diff --git a/.changeset/workflow-column-agent-assignment.md b/.changeset/workflow-column-agent-assignment.md
new file mode 100644
index 0000000000..249914fcce
--- /dev/null
+++ b/.changeset/workflow-column-agent-assignment.md
@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": minor
+---
+
+Add per-column agent assignment for workflow columns, behind the combined `experimentalFeatures.workflowColumns` + `experimentalFeatures.workflowGraphExecutor` flags.
+
+A workflow column can now name a permanent agent from the registry plus a mode — `defer` (the column agent is the default for work in that column that carries no agent/model settings of its own) or `override` (the column agent supersedes node- and task-level agent/model settings). The binding applies to all session-running work attributable to the column's nodes: custom prompt/gate/script nodes, the execute seam's coding session, and step-execute sessions. Precedence is resolved by one shared `@fusion/core` resolver (`resolveColumnAgentBinding` + `resolveEffectiveAgent`) consumed by every reader, with defer/override expressed as explicit named rules and defer granularity all-or-nothing (an own agent identity OR a complete `modelProvider`+`modelId` pair suppresses the column agent). The binding keys off the node's declared IR column; foreach template nodes inherit the enclosing foreach node's column. A missing/deleted agent at resolution time logs and falls back to normal resolution — a live session is never aborted. The built-in default workflow carries no column agents and stays byte-identical (parity oracle); with either flag off, column agents are inert.
diff --git a/packages/engine/src/__tests__/executor-column-agent-seams.test.ts b/packages/engine/src/__tests__/executor-column-agent-seams.test.ts
new file mode 100644
index 0000000000..981b6cd2e3
--- /dev/null
+++ b/packages/engine/src/__tests__/executor-column-agent-seams.test.ts
@@ -0,0 +1,398 @@
+// Column-agent coding seams: execute + step-execute sessions (plan U4,
+// R2/R3/R4/R8, KTD-2/KTD-3/KTD-5/KTD-6).
+//
+// The graph EXECUTE seam (single coding session) and STEP-EXECUTE seam
+// (StepSessionExecutor per-step sessions) must run as the column agent when the
+// governing seam node's DECLARED column carries a binding. Session identity =
+// model + persona + attribution (gating/heartbeat/restart are U5, untouched here).
+//
+// Harness: mirrors executor-step-session.test.ts / executor-column-agent-custom-
+// node.test.ts — a real TaskExecutor over a mock store with `createFnAgent`
+// (the outermost session-spawn boundary) mocked, plus the entirely-mocked
+// StepSessionExecutor from executor-test-helpers so the step-session branch's
+// constructor options are observable.
+//
+// The two per-run seam slots the executor reads — `graphSeamGoverningNodeId` and
+// `graphColumnAgentResolver` — are normally stamped by the graph seam wiring
+// (createPromptLikeHandler → execute/stepExecute seams). We seed them directly and
+// drive `runImplementationPhase` (the exact call the execute seam makes, which
+// registers a completion interceptor so graph routing is skipped) so the session
+// build runs the production resolution path with no scripted session layer.
+
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import "./executor-test-helpers.js";
+import { TaskExecutor } from "../executor.js";
+import {
+  createMockStore,
+  mockedCreateFnAgent,
+  mockedStepSessionExecutor,
+  mockExecuteAll,
+  resetExecutorMocks,
+} from "./executor-test-helpers.js";
+import type { WorkflowColumnAgent } from "@fusion/core";
+
+// The mocked resolveExecutorSessionModel (executor-test-helpers) reads
+// `runtimeConfig.model` in "provider/modelId" form, so the column agent advertises
+// its model that way; the assigned agent advertises a different one so we can prove
+// which one reached the session.
+function makeColumnAgent(overrides: Record<string, unknown> = {}) {
+  return {
+    id: "agent-col",
+    name: "Senior Reviewer",
+    soul: "I am the senior reviewer.",
+    instructionsText: "Always be thorough.",
+    memory: undefined,
+    runtimeConfig: { model: "anthropic/claude-col", runtimeHint: "col-hint" },
+    ...overrides,
+  };
+}
+
+function makeAssignedAgent(overrides: Record<string, unknown> = {}) {
+  return {
+    id: "agent-Y",
+    name: "Assigned Agent",
+    soul: "I am the assigned agent.",
+    instructionsText: "Assigned persona.",
+    memory: undefined,
+    runtimeConfig: { model: "openai/gpt-assigned", runtimeHint: "assigned-hint" },
+    ...overrides,
+  };
+}
+
+/** A mock fn agent that immediately calls fn_task_done so execute() completes. */
+function installTaskDoneAgent() {
+  mockedCreateFnAgent.mockImplementation((async (opts: any) => {
+    const tools = opts.customTools || [];
+    return {
+      session: {
+        prompt: vi.fn().mockImplementation(async () => {
+          const done = tools.find((t: any) => t.name === "fn_task_done");
+          if (done) await done.execute("tool-1", {});
+        }),
+        dispose: vi.fn(),
+        subscribe: vi.fn(),
+        on: vi.fn(),
+        sessionManager: { getLeafId: vi.fn().mockReturnValue("leaf-1") },
+        state: {},
+      },
+    };
+  }) as any);
+}
+
+function makeExecutor(store: ReturnType<typeof createMockStore>, agentsById: Record<string, unknown>) {
+  const agentStore = {
+    getAgent: vi.fn(async (id: string) => agentsById[id] ?? null),
+  };
+  const executor = new TaskExecutor(store as any, "/tmp/test", { agentStore } as any);
+  return { executor, agentStore };
+}
+
+/**
+ * Seed the per-run column-agent seam slots the executor reads at session-build
+ * time, then drive the implementation phase the way the execute seam does.
