diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 3727fffa..001a86aa 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -1341,6 +1341,70 @@ jobs:
           fi
           echo "integration-split under strict is GREEN, two-sided, with the arming control present"
 
+      # ---- P5f (f1), P5F-WIRE-COMPLETENESS.md §4: THE DUAL-BLOCK CENSUS LANE ---------------
+      #
+      # The same integration-split entries (plus Magma's own lane), re-run under
+      # MOBILEGL_IPC_ROLE_SPLIT_STATE=1: the client's residual fill writes a client-role block
+      # and the backend reads a server-role one, so every BARRIER-PULLED read aborts with a
+      # named Fatal{UnmigratedPipeInput, "<field>@<verb>"}. THE RED IS THE CENSUS - it is the
+      # machine-checked list of what still depends on the two roles sharing one object, and it
+      # is ratcheted two-sided against Harness/dualblock-expected-fatals.txt: a new pair is a
+      # debt somebody now owes, a vanished pair must be removed in the commit that retired it.
+      #
+      # THE STEP ALSO FAILS WHEN THE LANE IS GREEN while the expected file still has rows -
+      # "green" would mean the knob stopped arming, which is exactly the decay ID-122 assigns
+      # to a control rather than to a lane's good mood. At P5f's exit the file is empty and the
+      # green arm is the correct one; delete the red expectation in the same commit.
+      - name: Integration-split under the dual-block rehearsal - expected red, census ratcheted (P5f f1)
+        working-directory: build-split
+        env:
+          MOBILEGL_ITEST_REQUIRE_GPU: "1"
+          MOBILEGL_TRANSPORT: "inproc"
+          MOBILEGL_IPC_ROLE_SPLIT_STATE: "1"
+          MOBILEGL_IPC_STRICT_ERRORS: "1"
+        run: |
+          ulimit -c unlimited
+          helper="${GITHUB_WORKSPACE}/MobileGL/MG_IntegrationTest/Harness/split_log_paths.py"
+          expected="${GITHUB_WORKSPACE}/MobileGL/MG_IntegrationTest/Harness/dualblock-expected-fatals.txt"
+
+          ctest -L 'integration-split|integration-magma-split' --show-only=json-v1 \
+            > "${RUNNER_TEMP}/dualblock-lane.json"
+
+          # Stale logs from an earlier step would be read as this run's evidence (ID-119's rule).
+          python3 - "${RUNNER_TEMP}/dualblock-lane.json" "${helper}" <<'PY'
+          import importlib.util, json, os, sys
+          spec = importlib.util.spec_from_file_location("slp", sys.argv[2])
+          slp = importlib.util.module_from_spec(spec); spec.loader.exec_module(slp)
+          logs = slp.marker_log_paths(json.load(open(sys.argv[1])))
+          for path in set(logs.values()):
+              try:
+                  os.unlink(path)
+              except OSError:
+                  pass
+          print(f"reset {len(set(logs.values()))} private log(s) across {len(logs)} entries")
+          PY
+
+          rc=0
+          ctest -L 'integration-split|integration-magma-split' --no-tests=error -j 4 \
+            --output-junit "${RUNNER_TEMP}/dualblock.xml" \
+            > "${RUNNER_TEMP}/dualblock-split.txt" 2>&1 || rc=$?
+          tail -5 "${RUNNER_TEMP}/dualblock-split.txt"
+          echo "dual-block census run: rc=${rc}"
+
+          rows=$(grep -cvE '^\s*(#|$)' "${expected}")
+          if [ "${rc}" -eq 0 ] && [ "${rows}" -gt 0 ]; then
+            echo "::error::the dual-block census lane went GREEN with ${rows} expected pair(s) outstanding - the knob stopped arming, or the fields retired and dualblock-expected-fatals.txt was not emptied in the same commit"
+            exit 1
+          fi
+          python3 "${helper}" expect-fatal "${RUNNER_TEMP}/dualblock-lane.json" \
+            "${RUNNER_TEMP}/dualblock.xml" "${expected}"
+
+      # The mechanism's own negative control (P5F §6): the same named case must be green with
+      # the knob on and red - on its own assertion text - with it off.
+      - name: Dual-block negative control (P5f f1, P5F section 6)
+        working-directory: build-split
+        run: bash "${GITHUB_WORKSPACE}/scripts/ci/dualblock_negative_control.sh"
+
       # ---- P5e (gl), CONTRACT-P5E §7: MAGMA'S OWN LANE, EXPECTED RED -----------------------
       #
       # The two DirectVulkan.Split.NamedBlit entries abort on GetFramebufferBindingSlot@Clear
diff --git a/MobileGL/Config.h b/MobileGL/Config.h
index c4bb4db6..27807e22 100644
--- a/MobileGL/Config.h
+++ b/MobileGL/Config.h
@@ -535,6 +535,17 @@ namespace MobileGL::MG_Config {
         // build, the seven sticky forwards that are otherwise exempt - from "count it in
         // rsp" to Fatal (R-7.3).
         Bool StrictErrors = false;
+        // MOBILEGL_IPC_ROLE_SPLIT_STATE (P5f f1, P5F-WIRE-COMPLETENESS.md §4): the dual-block
+        // rehearsal. 1 = the client's residual fill writes a CLIENT-ROLE PipeInputs block and
+        // the backend/applier keep reading the SERVER-ROLE one, so every path that today works
+        // only because the two roles share one object turns into a named
+        // Fatal{UnmigratedPipeInput, "<field>@<verb>"} instead of a silent cross-role read.
+        // Meaningless under monolith transport (the two roles are one thread there, so the
+        // selection folds to the single shared block) and forced off by MOBILEGL_PIPE_VERIFY
+        // (the comparator owns the one fill block it compares against). 0 is not merely the
+        // default, it is the negative control: the lane's distinctness case must go red
+        // without it (P5F §6).
+        Bool RoleSplitState = false;
         // MOBILEGL_IPC_AUDIT: after a record retires, the server fills the SEG_STAGE bytes
         // it referenced with 0xDD (R-2.5). This is the ONLY mechanical control that an
         // inproc implementation did not quietly keep using a pointer past its lifetime.
diff --git a/MobileGL/ConfigLoader.cpp b/MobileGL/ConfigLoader.cpp
index 5d4d6a9d..a9686a22 100644
--- a/MobileGL/ConfigLoader.cpp
+++ b/MobileGL/ConfigLoader.cpp
@@ -393,17 +393,28 @@ namespace MobileGL::MG_ConfigLoader {
         ipc.StrictErrors = QueryEnvFlag("MOBILEGL_IPC_STRICT_ERRORS");
         ipc.Audit = QueryEnvFlag("MOBILEGL_IPC_AUDIT");
         QueryEnvVariable("MOBILEGL_IPC_SERVER_AFFINITY", ipc.ServerAffinity, "auto");
+        // P5f f1: the dual-block rehearsal (P5F-WIRE-COMPLETENESS.md §4). Forced OFF by the
+        // verify harness: the comparator's entry compare and compare-at-read hook are built on
+        // there being ONE filled block (the hook pins itself to &gPipeInputs,
+        // PipeFill.cpp's MGPipeVerifyReadHook), which is exactly what the rehearsal splits.
+        ipc.RoleSplitState = QueryEnvFlag("MOBILEGL_IPC_ROLE_SPLIT_STATE");
+        if (MG_Config::Features.PipeVerify && ipc.RoleSplitState) {
+            MGLOG_W("Config: MOBILEGL_IPC_ROLE_SPLIT_STATE=1 is incompatible with "
+                    "MOBILEGL_PIPE_VERIFY (the comparator owns the single fill block); "
+                    "the dual-block rehearsal is OFF for this run");
+            ipc.RoleSplitState = false;
+        }
 
