diff --git a/app/api/focused-investigation/deconstruct/route.js b/app/api/focused-investigation/deconstruct/route.js
index 32a0cd4..e9ce041 100644
--- a/app/api/focused-investigation/deconstruct/route.js
+++ b/app/api/focused-investigation/deconstruct/route.js
@@ -50,6 +50,23 @@ async function post(request) {
       );
     }
 
+    const REQUEST_LENGTH_LIMITS = {
+      answer: 10000,
+      question: 2048,
+      centralStatement: 2048,
+      targetLabel: 2048,
+      targetDescription: 2048,
+    };
+
+    for (const [field, limit] of Object.entries(REQUEST_LENGTH_LIMITS)) {
+      if (body[field] && body[field].length > limit) {
+        return Response.json(
+          { error: `Request field "${field}" exceeds maximum length of ${limit} characters` },
+          { status: 400 },
+        );
+      }
+    }
+
     const prompt = buildFocusedDeconstructPrompt({
       targetLabel: body.targetLabel,
       targetDescription: body.targetDescription,
diff --git a/components/reasoning-workspace.jsx b/components/reasoning-workspace.jsx
index 9d4a0e7..647d83a 100644
--- a/components/reasoning-workspace.jsx
+++ b/components/reasoning-workspace.jsx
@@ -30,6 +30,9 @@ function isTechnicalSummary(summary) {
   return false;
 }
 
+// ── Focused answer contract ──────────────────────────────────
+const FOCUSED_ANSWER_MAX_LENGTH = 10000;
+
 // ── Recovery state components (Phase 2) ───────────────────────
 
 function ProviderUnavailableCard({ onRestart }) {
@@ -238,8 +241,11 @@ function FocusedQuestionBody({
             {focused?.question?.trim() && processingStep !== "active" && focused.status === "formulated" && !hasAnswer && !hasActiveFollowUp && (
               <div data-testid="completed-narrative">
                 <label htmlFor={`rw-answer-${nodeId}`} className="mb-2 block text-sm font-medium text-gray-700">Your response</label>
-                <textarea id={`rw-answer-${nodeId}`} value={focusedAnswer} onChange={(e) => setFocusedAnswer(e.target.value)} rows={4} data-testid="response-textarea" className="w-full rounded-lg border border-gray-300 px-4 py-3 text-sm focus:border-gray-500 focus:outline-none focus:ring-2 focus:ring-gray-400 disabled:cursor-not-allowed disabled:opacity-60" placeholder="What do you know about this?" />
-                <button onClick={(e) => { e.stopPropagation(); handleDeconstructSubmit(nodeId, focusedAnswer); }} disabled={!focusedAnswer.trim() || processingStep === "active"} style={{ cursor: !focusedAnswer.trim() || processingStep === "active" ? "not-allowed" : "pointer" }} className="mt-3 rounded-lg border border-green-600 bg-white px-4 py-2 text-sm font-medium text-green-700 hover:bg-green-50 transition disabled:opacity-50">Submit response</button>
+                <textarea id={`rw-answer-${nodeId}`} value={focusedAnswer} onChange={(e) => setFocusedAnswer(e.target.value)} rows={4} maxLength={FOCUSED_ANSWER_MAX_LENGTH} data-testid="response-textarea" className="w-full rounded-lg border border-gray-300 px-4 py-3 text-sm focus:border-gray-500 focus:outline-none focus:ring-2 focus:ring-gray-400 disabled:cursor-not-allowed disabled:opacity-60" placeholder="What do you know about this?" />
+                <div className="flex items-center justify-between mt-2">
+                  <span className="text-xs text-gray-400">{focusedAnswer.length}/{FOCUSED_ANSWER_MAX_LENGTH}</span>
+                  <button onClick={(e) => { e.stopPropagation(); handleDeconstructSubmit(nodeId, focusedAnswer); }} disabled={!focusedAnswer.trim() || processingStep === "active"} style={{ cursor: !focusedAnswer.trim() || processingStep === "active" ? "not-allowed" : "pointer" }} className="rounded-lg border border-green-600 bg-white px-4 py-2 text-sm font-medium text-green-700 hover:bg-green-50 transition disabled:opacity-50">Submit response</button>
+                </div>
               </div>
             )}
 
diff --git a/docs/current-handoff.md b/docs/current-handoff.md
index 98f762c..c62509a 100644
--- a/docs/current-handoff.md
+++ b/docs/current-handoff.md
@@ -125,6 +125,15 @@ Jenkins SCM branch used to load the Jenkinsfile is conceptually separate from th
 - CSS Grid with responsive column placement replaces original flexbox; three DOM sections ensure correct mobile stacking without duplicating content
 - desktop two-column presentation (explanation left / sign-in right) preserved unchanged at `md` breakpoint and above
 
+## Focused-investigation input hardening
+
+- Focused answer now has a visible 10,000-character UI limit (maxLength + character counter).
+- Server enforces matching 10,000-character bound on `/api/focused-investigation/deconstruct`.
+- Other prompt-bearing fields retain explicit defensive bounds (2048 characters each).
+- Oversized/malformed requests are rejected before reasoning with controlled HTTP 400.
+- No punctuation/HTML/SQL-style content stripping introduced.
+- Reasoning semantics unchanged.
+
 ## CURRENT MVP DIRECTION
 
 Initial-decomposition hardening is frozen for the current MVP stage.
diff --git a/tests/focused-deconstruct-boundary.test.js b/tests/focused-deconstruct-boundary.test.js
index 9f87844..9fbea25 100644
--- a/tests/focused-deconstruct-boundary.test.js
+++ b/tests/focused-deconstruct-boundary.test.js
@@ -393,6 +393,174 @@ describe("focused-deconstruct targetNodeId identity boundary", () => {
     }
   });
 