+ */
+async function runExecuteSeam(
+  executor: TaskExecutor,
+  task: any,
+  governingNodeId: string,
+  binding: WorkflowColumnAgent | undefined,
+) {
+  (executor as any).graphSeamGoverningNodeId.set(task.id, governingNodeId);
+  (executor as any).graphColumnAgentResolver.set(task.id, (nodeId: string) =>
+    nodeId === governingNodeId ? binding : undefined,
+  );
+  return (executor as any).runImplementationPhase(task);
+}
+
+/** Force the step-session physics path and seed the seam slots, then run. */
+async function runStepSessionSeam(
+  executor: TaskExecutor,
+  task: any,
+  governingNodeId: string,
+  binding: WorkflowColumnAgent | undefined,
+) {
+  (executor as any).graphStepSessionPinned.add(task.id);
+  (executor as any).graphSeamGoverningNodeId.set(task.id, governingNodeId);
+  (executor as any).graphColumnAgentResolver.set(task.id, (nodeId: string) =>
+    nodeId === governingNodeId ? binding : undefined,
+  );
+  return (executor as any).runImplementationPhase(task);
+}
+
+function singleSessionTask(overrides: Record<string, unknown> = {}) {
+  return {
+    id: "FN-001",
+    title: "Test",
+    description: "Test task",
+    column: "in-progress",
+    dependencies: [],
+    steps: [{ name: "Implement", status: "in-progress" }],
+    currentStep: 0,
+    log: [],
+    prompt: "# test\n## Steps\n### Step 0: Implement\n- [ ] implement",
+    createdAt: new Date().toISOString(),
+    updatedAt: new Date().toISOString(),
+    ...overrides,
+  };
+}
+
+function lastFnAgentOpts() {
+  const calls = mockedCreateFnAgent.mock.calls;
+  return calls[calls.length - 1]?.[0] as any;
+}
+
+function lastStepExecutorOpts() {
+  const calls = mockedStepSessionExecutor.mock.calls;
+  return calls[calls.length - 1]?.[0] as any;
+}
+
+function loggedLines(store: ReturnType<typeof createMockStore>): string[] {
+  return store.logEntry.mock.calls.map((call: any[]) => String(call[1] ?? ""));
+}
+
+const OVERRIDE_COL: WorkflowColumnAgent = { agentId: "agent-col", mode: "override" };
+const DEFER_COL: WorkflowColumnAgent = { agentId: "agent-col", mode: "defer" };
+
+describe("column-agent coding seams (plan U4)", () => {
+  beforeEach(() => {
+    resetExecutorMocks();
+  });
+
+  // ── Characterization (pre-substitution behavior) ──────────────────────────
+  // These pin the assignedAgentId-driven session identity that exists today and
+  // MUST stay byte-identical on the no-binding path after substitution.
+
+  describe("characterization: no binding → assignedAgentId session identity unchanged", () => {
+    it("execute seam: session model/persona built from the assigned agent, no column-agent log", async () => {
+      const store = createMockStore();
+      const task = singleSessionTask({ assignedAgentId: "agent-Y" });
+      store.getTask.mockResolvedValue(task as any);
+      const { executor } = makeExecutor(store, { "agent-Y": makeAssignedAgent() });
+      installTaskDoneAgent();
+
+      // No governing node / no binding seeded → legacy path.
+      await (executor as any).runImplementationPhase(task);
+
+      const opts = lastFnAgentOpts();
+      // Model resolved from the ASSIGNED agent's runtimeConfig.model.
+      expect(opts.defaultProvider).toBe("openai");
+      expect(opts.defaultModelId).toBe("gpt-assigned");
+      // No column-agent adoption logged.
+      expect(loggedLines(store).some((l) => l.includes("running as column agent"))).toBe(false);
+    });
+
+    it("step session: attribution falls back to assignedAgentId; no effectiveAgentId override", async () => {
+      const store = createMockStore();
+      const task = singleSessionTask({ assignedAgentId: "agent-Y" });
+      store.getTask.mockResolvedValue(task as any);
+      const { executor } = makeExecutor(store, { "agent-Y": makeAssignedAgent() });
+      installTaskDoneAgent();
+
+      (executor as any).graphStepSessionPinned.add(task.id);
+      await (executor as any).runImplementationPhase(task);
+
+      const opts = lastStepExecutorOpts();
+      // No column agent governs → no attribution override (StepSessionExecutor
+      // falls back to taskDetail.assignedAgentId ?? "executor").
+      expect(opts.effectiveAgentId).toBeUndefined();
+      // Model precedence input is the assigned agent's runtimeConfig.