         if (MG_Config::Transport == MG_Config::TransportMode::Monolith) return;
         // One line, on the arm where these numbers decide behaviour, because every one of
         // them is a number a bug report has to quote.
         MGLOG_I("Config: IPC ring=%uMiB stage=%uMiB spin=%uus persistent-block=%uKiB "
                 "adopt-tier=%u verb-barrier=%u run-ahead=%u present-credit=%u strict=%d "
-                "audit=%d affinity='%s'",
+                "audit=%d role-split-state=%d affinity='%s'",
                 ipc.RingMb, ipc.StageMb, ipc.SpinUs, ipc.PersistentBlockKb, ipc.AdoptTier,
                 ipc.VerbBarrier, ipc.RunAhead, ipc.PresentCredit,
                 static_cast<int>(ipc.StrictErrors), static_cast<int>(ipc.Audit),
-                ipc.ServerAffinity.c_str());
+                static_cast<int>(ipc.RoleSplitState), ipc.ServerAffinity.c_str());
         if (ipc.VerbBarrier == 0) {
             MGLOG_W("Config: MOBILEGL_IPC_VERB_BARRIER=0 is the R-1 NEGATIVE CONTROL and is "
                     "expected to fail: the client still pulls 31 of 63 PipeInputs fields from a "
diff --git a/MobileGL/MG_Backend/MGPipe/PipeInputs.cpp b/MobileGL/MG_Backend/MGPipe/PipeInputs.cpp
index 40d37938..d8bcba59 100644
--- a/MobileGL/MG_Backend/MGPipe/PipeInputs.cpp
+++ b/MobileGL/MG_Backend/MGPipe/PipeInputs.cpp
@@ -160,6 +160,20 @@ namespace MobileGL::MG_Pipe {
         // MGPipeBarrierPullAdmitted is generated from those three tables (ID-116); there is no
         // list here to drift.
         void CountBarrierPull(MGPipeInputField field, MGPipeVerb verb) {
+            // P5f (f1), P5F-WIRE-COMPLETENESS.md §4: UNDER THE DUAL-BLOCK REHEARSAL EVERY
+            // BARRIER-PULLED READ IS A NAMED FATAL, unconditionally - ahead of the barriered
+            // question, ahead of the strict knob, ahead of the counter. The server block has no
+            // residual fill to answer from, so the value the read would return is not "torn"
+            // or "stale" but ABSENT: the field's stamp was withdrawn by the server's own verb
+            // boundary and nothing on this side of the role split ever writes it. Letting the
+            // read proceed would hand the backend default storage - or, for the O-class rows
+            // with no null check, an empty SharedPtr to dereference - which is an UNNAMED
+            // crash exactly where the rehearsal exists to produce a named one. The red is the
+            // census (Harness/dualblock-expected-fatals.txt), so the marker keeps the strict
+            // grammar with the knob's own name as the reason.
+            if (MGPipeRoleSplitActive()) {
+                StrictBarrierPullFatal(field, verb, "MOBILEGL_IPC_ROLE_SPLIT_STATE=1");
+            }
             if (!MGPipeApplierCurrentRecordIsBarriered()) {
                 StrictBarrierPullFatal(field, verb, "UNBARRIERED, the client did not fill it");
             }
@@ -260,8 +274,50 @@ namespace MobileGL::MG_Pipe {
         static void SetServerStamped(PipeInputs& inputs, Bool stamped) {
             inputs.m_serverStampedVerb = stamped;
         }
+        // P5f (f1): the server block's identity half of the stamp door (CONTRACT-P5E §3.2).
+        // The client's SetIdentity lives in MG_Impl's MGPipeFillAccess and writes the CLIENT
+        // block now; this one is how the server block learns which served context it describes
+        // without MG_Impl being in the process at all.
+        static void SetIdentity(PipeInputs& inputs, Bool live, const void* identity) {
+            inputs.m_live = live;
+            inputs.m_contextIdentity = identity;
+        }
     };
 
+    // ---- P5f (f1): the dual-block rehearsal's selection functions (PipeInputs.h) -----------
+    //
+    // MGPipeRoleSplitActive is deliberately NOT latched: it is two global loads, asked once per
+    // verb on the fill side and once per verb boundary on the stamp side, and a latch is a
+    // second thing a test that flips the knob mid-process would have to know about.
+    Bool MGPipeRoleSplitActive() {
+        return MG_Config::Ipc.RoleSplitState &&
+               MG_Config::Transport != MG_Config::TransportMode::Monolith;
+    }
+
+    PipeInputs& MGPipeClientInputs() {
+        return MGPipeRoleSplitActive() ? gPipeInputsClientBlock : gPipeInputs;
+    }
+
+    void MGPipeClientClearVerbBoundary() {
+        MGPipeStampAccess::SetServerStamped(MGPipeClientInputs(), false);
+    }
+
+    void MGPipeServerBlockNoteIdentity() {
+        if (!MGPipeRoleSplitActive()) return;
+        // The server cannot name the client's GLContext - under a real transport it is in
+        // another process - so the identity the server block carries is the server's OWN
+        // served-context clock: MGPipeApplierContextSerial moves exactly when the served
+        // context does (MGPipeApplierReset), which is all the backends' per-context memo keys
+        // ask of an identity. The odd-encoding keeps the token non-null at serial 0: a null
+        // identity reads as a hit against DirectGLES' zero-initialised fb-slot memo cache and
+        // hands out a null slot - an unnamed crash where the rehearsal exists to produce a
+        // named one.
+        const Uint64 serial = MGPipeApplierContextSerial();
+        MGPipeStampAccess::SetIdentity(gPipeInputs, true,
+                                       reinterpret_cast<const void*>(static_cast<std::uintptr_t>(
+                                           (serial << 1) | Uint64{1})));
+    }
+
     void MGPipeServerStampVerbBoundary(MGPipeVerb verb) {
         PipeInputs& inputs = gPipeInputs;
         // P5e (gl), ID-115: THE POSITIVE CONTROL'S ONLY HONEST SIGNAL, and it belongs HERE
@@ -302,6 +358,11 @@ namespace MobileGL::MG_Pipe {
             filled.FilledGen[i] = serial & (Uint64{0} - bit);
         }
         MGPipeStampAccess::SetServerStamped(inputs, true);
+        // P5f (f1): the server block's identity rides the stamp (CONTRACT-P5E §3.2's
+        // "SetIdentity moves to ApplyOne" - the stamp is the per-verb half of ApplyOne, and
+        // refreshing here rather than once at Attach keeps the token in step with
+        // MGPipeApplierContextSerial across a context switch). A no-op with the rehearsal off.
+        MGPipeServerBlockNoteIdentity();
     }
 
     void MGPipeServerClearVerbBoundary() { MGPipeStampAccess::SetServerStamped(gPipeInputs, false); }
diff --git a/MobileGL/MG_Backend/MGPipe/PipeInputs.h b/MobileGL/MG_Backend/MGPipe/PipeInputs.h
index 26d3daf9..6f7cf80f 100644
--- a/MobileGL/MG_Backend/MGPipe/PipeInputs.h
+++ b/MobileGL/MG_Backend/MGPipe/PipeInputs.h
@@ -834,6 +834,61 @@ namespace MobileGL::MG_Pipe {
     // (P3a; the exit-time heap corruption this closes is p3a-results/exit-order-v1.md.)
     inline PipeInputs& gPipeInputs = *new PipeInputs();
 