+  it("rejects oversized focused answer with 400 and does not reach reasoning seam", async () => {
+    const generateReconstruction = vi.fn().mockResolvedValue({
+      response: {},
+      providerApiPath: "/api/chat",
+    });
+
+    vi.doMock("@/lib/llm/provider", () => ({
+      getProvider: () => ({ generateReconstruction }),
+      getProviderModelName: () => "configured-model",
+    }));
+
+    const largeAnswer = "x".repeat(10001);
+
+    const { POST } = await import("../app/api/focused-investigation/deconstruct/route.js");
+
+    const response = await POST(new Request("http://localhost/api/focused-investigation/deconstruct", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({
+        targetNodeId: "node-id",
+        targetLabel: "label",
+        targetDescription: "description",
+        centralStatement: "central statement",
+        question: "question?",
+        answer: largeAnswer,
+      }),
+    }));
+
+    expect(response.status).toBe(400);
+    const json = await response.json();
+    expect(json.error).toMatch(/answer.*exceeds maximum length/i);
+    expect(generateReconstruction).not.toHaveBeenCalled();
+  });
+
+  it("accepts focused answer at exact server max (10000) and reaches reasoning seam", async () => {
+    const generateReconstruction = vi.fn().mockResolvedValue({
+      response: {
+        targetNodeId: "node-id",
+        observations: [],
+        uncertainties: [],
+        assumptions: [],
+        relationships: [],
+        possibleFollowUpQuestions: [],
+      },
+      providerApiPath: "/api/chat",
+    });
+
+    vi.doMock("@/lib/llm/provider", () => ({
+      getProvider: () => ({ generateReconstruction }),
+      getProviderModelName: () => "configured-model",
+    }));
+
+    const exactMaxAnswer = "x".repeat(10000);
+
+    const { POST } = await import("../app/api/focused-investigation/deconstruct/route.js");
+
+    const response = await POST(new Request("http://localhost/api/focused-investigation/deconstruct", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({
+        targetNodeId: "node-id",
+        targetLabel: "label",
+        targetDescription: "description",
+        centralStatement: "central statement",
+        question: "question?",
+        answer: exactMaxAnswer,
+      }),
+    }));
+
+    expect(response.status).toBe(200);
+    const json = await response.json();
+    expect(json.success).toBe(true);
+    expect(generateReconstruction).toHaveBeenCalled();
+  });
+
+  it("rejects malformed required field (wrong type) with 400", async () => {
+    const generateReconstruction = vi.fn().mockResolvedValue({
+      response: {}, providerApiPath: "/api/chat",
+    });
+
+    vi.doMock("@/lib/llm/provider", () => ({
+      getProvider: () => ({ generateReconstruction }),
+      getProviderModelName: () => "configured-model",
+    }));
+
+    const { POST } = await import("../app/api/focused-investigation/deconstruct/route.js");
+
+    const response = await POST(new Request("http://localhost/api/focused-investigation/deconstruct", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({
+        targetNodeId: ["not-a-string"],
+        targetLabel: "label",
+        targetDescription: "description",
+        centralStatement: "central statement",
+        question: "question?",
+        answer: "answer.",
+      }),
+    }));
+
+    expect(response.status).toBe(400);
+    const json = await response.json();
+    expect(json.error).toMatch(/targetNodeId.*string/i);
+    expect(generateReconstruction).not.toHaveBeenCalled();
+  });
+
+  it("rejects oversized targetDescription with 400", async () => {
+    const generateReconstruction = vi.fn().mockResolvedValue({
+      response: {}, providerApiPath: "/api/chat",
+    });
+
+    vi.doMock("@/lib/llm/provider", () => ({
+      getProvider: () => ({ generateReconstruction }),
+      getProviderModelName: () => "configured-model",
+    }));
+
+    const { POST } = await import("../app/api/focused-investigation/deconstruct/route.js");
+
+    const response = await POST(new Request("http://localhost/api/focused-investigation/deconstruct", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({
+        targetNodeId: "node-id",
+        targetLabel: "label",
+        targetDescription: "x".repeat(2049),
+        centralStatement: "central statement",
+        question: "question?",
+        answer: "answer.",
+      }),
+    }));
+
+    expect(response.status).toBe(400);
+    const json = await response.json();
+    expect(json.error).toMatch(/targetDescription.*exceeds maximum length/i);
+    expect(generateReconstruction).not.toHaveBeenCalled();
+  });
+
+  it("rejects malformed centralStatement (number) with 400", async () => {
+    const generateReconstruction = vi.fn().mockResolvedValue({
+      response: {}, providerApiPath: "/api/chat",
+    });
+
+    vi.doMock("@/lib/llm/provider", () => ({
+      getProvider: () => ({ generateReconstruction }),
+      getProviderModelName: () => "configured-model",
+    }));
+
+    const { POST } = await import("../app/api/focused-investigation/deconstruct/route.js");
+
+    const response = await POST(new Request("http://localhost/api/focused-investigation/deconstruct", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({
+        targetNodeId: "node-id",
+        targetLabel: "label",
+        targetDescription: "description",
+        centralStatement: 12345,
+        question: "question?",
+        answer: "answer.",
+      }),
+    }));
+
+    expect(response.status).toBe(400);
+    const json = await response.json();
+    expect(json.error).toMatch(/centralStatement.*string/i);
+    expect(generateReconstruction).not.toHaveBeenCalled();
+  });
+
   it("returns a sanitized 503 when the provider is unavailable", async () => {
     const errorSpy = vi.spyOn(console, "error").mockImplementation(() => {});
     vi.doMock("@/lib/llm/provider", () => ({