+      expect(opts.assignedAgentRuntimeConfig).toEqual(makeAssignedAgent().runtimeConfig);
+      expect(loggedLines(store).some((l) => l.includes("running as column agent"))).toBe(false);
+    });
+  });
+
+  // ── Execute seam (single coding session) ──────────────────────────────────
+
+  describe("execute seam", () => {
+    it("override column, task assigned to Y → session uses column agent X's model/persona/identity + audit", async () => {
+      const store = createMockStore();
+      const task = singleSessionTask({ assignedAgentId: "agent-Y" });
+      store.getTask.mockResolvedValue(task as any);
+      const { executor, agentStore } = makeExecutor(store, {
+        "agent-Y": makeAssignedAgent(),
+        "agent-col": makeColumnAgent(),
+      });
+      installTaskDoneAgent();
+
+      await runExecuteSeam(executor, task, "execute-node", OVERRIDE_COL);
+
+      const opts = lastFnAgentOpts();
+      // Column agent X's model supersedes the assigned agent Y's.
+      expect(opts.defaultProvider).toBe("anthropic");
+      expect(opts.defaultModelId).toBe("claude-col");
+      // Persona: column agent's soul + instructionsText reach the session system
+      // prompt layers (KTD-6 typed fields).
+      const promptText = JSON.stringify(opts.systemPromptLayers ?? "") + (opts.systemPrompt ?? "");
+      expect(promptText).toContain("I am the senior reviewer.");
+      expect(promptText).toContain("Always be thorough.");
+      // The column agent was fetched (identity), not just the assigned agent.
+      expect(agentStore.getAgent).toHaveBeenCalledWith("agent-col");
+      // Audit names the substitution + mode.
+      expect(
+        loggedLines(store).some(
+          (l) => l.includes("running as column agent 'agent-col' (override)") && l.includes("execute-node"),
+        ),
+      ).toBe(true);
+    });
+
+    it("defer column, task with complete modelProvider/modelId → task settings win", async () => {
+      const store = createMockStore();
+      // Task carries a complete own model pair → defer must yield own settings.
+      const task = singleSessionTask({ modelProvider: "task-prov", modelId: "task-model" });
+      store.getTask.mockResolvedValue(task as any);
+      const { executor, agentStore } = makeExecutor(store, { "agent-col": makeColumnAgent() });
+      installTaskDoneAgent();
+
+      await runExecuteSeam(executor, task, "execute-node", DEFER_COL);
+
+      const opts = lastFnAgentOpts();
+      // The task's own complete pair wins (mocked resolver: no agent runtimeConfig
+      // model, falls through to the task pair).
+      expect(opts.defaultProvider).toBe("task-prov");
+      expect(opts.defaultModelId).toBe("task-model");
+      // Column agent never fetched/adopted.
+      expect(agentStore.getAgent).not.toHaveBeenCalledWith("agent-col");
+      expect(loggedLines(store).some((l) => l.includes("running as column agent"))).toBe(false);
+    });
+
+    it("defer column, bare task (no own settings) → column agent adopted", async () => {
+      const store = createMockStore();
+      const task = singleSessionTask(); // no assignedAgentId, no model pair
+      store.getTask.mockResolvedValue(task as any);
+      const { executor, agentStore } = makeExecutor(store, { "agent-col": makeColumnAgent() });
+      installTaskDoneAgent();
+
+      await runExecuteSeam(executor, task, "execute-node", DEFER_COL);
+
+      const opts = lastFnAgentOpts();
+      expect(opts.defaultProvider).toBe("anthropic");
+      expect(opts.defaultModelId).toBe("claude-col");
+      expect(agentStore.getAgent).toHaveBeenCalledWith("agent-col");
+      expect(
+        loggedLines(store).some((l) => l.includes("running as column agent 'agent-col' (defer)")),
+      ).toBe(true);
+    });
+
+    it("column agent missing from registry at seam time → fallback to assignedAgentId path, logged, run proceeds", async () => {
+      const store = createMockStore();
+      const task = singleSessionTask({ assignedAgentId: "agent-Y" });
+      store.getTask.mockResolvedValue(task as any);
+      // Column agent absent from the registry; assigned agent present.
+      const { executor } = makeExecutor(store, { "agent-Y": makeAssignedAgent() });
+      installTaskDoneAgent();
+
+      await runExecuteSeam(executor, task, "execute-node", OVERRIDE_COL);
+
+      const opts = lastFnAgentOpts();
+      // Fell back to the assigned agent's model.
+      expect(opts.defaultProvider).toBe("openai");
+      expect(opts.defaultModelId).toBe("gpt-assigned");
+      // Fallback audited; no adoption claim.
+      expect(
+        loggedLines(store).some(
+          (l) => l.includes("column agent 'agent-col' not found") && l.includes("falling back"),
+        ),
+      ).toBe(true);
+      expect(loggedLines(store).some((l) => l.includes("running as column agent"))).toBe(false);
+      // Run still proceeded: a session was built and the task done tool fired
+      // (the missing column agent never aborted the session — R8).
+      expect(mockedCreateFnAgent).toHaveBeenCalled();
+    });
+
+    it("integration: the column agent's executor model reaches createResolvedAgentSession options end-to-end", async () => {
+      // Per the plugin-skills learning — prove with the REAL resolution layers
+      // (only the outermost createFnAgent/session-spawn boundary is mocked).