+#if MOBILEGL_BUILD_DISAGGREGATED
+    // ============================================================================
+    // P5f (f1), P5F-WIRE-COMPLETENESS.md §4: THE DUAL-BLOCK REHEARSAL
+    // ============================================================================
+    //
+    // One gPipeInputs served both roles because the verb barrier (R-1) made "the client filled
+    // it" and "the server is reading it" mutually exclusive in TIME on the same object. The
+    // rehearsal removes the same-object half of that sentence: under
+    // MOBILEGL_IPC_ROLE_SPLIT_STATE=1 the client's residual fill writes the CLIENT block below
+    // and the backend/applier keep reading gPipeInputs, which is the SERVER block. Every path
+    // that worked only because the two were one object - a BARRIER-PULLED field, a sticky
+    // forward's read of the fill side's stamps - then has no value to read, and the read
+    // becomes a NAMED Fatal{UnmigratedPipeInput, "<field>@<verb>"} (CountBarrierPull's
+    // dual-block arm, PipeInputs.cpp) instead of a silent cross-role answer.
+    //
+    // THE READ SIDE'S SPELLING DOES NOT MOVE. gPipeInputs remains the server-role block, so the
+    // 379 MGB_CTX sites and PipeApply.cpp's applier writes are untouched; the only new spelling
+    // is on the fill side (MG_Impl/Pipe/PipeFill.cpp), which asks MGPipeClientInputs(). Under
+    // monolith transport - every unit and integration-gpu lane of a split build - the selection
+    // folds back to the single shared block and behaviour is byte-for-byte the old one.
+    //
+    // Same leak-at-exit storage as gPipeInputs above, for the same reason.
+    inline PipeInputs& gPipeInputsClientBlock = *new PipeInputs();
+
+    // PipeInputs.cpp. Whether the rehearsal is armed: the knob AND a real transport (and not
+    // the verify build, which ConfigLoader forces off). Constant for the life of the process.
+    Bool MGPipeRoleSplitActive();
+    // PipeInputs.cpp. THE FILL SIDE'S ONE NEW SPELLING: the client block when the rehearsal is
+    // armed, gPipeInputs otherwise. Everything in MG_Impl/Pipe/PipeFill.cpp that used to spell
+    // gPipeInputs spells this instead.
+    PipeInputs& MGPipeClientInputs();
+    // PipeInputs.cpp. The client-role half of MGPipeServerClearVerbBoundary: clears the stamp
+    // flag on the FILL side's block. With the rehearsal off that IS gPipeInputs, so the two
+    // client call sites keep their old semantics exactly; with it on the client block's flag is
+    // never raised (nothing server-stamps it) and the clear is a no-op - which is the point:
+    // the client no longer reaches into the server's block at all.
+    void MGPipeClientClearVerbBoundary();
+    // PipeInputs.cpp. CONTRACT-P5E §3.2's other half: the server block's identity is
+    // SERVER-OWNED. Called from PipeApplier::Attach and from MGPipeServerStampVerbBoundary;
+    // with the rehearsal armed it sets m_live and points m_contextIdentity at a token derived
+    // from MGPipeApplierContextSerial() - the server's own served-context clock, which is what
+    // moves when the served context does. Without it the server block's ContextIdentity() would
+    // stay nullptr, and DirectGLES' fb-slot memo cache compares identity FIRST (a nullptr
+    // against its own nullptr initialiser reads as a hit and hands out a null slot): an
+    // unnamed crash where the rehearsal exists to produce a named one. A no-op with the
+    // rehearsal off, so the client's per-verb SetIdentity keeps owning the shared block there.
+    void MGPipeServerBlockNoteIdentity();
+#else
+    // The push-without-transport build has one role and one block, so the fill side's spelling
+    // folds onto gPipeInputs and PipeFill.cpp reads identically in both build flavours. An
+    // inline that no caller in such a build ever has a reason to call twice - the disaggregated
+    // arm above is the real one.
+    inline PipeInputs& MGPipeClientInputs() { return gPipeInputs; }
+#endif
+
     // Every field has storage or is forwarded, and nothing else.
 #define MGP_INPUT_COUNT_ONE(Field, Member) +1
     static_assert(0 MGP_INPUT_STORAGE_LIST(MGP_INPUT_COUNT_ONE) + kMGPipeForwardedFieldCount == kMGPipeInputFieldCount,
diff --git a/MobileGL/MG_Impl/Pipe/PipeFill.cpp b/MobileGL/MG_Impl/Pipe/PipeFill.cpp
index 72ff9c24..3769e5f6 100644
--- a/MobileGL/MG_Impl/Pipe/PipeFill.cpp
+++ b/MobileGL/MG_Impl/Pipe/PipeFill.cpp
@@ -624,9 +624,9 @@ namespace MobileGL::MG_Pipe {
         }
 
         void ReportDivergence(MGPipeInputField field, const char* where) {
-            const Uint64 serial = MGPipeFillAccess::Filled(gPipeInputs).CurrentVerbSerial;
+            const Uint64 serial = MGPipeFillAccess::Filled(MGPipeClientInputs()).CurrentVerbSerial;
             MGLOG_F("MGPipe: Fatal{PipeVerifyDiffer, \"%s@%s\", verb=%llu, where=%s}",
-                    kMGPipeInputFieldNames[static_cast<SizeT>(field)], MGPipeVerbName(gPipeInputs.CurrentVerb()),
+                    kMGPipeInputFieldNames[static_cast<SizeT>(field)], MGPipeVerbName(MGPipeClientInputs().CurrentVerb()),
                     static_cast<unsigned long long>(serial), where);
             if (g_verify.Fatal) std::abort();
             g_verify.Divergences.fetch_add(1, std::memory_order_relaxed);
@@ -650,7 +650,7 @@ namespace MobileGL::MG_Pipe {
     void SnapshotFromGLContext(PipeInputs& snapshot, const MGPipeFieldMask& mask) {
         auto* ctx = LiveContext();
         MGPipeFillAccess::SetIdentity(snapshot, ctx);
-        MGPipeFillAccess::SetVerb(snapshot, gPipeInputs.CurrentVerb());
+        MGPipeFillAccess::SetVerb(snapshot, MGPipeClientInputs().CurrentVerb());
         if (ctx == nullptr) return;
         for (SizeT i = 0; i < kMGPipeInputFieldCount; ++i) {
             const auto field = static_cast<MGPipeInputField>(i);
@@ -700,7 +700,7 @@ namespace MobileGL::MG_Pipe {
     // filled must stay Fatal{UnmigratedPipeInput} on the next read rather than be healed by
     // an unrelated frontend write.
     void MGPipeNoteFrontendMutation(MGPipeInputField field) {
-        PipeInputs& inputs = gPipeInputs;
+        PipeInputs& inputs = MGPipeClientInputs();
 #if MOBILEGL_BUILD_DISAGGREGATED
         // P5e (ra, CONTRACT-P5E §3.1): THIS REFRESHES ONE FIELD OF THE LAST FILL, so with no
         // fill behind it there is nothing to refresh. Under run-ahead the last verb may have
@@ -1214,7 +1214,7 @@ namespace MobileGL::MG_Pipe {
     // fill only for UNBARRIERED records); an unbarriered apply may not read it at all, and
     // §3.3's detector is what says so by name. Clearing the stamps as well would trade that
     // named abort for the poison Fatal, which names the field but not the rule it broke.
-    void MGPipeReleaseResidualFillPins() { MGPipeFillAccess::ReleaseObjectPins(gPipeInputs); }
+    void MGPipeReleaseResidualFillPins() { MGPipeFillAccess::ReleaseObjectPins(MGPipeClientInputs()); }
 
     // P5e (ra), CONTRACT-P5E §2.5. Declared in PipeMutation.h; see there for why it is not a
     // ClientSession call at the GL entry point.
@@ -2202,7 +2202,7 @@ namespace MobileGL::MG_Pipe {
     Uint32 MGPipePendingBaseInstance() { return MGPipeTrackerInstance().PendingBaseInstance(); }
 
     void MGPipeLeaveVerb() {
-        PipeInputs& inputs = gPipeInputs;
+        PipeInputs& inputs = MGPipeClientInputs();
 #if MOBILEGL_BUILD_DISAGGREGATED
         // P5e (ra, §3.1): a verb whose fill was skipped has no stamps of this thread's to
         // retire, and the bump below would move a serial the SERVER's stamp owns. Leave
@@ -2225,7 +2225,7 @@ namespace MobileGL::MG_Pipe {
         ++MGPipeFillAccess::Filled(inputs).CurrentVerbSerial;
 #endif
 #if MOBILEGL_BUILD_DISAGGREGATED
-        MGPipeServerClearVerbBoundary();
+        MGPipeClientClearVerbBoundary();
 #endif
         MGPipeFillAccess::SetVerb(inputs, MGPipeVerb::kVerbCount);
         // The pending base instance belongs to the verb that was about to run, so leaving
@@ -3013,7 +3013,7 @@ namespace MobileGL::MG_Pipe {
 #if MOBILEGL_BUILD_DISAGGREGATED
             if (ClientRunsAhead()) return sizeof(MGPVertexAttribDefaults) + header.Count * sizeof(MGPAttribValue);
 #endif
-            const auto* mirror = MGPipeFillAccess::VertexAttribDefaultsOf(gPipeInputs);
+            const auto* mirror = MGPipeFillAccess::VertexAttribDefaultsOf(MGPipeClientInputs());
             Bool reproduced = true;
             for (SizeT i = 0; i < kAttribs && reproduced; ++i) {
                 if ((header.Mask & (Uint32{1} << static_cast<Uint32>(i))) == 0) continue;
@@ -3024,7 +3024,7 @@ namespace MobileGL::MG_Pipe {
                 MGLOG_W_ONCE("MGPipe: MGPipeApplySetVertexAttribDefaults did not reproduce the "
                              "carried three views on this build - the client is keeping "
                              "m_currentVertexAttribute authoritative");
-                MGPipeFillAccess::CopyField(gPipeInputs, ctx, MGPipeInputField::GetCurrentVertexAttribute);
+                MGPipeFillAccess::CopyField(MGPipeClientInputs(), ctx, MGPipeInputField::GetCurrentVertexAttribute);
             }
             return sizeof(MGPVertexAttribDefaults) + header.Count * sizeof(MGPAttribValue);
         }
@@ -3345,7 +3345,10 @@ namespace MobileGL::MG_Pipe {
 
