\n\t<meta charset=\"utf-8\" />\n\t<meta name=\"viewport\" content=\"width=device-width, initial-scale=1\" />\n\t<link rel=\"manifest\" href=\"/manifest.json\" />\n\t<link rel=\"icon\" href=\"/dragnes-icon-192.svg\" />\n\t<title>DrAgnes -- Dermatology Intelligence</title>\n\t" + head + "\n</head>\n<body data-sveltekit-preload-data=\"hover\">\n\t<div style=\"display: contents\">" + body + "</div>\n</body>\n</html>\n",
 		error: ({ status, message }) => "<!doctype html>\n<html lang=\"en\">\n\t<head>\n\t\t<meta charset=\"utf-8\" />\n\t\t<title>" + message + "</title>\n\n\t\t<style>\n\t\t\tbody {\n\t\t\t\t--bg: white;\n\t\t\t\t--fg: #222;\n\t\t\t\t--divider: #ccc;\n\t\t\t\tbackground: var(--bg);\n\t\t\t\tcolor: var(--fg);\n\t\t\t\tfont-family:\n\t\t\t\t\tsystem-ui,\n\t\t\t\t\t-apple-system,\n\t\t\t\t\tBlinkMacSystemFont,\n\t\t\t\t\t'Segoe UI',\n\t\t\t\t\tRoboto,\n\t\t\t\t\tOxygen,\n\t\t\t\t\tUbuntu,\n\t\t\t\t\tCantarell,\n\t\t\t\t\t'Open Sans',\n\t\t\t\t\t'Helvetica Neue',\n\t\t\t\t\tsans-serif;\n\t\t\t\tdisplay: flex;\n\t\t\t\talign-items: center;\n\t\t\t\tjustify-content: center;\n\t\t\t\theight: 100vh;\n\t\t\t\tmargin: 0;\n\t\t\t}\n\n\t\t\t.error {\n\t\t\t\tdisplay: flex;\n\t\t\t\talign-items: center;\n\t\t\t\tmax-width: 32rem;\n\t\t\t\tmargin: 0 1rem;\n\t\t\t}\n\n\t\t\t.status {\n\t\t\t\tfont-weight: 200;\n\t\t\t\tfont-size: 3rem;\n\t\t\t\tline-height: 1;\n\t\t\t\tposition: relative;\n\t\t\t\ttop: -0.05rem;\n\t\t\t}\n\n\t\t\t.message {\n\t\t\t\tborder-left: 1px solid var(--divider);\n\t\t\t\tpadding: 0 0 0 1rem;\n\t\t\t\tmargin: 0 0 0 1rem;\n\t\t\t\tmin-height: 2.5rem;\n\t\t\t\tdisplay: flex;\n\t\t\t\talign-items: center;\n\t\t\t}\n\n\t\t\t.message h1 {\n\t\t\t\tfont-weight: 400;\n\t\t\t\tfont-size: 1em;\n\t\t\t\tmargin: 0;\n\t\t\t}\n\n\t\t\t@media (prefers-color-scheme: dark) {\n\t\t\t\tbody {\n\t\t\t\t\t--bg: #222;\n\t\t\t\t\t--fg: #ddd;\n\t\t\t\t\t--divider: #666;\n\t\t\t\t}\n\t\t\t}\n\t\t</style>\n\t</head>\n\t<body>\n\t\t<div class=\"error\">\n\t\t\t<span class=\"status\">" + status + "</span>\n\t\t\t<div class=\"message\">\n\t\t\t\t<h1>" + message + "</h1>\n\t\t\t</div>\n\t\t</div>\n\t</body>\n</html>\n"
 	},
-	version_hash: "3oxd9x"
+	version_hash: "a9s2ra"
 };
 
 export async function get_hooks() {