+      const store = createMockStore();
+      const task = singleSessionTask({ assignedAgentId: "agent-Y" });
+      store.getTask.mockResolvedValue(task as any);
+      const { executor } = makeExecutor(store, {
+        "agent-Y": makeAssignedAgent(),
+        "agent-col": makeColumnAgent({ runtimeConfig: { model: "anthropic/claude-e2e", runtimeHint: "e2e-hint" } }),
+      });
+      installTaskDoneAgent();
+
+      await runExecuteSeam(executor, task, "execute-node", OVERRIDE_COL);
+
+      const opts = lastFnAgentOpts();
+      expect(opts.defaultProvider).toBe("anthropic");
+      expect(opts.defaultModelId).toBe("claude-e2e");
+      // Runtime hint also follows the column agent end-to-end.
+      expect(opts.runtimeHint).toBe("e2e-hint");
+    });
+  });
+
+  // ── Step-execute seam (StepSessionExecutor per-step sessions) ─────────────
+
+  describe("step-execute seam", () => {
+    it("foreach instance node inherits the foreach's bound column → instance session carries column agent identity (attribution asserted)", async () => {
+      const store = createMockStore();
+      const task = singleSessionTask({ assignedAgentId: "agent-Y" });
+      store.getTask.mockResolvedValue(task as any);
+      const { executor, agentStore } = makeExecutor(store, {
+        "agent-Y": makeAssignedAgent(),
+        "agent-col": makeColumnAgent(),
+      });
+      installTaskDoneAgent();
+
+      // Governing node is the foreach INSTANCE id; the resolver (which the real
+      // core resolver implements via template inheritance) returns the foreach's
+      // bound column binding for that instance id.
+      const instanceNodeId = "foreach-1#0:step-exec";
+      await runStepSessionSeam(executor, task, instanceNodeId, OVERRIDE_COL);
+
+      const opts = lastStepExecutorOpts();
+      // Attribution: the per-step session is attributed to the column agent.
+      expect(opts.effectiveAgentId).toBe("agent-col");
+      // Model precedence input is the column agent's runtimeConfig (not the
+      // assigned agent's).
+      expect(opts.assignedAgentRuntimeConfig).toEqual(makeColumnAgent().runtimeConfig);
+      expect(agentStore.getAgent).toHaveBeenCalledWith("agent-col");
+      expect(mockExecuteAll).toHaveBeenCalled();
+      expect(
+        loggedLines(store).some((l) => l.includes("running as column agent 'agent-col' (override)")),
+      ).toBe(true);
+    });
+
+    it("defer column with task own complete model pair → step session keeps assigned-agent attribution", async () => {
+      const store = createMockStore();
+      const task = singleSessionTask({
+        assignedAgentId: "agent-Y",
+        modelProvider: "task-prov",
+        modelId: "task-model",
+      });
+      store.getTask.mockResolvedValue(task as any);
+      const { executor, agentStore } = makeExecutor(store, {
+        "agent-Y": makeAssignedAgent(),
+        "agent-col": makeColumnAgent(),
+      });
+      installTaskDoneAgent();
+
+      await runStepSessionSeam(executor, task, "foreach-1#0:step-exec", DEFER_COL);
+
+      const opts = lastStepExecutorOpts();
+      // Own settings (complete model pair) suppress the defer column agent.
+      expect(opts.effectiveAgentId).toBeUndefined();
+      expect(opts.assignedAgentRuntimeConfig).toEqual(makeAssignedAgent().runtimeConfig);
+      expect(agentStore.getAgent).not.toHaveBeenCalledWith("agent-col");
+    });
+
+    it("column agent missing from registry at step-execute seam → fallback to assigned-agent attribution, logged", async () => {
+      const store = createMockStore();
+      const task = singleSessionTask({ assignedAgentId: "agent-Y" });
+      store.getTask.mockResolvedValue(task as any);
+      const { executor } = makeExecutor(store, { "agent-Y": makeAssignedAgent() });
+      installTaskDoneAgent();
+
+      await runStepSessionSeam(executor, task, "foreach-1#0:step-exec", OVERRIDE_COL);
+
+      const opts = lastStepExecutorOpts();
+      expect(opts.effectiveAgentId).toBeUndefined();
+      expect(opts.assignedAgentRuntimeConfig).toEqual(makeAssignedAgent().runtimeConfig);
+      expect(
+        loggedLines(store).some(
+          (l) => l.includes("column agent 'agent-col' not found") && l.includes("falling back"),
+        ),
+      ).toBe(true);
+    });
+  });
+});
diff --git a/packages/engine/src/executor.ts b/packages/engine/src/executor.ts
index e1f3028606..521a154928 100644
--- a/packages/engine/src/executor.ts
+++ b/packages/engine/src/executor.ts
@@ -28,6 +28,7 @@ import type {
 import { observeWorkflowParity, WORKFLOW_INTERPRETER_DUAL_OBSERVE_FLAG } from "./workflow-parity-observer.js";
 import {
   FOREACH_ACTIVE_CONTEXT_KEY,
+  SEAM_GOVERNING_NODE_CONTEXT_KEY,
   type ForeachActiveContext,
   type WorkflowLegacySeams,
 } from "./workflow-node-handlers.js";
@@ -3259,6 +3260,22 @@ export class TaskExecutor {
     return `${taskId}:${instanceId}`;
   }
 
+  /** Column-agent seam wiring (column-agent plan U4, R2/R3/R4). Per-run binding