     // ---- the validate point (P2 brief D1) ----
     void MGPipeValidateForVerb(MGPipeVerb verb) {
-        PipeInputs& inputs = gPipeInputs;
+        // P5f (f1): the fill side's one new spelling. With MOBILEGL_IPC_ROLE_SPLIT_STATE=1 this
+        // is the CLIENT-role block and gPipeInputs is the server's alone; off, or under
+        // monolith transport, it folds back onto gPipeInputs and nothing below changes.
+        PipeInputs& inputs = MGPipeClientInputs();
 #if MOBILEGL_BUILD_DISAGGREGATED
         // ---- P5e (ra), CONTRACT-P5E §3: WHO OWNS gPipeInputs FOR THIS VERB ----------------
         //
@@ -3434,7 +3437,14 @@ namespace MobileGL::MG_Pipe {
             // while the apply thread is inside a record that depends on it. For an unbarriered
             // verb the stamp is not touched at all, and the applier's own LeaveApplier is then
             // its only writer.
-            MGPipeServerClearVerbBoundary();
+            //
+            // P5f (f1): this is now the CLIENT block's flag (MGPipeClientClearVerbBoundary).
+            // With the rehearsal off both names denote the one shared block and the semantics
+            // above are unchanged; with it on the client block's flag is never raised and the
+            // clear is a no-op, and withdrawing the SERVER's stamp is the applier's job alone
+            // (PipeApplier::LeaveApplier) - which is CONTRACT-P5E §3.2's per-role stamp
+            // ownership, landed as the dual block rather than as moved fields.
+            MGPipeClientClearVerbBoundary();
 #endif
             MGPipeFillAccess::SetVerb(inputs, verb);
         }
diff --git a/MobileGL/MG_IntegrationTest/CMakeLists.txt b/MobileGL/MG_IntegrationTest/CMakeLists.txt
index 87e49e2e..1f2a80d4 100644
--- a/MobileGL/MG_IntegrationTest/CMakeLists.txt
+++ b/MobileGL/MG_IntegrationTest/CMakeLists.txt
@@ -2496,3 +2496,58 @@ if (MOBILEGL_BUILD_DISAGGREGATED)
             ENVIRONMENT "${MGL_ITEST_GLES_SPLIT_STRICT_ARMING_ENVIRONMENT}"
     )
 endif()
+
+# =====================================================================================
+# P5f (f1), P5F-WIRE-COMPLETENESS.md §4/§6: THE DUAL-BLOCK REHEARSAL'S OWN LANE
+# =====================================================================================
+#
+# The lane has TWO halves, and only one of them registers entries here:
+#
+#   - the CENSUS half is an environment-variable lane, exactly like strict: the gate runs the
+#     EXISTING `integration-split` entries under MOBILEGL_IPC_ROLE_SPLIT_STATE=1 +
+#     MOBILEGL_IPC_STRICT_ERRORS=1, and every BARRIER-PULLED read there aborts with a named
+#     Fatal{UnmigratedPipeInput, "<field>@<verb>"} - the red IS the census, collected and
+#     ratcheted against Harness/dualblock-expected-fatals.txt by split_log_paths.py's
+#     `expect-fatal` mode. Re-registering the whole set under a second prefix would double the
+#     discovered test count for zero new information.
+#
+#   - the CONTROL half is the one entry below: DualBlockScenario is the mechanism's negative
+#     control (§6: turning the knob OFF must turn a named case from green to red), so it needs
+#     an entry of its own, under a label of its own.
+#
+# THE LABEL IS `integration-dualblock-split`, and the spelling is load-bearing twice over
+# (ID-131): `ctest -L` is a REGEX, so a label CONTAINING `integration-split` as a substring
+# would be selected by the main lane's runs - and this one is expected-red there, which must
+# never poison the hard-green gate. Checked both directions: `-L integration-split` does not
+# match it (`dualblock` stands between the two words), and `-L integration-dualblock-split`
+# matches nothing else in the tree. It does not carry `integration-gpu` either: that label must
+# be able to go green, and this entry goes green only under the armed knob.
+#
+# THE KNOB IS NOT PINNED IN THE ENTRY'S ENVIRONMENT, deliberately, and that is the ONE deviation
+# from the N-6 pinning rule every other split entry follows: a ctest ENVIRONMENT property
+# overrides the process environment, so pinning MOBILEGL_IPC_ROLE_SPLIT_STATE=1 here would make
+# the knob=0 form - which IS the negative control - unreachable through ctest. The failure
+# direction is safe: a bare `-L integration-dualblock-split` run without the knob is RED by
+# name, never silently green.
+#
+# MOBILEGL_IPC_RUN_AHEAD=0 IS pinned: the serial-separation half asserts on the fill side's
+# behaviour per verb boundary, and under run-ahead an unbarriered verb fills nothing, which
+# would make the assertion timing-dependent instead of deterministic.
+if (MOBILEGL_BUILD_DISAGGREGATED)
+    target_sources(MobileGLIntegrationTest PRIVATE
+        Scenarios/DualBlockScenario.cpp Harness/DualBlockPeek.cpp)
+    mgl_itest_join_environment(MGL_ITEST_GLES_DUALBLOCK_ENVIRONMENT
+        "MOBILEGL_BACKEND_TYPE=DirectGLES" "MOBILEGL_TRANSPORT=inproc"
+        "MOBILEGL_IPC_SERVER_PATH=${MGL_ITEST_SPLIT_SERVER_PATH}" "MGITEST_SPLIT_LANE=1"
+        "MOBILEGL_IPC_RUN_AHEAD=0"
+        ${MGL_ITEST_CAPABILITY_ENV} ${MGL_ITEST_COMMON_ENV})
+    gtest_discover_tests(MobileGLIntegrationTest
+        TEST_PREFIX "DirectGLES.Split.DualBlock."
+        TEST_FILTER "DualBlockScenario.*"
+        DISCOVERY_TIMEOUT 30
+        PROPERTIES
+            LABELS "integration-dualblock-split"
+            TIMEOUT ${MGL_ITEST_TIMEOUT}
+            ENVIRONMENT "${MGL_ITEST_GLES_DUALBLOCK_ENVIRONMENT}\;MOBILEGL_LOG_FILE_PATH=${CMAKE_CURRENT_BINARY_DIR}/split-logs/DirectGLES.Split.DualBlock.TheTwoRolesHaveDistinctBlocks.log"
+    )
+endif()
diff --git a/MobileGL/MG_IntegrationTest/Harness/DualBlockPeek.cpp b/MobileGL/MG_IntegrationTest/Harness/DualBlockPeek.cpp
new file mode 100644
index 00000000..87d062aa
--- /dev/null
+++ b/MobileGL/MG_IntegrationTest/Harness/DualBlockPeek.cpp
@@ -0,0 +1,60 @@
+// MobileGL - MobileGL/MG_IntegrationTest/Harness/DualBlockPeek.cpp
+// Copyright (c) 2026 MobileGL-Dev
+// Licensed under the GNU Lesser General Public License v3.0:
+//   https://www.gnu.org/licenses/gpl-3.0.txt
+//   https://www.gnu.org/licenses/lgpl-3.0.txt
+// SPDX-License-Identifier: LGPL-3.0-only
+// End of Source File Header
+
+#include "DualBlockPeek.h"
+
+#if defined(MGITEST_SPLIT_RUNTIME_PEEK) && !defined(__ANDROID__)
+#include <MG_Pipe/MGPipe.h>
+#if MOBILEGL_PIPE_PUSH
+#include <MG_Backend/MGPipe/PipeInputs.h>
+#include <MG_Impl/Pipe/PipeFill.h>
+// The serials live behind FilledState(), which exists only where the poison build flag is on -
+// and PipeInputs.h derives that flag, so the question can only be asked AFTER the include. A
+// disaggregated build always has it (MOBILEGL_BUILD_DISAGGREGATED is one of its three arms).
+#if MOBILEGL_PIPE_POISON
+#define MGITEST_DUAL_BLOCK_PEEK_LIVE 1
+#endif
+#endif
+#endif
+
+namespace MGITest {
+
+#if defined(MGITEST_DUAL_BLOCK_PEEK_LIVE)
+    // The serials live behind the poison build's FilledState(); the peek is live exactly in
+    // the configurations where MOBILEGL_PIPE_POISON is (a disaggregated build always has it,
+    // which is what the #if above says), so an answer here is never a different build's guess.
+    bool PeekDualBlockState(DualBlockPeekState* out) {
+        if (out == nullptr) return false;
+        namespace MGP = MobileGL::MG_Pipe;
+        out->peekAvailable = true;
+        out->roleSplitActive = MGP::MGPipeRoleSplitActive();
+        MGP::PipeInputs& client = MGP::MGPipeClientInputs();
+        out->blocksDistinct = &client != &MGP::gPipeInputs;
+        out->clientSerial = client.FilledState().CurrentVerbSerial;
+        out->serverSerial = MGP::gPipeInputs.FilledState().CurrentVerbSerial;
+        out->serverIdentity = MGP::gPipeInputs.ContextIdentity();
+        return true;
+    }
+
+    bool DualBlockDriveClientVerbBoundary() {
+        // ReadPixels: a verb whose class mask carries the pixel-pack state the scenario dirties
+        // (so the emission step has a record to publish and the fixture's wire-moved assertion
+        // is fed), while the read_pixels VERB RECORD itself is never emitted - that is the GL
+        // entry point's job, not the validate point's - so nothing here reaches the server's
+        // stamp path (Clear / DrawVbo / ReadPixels / Blit) and the server block's serial is
+        // provably not this call's to move.
+        MobileGL::MG_Pipe::MGPipeValidateForVerb(MobileGL::MG_Pipe::MGPipeVerb::ReadPixels);
+        MobileGL::MG_Pipe::MGPipeLeaveVerb();
+        return true;
+    }
+#else
+    bool PeekDualBlockState(DualBlockPeekState*) { return false; }
+    bool DualBlockDriveClientVerbBoundary() { return false; }
+#endif
+
+} // namespace MGITest
diff --git a/MobileGL/MG_IntegrationTest/Harness/DualBlockPeek.h b/MobileGL/MG_IntegrationTest/Harness/DualBlockPeek.h
new file mode 100644
index 00000000..20c475e9
--- /dev/null
+++ b/MobileGL/MG_IntegrationTest/Harness/DualBlockPeek.h
@@ -0,0 +1,54 @@
+// MobileGL - MobileGL/MG_IntegrationTest/Harness/DualBlockPeek.h
+// Copyright (c) 2026 MobileGL-Dev
+// Licensed under the GNU Lesser General Public License v3.0:
+//   https://www.gnu.org/licenses/gpl-3.0.txt
+//   https://www.gnu.org/licenses/lgpl-3.0.txt
+// SPDX-License-Identifier: LGPL-3.0-only
+// End of Source File Header
+//
+// P5f (f1), P5F-WIRE-COMPLETENESS.md §4/§6: THE DUAL-BLOCK REHEARSAL, READ FROM THE RUNNING
+// PROCESS. The lane's negative control is a scenario that is green exactly when the two roles
+// hold two DISTINCT PipeInputs blocks, and "the blocks are distinct" is a fact about pointers
+// and serials inside the library - so, like SplitRuntimePeek, the reading is taken from the
+// process and never inferred from the environment the entry was launched with.
+//
+// A SEPARATE TRANSLATION UNIT for PipeApplyPeek's reason, verbatim: the scenario sources
+// include the GL headers with prototypes and PipeInputs.h is not meant to meet them in one
+// file.
+//
+// PeekDualBlockState returns false, touching nothing, where it cannot look: a build without
+// MOBILEGL_BUILD_DISAGGREGATED (the block split is compiled there only), or Android, where
+// this module links the shipping libMobileGL.so built -fvisibility=hidden. A caller that gets
+// false has learned NOTHING and must skip, not fail.
+
+#pragma once
+
+namespace MGITest {
+
+    struct DualBlockPeekState {
+        // False wherever the peek cannot look (see the header above). Everything else is
+        // meaningful only when this is true.
+        bool peekAvailable = false;
+        // MGPipeRoleSplitActive(): the knob AND a real transport. The control's red half turns
+        // on this answer alone.
+        bool roleSplitActive = false;
+        // &MGPipeClientInputs() != &gPipeInputs - the fill side and the read side are two
+        // different objects.
+        bool blocksDistinct = false;
+        // The two blocks' CurrentVerbSerial. The client block's advances with the fill side's
+        // verb boundaries; the server block's moves only on a server verb stamp.
+        unsigned long long clientSerial = 0;
+        unsigned long long serverSerial = 0;
+        // gPipeInputs.ContextIdentity(): non-null once the server has stamped a verb boundary
+        // under the rehearsal (CONTRACT-P5E §3.2's server-owned identity).
+        const void* serverIdentity = nullptr;
+    };
+
+    bool PeekDualBlockState(DualBlockPeekState* out);
+
+    // Drives ONE client-side verb boundary - MGPipeValidateForVerb + MGPipeLeaveVerb - with no
+    // draw behind it: exactly the fill side's write shape (serial bump, fill, verb set), and
+    // nothing that stamps the server block. Returns false where the peek cannot look.
+    bool DualBlockDriveClientVerbBoundary();
+
+} // namespace MGITest
diff --git a/MobileGL/MG_IntegrationTest/Harness/dualblock-expected-fatals.txt b/MobileGL/MG_IntegrationTest/Harness/dualblock-expected-fatals.txt
new file mode 100644
index 00000000..0b230f0c
--- /dev/null
+++ b/MobileGL/MG_IntegrationTest/Harness/dualblock-expected-fatals.txt
@@ -0,0 +1,30 @@
+# The Fatal{UnmigratedPipeInput} pairs the dual-block lane ACTUALLY PRODUCES under
+# MOBILEGL_IPC_ROLE_SPLIT_STATE=1 + MOBILEGL_IPC_STRICT_ERRORS=1, one `<field>@<verb>` per line.
+#
+# P5f (f1), P5F-WIRE-COMPLETENESS.md §4. This is the strict-expected-markers.txt shape turned
+# around: the strict lane ratchets ADMITTED pulls (debts a phase has agreed to leave standing);
+# this file ratchets the FATAL census of the dual-block rehearsal, which is the full list of
+# "this read only ever worked because both roles shared one gPipeInputs". Every entry in the
+# census lane aborts on its FIRST such read, so the pairs below are first-blockers, and retiring
+# one advances the entry to its next one - the file shrinks package by package (fm/fs/fr/fv)
+# and P5f's exit is this file empty with the lane green.
+#
+# TWO-SIDED, same rule as the strict file (ID-119): split_log_paths.py's `expect-fatal` mode
+# fails when a pair appears that is not listed here AND when a pair listed here no longer
+# appears. Remove a row in the commit that retires it.
+#
+# HOW TO REFRESH (never by hand from a guess):
+#   ctest --test-dir build-split -L 'integration-split|integration-magma-split' \
+#       --show-only=json-v1 > lane.json
+#   # reset the private logs, then:
+#   MOBILEGL_IPC_ROLE_SPLIT_STATE=1 MOBILEGL_IPC_STRICT_ERRORS=1 \
+#       ctest --test-dir build-split -L 'integration-split|integration-magma-split' -j 4 \
+#       --output-junit dualblock.xml
+#   python3 MobileGL/MG_IntegrationTest/Harness/split_log_paths.py expect-fatal \
+#       lane.json dualblock.xml this-file
+# and put the census it prints here.
+#
+# ---------------------------------------------------------------------------------------------
+# INITIAL CENSUS (f1, measured on the WSL gate tree - see docs/Disaggregated/notes/p5f/
+# f1-report.md for the run):
+# ---------------------------------------------------------------------------------------------
diff --git a/MobileGL/MG_IntegrationTest/Harness/split_log_paths.py b/MobileGL/MG_IntegrationTest/Harness/split_log_paths.py
index 9299a5ae..988bfbea 100644
--- a/MobileGL/MG_IntegrationTest/Harness/split_log_paths.py
+++ b/MobileGL/MG_IntegrationTest/Harness/split_log_paths.py
@@ -113,6 +113,107 @@ def print_marker_table(title, table):
         print(f"    {len(table[pair]):4d}  {pair}")
 
 
+def expect_fatal(document, junit_path, expected_path):
+    """THE DUAL-BLOCK LANE'S CENSUS AND TWO-SIDED RATCHET (P5f f1, P5F §4/§6).
+
+    The markers mode above ratchets the strict lane's ADMITTED pairs; the dual-block lane's
+    currency is the FATAL pair, because under MOBILEGL_IPC_ROLE_SPLIT_STATE=1 every
+    BARRIER-PULLED read aborts unconditionally (CountBarrierPull's dual-block arm) - the red
+    IS the list of fields that still read the other role's memory.
+
+    Three checks, all load-bearing:
+
+      1. EVERY failed entry with a private log carries at least one Fatal{UnmigratedPipeInput}
+         marker in it. A red entry without one is an UNNAMED crash - a segfault or an abort
+         from something else - which is precisely what the rehearsal must make impossible
+         (the O-class fields have no null check; without the Fatal arm they die nameless).
+      2. No ADMITTED marker anywhere: under the dual-block arm the Fatal fires before the
+         admission question is asked, so an Admitted line means the arm did not fire.
+      3. The Fatal pair set equals the committed expected file, BOTH ways - a new pair is a
+         debt somebody now owes, a vanished pair means a later f package retired it and the
+         file must lose the row in the same commit (the strict file's rule, ID-119).
+
+    A green entry needs no marker and asserts nothing; the lane turning fully green with an
+    empty expected file IS the P5f exit state, which this mode then verifies rather than
+    obstructs."""
+    logs = marker_log_paths(document)
+    cases = ET.parse(junit_path).getroot().findall(".//testcase")
+    by_name = {}
+    for case in cases:
+        by_name.setdefault(case.get("name"), []).append(case)
+
+    problems = []
+    fatal, admitted, escalated = {}, {}, {}
+    scanned = 0
+    skipped = 0
+    green = 0
+    red = 0
+    for name, entries in sorted(by_name.items()):
+        if any(c.find("skipped") is not None for c in entries):
+            skipped += 1
+            continue
+        failed = any(c.find("failure") is not None or c.get("status") == "fail"
+                     for c in entries)
+        if not failed:
+            green += 1
+            continue
+        red += 1
+        path = logs.get(name)
+        if path is None:
+            problems.append(f"{name} is RED and declares no MOBILEGL_LOG_FILE_PATH, so the "
+                            "red cannot be shown to be a named Fatal - an unnamed crash is "
+                            "exactly what this lane exists to forbid")
+            continue
+        if not Path(path).is_file():
+            problems.append(f"{name} is RED and its private log {path} does not exist - the "
+                            "process died before it could write one, which is an unnamed crash")
+            continue
+        scanned += 1
+        text = Path(path).read_text(errors="replace")
+        entry_fatal = set()
+        for tag, field, verb, why in MARKER_RE.findall(text):
+            pair = f"{field}@{verb}"
+            if tag == "Fatal":
+                entry_fatal.add(pair)
+                fatal.setdefault(pair, set()).add(name)
+            elif (why or "").strip() == "ADMITTED-ESCALATED":
+                escalated.setdefault(pair, set()).add(name)
+            else:
+                admitted.setdefault(pair, set()).add(name)
+        if not entry_fatal:
+            problems.append(f"{name} is RED but its private log carries no "
+                            "Fatal{UnmigratedPipeInput} marker - an unnamed failure, not a "
+                            "census entry")
+
+    print(f"SplitLogPaths expect-fatal: {len(by_name)} entrie(s) in the run - "
+          f"{green} green, {skipped} skipped, {red} red ({scanned} with a private log scanned)")
+    print_marker_table("Fatal{UnmigratedPipeInput", fatal)
+    if admitted or escalated:
+        problems.append("%d Admitted marker(s) under the dual-block knob - the unconditional "
+                        "Fatal arm in CountBarrierPull did not fire first: %s"
+                        % (len(admitted) + len(escalated),
+                           ", ".join(sorted(set(admitted) | set(escalated)))))
+
+    expected = read_pair_set(expected_path)
+    observed = set(fatal)
+    appeared = sorted(observed - expected)
+    vanished = sorted(expected - observed)
+    if appeared:
+        problems.append("%d Fatal pair(s) the lane has not seen before: %s. Each is a field "
+                        "that reads the other role's memory - add it to %s with the package "
+                        "that retires it, or retire it."
+                        % (len(appeared), ", ".join(appeared), Path(expected_path).name))
+    if vanished:
+        problems.append("%d expected pair(s) no longer appear: %s. Remove them from %s in the "
+                        "commit that retired them - an expected set that keeps rows nothing "
+                        "writes any more is how this lane rots green."
+                        % (len(vanished), ", ".join(vanished), Path(expected_path).name))
+    if problems:
+        raise ValueError("the dual-block lane's fatal ratchet: " + " | ".join(problems))
+    print(f"SplitLogPaths expect-fatal: ratchet OK - {len(observed)} fatal pair(s), "
+          f"zero admitted, every red entry named")
+
+
 def markers(document, selector, allowed_path, expected_path, require_no_fatal):
     """THE TWO-SIDED RATCHET (ID-119).
 