+   *  resolver keyed by task id: maps a governing node id to its column-agent
+   *  binding (if any), computed once per run in maybeExecuteWorkflowGraph from the
+   *  resolved IR. The execute / step-execute seams consume it to decide whether the
+   *  coding/step session runs as a column agent. Cleared in the run's finally. */
+  private graphColumnAgentResolver = new Map<string, (nodeId: string) => WorkflowColumnAgent | undefined>();
+
+  /** Column-agent seam wiring (column-agent plan U4). The governing graph node id
+   *  for the implementation pass currently in flight for a task — the execute-seam
+   *  prompt node's id (execute seam), or the foreach instance node id (step-execute
+   *  seam, which the core resolver maps through template inheritance). Stamped by
+   *  the seam from the reserved {@link SEAM_GOVERNING_NODE_CONTEXT_KEY} context key
+   *  right before it drives the implementation phase, read inside execute()'s
+   *  session build, and cleared by the seam afterward. Keyed by task id. */
+  private graphSeamGoverningNodeId = new Map<string, string>();
+
   /** Tasks currently being orchestrated by the graph runner. Process-wide for
    *  the same reason as executingTaskLock (FN-4811): duplicate execute()
    *  invocations can arrive from different TaskExecutor instances in one
@@ -3334,6 +3351,11 @@ export class TaskExecutor {
       }
       const resolveBindingForNode = (nodeId: string): WorkflowColumnAgent | undefined =>
         columnAgentIr ? resolveColumnAgentBinding(columnAgentIr, nodeId) : undefined;
+      // Column-agent seam wiring (U4): expose the same per-run resolver to the
+      // execute / step-execute seams (which key off a governing node id stamped
+      // into context), so the coding/step session runs as the column agent under
+      // the SAME binding lookup the custom-node seam uses (KTD-2 single resolver).
+      this.graphColumnAgentResolver.set(task.id, resolveBindingForNode);
 
       const runner = new WorkflowGraphTaskRunner({
         store: this.store,
@@ -3401,6 +3423,10 @@ export class TaskExecutor {
       // Clear per-run step-inversion pins (KTD-8: pinned only for the run's life).
       this.graphStepSessionPinned.delete(task.id);
       this.graphStepRunOnce.delete(task.id);
+      // Clear per-run column-agent seam wiring (U4): the resolver and any dangling
+      // governing-node-id are scoped to this run only.
+      this.graphColumnAgentResolver.delete(task.id);
+      this.graphSeamGoverningNodeId.delete(task.id);
       // Per-instance keys: clear every instance slot owned by this task.
       const ctxPrefix = `${task.id}:`;
       for (const key of this.graphStepActiveContext.keys()) {
@@ -4157,8 +4183,22 @@ export class TaskExecutor {
       // so planning is a no-op for already-specified tasks. Custom planning
       // behavior is expressed as a custom prompt node before the execute seam.
       planning: async () => ({ outcome: "success", value: "pre-specified" }),
-      execute: async (seamTask) => {
-        const result = await this.runImplementationPhase(seamTask);
+      execute: async (seamTask, context) => {
+        // Column-agent seam wiring (U4, R4): record the governing node id (the
+        // execute-seam prompt node, stamped into context by createPromptLikeHandler)
+        // so execute()'s session build can resolve the column-agent binding for the
+        // node's DECLARED column. Cleared after the pass so a later seam without a
+        // binding cannot inherit a stale node id.
+        const governingNodeId = context?.[SEAM_GOVERNING_NODE_CONTEXT_KEY];
+        if (typeof governingNodeId === "string") {
+          this.graphSeamGoverningNodeId.set(seamTask.id, governingNodeId);
+        }
+        let result: { taskDone: boolean; modifiedFiles: string[] };
+        try {
+          result = await this.runImplementationPhase(seamTask);
+        } finally {
+          this.graphSeamGoverningNodeId.delete(seamTask.id);
+        }
         if (result.taskDone) {
           return { outcome: "success", value: "implemented" };
         }
@@ -4236,6 +4276,17 @@ export class TaskExecutor {
         // Stamp the active instance so `runGraphTaskStep` can honor
         // `deferDoneToReview` when judging a non-terminal step (FIX 3).
         this.graphStepActiveContext.set(this.graphActiveContextKey(seamTask.id, active.instanceId), active);
+        // Column-agent seam wiring (U4, R4): record the governing node id — the
+        // foreach INSTANCE node id (`<foreachId>#<i>:<templateNodeId>`) stamped into
+        // context by createPromptLikeHandler — so the (once-per-run, KTD-2/KTD-8)
+        // step-session implementation pass resolves the column-agent binding for the
+        // step-execute node's effective column (template-node column, else inherited
+        // foreach column). All instances share the same template node and thus the
+        // same binding, so the first instance to drive the pass sets it correctly.