@@ -190,6 +291,9 @@ def main():
         markers(json.loads(Path(sys.argv[2]).read_text()), sys.argv[3], sys.argv[4], sys.argv[5],
                 len(sys.argv) > 6 and sys.argv[6] == "no-fatal")
         return
+    if mode == "expect-fatal":
+        expect_fatal(json.loads(Path(sys.argv[2]).read_text()), sys.argv[3], sys.argv[4])
+        return
     selected = paths(json.loads(Path(sys.argv[2]).read_text()))
     selected = {name: path for name, path in selected.items() if re.search(sys.argv[3], name)}
     # The SAME exclusion run_control hands ctest -E. Without it this helper and ctest disagree
diff --git a/MobileGL/MG_IntegrationTest/Scenarios/DualBlockScenario.cpp b/MobileGL/MG_IntegrationTest/Scenarios/DualBlockScenario.cpp
new file mode 100644
index 00000000..3206232c
--- /dev/null
+++ b/MobileGL/MG_IntegrationTest/Scenarios/DualBlockScenario.cpp
@@ -0,0 +1,93 @@
+// MobileGL - MobileGL/MG_IntegrationTest/Scenarios/DualBlockScenario.cpp
+// Copyright (c) 2026 MobileGL-Dev
+// Licensed under the GNU Lesser General Public License v3.0:
+//   https://www.gnu.org/licenses/gpl-3.0.txt
+//   https://www.gnu.org/licenses/lgpl-3.0.txt
+// SPDX-License-Identifier: LGPL-3.0-only
+// End of Source File Header
+//
+// P5f (f1), P5F-WIRE-COMPLETENESS.md §6: THE DUAL-BLOCK MECHANISM'S NEGATIVE CONTROL, and it is
+// deterministic rather than observational. "Give the two roles one PipeInputs block each"
+// proves nothing if no case can tell one block from two, so this scenario asserts exactly that,
+// from inside the process:
+//
+//   1. MGPipeRoleSplitActive() - the knob is armed AND the transport is real;
+//   2. &MGPipeClientInputs() != &gPipeInputs - two distinct objects;
+//   3. a client verb boundary moves the CLIENT block's serial and leaves the SERVER block's
+//      serial alone - the fill side's writes no longer reach the read side.
+//
+// With MOBILEGL_IPC_ROLE_SPLIT_STATE unset every one of the three fails by name - the block is
+// shared, so (2) is false by construction and (3)'s "server serial does not move" fails because
+// there is no second serial. The lane entry deliberately does NOT pin the knob in its own
+// ENVIRONMENT: the gate's dualblock step exports MOBILEGL_IPC_ROLE_SPLIT_STATE=1 for the green
+// form and scripts/ci/dualblock_negative_control.sh runs the same entry with it off for the red
+// form, and a ctest ENVIRONMENT property would override the process environment in BOTH runs and
+// make the red form unreachable. A bare `ctest -L integration-dualblock-split` without the knob
+// is red, which is the honest answer for a lane whose name promises the split.
+//
+// No draw, clear or readback: under the armed knob every BARRIER-PULLED read is a named Fatal
+// by design (that census is the lane's other half), and this case is the control - it must be
+// GREEN on the armed arm. The lane pins MOBILEGL_IPC_RUN_AHEAD=0 so the fill decision is not a
+// third variable: every verb boundary fills, deterministically.
+
+#include "../Harness/DualBlockPeek.h"
+#include "../Harness/ScenarioFixture.h"
+#include "../Harness/SplitRuntimePeek.h"
+#ifdef GLAPI
+#undef GLAPI
+#endif
+#define GL_GLEXT_PROTOTYPES
+#include <GL/gl.h>
+#include <GL/glcorearb.h>
+#undef GL_GLEXT_PROTOTYPES
+
+namespace MGITest {
+namespace {
+
+class DualBlockScenario : public ScenarioTest {
+protected:
+    void SetUp() override {
+        ScenarioTest::SetUp();
+        if (!Ready()) return;
+        const auto why = SplitRuntimeSkipReason();
+        if (!why.empty()) GTEST_SKIP() << why;
+    }
+};
+
+TEST_F(DualBlockScenario, TheTwoRolesHaveDistinctBlocks) {
+    if (!Ready()) return;
+    DualBlockPeekState before;
+    ASSERT_TRUE(PeekDualBlockState(&before))
+        << "the dual-block peek is compiled out: this entry is only ever registered in a "
+           "disaggregated build, so a false here means the registration and the build disagree";
+    ASSERT_TRUE(before.roleSplitActive)
+        << "dual-block control: MGPipeRoleSplitActive() is false - MOBILEGL_IPC_ROLE_SPLIT_STATE "
+           "is not armed in this process (or the transport is monolith), so the two roles share "
+           "one PipeInputs block and this case is the knob's negative control";
+    EXPECT_TRUE(before.blocksDistinct)
+        << "dual-block control: MGPipeClientInputs() and gPipeInputs are the SAME object with "
+           "the rehearsal armed - the fill side and the read side were never split";
+    EXPECT_NE(before.serverIdentity, nullptr)
+        << "dual-block control: the server block's identity was never set (CONTRACT-P5E §3.2's "
+           "server-owned SetIdentity) - the backend's identity-keyed memo caches read nullptr "
+           "as a hit on their zero-initialised slot";
+
+    // One client verb boundary, driven directly so no draw reaches the server: the CLIENT
+    // block's serial must move and the SERVER block's must not. Under a shared block the second
+    // half is impossible, which is the control's other edge. The glPixelStorei dirties the
+    // pixel-pack state so the boundary's emission step has a record to publish - the fixture's
+    // destructor asserts the wire moved on every armed split case.
+    glPixelStorei(GL_PACK_ALIGNMENT, 4);
+    ASSERT_TRUE(DualBlockDriveClientVerbBoundary());
+    DualBlockPeekState after;
+    ASSERT_TRUE(PeekDualBlockState(&after));
+    EXPECT_GT(after.clientSerial, before.clientSerial)
+        << "dual-block control: a client verb boundary did not move the client block's serial";
+    EXPECT_EQ(after.serverSerial, before.serverSerial)
+        << "dual-block control: a fill-side verb boundary moved the SERVER block's serial "
+           << before.serverSerial << " -> " << after.serverSerial
+           << " - the two roles are still writing one block";
+}
+
+} // namespace
+} // namespace MGITest
diff --git a/MobileGL/MG_Remote/Server/PipeApplier.cpp b/MobileGL/MG_Remote/Server/PipeApplier.cpp
index f9c4fa22..3c9f557f 100644
--- a/MobileGL/MG_Remote/Server/PipeApplier.cpp
+++ b/MobileGL/MG_Remote/Server/PipeApplier.cpp
@@ -1145,6 +1145,12 @@ namespace MobileGL::MG_Remote::Server {
         m_verbs.SetBackend(backend);
         m_decoder = Wire::PipeWireDecoder(control, m_segments, m_replies);
         m_decoder.SetVerbSink(&m_verbs);
+        // P5f (f1), CONTRACT-P5E §3.2: the server block's identity, once per session (the
+        // per-verb stamp refreshes it against the served-context serial). A no-op unless the
+        // dual-block rehearsal is armed; without it the server block's ContextIdentity() stays
+        // nullptr and the backend's identity-keyed memo caches read that as a HIT on their
+        // zero-initialised slot - an unnamed null dereference instead of a named marker.
+        MG_Pipe::MGPipeServerBlockNoteIdentity();
         m_attached = true;
     }
 
diff --git a/MobileGL/MG_Test/Pipe/FieldOwnershipTest.cpp b/MobileGL/MG_Test/Pipe/FieldOwnershipTest.cpp
index 1dc143e2..aabd719d 100644
--- a/MobileGL/MG_Test/Pipe/FieldOwnershipTest.cpp
+++ b/MobileGL/MG_Test/Pipe/FieldOwnershipTest.cpp
@@ -865,6 +865,104 @@ TEST_F(FieldOwnershipTest, ResidualPullsReachThePublishedPerFrameCounter) {
     PS::SetEnabledForTesting(false);
 }
 
+// ================================================================================
+// P5f (f1): THE DUAL-BLOCK REHEARSAL AT THE BLOCK LEVEL (P5F-WIRE-COMPLETENESS.md §4)
+// ================================================================================
+//
+// MOBILEGL_IPC_ROLE_SPLIT_STATE=1 gives the fill side its own PipeInputs block
+// (MGPipeClientInputs()) and leaves gPipeInputs to the server alone. These cases pin the
+// mechanism's three arms without a transport running: the selection folds to the shared block
+// when the knob is off OR the transport is monolith, the two blocks are distinct objects when
+// it is armed, and a BARRIER-PULLED read under it is a NAMED Fatal with no strict knob
+// involved (the fork cases below).
+
+namespace {
+    // Arms the rehearsal by hand - the knob is parsed from the environment once per process,
+    // so a case sets the two globals directly, and the guard puts them back on every exit
+    // path, ASSERT death included.
+    struct RoleSplitArm {
+        RoleSplitArm() {
+            m_previousTransport = MG_Config::Transport;
+            m_previousKnob = MG_Config::Ipc.RoleSplitState;
+        }
+        ~RoleSplitArm() {
+            MG_Config::Transport = m_previousTransport;
+            MG_Config::Ipc.RoleSplitState = m_previousKnob;
+        }
+        void Arm(Bool arm) {
+            MG_Config::Ipc.RoleSplitState = arm;
+            MG_Config::Transport = arm ? MG_Config::TransportMode::InProcess
+                                       : MG_Config::TransportMode::Monolith;
+        }
+        MG_Config::TransportMode m_previousTransport;
+        Bool m_previousKnob;
+    };
+} // namespace
+
+TEST_F(FieldOwnershipTest, RoleSplitOffFoldsTheFillSideOntoTheSharedBlock) {
+    EXPECT_FALSE(MGPipeRoleSplitActive());
+    EXPECT_EQ(&MGPipeClientInputs(), &gPipeInputs);
+}
+
+// The knob alone is not the arm: under monolith transport the two roles are one thread and
+// there is exactly one block, or every read would starve. This is the fold the whole unit
+// lane and the integration-gpu lane of a split build stand on.
+TEST_F(FieldOwnershipTest, RoleSplitUnderMonolithTransportIsStillOneBlock) {
+    RoleSplitArm guard;
+    MG_Config::Ipc.RoleSplitState = true;
+    MG_Config::Transport = MG_Config::TransportMode::Monolith;
+    EXPECT_FALSE(MGPipeRoleSplitActive());
+    EXPECT_EQ(&MGPipeClientInputs(), &gPipeInputs);
+}
+
+TEST_F(FieldOwnershipTest, RoleSplitGivesTheFillSideADistinctBlockTheStampNeverTouches) {
+    RoleSplitArm guard;
+    guard.Arm(true);
+    ASSERT_TRUE(MGPipeRoleSplitActive());
+    ASSERT_NE(&MGPipeClientInputs(), &gPipeInputs);
+
+    const Uint64 serverSerialBefore = gPipeInputs.FilledState().CurrentVerbSerial;
+    const Uint64 clientSerialBefore = MGPipeClientInputs().FilledState().CurrentVerbSerial;
+    MGPipeServerStampVerbBoundary(MGPipeVerb::DrawArrays);
+    EXPECT_EQ(gPipeInputs.FilledState().CurrentVerbSerial, serverSerialBefore + 1);
+    EXPECT_EQ(MGPipeClientInputs().FilledState().CurrentVerbSerial, clientSerialBefore)
+        << "the server's verb stamp reached into the client block";
+    EXPECT_TRUE(gPipeInputs.ServerStampedVerb());
+    EXPECT_FALSE(MGPipeClientInputs().ServerStampedVerb());
+
+    // The fill side's clear is the client block's own: it must not disarm the server's stamp,
+    // which is CONTRACT-P5E §3.2's "the server's stamp is server-private" as a fact rather
+    // than as a comment.
+    MGPipeClientClearVerbBoundary();
+    EXPECT_TRUE(gPipeInputs.ServerStampedVerb())
+        << "the client-role clear withdrew the SERVER's stamp";
+    MGPipeServerClearVerbBoundary();
+    EXPECT_FALSE(gPipeInputs.ServerStampedVerb());
+
+    // And the server block's identity is server-owned under the rehearsal (§3.2's other half):
+    // the stamp set it, and it is non-null - a null identity reads as a hit against
+    // DirectGLES' zero-initialised fb-slot memo cache, which is the unnamed crash this line
+    // exists to preclude.
+    EXPECT_NE(gPipeInputs.ContextIdentity(), nullptr);
+}
+
+// The fill side's writers land in the client block alone. MGPipeLeaveVerb rather than
+// MGPipeValidateForVerb: the validate point's step 3 EMITS, which is session machinery this
+// process does not have, while LeaveVerb's serial bump and verb reset are exactly the fill
+// side's write shape with nothing else in the way.
+TEST_F(FieldOwnershipTest, TheClientVerbLeaveWritesTheClientBlockAlone) {
+    RoleSplitArm guard;
+    guard.Arm(true);
+    ASSERT_NE(&MGPipeClientInputs(), &gPipeInputs);
+    const Uint64 serverSerialBefore = gPipeInputs.FilledState().CurrentVerbSerial;
+    const Uint64 clientSerialBefore = MGPipeClientInputs().FilledState().CurrentVerbSerial;
+    MGPipeLeaveVerb();
+    EXPECT_EQ(MGPipeClientInputs().FilledState().CurrentVerbSerial, clientSerialBefore + 1);
+    EXPECT_EQ(gPipeInputs.FilledState().CurrentVerbSerial, serverSerialBefore)
+        << "a fill-side write reached the server block";
+    EXPECT_EQ(MGPipeClientInputs().CurrentVerb(), MGPipeVerb::kVerbCount);
+}
+
 #if MGTEST_HAVE_FORK
 