+        const stepGoverningNodeId = context[SEAM_GOVERNING_NODE_CONTEXT_KEY];
+        if (typeof stepGoverningNodeId === "string" && !this.graphSeamGoverningNodeId.has(seamTask.id)) {
+          this.graphSeamGoverningNodeId.set(seamTask.id, stepGoverningNodeId);
+        }
         const result = await runTaskStep(
           {
             store: this.store,
@@ -4570,6 +4621,75 @@ export class TaskExecutor {
     }
   }
 
+  /**
+   * Resolve the effective COLUMN AGENT governing the coding/step session currently
+   * being built for a task (column-agent plan U4, R2/R3/R4/R8).
+   *
+   * Reads the governing node id stamped by the active seam ({@link
+   * graphSeamGoverningNodeId}) and the per-run binding resolver ({@link
+   * graphColumnAgentResolver}), both scoped to a graph-owned run. Feeds the task's
+   * OWN settings (`assignedAgentId` + complete `modelProvider`/`modelId` pair) into
+   * the shared core resolver (`resolveEffectiveAgent`, KTD-2/KTD-5) so defer/override
+   * precedence is never reimplemented here. When the verdict is `column-agent`,
+   * fetches the full Agent best-effort and audits the adoption; on a missing/deleted
+   * agent it logs and returns undefined so the caller falls back to the
+   * `assignedAgentId` path (R8). Returns undefined for the legacy/no-binding path so
+   * the session build is byte-identical (characterization parity).
+   *
+   * Exposes the resolved Agent object (not just an id) so U5 can consume the same
+   * effective principal for gating/heartbeat/restart without re-resolving.
+   */
+  private async resolveSeamColumnAgent(
+    task: Task,
+    detail: TaskDetail,
+  ): Promise<{ agent: Agent; mode: WorkflowColumnAgent["mode"] | undefined } | undefined> {
+    const governingNodeId = this.graphSeamGoverningNodeId.get(task.id);
+    const resolveBinding = this.graphColumnAgentResolver.get(task.id);
+    if (!governingNodeId || !resolveBinding) return undefined;
+
+    const binding = resolveBinding(governingNodeId);
+    if (!binding) return undefined;
+
+    // The task's OWN settings: its assigned agent identity and a COMPLETE model
+    // pair (an incomplete pair does not count — KTD-5, mirrors
+    // resolveExecutorSessionModel's both-present rule).
+    const ownAgentId = typeof detail.assignedAgentId === "string" && detail.assignedAgentId.trim()
+      ? detail.assignedAgentId.trim()
+      : undefined;
+    const ownModelComplete = Boolean(detail.modelProvider && detail.modelId);
+    const effective = resolveEffectiveAgent({
+      binding,
+      ownAgentId,
+      ownModelProvider: ownModelComplete ? detail.modelProvider : undefined,
+      ownModelId: ownModelComplete ? detail.modelId : undefined,
+    });
+    if (effective.source !== "column-agent") return undefined;
+
+    // Column agent governs: fetch the full Agent (best-effort, R8 fallback).
+    let agent: Agent | null = null;
+    try {
+      agent = (await this.options.agentStore?.getAgent(effective.agentId)) ?? null;
+    } catch {
+      agent = null;
+    }
+    if (!agent) {
+      await this.store.logEntry(
+        task.id,
+        `Workflow seam node '${governingNodeId}': column agent '${effective.agentId}' not found — falling back to assigned-agent resolution`,
+        undefined,
+        this.getRunContextFor(task.id),
+      );
+      return undefined;
+    }
+    await this.store.logEntry(
+      task.id,
+      `Workflow seam node '${governingNodeId}': running as column agent '${effective.agentId}' (${binding.mode})`,
+      undefined,
+      this.getRunContextFor(task.id),
+    );
+    return { agent, mode: binding.mode };
+  }
+
   /** Run a custom (non-seam) graph node on the proven WorkflowStep machinery.
    *
    *  `columnBinding` (plan U3) is the agent binding governing this node's
@@ -5289,7 +5409,17 @@ export class TaskExecutor {
         const stepSessionAgent = detail.assignedAgentId && this.options.agentStore
           ? await this.options.agentStore.getAgent(detail.assignedAgentId).catch(() => null)
           : null;
-        const stepSessionRuntimeHint = extractRuntimeHint(stepSessionAgent?.runtimeConfig);
+
+        // Column-agent SESSION IDENTITY (U4, R2/R3/R4/R8): when the governing
+        // step-execute node's declared column binds an agent that supersedes the
+        // task's assigned agent, the per-step session's MODEL, runtime hint, and
+        // attribution adopt the column agent. The core resolver decides defer vs
+        // override (KTD-2); a missing agent logs + falls back (R8). Gating contexts
+        // still key off the ASSIGNED agent here — principal substitution for
+        // gating/heartbeat is U5 (kept out of this unit deliberately).