 // R-7.3's proof that the instrumentation can go red. An instrumentation that cannot is
@@ -1117,6 +1215,42 @@ TEST_F(FieldOwnershipTest, AnUnmigratedEmulationIsFatalUnderARealTransportAndIne
         << split.Log;
 }
 
+// P5f (f1): THE REHEARSAL'S FATAL ARM, AND THE PROOF IT NEEDS NO OTHER KNOB. Under
+// MOBILEGL_IPC_ROLE_SPLIT_STATE=1 a BARRIER-PULLED read is Fatal{UnmigratedPipeInput} with
+// StrictErrors FALSE - the server block has no residual fill to answer from, so the read has
+// no legal value whatever the strict knob says, and the marker names the knob that armed it.
+// This is also the red-once for the lane's "no unnamed crashes" gate: the read below is an
+// O-class SharedPtr field, which without this arm would hand the backend an empty pointer to
+// dereference.
+TEST_F(FieldOwnershipTest, DualBlockMakesABarrierPulledReadANamedAbortWithoutStrict) {
+    const ChildResult r = RunInChild([] {
+        MG_Config::Transport = MG_Config::TransportMode::InProcess;
+        MG_Config::Ipc.RoleSplitState = true;
+        MGPipeServerStampVerbBoundary(MGPipeVerb::DrawArrays);
+        (void)gPipeInputs.GetBoundVertexArray();
+    });
+    ASSERT_TRUE(DiedOfAbort(r)) << DescribeStatus(r) << "\n" << r.Log;
+    EXPECT_NE(r.Log.find("Fatal{UnmigratedPipeInput, \"GetBoundVertexArray@DrawArrays\"}"),
+              std::string::npos)
+        << r.Log;
+    EXPECT_NE(r.Log.find("BARRIER-PULLED"), std::string::npos) << r.Log;
+    EXPECT_NE(r.Log.find("MOBILEGL_IPC_ROLE_SPLIT_STATE=1"), std::string::npos) << r.Log;
+}
+
+// ... and the same read with the knob set but the transport MONOLITH is the folded arm: one
+// block, the pull counted, no Fatal. The knob is not the mechanism; the knob AND a real
+// transport are.
+TEST_F(FieldOwnershipTest, DualBlockDoesNotArmUnderMonolithTransport) {
+    const ChildResult r = RunInChild([] {
+        MG_Config::Ipc.RoleSplitState = true; // the knob, with no transport behind it
+        MGPipeServerStampVerbBoundary(MGPipeVerb::DrawArrays);
+        (void)gPipeInputs.GetBoundVertexArray();
+        if (MGPipeResidualPullCount() != 1) ::_exit(7);
+    });
+    ASSERT_TRUE(ExitedWith(r, 0)) << DescribeStatus(r) << "\n" << r.Log;
+    EXPECT_EQ(r.Log.find("Fatal{"), std::string::npos) << r.Log;
+}
+
 #endif // MGTEST_HAVE_FORK
 #endif // MOBILEGL_BUILD_DISAGGREGATED
 #endif // MOBILEGL_PIPE_PUSH
diff --git a/scripts/ci/dualblock_negative_control.sh b/scripts/ci/dualblock_negative_control.sh
new file mode 100644
index 00000000..1e7335b3
--- /dev/null
+++ b/scripts/ci/dualblock_negative_control.sh
@@ -0,0 +1,72 @@
+#!/bin/bash
+# P5f (f1)'s NEGATIVE CONTROL (P5F-WIRE-COMPLETENESS.md §6): the dual-block rehearsal must be
+# the thing the dual-block lane measures.
+#
+# The contract is split_negative_controls.sh's, shrunk to one control because there is one
+# knob:
+#
+#   1. THE BASELINE MUST BE GREEN. DirectGLES.Split.DualBlock.* runs with
+#      MOBILEGL_IPC_ROLE_SPLIT_STATE=1 exported (the entry deliberately does NOT pin the knob in
+#      its ctest ENVIRONMENT - a property would override the process environment and make the
+#      red form below unreachable, see its registration block). A skipped baseline is not a
+#      green one (ID-62), so skips are a hard error here, not a pass.
+#   2. THE RED MUST CARRY THE SELECTED CASE'S OWN FAILURE TEXT. With the knob off the two roles
+#      share one block again and DualBlockScenario.TheTwoRolesHaveDistinctBlocks must fail on
+#      its own "dual-block control:" assertion - a timeout, a setup abort or an unrelated
+#      failure proves nothing about the knob (ID-46 finding 8).
+#
+# Usage:  dualblock_negative_control.sh
+#   CTEST          ctest binary                       (default: ctest)
+#   CONTROL_TMPDIR scratch dir for the junit + output  (default: ${RUNNER_TEMP:-/tmp})
+set -u
+
+CTEST="${CTEST:-ctest}"
+CONTROL_TMPDIR="${CONTROL_TMPDIR:-${RUNNER_TEMP:-/tmp}}"
+mkdir -p "${CONTROL_TMPDIR}"
+
+label="integration-dualblock-split"
+tally_tool="$(dirname "$0")/junit_tally.py"
+
+matched=$("${CTEST}" -N -L "${label}" | grep -cE '^ *Test *#[0-9]+:')
+if [ "${matched}" -lt 1 ]; then
+  echo "::error::the dual-block lane selected ${matched} tests; its label no longer matches anything"
+  exit 1
+fi
+
+# ---- the baseline: knob ON, and it must be GREEN ------------------------------------------
+baseline_xml="${CONTROL_TMPDIR}/dualblock-baseline.xml"
+rm -f "${baseline_xml}"
+MOBILEGL_IPC_ROLE_SPLIT_STATE=1 "${CTEST}" -L "${label}" --no-tests=error \
+  --output-on-failure --output-junit "${baseline_xml}"
+baseline_rc=$?
+tally=$(python3 "${tally_tool}" "${baseline_xml}")
+baseline_passed=$(echo "${tally}" | cut -d' ' -f1)
+baseline_failed=$(echo "${tally}" | cut -d' ' -f2)
+baseline_skipped=$(echo "${tally}" | cut -d' ' -f3)
+echo "dual-block baseline (knob on) - passed: ${baseline_passed}, failed: ${baseline_failed}, skipped: ${baseline_skipped}"
+if [ "${baseline_rc}" -ne 0 ] || [ "${baseline_failed}" -gt 0 ]; then
+  echo "::error::the dual-block lane is ALREADY RED with the knob armed, so turning it off below proves nothing. Fix the lane first."
+  exit 1
+fi
+if [ "${baseline_passed}" -lt 1 ] || [ "${baseline_skipped}" -gt 0 ]; then
+  echo "::error::the dual-block baseline skipped or ran nothing - a control whose armed arm never executed is not a control (ID-62)"
+  exit 1
+fi
+
+# ---- the control: knob OFF, and the red must be the case's own assertion -------------------
+control_xml="${CONTROL_TMPDIR}/dualblock-control.xml"
+out="${CONTROL_TMPDIR}/dualblock-control-output.txt"
+rm -f "${control_xml}"
+control_rc=0
+MOBILEGL_IPC_ROLE_SPLIT_STATE=0 "${CTEST}" -L "${label}" --no-tests=error \
+  --output-on-failure --output-junit "${control_xml}" > "${out}" 2>&1 || control_rc=$?
+cat "${out}"
+if [ "${control_rc}" -eq 0 ]; then
+  echo "::error::MOBILEGL_IPC_ROLE_SPLIT_STATE=0 left the dual-block lane GREEN: the two roles shared one block and nothing noticed - the mechanism is not load-bearing (P5F §6)"
+  exit 1
+fi
+if ! grep -q 'dual-block control:' "${out}"; then
+  echo "::error::the knob-off run is red but carries none of the case's own 'dual-block control:' assertions - the failure is something else wearing the control's name"
+  exit 1
+fi
+echo "negative control P5f-f1 (MOBILEGL_IPC_ROLE_SPLIT_STATE=0) turned ${matched} dual-block entries red, and the red is the case's own assertion, as it must"