+        const stepColumnAgent = await this.resolveSeamColumnAgent(task, detail);
+        const stepIdentityAgent = stepColumnAgent?.agent ?? stepSessionAgent;
+        const stepSessionRuntimeHint = extractRuntimeHint(stepIdentityAgent?.runtimeConfig);
 
         let accumulatedStepTokenUsage = detail.tokenUsage;
         const tokenUsageRecordedSteps = new Set<number>();
@@ -5304,7 +5434,10 @@ export class TaskExecutor {
           stuckTaskDetector: this.options.stuckTaskDetector,
           pluginRunner: this.options.pluginRunner,
           runtimeHint: stepSessionRuntimeHint,
-          assignedAgentRuntimeConfig: (stepSessionAgent?.runtimeConfig ?? undefined) as Record<string, unknown> | undefined,
+          assignedAgentRuntimeConfig: (stepIdentityAgent?.runtimeConfig ?? undefined) as Record<string, unknown> | undefined,
+          // Attribute the per-step run auditor to the column agent when it governs
+          // (U4); absent → StepSessionExecutor falls back to assignedAgentId.
+          effectiveAgentId: stepColumnAgent?.agent.id,
           actionGateContext: this.buildActionGateContext(task.id, stepSessionAgent, settings.defaultAgentPermissionPolicy),
           permanentAgentGating: this.buildPermanentAgentGatingContext(task.id, stepSessionAgent, settings.defaultAgentPermissionPolicy),
           // Pass skill selection context from the main executor session
@@ -5784,7 +5917,18 @@ export class TaskExecutor {
       const assignedAgent = assignedAgentId && this.options.agentStore
         ? await this.options.agentStore.getAgent(assignedAgentId).catch(() => null)
         : null;
-      const executorRuntimeHint = extractRuntimeHint(assignedAgent?.runtimeConfig);
+
+      // Column-agent SESSION IDENTITY (U4, R2/R3/R4/R8): when the governing execute
+      // seam node's declared column binds an agent that supersedes the task's
+      // assigned agent, the coding session's MODEL, runtime hint, persona, and
+      // memory tools adopt the column agent. The core resolver decides defer vs
+      // override (KTD-2); a missing agent logs + falls back (R8). No binding →
+      // `columnAgentSeam` is undefined and every line below is byte-identical to the
+      // assigned-agent path (characterization parity). Gating contexts still key off
+      // the ASSIGNED agent — principal substitution for gating/heartbeat is U5.
+      const columnAgentSeam = await this.resolveSeamColumnAgent(task, detail);
+      const identityAgent = columnAgentSeam?.agent ?? assignedAgent;
+      const executorRuntimeHint = extractRuntimeHint(identityAgent?.runtimeConfig);
 
       // Log fast mode status
       if (executionMode === "fast") {
@@ -5831,11 +5975,11 @@ export class TaskExecutor {
           })
           : []),
         createWebFetchTool(),
-        ...createMemoryTools(this.rootDir, settings, assignedAgent ? {
+        ...createMemoryTools(this.rootDir, settings, identityAgent ? {
           agentMemory: {
-            agentId: assignedAgent.id,
-            agentName: assignedAgent.name,
-            memory: assignedAgent.memory,
+            agentId: identityAgent.id,
+            agentName: identityAgent.name,
+            memory: identityAgent.memory,
           },
         } : undefined),
         // Conditionally add agent self-reflection when enabled and task has an assigned agent.
@@ -5890,11 +6034,14 @@ export class TaskExecutor {
         // 3. Global execution lane pair (executionGlobalProvider + executionGlobalModelId)
         // 4. Project default override pair (defaultProviderOverride + defaultModelIdOverride)
         // 5. Global default pair (defaultProvider + defaultModelId)
+        // Column-agent session identity (U4): the model precedence input is the
+        // EFFECTIVE identity agent's runtimeConfig (column agent when it governs,
+        // else the assigned agent — byte-identical no-binding path).
         const { provider: executorProvider, modelId: executorModelId } = resolveExecutorSessionModel(
           detail.modelProvider,
           detail.modelId,
           settings,
-          (assignedAgent?.runtimeConfig ?? undefined) as Record<string, unknown> | undefined,
+          (identityAgent?.runtimeConfig ?? undefined) as Record<string, unknown> | undefined,
         );
         const executorFallbackProvider = settings.fallbackProvider;
         const executorFallbackModelId = settings.fallbackModelId;
@@ -5928,8 +6075,15 @@ export class TaskExecutor {
 
         executorLog.log(`${task.id}: creating agent session (provider=${executorProvider ?? "default"}, model=${executorModelId ?? "default"}, resuming=${isResuming})`);
 
-        // Resolve per-agent custom instructions for the executor role
-        const executorInstructions = await this.resolveInstructionsForRole("executor", settings);
+        // Resolve per-agent custom instructions for the executor role.
+        // Column-agent session identity (U4, R3/KTD-6): when a column agent governs,
+        // its TYPED persona (soul/instructionsText, via buildAgentPersona — the same
+        // source the custom-node path uses) supersedes the role-resolved executor
+        // instructions, so the coding session speaks AS the column agent. No binding
+        // → role instructions unchanged (characterization parity).
+        const columnAgentPersona = columnAgentSeam ? this.buildAgentPersona(columnAgentSeam.agent) : undefined;
+        const executorInstructions = columnAgentPersona
+          ?? (await this.resolveInstructionsForRole("executor", settings));
 
         // Build structured layers for cross-session prompt caching.
         const executorPluginContributions = buildPluginPromptSection(
diff --git a/packages/engine/src/step-session-executor.ts b/packages/engine/src/step-session-executor.ts
index 0971c6983e..6fe611a4ba 100644
--- a/packages/engine/src/step-session-executor.ts
+++ b/packages/engine/src/step-session-executor.ts
@@ -125,6 +125,18 @@ export interface StepSessionExecutorOptions {
   permanentAgentGating?: PermanentAgentGatingContext;
   /** Task-scoped environment injected into non-git subprocesses. */
   taskEnv?: NodeJS.ProcessEnv;
+  /**
+   * Column-agent identity override for session attribution (column-agent plan U4,
+   * R2/R3/R4). When the governing foreach/step-execute node's declared column
+   * binds an agent that supersedes the task's `assignedAgentId` (override, or
+   * defer with no own settings), the executor passes the column agent's id here so
+   * the per-step run auditor attributes the session to who actually ran — not
+   * `taskDetail.assignedAgentId`. Absent → attribution falls back to
+   * `taskDetail.assignedAgentId ?? "executor"` (byte-identical legacy path). The
+   * column agent's MODEL flows separately via {@link assignedAgentRuntimeConfig}
+   * (the executor swaps it to the column agent's `runtimeConfig` at the seam).
+   */
+  effectiveAgentId?: string;
 }
 
 // ── File Scope Extraction ─────────────────────────────────────────────
@@ -1018,7 +1030,10 @@ Follow instructions precisely and avoid unrelated changes.`,
             defaultThinkingLevel: taskDetail.thinkingLevel ?? settings.defaultThinkingLevel,
             runAuditor: createRunAuditor(this.store, {
               runId: generateSyntheticRunId("workflow-step", taskDetail.id),
-              agentId: taskDetail.assignedAgentId ?? "executor",
+              // Column-agent attribution (U4): the effective column agent is the
+              // principal that actually ran when the seam node's column governs;
+              // fall back to the task's assigned agent (legacy, byte-identical).
+              agentId: this.options.effectiveAgentId ?? taskDetail.assignedAgentId ?? "executor",
               taskId: taskDetail.id,
               taskLineageId: taskDetail.lineageId,
               phase: "execute",
diff --git a/packages/engine/src/workflow-node-handlers.ts b/packages/engine/src/workflow-node-handlers.ts
index 40ab61869f..475626363a 100644
--- a/packages/engine/src/workflow-node-handlers.ts
+++ b/packages/engine/src/workflow-node-handlers.ts
@@ -1,4 +1,4 @@
-import { WorkflowIrError, getStepParser } from "@fusion/core";
+import { WorkflowIrError, getStepParser, instanceNodeId } from "@fusion/core";
 import type { TaskDetail, TaskStep, WorkflowIrNode } from "@fusion/core";
 
 import type { WorkflowNodeHandler, WorkflowNodeResult } from "./workflow-graph-executor.js";
@@ -71,6 +71,19 @@ export interface StepReviewSeamResult {
  *  which step they operate on and the per-instance baseline/checkpoint state. */
 export const FOREACH_ACTIVE_CONTEXT_KEY = "foreach:active";
 
+/**
+ * Reserved context key carrying the GOVERNING graph node id into the legacy
+ * coding seams (column-agent plan U4, R4). The execute seam reads the seam node's
+ * own id; the step-execute seam reads the foreach INSTANCE node id
+ * (`<foreachId>#<i>:<templateNodeId>`) so the core column-agent resolver can map
+ * it through template inheritance to the governing column's binding. The seam
+ * stamps it into a per-run executor slot before driving the implementation pass,
+ * so the binding the session runs under keys off the node's DECLARED IR column
+ * — never the task's current board lane. Custom (non-seam) nodes never use this:
+ * runGraphCustomNode receives its binding directly as a parameter (U3).
+ */
+export const SEAM_GOVERNING_NODE_CONTEXT_KEY = "workflow:seam-governing-node-id";
+
 /**
  * Reserved context marker set by the split sub-walk (`runSplitJoin`) for the
  * duration of its branches' execution and cleared at the join (KTD-4, U5). A
@@ -179,9 +192,24 @@ export function createPromptLikeHandler(
         // succeed — that would merge a task with no step work done.
         return { outcome: "failure", value: "step-execute-unwired" };
       }
+      // Column-agent seam wiring (U4, R4): the GOVERNING node for a step-execute
+      // session is the foreach INSTANCE node id, so the core resolver can map it
+      // through template inheritance to the enclosing foreach's bound column (or
+      // the template node's own column when it declares one). The template node id
+      // is THIS node's id; the foreach node id + step index come from the active
+      // instance context. Stamped so the seam threads it into the session build.
+      context.context[SEAM_GOVERNING_NODE_CONTEXT_KEY] = instanceNodeId(
+        active.foreachNodeId,
+        active.stepIndex,
+        node.id,
+      );
       return seams.stepExecute(context.task, context.context);
     }
     if (seam) {
+      // Column-agent seam wiring (U4, R4): for the execute seam the governing node
+      // IS the seam node, so its declared column drives the binding. (Other seams
+      // — planning/review/merge/schedule — stamp it too; only execute reads it.)
+      context.context[SEAM_GOVERNING_NODE_CONTEXT_KEY] = node.id;
       return seams[seam]!(context.task, context.context);
     }
     if (!runCustomNode) {